Impulse 3.0 Users Guide

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AFT ImpulseTM

User’s Guide

AFT Impulse version 3.0Waterhammer Modeling in Piping Systems

Applied Flow Technology

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CAUTION!AFT Impulse is a sophisticated waterhammer and surge transientmodeling program designed for qualified engineers with experience inwaterhammer analysis and should not be used by untrained individuals.AFT Impulse is intended solely as an aide for pipe flow analysisengineers and not as a replacement for other design and analysismethods, including hand calculations and sound engineering judgment.All data generated by AFT Impulse should be independently verifiedwith other engineering methods.

AFT Impulse is designed to be used only by persons who possess a levelof knowledge consistent with that obtained in an undergraduateengineering course in the analysis of pipe system fluid mechanics and isfamiliar with standard industry practice in waterhammer analysis.

AFT Impulse is intended to be used only within the boundaries of itsengineering assumptions. The user should consult the manual for adiscussion of all engineering assumptions made by AFT Impulse.

Information in this document is subject to change without notice. No part of this User’sGuide may be reproduced or transmitted in any form or by any means, electronic ormechanical, for any purpose, without the express written permission of Applied FlowTechnology.

© 2003 Applied Flow Technology Corporation. All rights reserved.

Printed in the United States of America.

"AFT Impulse", "AFT Fathom", "AFT Mercury", "Applied Flow Technology", and theAFT logo are trademarks of Applied Flow Technology Corporation

Chempak is a trademark of Madison Technical Software, Inc.

Microsoft, "Visual Basic", Excel and Windows are trademarks or registered trademarksof Microsoft Corporation.


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Contents

Summary

Summary..................................................................................................iiiDetailed Contents ..................................................................................... vNomenclature....................................................................................... xxix

1. Introduction .................................................................... 1

2. A Walk Through AFT Impulse...................................... 15

3. Fundamental Concepts................................................ 59

4. The Five Primary Windows.......................................... 71

5. Building and Running Models................................... 121

6. Pipe and Junction Specifications Windows ............ 179

7. Customizing AFT Impulse ......................................... 271

8. Steady-State Hydraulic Theory and Loss Models.... 311

9. Theory of Waterhammer & Solution Methodology .. 355

10. Modeling Time and Event Based Transients ......... 413


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11. Reducing Surge Pressures ..................................... 425

12. Special Topics & Troubleshooting Models ............ 429

Appendix A. Keyboard Modifiers and Shortcuts ......... 461

Appendix B. Limitations ................................................ 467

Appendix C. Installation Issues .................................... 469

Appendix D. Chempak Technical Information ............. 477

Appendix E. Obtaining Technical Support................... 479

References...................................................................... 481

Glossary.......................................................................... 483


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Detailed Contents

Summary..................................................................................................iiiDetailed Contents ..................................................................................... vNomenclature....................................................................................... xxix

1. Introduction .................................................................... 1Terminology ............................................................................................. 1Modeling capabilities ............................................................................... 2Interface features ...................................................................................... 3Thermophysical property data .................................................................. 4Who can use AFT Impulse ....................................................................... 4Engineering assumptions in AFT Impulse ............................................... 4Installing AFT Impulse............................................................................. 5

Check hardware and system requirements ......................................... 5Read the README.TXT file............................................................. 5Run the setup program ....................................................................... 6

Getting started with AFT Impulse ............................................................ 6Example models ................................................................................. 7

Using online help...................................................................................... 7Verification models .................................................................................. 7AFT Impulse overview ............................................................................. 7

Input windows.................................................................................... 8Output windows ................................................................................. 8

Converting models and databases from AFT Impulse 1.0 and 2.0........... 9Pipe friction........................................................................................ 9Infinite pipe junctions ........................................................................ 9Transient junction data....................................................................... 9Junction loss models .......................................................................... 9Reservoir transients............................................................................ 9Databases ......................................................................................... 10


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Converting models and databases from AFT Fathom or Mercury ......... 10Combining pipes .............................................................................. 10Junctions with polynomial loss data or interpolated x-y data.......... 10Databases ......................................................................................... 10

What’s new in version 3.0 ...................................................................... 11General ............................................................................................. 11Pipes and Junctions .......................................................................... 11Workspace........................................................................................ 12Output............................................................................................... 12Graph Results ................................................................................... 12Visual Report ................................................................................... 13Miscellaneous................................................................................... 13

2. A Walk Through AFT Impulse...................................... 15Step 1. Start AFT Impulse ...................................................................... 15

The Workspace window ............................................................ 16Step 2. Lay out the model ....................................................................... 17

A. Place the first reservoir ............................................................... 17Objects and ID numbers ............................................................ 17Editing on the Workspace.......................................................... 18

B. Place the first branch ................................................................... 18C. Place surge tank........................................................................... 20D. Place the second reservoir ........................................................... 20E. Add the remaining junctions........................................................ 21F. Draw a pipe between J1 and J2.................................................... 22

Reference positive flow direction.............................................. 23G. Add the remaining pipes ............................................................. 24

Step 3. Set the unit preferences .............................................................. 25Step 4. Complete the first three checklist requirements ......................... 26

A. Specify Steady Solution Control ................................................. 27B. Specify output control ................................................................. 27C. Specify system properties............................................................ 28

Step 5. Define the model components (checklist item #4) ..................... 30


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Object status .............................................................................. 31A. Define Reservoir J1..................................................................... 31

The Inspection feature ............................................................... 33B. Define Branch J2 ......................................................................... 34C. Define Surge Tank J3 .................................................................. 35D. Define Reservoir J4 and Branches J5 and J6 .............................. 36E. Define Valve J7 ........................................................................... 36F. Define Relief Valve J8................................................................. 37G. Define Pipe P1............................................................................. 38

The Pipe Specifications window ............................................... 39H. Define Pipes P2 through P7 ........................................................ 40

Reviewing input in Model Data window................................... 41Step 6. Complete the last two checklist requirements ............................ 43

A. Specify pipe sectioning ............................................................... 43B. Specify transient control.............................................................. 45

Step 7. Run the Solver ............................................................................ 46The Two Solvers .............................................................................. 46The transient output file ................................................................... 47

Step 8. Review the output....................................................................... 48A. Modify the output format ............................................................ 50B. Graph the results.......................................................................... 51C. Animate the Results..................................................................... 53D. View the Visual Report............................................................... 55

Conclusion .............................................................................................. 57

3. Fundamental Concepts................................................ 59Pipes and junctions ................................................................................. 59Convention for flow entering and exiting............................................... 60Features for modeling irrecoverable losses ............................................ 60

Incorporating equivalent length data ......................................... 61Convention for specifying junction base area.................................. 61Specifying losses.............................................................................. 62Modeling valve closures with K factors .......................................... 62


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Specifying frictional losses in pipes ................................................ 62Comment on importance of hydraulic losses during transients ....... 63

Static vs. dynamic losses ........................................................... 63Variable resistance .................................................................... 63

Introduction to wavespeed and waterhammer phenomenon................... 64Description of waterhammer............................................................ 64Instantaneous waterhammer............................................................. 65Wavespeed ....................................................................................... 66Communication time in pipes .......................................................... 66Conceptual example ......................................................................... 66

Phase A: 0 < t < L/a................................................................... 67Phase B: L/a < t < 2L/a.............................................................. 69Phase C: 2L/a < t < 3L/a............................................................ 69Phase D: 3L/a < t < 4L/a............................................................ 69

4. The Five Primary Windows.......................................... 71Overview................................................................................................. 71The Workspace window ......................................................................... 72

The Selection Drawing Tool ............................................................ 73Selecting objects completely or partially inside the box........... 73

The Pipe Drawing Tool.................................................................... 74Pipe handles and segmenting a pipe .......................................... 75

The Zoom Select Tool...................................................................... 77The Annotation Tool........................................................................ 78Junction icons................................................................................... 79Editing features ................................................................................ 79

Selecting groups of objects........................................................ 80Selecting objects in a flow path................................................. 80Aligning objects......................................................................... 81

Creating and Managing Saved Groups ............................................ 81Bookmarks on the Workspace.......................................................... 82Last View ......................................................................................... 82Panning the Workspace.................................................................... 82


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Customizing features........................................................................ 82Workspace Preferences ............................................................. 82Adjusting the Workspace size ................................................... 83Selective display of pipes and junctions.................................... 83Scale/flip workspace.................................................................. 84Renumbering the Workspace objects ........................................ 84Using the Renumber Wizard ..................................................... 85

Background Graphic ........................................................................ 85Specifications windows.................................................................... 86

The Model Data window ........................................................................ 86Display and printing features ........................................................... 88The Model Data Control window .................................................... 88

Pipe data display........................................................................ 88Junction data display ................................................................. 88Show selected pipes and junctions ............................................ 89Database connections ................................................................ 89

The Output window................................................................................ 90Transient output files ....................................................................... 91The Output Control window ............................................................ 92

Pipe and Junction parameters .................................................... 92Pipe and Junction Transient parameters.................................... 94General output ........................................................................... 94Summaries ................................................................................. 94Format and action ...................................................................... 95Show selected pipes/junctions................................................... 96Database connections ................................................................ 97Command buttons...................................................................... 97Output parameter descriptions................................................... 98

Steady flow results ........................................................................... 98Transient flow results for pipes ....................................................... 99

Transient Max/Min results for pipes ....................................... 100Summary Max/Min results ...................................................... 101

Transient flow results for junctions ............................................... 103


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Transient event messages ........................................................ 103Output window updates ................................................................. 104

The Graph Results window .................................................................. 104The Select Graph Data window ..................................................... 105

Transient pipe results............................................................... 106Transient junction results ........................................................ 106Profile Along a Flow Path and EGL, HGL and Elevation Profile106Animating profile results ......................................................... 107Other graph controls ................................................................ 110

The Customize Graph window....................................................... 110The Auxiliary Graph Formatting window...................................... 111

The Visual Report window................................................................... 111The Visual Report Control window............................................... 112

Display parameters .................................................................. 113General display........................................................................ 113Show selected pipes and junctions .......................................... 115Color map ................................................................................ 115Command buttons.................................................................... 118

Visual Report annotations.............................................................. 118Annotations for the Workspace ............................................... 118Creating Visual Report annotations ........................................ 119

The Toolbars......................................................................................... 119

5. Building and Running Models................................... 121Creating objects .................................................................................... 121

Pipes ............................................................................................... 121Junctions......................................................................................... 124

Morphing junctions ................................................................. 124Splitting pipes .......................................................................... 124

Annotations .................................................................................... 125Moving objects ..................................................................................... 125

Keyboard modifiers........................................................................ 126Editing objects ...................................................................................... 126


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Connecting objects ............................................................................... 126Defining objects.................................................................................... 128

Specifying required property data.................................................. 128Highlighting required information ................................................. 129Use status feature ........................................................................... 129Satisfying connectivity requirements............................................. 129

Inspecting objects ................................................................................. 130Using the Checklist............................................................................... 132

Analysis Type................................................................................. 133Specify Steady Solution Control.................................................... 134Specify Output Control .................................................................. 134Specify System Properties.............................................................. 134Define All Pipes and Junctions ...................................................... 134Section Pipes .................................................................................. 135Transient Control ........................................................................... 135

Steady Solution Control........................................................................ 135Output Control ...................................................................................... 136System Properties ................................................................................. 138

System fluid property variation...................................................... 139Density and dynamic viscosity....................................................... 140Bulk modulus of elasticity ............................................................. 140Vapor pressure and cavitation........................................................ 140The fluid databases ........................................................................ 141

The AFT Standard Database ................................................... 141The Chempak database............................................................ 141Accuracy option....................................................................... 142

Viscosity models ............................................................................ 143Non-Newtonian flow in non-pipe elements............................. 144

Atmospheric pressure..................................................................... 144Gravitational acceleration .............................................................. 145Transition Reynolds Numbers ....................................................... 145Editing the AFT Standard fluid database....................................... 146

Finding object definition status ............................................................ 146


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Section Pipes......................................................................................... 147Numbering convention................................................................... 149

Transient Control.................................................................................. 151Start and stop time.......................................................................... 151Save output to file… ...................................................................... 152Use variable pipe resistance........................................................... 152Model transient cavitation.............................................................. 153Stop run if artificial transient detected........................................... 153Include in transient output file ....................................................... 154

Saving pipe output ................................................................... 154Disadvantages to saving all stations ........................................ 154Advantages to saving all stations............................................. 155

Saving junction output ................................................................... 155Estimated file size and run time..................................................... 155

Defining pressures for the system ........................................................ 155Using scenario manager........................................................................ 156

The Scenario Manager window ..................................................... 156Creating, organizing and editing scenarios .................................... 157Modifying individual scenarios ..................................................... 159Passing changes to child scenarios................................................. 160

Re-establishing broken links ................................................... 161Fast Scenario Changes ................................................................... 161

Special modeling features..................................................................... 162Waterhammer Assistant ................................................................. 162Workspace Find ............................................................................. 163Reverse Direction........................................................................... 164Select Special ................................................................................. 164Special Conditions ......................................................................... 165

Special conditions with no transient data ................................ 167Special conditions with transient data ..................................... 167Pump special conditions .......................................................... 167Special conditions only applied to junctions........................... 168

Transient special conditions........................................................... 169


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No reflections – infinite pipe................................................... 169Special Condition Ignore................................................................ 169Merging models ............................................................................. 170

Merging models with multiple scenarios................................. 170Print Preview/Special ..................................................................... 170Transfer Results to Initial Guesses ................................................ 172Batch mode..................................................................................... 172

Running Scenarios in Batch Mode .......................................... 173Print Format ................................................................................... 174Extended Model Check.................................................................. 175Math calculator .............................................................................. 175

Working with the steady and transient solvers..................................... 175Steady-state solver ......................................................................... 175

Recommendations on Steady Solver ....................................... 176Transient solver.............................................................................. 176

Artificial transients .................................................................. 176

6. Pipe and Junction Specifications Windows ............ 179Highlight feature ............................................................................ 180Jump feature ................................................................................... 180

Pipe Specifications window ................................................................. 180Common input parameters ............................................................. 183

Pipe number ............................................................................. 183Pipe name ................................................................................ 183Same As Pipe........................................................................... 184Scenario same as parent........................................................... 184Connected junctions ................................................................ 184

Pipe Model ..................................................................................... 184Pipe length ............................................................................... 185Pipe material ............................................................................ 185Pipe diameter ........................................................................... 186Pipe wall thickness .................................................................. 186Pipe modulus of elasticity ....................................................... 187


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Pipe Poisson ratio .................................................................... 187Friction Model ......................................................................... 187Pipe Support ............................................................................ 189Pipe Wavespeed....................................................................... 189

Additional Losses........................................................................... 189Design Alerts.................................................................................. 191Optional input ................................................................................ 191

Initial Flow Rate Guess ........................................................... 191Workspace display................................................................... 192Design Factor........................................................................... 192Pipe Line Size and Color ......................................................... 192Parallel Pipes ........................................................................... 192Support for Partially Full Pipe................................................. 192Intermediate Pipe Elevations ................................................... 193

Fluid Properties .............................................................................. 194Notes .............................................................................................. 194Status .............................................................................................. 194

Junction Specifications windows ......................................................... 194Format #1: Junctions with one or two connecting pipes............... 197Format #2: Junctions with more than two connecting pipes.......... 198

Parameters common to all junction Specifications windows ............... 199Junction Number ............................................................................ 199Junction Name................................................................................ 199Junction Elevation.......................................................................... 200Database List.................................................................................. 201Same As Junction list ..................................................................... 201

Scenario same as parent........................................................... 201Pipe connectivity............................................................................ 201Jumping to another junction........................................................... 202Transient Data ................................................................................ 202

Repeat Transient...................................................................... 203Transient Special Conditions................................................... 204Time based and event based transients.................................... 204


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Initial Pressure/Head Guess ........................................................... 204Displaying junction names and numbers ....................................... 206Special Conditions ......................................................................... 206Design Factor ................................................................................. 206Changing the icon graphic ............................................................. 207Changing the icon size ................................................................... 207Specifying Base Area..................................................................... 207Notes .............................................................................................. 207Status .............................................................................................. 208

Area Change Specifications window.................................................... 208Assigned Flow Specifications window................................................. 209

Transient Data ................................................................................ 211Special Conditions ......................................................................... 211Special features for steady flow..................................................... 211

Assigned Pressure Specifications window ........................................... 212Transient Data ................................................................................ 214

Branch Specifications window ............................................................. 214Transient Data ................................................................................ 216

Check Valve Specifications window.................................................... 217Specifying losses............................................................................ 218Forward Velocity to Close Valve................................................... 218Delta Pressure to Re-Open............................................................. 218Transient Data ................................................................................ 218

Control Valve Specifications window.................................................. 219Control Valve types ....................................................................... 220

PRV/PSV static vs. stagnation pressure .................................. 220Action if control is lost .................................................................. 221Special Conditions ......................................................................... 221Open Percentage Table .................................................................. 222Transient Data ................................................................................ 222

Failure Transient...................................................................... 222Control Transient..................................................................... 223

Dead End Specifications window......................................................... 223


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Gas Accumulator Specifications window............................................. 224Initial conditions and polytropic process ....................................... 225Maximum and minimum volumes.................................................. 226Interface with pipe system ............................................................. 226Special features for steady flow..................................................... 228Graphing Gas Accumulator data.................................................... 229

General Component Specifications window ........................................ 229Liquid Accumulator Specifications window ........................................ 231

Initial conditions and elastic process ............................................. 231Pump Specifications window ............................................................... 232

Pump Model ................................................................................... 232Submerged pump ..................................................................... 233Do not allow backwards rotation............................................. 234Check valve at pump discharge ............................................... 234

Pump configurations ...................................................................... 234Controlled discharge pressure or flow (variable speed) ................ 234NPSHR and NPSPR (optional in pump configuration) ................. 237Efficiency and power usage (optional in pump configuration)...... 238Transient Data ................................................................................ 238

Pumps with curves................................................................... 238No transient from pump, no back flow.................................... 238No transient from pump, but back flow possible..................... 238Known speed transients, no back flow.................................... 239Pump startup transient with possible reverse flow.................. 240Pump transients with inertia .................................................... 240Trip with inertia and no back flow or reverse speed ............... 240Startup with inertia and no back flow or reverse speed .......... 241Trip with inertia – four quadrant ............................................. 241Startup with inertia - four quadrant, known motor torque/speed243Estimating the pump inertia..................................................... 243Event modeling – including trips followed by restarts or viceversa......................................................................................... 243Controller transient.................................................................. 244


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Pumps as assigned flow........................................................... 246Special Conditions ......................................................................... 246Representing Multiple Pumps With One Junction......................... 247Viscosity corrections...................................................................... 247Graphing pump data ....................................................................... 248

Relief Valve Specifications window .................................................... 248Three relief valve configurations: internal, exit and inline............ 249Specifying losses............................................................................ 249Transient valve cracking ................................................................ 249Transient Data ................................................................................ 250Transients with variable Cv ........................................................... 250

Reservoir Specifications window ......................................................... 250Transient Data ................................................................................ 253

Spray Discharge Specifications window .............................................. 253Loss Model..................................................................................... 254Transient Data ................................................................................ 255Graphing Spray Discharge data ..................................................... 255

Surge Tank Specifications window ...................................................... 255Tank geometry................................................................................ 256Interface with pipe system ............................................................. 257One-Way Surge Tank..................................................................... 258Transient Data ................................................................................ 258Special features for steady flow..................................................... 258

Surge tank as a branch during steady-state.............................. 259Surge tank as a pressurizer during steady-state ....................... 259Surge Tank as a finite size reservoir during steady-state ........ 260Difference between a Reservoir and Surge Tank .................... 260

Model as dipping tube vessel ......................................................... 260Graphing Surge Tank data ............................................................. 261

Tee/Wye Specifications window.......................................................... 261Loss factors .................................................................................... 261

Valve Specifications window ............................................................... 263Transient Data ................................................................................ 264


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Special Conditions ......................................................................... 264Vacuum Breaker Valve Specifications window................................... 264

Graphing Vacuum Breaker Valve data .......................................... 266Volume Balance Specifications window.............................................. 266Global Edit windows ............................................................................ 267

Global Pipe editing......................................................................... 267Global Junction editing .................................................................. 268

Editing specific junction data .................................................. 269

7. Customizing AFT Impulse ......................................... 271Parameter and Unit Preferences ........................................................... 271

Parameter Preferences.................................................................... 272Unit Preferences............................................................................. 273

Setting preferred units ............................................................. 275Description of selected unit ........................................................... 275Database connections..................................................................... 275Command buttons .......................................................................... 276

Workspace Preferences......................................................................... 276Pipes and Junctions ........................................................................ 276

Pipe Line Options .................................................................... 277Special Conditions Options ..................................................... 278Action When Drawing Selection Right To Left...................... 278Modifying icon size ................................................................. 278Action when dragging junctions.............................................. 279Auto increment labels.............................................................. 279Icon source............................................................................... 279

Display Options.............................................................................. 279Workspace grid........................................................................ 279Displaying source/sink indicator ............................................. 280Special condition junction symbol .......................................... 281Pipe direction arrows............................................................... 281Displaying name and/or ID numbers ....................................... 281Setting the default junction label position............................... 281


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Allowable Workspace label movements ................................. 281Junction transient symbols ...................................................... 282

Colors and Fonts ............................................................................ 282Sample Workspace......................................................................... 283Database connections..................................................................... 283Command buttons .......................................................................... 284

Toolbox Preferences............................................................................. 284Customizing Toolbox contents ...................................................... 285Database connections..................................................................... 286

General Preferences.............................................................................. 286Customizing the Output........................................................................ 287Specifying graph style preferences....................................................... 288Specifying Visual Report preferences .................................................. 288Customizing Additional Pipe Losses.................................................... 288Building custom databases ................................................................... 290

Adding custom fluid properties to the Fluid Database .................. 292Adding custom pipe materials to the Pipe Material Database ....... 294

Friction Data Sets .................................................................... 295Pipe Physical Properties .......................................................... 296

Adding custom junction data to the Component Database ............ 296Editing the Component Database................................................... 297

Database Manager ................................................................................ 297Setting available databases............................................................. 298Connecting and disconnecting to a database.................................. 298Editing databases............................................................................ 300Benefits of shared databases .......................................................... 302

Creating an enterprise-wide, network database system........................ 304Overview ........................................................................................ 304Creating database files ................................................................... 305Sharing database files using DATABASE.LIB ............................. 307Connecting to the external shared databases ................................. 309

8. Steady-State Hydraulic Theory and Loss Models.... 311


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The Steady-State Solver ....................................................................... 311Stagnation vs. static pressure boundaries....................................... 315

When to use static pressure ..................................................... 317Static pressure at pressure control valve ................................. 318

Open vs. closed systems ....................................................................... 318Verifying network solutions ................................................................. 321Pressure drop calculation methods ....................................................... 322

Pressure drop calculation methods for Newtonian fluids .............. 323Roughness-based methods ............................................................. 323

Laminar flow ........................................................................... 323Turbulent flow......................................................................... 324Transition flow ........................................................................ 324

Hydraulically smooth ..................................................................... 324Hazen-Williams method................................................................. 324MIT Equation for crude oil ............................................................ 324Miller Turbulent method................................................................ 325Frictionless pipes ........................................................................... 326

Pressure drop calculation methods for non-Newtonian fluids ............. 326Duffy method for pulp and paper stock ......................................... 326Brecht & Heller method for pulp and paper stock ......................... 327Power Law non-Newtonian............................................................ 328Bingham Plastic non-Newtonian.................................................... 328

Non-Newtonian flow through non-pipes ................................. 329Design factors ....................................................................................... 329Steady Solution Control parameters ..................................................... 329

Solution tolerance specification..................................................... 330How tolerances relate to solution accuracy............................. 333

Relaxation ...................................................................................... 333Flow Rate Relaxation .............................................................. 334Pressure Relaxation ................................................................. 335Avoiding false convergence .................................................... 335

Maximum Iterations ....................................................................... 336Matrix Method ............................................................................... 336


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Solution Progress window and iteration history................................... 336Warnings in the solution....................................................................... 337Junction types and the Steady-State Solver.......................................... 338

Pressure specification junctions..................................................... 338Flow specification junctions .......................................................... 339Pressure modifying junctions......................................................... 340Pipe joining junctions .................................................................... 340Special junction.............................................................................. 340

Modeling irrecoverable losses .............................................................. 340Loss models and reference material ..................................................... 341

Design factors ................................................................................ 342Area change.................................................................................... 342Bend ............................................................................................... 343

90 degree bends ....................................................................... 343Non-90 degree bends ............................................................... 344

Valve .............................................................................................. 345Orifice ............................................................................................ 345Tee/Wye ......................................................................................... 346

Diverging Case ........................................................................ 348Converging Case...................................................................... 349

K and CV (valve coefficient) .......................................................... 350K for fire sprinklers........................................................................ 351

Pumps ................................................................................................... 352Chempak thermophysical property database ........................................ 353

Accuracy options ..................................................................... 353AFT Standard fluids ............................................................................. 353

Viscosity......................................................................................... 353

9. Theory of Waterhammer & Solution Methodology .. 355Description of waterhammer ................................................................ 355Instantaneous waterhammer ................................................................. 355Wavespeed............................................................................................ 356Communication time in pipes............................................................... 358


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Method of Characteristics..................................................................... 358Momentum equation ...................................................................... 359Mass continuity equation ............................................................... 359

Transient vapor cavitation .................................................................... 364Pipe sectioning...................................................................................... 365Effects of flow resistance ..................................................................... 367Assigned Pressure theory...................................................................... 367

Assigned Pressure vapor cavitation theory .................................... 368Reservoir theory ................................................................................... 368

Reservoir vapor cavitation theory .................................................. 368Assigned Flow theory........................................................................... 368

Assigned Flow vapor cavitation theory.......................................... 369Dead End theory ................................................................................... 369

Dead End vapor cavitation theory.................................................. 370Branch theory ....................................................................................... 370

Branch vapor cavitation theory ...................................................... 371Tee/Wye theory .................................................................................... 372Valve theory ......................................................................................... 372

Exit valves...................................................................................... 373Valve vapor cavitation theory ........................................................ 374

Infinite Pipe theory ............................................................................... 375Infinite Pipe vapor cavitation theory.............................................. 376

Spray Discharge theory......................................................................... 376Spray Discharge vapor cavitation theory ....................................... 378

Pump theory.......................................................................................... 378Modeling pump as a known speed ................................................. 378

Pumps with higher order curve fits.......................................... 380Backflow through the pump .................................................... 380Flow rates past the pump maximum........................................ 381Submerged pumps ................................................................... 381

Pump with known speed vapor cavitation theory .......................... 381Modeling pump as with unknown speed........................................ 382Trip With Inertia and No Back Flow or Reverse Speed ................ 383


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Startup With Inertia and No Back Flow or Reverse Speed............ 384Trip With Inertia – Four Quadrant................................................. 386Startup With Inertia – Four Quadrant, Known Motor Torque/Speed391Estimating the pump inertia ........................................................... 391

Liquid Accumulator theory .................................................................. 392Liquid Accumulator vapor cavitation theory ................................. 393

Relief Valve theory............................................................................... 393Internal relief valve ........................................................................ 394Exit relief valve.............................................................................. 394Inline Relief Valve ......................................................................... 395Relief Valve vapor cavitation theory ............................................. 395

Internal relief valves ................................................................ 395Exit relief valves...................................................................... 395

Check Valve theory .............................................................................. 395Check Valve vapor cavitation theory............................................. 395

Control Valve theory ............................................................................ 395Flow control valves ........................................................................ 396Pressure control valves................................................................... 396Pressure drop control valves .......................................................... 396

Surge Tank theory................................................................................. 396Surge Tank vapor cavitation theory ............................................... 398

Gas Accumulator theory....................................................................... 398Gas Accumulator vapor cavitation theory...................................... 401

Vacuum Breaker Valve theory ............................................................. 401Sonic inflow ................................................................................... 401Subsonic inflow.............................................................................. 402Sonic outflow ................................................................................. 402Subsonic outflow............................................................................ 402Equation of state............................................................................. 402

Sonic Inflow ............................................................................ 403Subsonic Inflow....................................................................... 404Subsonic Outflow .................................................................... 405Sonic Outflow.......................................................................... 407


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Lumped inertia pipe with orifice theory............................................... 407Interpreting cavitation results ............................................................... 409Instantaneous waterhammer equation exceptions ................................ 410

Cavitation ....................................................................................... 411Pipe size changes............................................................................ 411Multiple pressure surge sources..................................................... 412Line pack due friction .................................................................... 412

10. Modeling Time and Event Based Transients ......... 413Time-based transients ........................................................................... 413Event-based transients .......................................................................... 414

Single event transients ................................................................... 414Dual event transients: cyclic and sequential .................................. 415

Cyclic events............................................................................ 416Sequential events ..................................................................... 417

Junctions with inherent event logic ...................................................... 418Check valve .................................................................................... 418Relief valve .................................................................................... 418Vacuum breaker valve.................................................................... 418

Thought experiment to further clarify event transients ........................ 419Other transient features......................................................................... 419

Absolute vs. relative transient data ................................................ 419Repeat transient.............................................................................. 420Add time offset............................................................................... 420Graphing the transient data ............................................................ 420

Event messages..................................................................................... 420Transient indicators on the Workspace ................................................ 421Transient data in Model Data ............................................................... 422

11. Reducing Surge Pressures ..................................... 425General considerations ......................................................................... 425Larger pipe sizes ................................................................................... 426Flexible hose......................................................................................... 426


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Slower or modified valve closure profiles............................................ 426Parallel valves....................................................................................... 426Relief valves ......................................................................................... 426Vacuum breaker valves for low pressures ............................................ 427Surge tank ............................................................................................. 427Gas accumulator ................................................................................... 427

12. Special Topics & Troubleshooting Models ............ 429The philosophy of computer modeling................................................. 429Troubleshooting steady-state models ................................................... 430

Use the Model Data window.......................................................... 430Poor pump curve fits ...................................................................... 431Use the Output window Sort feature.............................................. 434Tee/Wye junction complexity........................................................ 434Lower the flow rate relaxation....................................................... 434Try absolute tolerance.................................................................... 435Make initial flow rate guesses for pipes ........................................ 435Review the Iteration History .......................................................... 435Change boundary conditions.......................................................... 436Turn off parts of the model ............................................................ 436Break a large model into submodels .............................................. 436

Control valve failure issues in steady-state .......................................... 436Infinite pipe junctions........................................................................... 437

Application example ...................................................................... 438How pressure junctions work in steady-state ....................................... 439The role of pressure junctions in steady-state ...................................... 442

Examples ........................................................................................ 443Sizing pumps with flow control valves .......................................... 448Pressure control valves................................................................... 453Closing parts of a system ............................................................... 456

Effect of pipe sectioning on transient run times................................... 457Artificial transients ............................................................................... 459

Manually detecting artificial transients.......................................... 460


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Appendix A. Keyboard Modifiers and Shortcuts ......... 461Selection Drawing Tool........................................................................ 461

Double-clicking tool ................................................................ 461Control key .............................................................................. 461Shift key................................................................................... 461Alt key ..................................................................................... 462Drag left to right ...................................................................... 462Drag right to left ...................................................................... 462

Pipe Drawing Tool................................................................................ 462Double-clicking tool ................................................................ 462Control key .............................................................................. 462Shift key................................................................................... 462

Zoom Select Tool ................................................................................. 463Shift key................................................................................... 463

Panning the Workspace ........................................................................ 463Dragging junctions from Toolbox ........................................................ 463

Control key .............................................................................. 463Shift key................................................................................... 463

Inspecting pipes and junctions.............................................................. 464Shift key................................................................................... 464Control key .............................................................................. 464

Moving junctions with connected pipes ............................................... 464Control key .............................................................................. 464

Pipe and junction specifications window ............................................. 464F2 function key........................................................................ 464F5 function key........................................................................ 464

Appendix B. Limitations ................................................ 467

Appendix C. Installation Issues .................................... 469Customization files ............................................................................... 469Special file control features .................................................................. 469


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What is the Control.aft File?.......................................................... 469Specifying where Chempak is located ........................................... 470Special network installations (remote tokens) ............................... 470

Network installations............................................................................ 471Problems loading the Graphics Server ................................................. 473

Appendix D. Chempak Technical Information ............. 477

Appendix E. Obtaining Technical Support................... 479AFT's three levels of support................................................................ 479

1. Free Software Support for 180 Days.......................................... 4792. Annual Support, Upgrade and Maintenance (SUM).................. 4793. Waterhammer Modeling Consulting Support ............................ 479

Contacting AFT .................................................................................... 480Telephone support.......................................................................... 480Web site.......................................................................................... 480E-Mail support ............................................................................... 480Mail support ................................................................................... 480

References...................................................................... 481

Glossary.......................................................................... 483


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Nomenclature

a wavespeedA cross-sectional flow area of a pipeBM parameter as defined after Equation 9.20

BP parameter as defined after Equation 9.20

CD discharge coefficientCM parameter as defined after Equation 9.20

CP parameter as defined after Equation 9.20CV valve coefficientCHW Hazen-Williams factorD diameter of a pipeE modulus of elasticity

e pipe wall thickness

f friction factorfT turbulent friction factor (used by Crane, 1988)F error value in mass balanceFB Dimensionless pump torque parameter (see Chapter 9)FW Dimensionless pump head parameter (see Chapter 9)g gravitational constanth head ratio for pumpH headHGL hydraulic gradeline (piezometric head)I moment of inertia of pump and entrained liquidJ Jacobian matrixK loss factorK constant for Power Law fluidKB liquid bulk modulus of elasticityKsprinkler fire sprinkler loss factorL length of a pipe


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m& mass flow raten general constantn constant for Power Law fluidN pump synchronous speed (rpm)P pressure, staticPo pressure, stagnationPw wetted perimeterP powerQ volumetric flow rater radiusr relaxationR, R’ resistanceRc coefficient of rigidityRe Reynolds Numbers pump speedSB parameter as defined in Equation 9.27

SC parameter as defined in Equation 9.28

Sy yield stresst timeT torque on pumpv pump flow ratio to referenceV velocityx distance along pipe centerlinez elevationα, µ, θ angleα pump speed ratioβ diameter ratioβ torque ratioλ method of characteristics multiplier

ν kinematic viscosityε roughness


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ω pump speedρ densityµ dynamic viscosity

Subscripts

f fluidi junction or pipe station at which solution is soughtj junctions with pipes connecting to junction Im motorR rated conditions for a pump, usually BEPo stagnation∞ infinity, far away, ambientnew current time for computation

old most recent time for computation


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C H A P T E R 1

Introduction

Welcome to AFT Impulse™ 3.0 for Windows. AFT Impulse is agraphical platform for modeling waterhammer and surge transients inpipe networks.

AFT Impulse’s advanced Windows graphical interface simplifies thecomplex process of building waterhammer models. The transientsolution engine employs the proven and well understood Method ofCharacteristics to solve the fundamental equations of waterhammer.

An integral part of the interface is AFT Impulse's Workspace window.Here you place icons and draw lines to represent the components of apipe system. You then enter data for the components in the associatedSpecifications windows.

AFT Impulse shows you both input data and analysis results in graphicalform, allowing rapid analysis of the model's validity. Identifying poorassumptions, catching typographical errors, and rerunning models are allaccelerated because of AFT Impulse's graphical environment. Thisreduces the possibility of modeling errors.

Whether your pipe system model will be used to evaluate and improvean existing system or to design a new one, AFT Impulse increases yourproductivity in the modeling process.

Terminology

The term “waterhammer” has been used for more than one-hundredyears, and describes a transient phenomenon in piping systems caused by


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2 AFT Impulse 3.0 User’s Guide


changing pressure or flow boundary conditions, or changing operatingconditions of pipe system equipment. The result is often a pressure surgethat builds much higher than the steady state pressure that preceded it.

There have been and are many terms employed to refer to thewaterhammer phenomenon. The oldest and probably most recognized isthe term waterhammer itself. One drawback of this term is that itsuggests a phenomenon associated only with water. This is misleading,because the surge pressures resulting from waterhammer occur in allliquid piping systems.

To help avoid this confusion, the term “fluidhammer” has beenadvocated and/or used in some of the literature. Fluidhammer does betterconvey the breadth of fluids that can experience waterhammer, but hasnot gained wide acceptance.

Another alternate term for waterhammer is “hydraulic transients”.Again, a suggestion of water-related events are present in this term.Another term, “fluid transients” was therefore put forth. This term alsohas problems. Specifically, a fluid system can have many kinds oftransient behavior, of which waterhammer is only one.

Finally, another commonly heard term is “surge”. This term is morespecific than fluid transients, but is still somewhat ambiguous.

The unfortunate situation, therefore, is that there is no universallyaccepted or recognized term to refer to this important phenomenon.

Arguably, the term waterhammer remains the most recognized term,even with its deficiencies. Therefore, with the qualification that it is notlimited solely to water systems, waterhammer is the preferred term andthe one that will be used most commonly when referring to AFTImpulse.

Modeling capabilities

AFT Impulse can be used to model a wide variety of pipe systemtransients, including:

• Transients in open and closed (recirculating) systems

• Network systems that branch or loop, with no limit on the number ofloops

• Systems with valve transients


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• Systems with pump transients

• Systems with pressure or flow control valve transients

• Systems with transient cavitation and liquid column separation

• Systems with surge suppression devices such as accumulators, surgetanks and vacuum breaker valves

• Systems with variable density and viscosity

• Multiple design cases in a single model file

• Non-Newtonian fluid behavior

AFT Impulse's steady-state solution engine is based on standardtechniques used for many years in industry. The Newton-Raphsonmethod is used to solve the fundamental equations of pipe flow thatgovern mass and momentum balance. Solutions are obtained by iteration,and matrix methods optimized for speed are employed to obtainconvergence.

Once a steady-state solution is obtained, AFT Impulse solves theequations of waterhammer using the Method of Characteristics (MOC).The MOC is the most widely used method for solving waterhammerproblems.

Interface features

AFT Impulse's graphical interface is based on drag-and-drop operations,which make it simple to build a model of a generalized pipe system. Youcontrol the arrangement, and you benefit from the direct visual feedbackregarding the layout of your model.

Data is entered for the components in Specifications windows, which areopened by double-clicking the component of interest. Additional globalediting features simplify making large-scale changes to the model.

AFT Impulse handles both traditional English and SI systems of units.You assign units to all input parameters by choosing from lists. Thishighly flexible approach removes the burden of hand calculated unitconversions.


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Numerous output reports can be generated, all of which arecustomizable. All printed output is of report quality. Effectivelyorganized output data also aids in verifying modeling accuracy.

Thermophysical property data

AFT Impulse derives physical properties from one of three sources. Thefirst is the standard AFT Impulse set of incompressible fluids whichcontains data for about 10 common fluids (called AFT Standard). Theseare the same fluids as were available in previous versions of AFTImpulse.

The second are Unspecified (by Impulse) Fluids, for which data isprovided by the user.

The third source is Chempak™. Chempak has a database ofapproximately 700 fluids, and supports user specified fluid mixtures.AFT Impulse is restricted to non-reacting mixture calculations.

In the remainder of this User’s Guide it will be assumed in generaldiscussion that the user has access to the Chempak Database. If the userdoes not, then the portions describing Chempak will not be applicable.

Who can use AFT Impulse

AFT Impulse assumes that the user possesses a good general knowledgeof engineering pipe system hydraulics and has at least a basicunderstanding of the waterhammer and surge phenomenon. See thecopyright page in this manual for cautionary information.

Engineering assumptions in AFT Impulse

AFT Impulse is based on the following fundamental fluid mechanicsassumptions:

• Liquid flow

• One-dimensional flow

• No chemical reactions


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• Wavespeed remains constant during transients

• Non-condensable gas release is negligible

• Bubbles that form during transient cavitation do not move

AFT Impulse allows you to change the gravitational acceleration of thesystem, a feature useful for aerospace vehicle applications or pipesystems that are designed for extraterrestrial environments.

Installing AFT Impulse

You install AFT Impulse on your computer by running the SETUP.EXEprogram located on the installation CD. The setup program will installAFT Impulse, the Help system, example models, verification models,and AFT Impulse support files to your hard disk.

Note: You must use the setup program to install AFT Impulse. Youcannot simply copy the files from the installation disks to your harddrive.

Before you run the setup program, make sure your computer platformmeets the minimum requirements.

Check hardware and system requirementsTo install and run AFT Impulse, your computer must have the featureslisted in Table 1.1.

Read the README.TXT fileThe README.TXT file is included on the installation CD.README.TXT contains information about installation and supportmaterial that is more up to date than this User’s Guide.


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Table 1.1 Computer hardware and software requirements to useAFT Impulse

Feature Required

Processor Pentium II or higher

Hard disk space 30 MB

Floppy drive Yes

CD-ROM drive Yes

Monitor resolution SVGA (800x600)

RAM 64 MB

MS-Windows Windows 95, 98, NT 4.0,2000, ME, XP or laterversions

Run the setup programWhen you run the setup program, you will be prompted for the directorywhere you want AFT Impulse installed. To run the setup program:

1. Insert the CD into your CD-ROM drive.

2. If Autorun is disabled, from the Start button choose Run. Assumingyour CD-ROM is drive “d”, type d:\ setup, or click the Browsebutton and search for the setup program on your CD-ROM drive inthe Impulse 3.0 directory.

3. Select Impulse 3.0 on the displayed menu.

4. Follow the instructions on the screen.

Getting started with AFT Impulse

If you are not familiar with basic techniques used in Windowsapplications (working with the mouse, selecting menus, using dialogboxes, and so on), first consult your Windows documentation.

To gain an understanding of the powerful features available in AFTImpulse, read Chapters 3-12 of this User’s Guide. For an overview of themodel building process and AFT Impulse’s capabilities, read Chapter 2and follow the instructions for building a simple model.


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Example modelsAn auxiliary help file (called ImpulseExamples.hlp) is installed withAFT Impulse and leads the user through modeling a number of realworld systems. This Help file can be accessed by choosing "ShowExamples" from the help menu. The example models discussed inImpulseExamples.hlp are installed in the EXAMPLES folder.

Using online help

To access AFT Impulse's online help, press the F1 function key or selectHelp from the menu bar. For convenient access, much of the content ofthis User’s Guide is included in the Help system. The Help button ineach dialog window provides context-sensitive help on the features ofthat window.

In the Help system you have the option of searching for information onspecific topics or searching through the hierarchical layout to find moregeneral information.

Verification models

A large number of verification models have been built and compared topublished results from the open literature. These are included in theVerification sub-folder below AFT Impulse. Along with the models youwill find documented comparisons in the Verification help file alsoinstalled in the Verification folder.

AFT Impulse overview

The AFT Impulse window has five subordinate windows that work in anintegrated fashion. You work exclusively from one of these windows atall times. For this reason they are referred to as primary windows.

Of the five primary windows, two are input windows, two are outputwindows, and one displays output and input information. Figure 1.1shows the relationship between the primary windows.


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Graph Results

Visual Report

Workspace Output

Model Data

Figure 1.1 Primary window workflow in AFT Impulse

Input windowsThe two windows that function exclusively as input windows are theWorkspace window and the Model Data window. These two windows,one graphical and the other text-based, work together to process modelinput data with immense flexibility. The tools provided in these twowindows allow you to model a large variety of pipe networks.

The Visual Report window can function in support of both input andoutput data. As an input window, it allows you to see the input datasuperimposed on the pipe system schematic created on the Workspace.

Output windowsThe two windows that function exclusively as output windows are theOutput window and the Graph Results window. The Output window istext-based, while the Graph Results window is graphical. These twowindows offer a powerful and diverse range of features for reviewinganalysis results for modeling errors, gaining a deeper understanding ofthe pipe system's flow behavior, and preparing the results fordocumentation.

As an output window, Visual Report allows you to see the output resultssuperimposed on the pipe system schematic created on the Workspace.

The five primary windows form a tightly integrated, highly efficientsystem for entering, processing, analyzing, and documentingwaterhammer analyses of pipe networks.


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Converting models and databases from AFT Impulse 1.0 and 2.0

AFT Impulse 1.0 and 2.0 models can be opened directly by choosingOpen from the File menu. Once saved, they are saved in the version 3.0format. Importing issues are discussed in the following.

Pipe frictionAFT Impulse 1.0 only worked with constant friction factor models.When these models are imported into version 3.0, the friction model forthe pipe is set to Explicit Friction Factor or Frictionless, depending onwhether the version 1.0 pipe had a zero or non-zero friction factor.

Infinite pipe junctionsAFT Impulse 1.0 supported a junction called an Infinite Pipe junction.This feature is still supported in version 3.0 but in a different way. Whenimporting into version 3.0, Infinite Pipe junctions are converted intoAssigned Pressure junctions, with the Transient Special Condition set toIgnore Reflections – Infinite Pipe. This was necessary because of theneed to first perform a steady-state solution.

Transient junction dataAFT Impulse 1.0 only supported time-based transients, and thus alltransient initiation data is specified as time-based.

Junction loss modelsA loss model found in several junction types in AFT Impulse 1.0 wasK/A^2. This is no longer supported in version 3.0. Such data, whethersteady or transient, is converted to a loss model that is supported such asK factor or Cv for valves.

Reservoir transientsAFT Impulse 1.0 allowed reservoir surface level transients as a deltafrom the steady-state. These will all get converted automatically to theversion 3.0 convention of specifying the actual liquid surface level.


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DatabasesAFT Impulse 1.0 databases can be converted using the AFT Convertutility provided with the software.

Converting models and databases from AFT Fathom or Mercury

AFT Fathom™ 4.0 and 5.0 models, and AFT Mercury™ 5.0 models canbe read directly by AFT Impulse 3.0 by choosing Open from the Filemenu and opening the file. Only the base scenario can be imported. Ifyou want to import scenarios that are not the base, you will need to firstuse the Scenario Manager in AFT Fathom or AFT Mercury to save thescenario into a separate model file.

When imported, all pipe specification data should be maintained,including friction data.

Combining pipesAFT Impulse 3.0 will offer to combine pipes for you when you importAFT Fathom models. This will allow models to run much moreefficiently. When combined, AFT Impulse will create intermediate pipeelevations at the points where junctions are deleted. All pipe resistanceand junction losses will be maintained in the model.

Junctions with polynomial loss data or interpolated x-ydataExcept for General Components, AFT Impulse 3.0 does not supportpolynomial or interpolated x-y loss data. When AFT Fathom or AFTMercury models are imported, all such data is automatically converted toK factors.

DatabasesAFT Fathom and AFT Mercury databases can be converted using theAFT Convert utility.


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What’s new in version 3.0

General• New animation feature for graphing improves interpretation of

results

Pipes and Junctions• Pumps can be tripped or started based on events

• Pumps can trip and then restart and vice versa

• Pump four quadrant data sets increased from three to twenty-one

• Anti-reverse rotation option for pumps

• New pump startup method using motor torque and speed data

• Pump pressure control on suction side implemented

• New Special Condition Ignore feature for vacuum breaker valves,surge tanks, gas accumulators, relief valves and liquid accumulatorssimplifies locating surge suppression equipment

• Reference gas volume for gas accumulators simplifies gas volumeinitialization

• Pump internal check valves can reopen

• Pump can control pressure only when set pressure is exceeded highor low

• One pump can represent multiple pumps in parallel or in series

• Vacuum Breaker Valves with one connecting pipe now allowed

• Vacuum Breaker Valve can model dual vacuum break/air release, oronly vacuum break or air release

• New feature allows modeling of three-stage air valves

• One-Way Surge Tank modeling implemented

• New relief valve pressure loss model with Cv vs. pressure differenceallows improved modeling of passive relief valves

• New feature for inline relief valves


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• Pipes can be modeled with diameter reduction because of scaling

• Pipe size and schedule can be shown on the Workspace

• Improved selection tool for junction icons

• Pipes connected to vacuum breaker valves, exit valves, spraydischarge or assigned pressure junctions can drain or start thetransient partially full

Workspace• Annotation Manager simplifies working with annotations across

multiple scenarios

• Renumber Wizard allows user to renumber pipes and junctions justby clicking on them

• Improved pipe segmenting features

• Workspace pan feature

• Toolbox shortcuts shown as text on Toolbox

• Selecting objects from right to left selects all objects completely orpartially in selection box

Output• Event messages now separated into two lists – one sorted by

junction and the other sorted by time

Graph Results• New animation feature allows user to replay transient with dynamic

graphics

• Multiple profile plots can be cross-plotted using Workspace Groups

• Cross-plotting for multiple junctions implemented

• Pipe Outlet station can be saved into graph sets, and when model isresectioned the outlet station is automatically updated

• User has improved control over graph colors and curve thicknessesand can now specify defaults


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• Graph x-y data can be copied directly to the clipboard

Visual Report• User can now go directly to Visual Report from Solution Progress

window once run is complete

• Visual Report window is automatically updated

• More font flexibility

Miscellaneous• Support for output in languages other than English

• New minimum absolute flowrate threshold for artificial transientsavoids artificial transients on high flowrate systems with somesections initially at zero flowrate

• Improved Global Junction Editing Tool allows pre-selection ofspecific types of junctions

• Brief explanation of Global Pipe Edit and Global Junction Editwindows shown directly on window

• More thorough use of junction and pipe names in all output

• Archival of transient data for large models changed to improvemodel loading time


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C H A P T E R 2

A Walk Through AFT Impulse

This chapter is designed to give you the big picture of AFT Impulse'slayout and structure. Some of the more basic concepts will be used tobuild an eight-pipe, eight-junction model to solve a waterhammerproblem published in the literature (Karney, et al., 1992). This systemhas reservoirs, a surge tank, a relief valve and an exit valve.

This chapter is not intended to replace the more in-depth discussionsgiven in later chapters. To acquire a more detailed understanding of AFTImpulse's menus and functionality before creating a model, skip over thischapter.

A number of other example model discussions are included in a Help filedistributed with AFT Impulse called ImpulseExamples.hlp. It can beopened from the Help menu by choosing "Show Examples".

Step 1. Start AFT Impulse

To start AFT Impulse, click Start on the Windows taskbar, choosePrograms, then AFT Products then AFT Impulse. (This refers to thestandard menu items created by setup. You may have chosen to specify adifferent menu item).

When you start AFT Impulse, the Workspace window is always theactive (large) window. The Workspace window is one of five primarywindows.


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After AFT Impulse loads, you will notice four windows in the lower partof the AFT Impulse window; these represent four of the five primarywindows that are currently minimized (see Figure 2.1). The AFTImpulse window acts as a container for the five primary windows.

Toolbars

Workspace

Toolbox

Minimizedprimary

windows

Status Bar

Figure 2.1 The Workspace window is where the model is built

The Workspace windowThe Workspace window is the primary vehicle for building your model.This window has two main areas: the Toolbox and the Workspace itself.The Toolbox is the bundle of tools on the far left. The Workspace takesup the rest of the window. See Chapter 4 for more detailed informationon the Workspace.

You will build your waterhammer model on the Workspace using theToolbox tools. At the top of the Toolbox are four drawing tools. TheSelection Drawing tool, on the upper left, is useful for selecting groupsof objects on the Workspace for editing or moving. The Pipe Drawing


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tool, on the upper right, is used to draw new pipes on the Workspace.Below these two tools are the Zoom Select tool and the Annotation tool.The Zoom Select tool allows you to draw a box on the Workspace afterwhich AFT Impulse will zoom into that area. The Annotation tool allowsyou to create annotations and auxiliary graphics.

Below the four drawing tools are nineteen icons that represent thedifferent types of junctions available in AFT Impulse. Junctions arecomponents that connect pipes and also influence the pressure or flowbehavior of the pipe system. The nineteen junction icons can be draggedfrom the Toolbox and dropped onto the Workspace.

When you pass your mouse pointer over any of the Toolbox tools, aTool Tip identifies the tool's function.

Step 2. Lay out the model

To lay out the walk-through model, you will place the two reservoirjunctions, three branch junctions, a surge tank junction, a relief valvejunction, and a valve junction on the Workspace. Then you will connectthe junctions with pipes.

A. Place the first reservoir

To start, drag a reservoir junction from the Toolbox and drop it onthe Workspace. Figure 2.2a shows the Workspace with one reservoir.

Objects and ID numbersItems placed on the Workspace are called objects. All objects arederived directly or indirectly from the Toolbox. AFT Impulse uses threetypes of objects: pipes, junctions and annotations.

All pipe and junction objects on the Workspace have an associated IDnumber. For junctions, this number is, by default, placed directly abovethe junction and prefixed with the letter “J”. Pipe ID numbers areprefixed with the letter “P”. You can optionally choose to display eitheror both the ID number and the name of a pipe or junction. You also candrag the ID number/name text to a different location to improvevisibility.


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The Reservoir you have created on the Workspace will take on thedefault ID number of 1. You can change this to any desired numbergreater than zero but less than 10,000.

Figure 2.2a Walk Through Model with one reservoir placed

Editing on the WorkspaceOnce on the Workspace, junction objects can be moved to new locationsand edited with the features on the Edit menu. Cutting, copying, andpasting are all supported. A single level of undo is available for allediting operations.

Note: The relative location of objects in AFT Impulse is not important.Distances and heights are defined through dialog boxes. The relativelocations on the Workspace establish the connectivity of the objects, buthave no bearing on the actual length or elevation relationships.

B. Place the first branch

Next, drag a branch junction from the Toolbox and drop it to thelower right of the reservoir J1. This becomes junction J2 (Figure 2.2b).


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Figure 2.2b Walk Through Model with one reservoir and one branchplaced

Figure 2.2c Walk Through Model after placing the Surge Tank


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C. Place surge tank

Now, drag a surge tank junction from the Toolbox and drop it to theright of the reservoir J1. This junction takes on the number J3 (Figure2.2c).

D. Place the second reservoirThe second reservoir can be created the same way as the first one or itcan be derived from the existing reservoir.

To create a second reservoir from the existing one, select junction J1by clicking it with the mouse. A red outline will surround the junction.Open the Edit menu and choose Duplicate. If you like, you can Undo theDuplicate operation and then Redo it to see how these editing featureswork.

Drag the new reservoir junction icon to the upper right of theWorkspace. This is junction J4 (Figure 2.2d).

Figure 2.2d Walk Through Model after placing the second Reservoir


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E. Add the remaining junctionsNext add the four remaining junctions - two branches, a regular valveand a relief valve (Figure 2.2e).

To add a branch junction, select a Branch from the Toolbox and placeit on the Workspace as shown in Figure 2.2e. The Branch will take onthe default number J5. Similarly, add another branch at junction J6.

Add a valve junction and a relief valve junction by selecting thejunction icons from the Toolbox and placing them on the Workspace asshown in Figure 2.2e. These junctions will become J7 and J8,respectively.

Before continuing, save the work you have done so far. Choose SaveAs from the File menu and enter a file name ("WalkThru", perhaps) andAFT Impulse will append the “.IMP” extension to the file name.

Figure 2.2e Walk Through Model after placing all junctions


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F. Draw a pipe between J1 and J2Now that you have the eight junctions, you need to connect them withpipes.

To create a pipe, click on the Pipe Drawing tool icon. The pointer willchange to a cross-hair when you move it over the Workspace. Draw apipe below the junctions, similar to that shown in Figure 2.2f.

The pipe object on the Workspace has an ID number (P1) shown nearthe center of the pipe.

Figure 2.2f Walk Through Model after drawing first pipe


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To place the pipe between J1 and J2, grab the pipe in the center withthe mouse and drag it so that its left endpoint falls within the J1Reservoir icon and drop it there. Next, grab the right endpoint of thepipe and stretch the pipe, dragging it until the endpoint terminates withinthe J2 Branch icon (see Figure 2.2g).

Figure 2.2g Walk Through Model after placing first pipe

Reference positive flow directionLocated on the pipe is an arrow that indicates the reference positive flowdirection for the pipe. AFT Impulse assigns a flow directioncorresponding to the direction in which the pipe is drawn. You canreverse the reference positive flow direction by choosing ReverseDirection from the Arrange menu or selecting the reverse directionbutton on the Toolbar.

The reference positive flow direction indicates which direction isconsidered positive. If the reference positive direction is the opposite ofthat obtained by the Solver, the output will show the flow rate as anegative number.


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G. Add the remaining pipesA quicker way to add a pipe is to draw it directly between the desiredjunctions.

Activate the pipe drawing tool again and press the mouse button downon the J2 Branch. Stretch the pipe up to the J3 Surge Tank and releasethe mouse button. Draw a third pipe from the J3 Surge Tank to the J4Reservoir. Draw a fourth pipe from the J3 Surge Tank to the J5 Branch.Draw a fifth pipe from the J6 Branch to the J5 Branch. Draw a sixth pipefrom the J2 Branch to the J6 Branch. Draw a seventh pipe from the J6Branch to the J7 Valve.. Finally, draw an eight pipe from the J6 Branchto the J8 Relief Valve. Your model should now look similar to Figure2.2h.

At this point all the objects in the model are graphically connected. Savethe model by selecting Save in the File menu or by clicking on thediskette button on the Toolbar.

Note: It is generally desirable to lock your objects to the Workspaceonce they have been placed. This prevents accidental movement anddisruption of the connections. You can lock all the objects by choosingSelect All from the Edit menu, then selecting Lock Object from theArrange menu. The lock button on the Toolbar will appear depressedindicating it is in an enabled state, and will remain so as long as anyselected object is locked. Alternatively, you can use the grid featureenabled on the Workspace Preferences window and specify that thepipes and junctions snap to grid.


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Figure 2.2h Walk Through Model with all pipes and junctionsplaced

Step 3. Set the unit preferences

Open the Parameter and Units Preferences window from the Optionsmenu. This Walk Through model is in metric units. The default in AFTImpulse is to offer both metric and traditional English units to the user,with English as the default. For consistency, we want to use metric asthe default for this model, and we can specify that here.

Select the Unit Preferences tab and then select the Default Unit Systemas SI. Click the Apply Default Units button. See Figure 2.3. Finally clickOK to close the window.


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Figure 2.3 The Parameter and Unit Preferences window allows youto specify the default unit system

Step 4. Complete the first three checklist requirements

Next, click the checkmark on the Toolbar that runs across the top ofthe AFT Impulse window. This opens the Checklist window (see Figure2.4). The checklist contains six items. Each item needs to be completedbefore AFT Impulse allows you to run the Solver.

The Status Bar at the bottom of the AFT Impulse window also reflectsthe state of each checklist item (see Figure 2.1). Once the checklist iscomplete, the Model Status light in the lower left corner turns from redto green.


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Figure 2.4 The Checklist tracks the model’s status

A. Specify Steady Solution ControlThe first item, Specify Steady Solution Control, is always checked whenyou start AFT Impulse because AFT Impulse assigns default solutioncontrol parameters for the steady-state part of the analysis. In general,you do not need to adjust Steady Solution Control values. If necessary,you can make adjustments by opening the Steady Solution Controlwindow from the Analysis menu.

B. Specify output controlThe second item on the checklist is Specify Output Control. LikeSolution Control, this item is always checked when you start AFTImpulse. Default Output Control parameters and a default title areassigned.

You may want to add a descriptive title for the model. To do this, openthe Output Control window (see Figure 2.5) and enter a title on theGeneral tab. In addition, this window allows you to select the specificoutput parameters you want in your output. You also can choose theunits for the output.

Close the checklist and select Output Control from the Analysis menu.(Figure 2.5 shows the Output Control window). Click the GeneralOutput tab, enter a new title (if you like you can title this “Transient


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With Surge Tank”), then click OK to accept the title and other defaultdata.

Figure 2.5 The Output Control window lets you customize theoutput

C. Specify system propertiesThe third item on the checklist is Specify System Properties. Tocomplete this item, you must open the System Properties window (seeFigure 2.6). This window allows you to specify your fluid properties(density, dynamic viscosity, bulk modulus and optional vapor pressure),viscosity model, gravitational acceleration and atmospheric pressure.

For models with variable fluid properties, the values for density,viscosity and bulk modulus are default fluid properties. You can thenenter different property values, if desired, for any pipe in the PipeSpecifications window.

You can model the fluid properties in one of four ways.

1. Unspecified fluid – This fluid model allows you to directly type inthe density, viscosity, bulk modulus and vapor pressure.


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2. AFT Standard fluid – This fluid model accesses fluid data from theAFT Standard database. These fluid properties are eithertemperature dependent or dependent on the solids concentration.You type in the desired condition (e.g., temperature), click theCalculate Properties button and the required properties arecalculated. Users can add their own fluids to this database. Customfluids are created by opening the Fluid Database window from theAFT Impulse Database menu or by clicking the Edit Fluid Listbutton in the System Properties window.

3. Chempak Fluid – This fluid model allows you to select a single fluidfrom the Chempak database list. These fluid properties are pressureand temperature dependent, although some are temperaturedependent only. Chempak is an optional add-on to AFT Impulse.

4. Chempak Mixture – This fluid model allows you to create a liquidmixture from among the Chempak database fluids. These fluidproperties are pressure and temperature dependent. Upon entering apressure, AFT Impulse will display a temperature range applicableto the liquid. Chempak is an optional add-on to AFT Impulse.

5. Water Data from ASME Steam Tables – As its name implies, thisfluid model obtains water from ASME steam tables. This model ispressure and temperature dependent.


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Figure 2.6 The System Properties window lets you enter physicalproperties of the fluid

Select System Properties from the Analysis menu to open the SystemProperties window. For this example, select the AFT Standard fluidoption, then choose “Water at 1 atm” from the list and click the Add toModel button. The properties for AFT Standard water are given only asa function of temperature. Enter 20° C in the temperature box, click theCalculate Properties button and click OK.

Open the checklist once more or observe the Status Bar and you shouldnow see the third item checked off.

Step 5. Define the model components (checklist item #4)

The fourth item on the checklist, Define All Pipes and Junctions, is notas straightforward to satisfy as the first three. This item encompasses theproper input data and connectivity for all pipes and junctions.


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Object statusEvery pipe and junction has an object status. The object status tells youwhether the object is defined according to AFT Impulse's requirements.To see the status of the objects in your model, click the floodlight on theToolbar (alternatively, you could choose Show Object Status from theView menu). Each time you click the floodlight, Show Object Status istoggled on or off.

When Show Object Status is on, the ID numbers for all undefined pipesand junctions are displayed in red on the Workspace. Objects that arecompletely defined have their ID numbers displayed in black. (Thesecolors are configurable through Workspace Preferences from theOptions menu.)

Because you have not yet defined the pipes and junctions in this sampleproblem, all the objects' ID numbers will change to red when you turn onShow Object Status.

A. Define Reservoir J1

To define the first reservoir, open the J1 Reservoir Specificationswindow by double-clicking on the J1 icon. Enter in a reservoir surfaceelevation of 200 meters. You can assign any unit of length found in theadjacent drop-down list box of units.

Note: You can also open an object's Specifications window by selectingthe object (clicking on it) and then either pressing the Enter key or byclicking the Open Pipe/Jct Window icon on the Toolbar.

Enter surface pressure of 1 atmosphere (atm) and a reservoir depth of50 meters in the table on the Pipe Depth and Loss Coefficient tab.

Note: You can specify preferred units for many parameters (such asmeters for length) in the Parameter & Unit Preferences window.

You can give the component a name, if desired, by entering it in theName field at the top of the window. In Figure 2.7, the name of thisreservoir is Supply Tank A. By default the junction’s name is thejunction type. The name can be displayed on the Workspace, VisualReport or in the Output.


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Most junction types can be entered into a custom database allowing thejunction to be used multiple times or shared between users. To select ajunction from the custom database, choose the desired junction from theDatabase list. The current junction will get the properties from thedatabase component.

The Same As Jct… list will show all the junctions of the same type inthe model. This will copy selected parameters from an existing junctionin the model to the current junction.

The pipe table on the Pipe Depth and Loss Coefficients tab allows you tospecify entrance and exit loss factors for each pipe connected to thereservoir (in this case there is one). You can enter standard losses byselecting the option buttons at the right. The default selection is theCustom option with loss factors specified as zero. To later change theloss factors, click within the pipe table and enter the loss. You can alsospecify a depth for the pipe.

The Optional tab allows you to enter different types of optional data.You can select whether the junction number, name, or both are displayedon the Workspace. Some junction types also allow you to specify aninitial pressure as well as other junction specific data. The junction icongraphic can be changed, as can the size of the icon. Design factors canbe entered for most junctions, which are applied to the pressure losscalculations for the junction in order to give additional safety margin tothe model.

Each junction has a tab for notes, allowing you to enter text describingthe junction or documenting any assumptions.

The highlight feature displays all the required information in theSpecifications window in light blue. The highlight is on by default. Youcan toggle the highlight off and on by double-clicking anywhere in thewindow or by pressing the F2 key. The highlight feature can also beturned on or off by selecting it on the Options menu.

Click OK. If Show Object Status is turned on, you should see the J1 IDnumber turn black again, telling you that J1 is now completely defined.


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Figure 2.7 The Reservoir Specifications window for J1

The Inspection featureYou can check the input parameters for J1 quickly, in read-only fashion,by using the Inspection feature. Position the mouse pointer on J1 andhold down the right mouse button. An information box appears, asshown in Figure 2.8.

Inspecting is a faster way of examining the input in an object thanopening the Specifications window.


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Inspectionwindow

Figure 2.8 Inspecting from the Workspace with right mouse button

B. Define Branch J2

Open the J2 Branch Specifications window. In this window, threeconnecting pipes will be displayed in the pipe table area. Branches areconnector points for up to twenty-five pipes.

Enter an elevation of 100 meters for the J2 Branch junction (anelevation must be defined for all junctions). You can also specify atransient flow source or sink at the junction.

Click the Optional tab and enter an imposed flow rate of -2 m3/sec (thenegative sign means that the flow is out of the junction – a flow sink).

Click OK to accept the input and exit the window.


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C. Define Surge Tank J3

Open the J3 Surge Tank Specifications window and enter an elevationof 150 meters. Enter a tank area of (a constant) 5 square meters. This isthe cross-sectional area of the surge tank, which allows AFT Impulse todetermine how the surface elevation changes with a given amount offlow into the tank. Leave the tank height blank. This is the physicalheight of the tank. If a height is entered, AFT Impulse will not allow theliquid height to exceed the tank height (which results in spillage over thetank edge). Figure 2.9 shows the Surge Tank window.

Click the check box to Model Short Connector Pipe. This allows youto model a short pipe which is lumped together with the surge tank foreach solution. The data for the connector pipe is as follows: Frictionfactor is 0.02, Pipe Diameter is 0.5 meters, Pipe Area is 0.1963 squaremeters (it is cylindrical), Pipe Length is 30 meters, and ElevationChange is 30 meters (i.e., the pipe is vertical because the elevationchange is the same as the length).

You also can model a flow restrictor (e.g., orifice) if one exists.

Figure 2.9 The Surge Tank specifications window for J3


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D. Define Reservoir J4 and Branches J5 and J6

Open the J4 Reservoir Specifications window. Enter a surfaceelevation of 175 meters and a depth of 25 meters (on the Pipe Depth…tab). Leave surface pressure as atmospheric.

Open the J5 Branch Specifications window. Enter an elevation of 100meters and an imposed flow of -1 m3/sec (on the Optional tab). This isan outflow type of branch similar to J2.

Open the J6 Branch Specifications window. Enter an elevation of 50meters. There is no imposed flow at this junction.

E. Define Valve J7

Open the J7 Valve Specifications window. Valve junctions connectwith two pipes if they are internal and one pipe if they are exit valves.This will be an exit valve. Enter an elevation of 25 meters and select theExit Valve option at the lower left. Choose Head and enter an exitpressure of 25 meters, which is equivalent to 1 atm (because exitatmospheric head is equal to elevation).

Choose the loss model as Cv. This valve will initiate the transient forthis system. Until the valve changes position, the entire pipe system is ina steady-state condition. Enter a Cv of 2392.6. This is the steady-statevalue.


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Figure 2.10 Valve Specifications for J7 initiates the transient for thesystem

Select the Transient Data tab. In this transient the valve partiallycloses over a period of 10 seconds, then holds steady at the new positionfor the remainder of the simulation. Enter the following Cv vs. time datain the table (see Figure 2.10):

Time Cv0 2392.6

10 797.4100 797.4

F. Define Relief Valve J8

Open the J8 Relief Valve Specifications window. Relief Valvejunctions can connect with one pipe if they relieve to an external source.If they are internal relief valves they can connect to two pipes. This


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valve is an exit valve, so choose the Valve Type option as Exit. Enter anelevation of 50 meters, a cracking pressure of 1.668 MPa, and an exitpressure of 1 atm.

Choose the loss model as Cv and enter a Cv value of 651.2. Thisrepresents the valve's Cv after it fully opens. The relief valve remainsclosed until the cracking pressure is exceeded, at which point the valveopens to relieve pressure.

Click the Transient Data tab and enter the data below for Cv.

Time Cv0 03 651.2

63 0100 0

The data represents the valve initially closed at time zero. Time zero isthe moment of cracking. The valve then opens and gradually closesagain over a period of 60 seconds.

G. Define Pipe P1The next step is to specify all the pipes. To open the Pipe Specificationswindow, double-click the pipe object on the Workspace.

First open the Pipe Specifications window for Pipe P1 (Figure 2.11).Choose the Pipe Material as Unspecified, choose the User SpecifiedWavespeed option, and select the Friction Model Data Set asUnspecified with the Friction Model as "Explicit Friction Factor". Entera length of 1001.2 meters, a diameter of 1.5 meters, and a friction factorof 0.012.

Note: This example from Karney assumes all friction factors areknown ahead of time. Normally this will not be the case. In most casesyou will access the roughness for the pipe from the pipe materialdatabase supplied with AFT Impulse, or input your own roughness


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value. Then AFT Impulse will calculate the friction factor using standardmethods (see Chapter 8).

The wavespeed is a very important parameter in waterhammer analysis.The wavespeed can be calculated with reasonable accuracy from fluidand pipe data, or it may be available from test data or industrypublications. In this example the wavespeed is known to be 996.3 metersper second. Therefore, the wavespeed can be entered directly. This iswhy we selected the User Specified Wavespeed option. If the wavespeedwere not known (which is typical), then the Calculated Wavespeedoption would be preferred. In this case, data would be required for pipewall thickness, modulus of elasticity, Poisson Ratio, and pipe supportdetails. Data for pipe wall thickness, modulus of elasticity, Poisson Ratioare built into the pipe material databases supplied with AFT Impulse.

The Pipe Specifications windowThe Pipe Specifications window offers control over all important flowsystem parameters related to pipes.

The Inspect feature also works within the Pipe Specifications window.To Inspect a connected junction, position the mouse pointer on theconnected junction's ID number and hold down the right mouse button.This is helpful when you want to quickly check the properties ofconnecting objects. (You can also use this feature in junctionSpecifications windows for checking connected pipe properties.)

By double-clicking the connected junction number, you can jumpdirectly to the junction's Specifications window. Or you can click theJump button to jump to any other part of your model.


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Figure 2.11 Pipe Specifications window for P1

H. Define Pipes P2 through P7

Open the Specifications window for each of the other pipes and enterthe following data (pipe 1 data was just entered so ignore it):

Pipe Length(meters)

Diameter(meters)

FrictionFactor

Wavespeed(meters/sec)

1 1001.2 1.5 0.012 996.32 2000 1 0.013 995.33 2000 0.75 0.014 9954 502.5 0.5 0.015 10005 502.5 0.5 0.015 10006 1001.2 1 0.014 996.37 2000.2 0.75 0.013 995.18 100.12 1 0.014 996.3


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After entering the data for all the pipes, the fourth checklist item shouldbe completed. If it is not, see if the Show Object Status is on. If not,select Show Object Status from the View menu or toolbar. If the fourthchecklist item is not completed at this point, see if any of the pipes orjunctions have their number displayed in red. If so, you did not enter allthe data for that item.

Before continuing the model, save it to file one more time. It is also agood idea to review the input using the Model Data window.

Reviewing input in Model Data windowThe Model Data window is shown in Figure 2.12. To change to thiswindow, you can select it from the Window menu, the Toolbar, pressingCtrl-M, clicking anywhere in the Model Data window if it has beenrestored or, if minimized, clicking on the minimized window at thebottom of the screen and restoring it. The Model Data window gives youa text-based perspective of your model. Selections can be copied to theclipboard and transferred into other Windows programs, or printed outfor review. Figure 2.13 shows an expanded view of the Transient Datatab from Figure 2.12. Here all transient input data for the model isshown.

The Model Data window allows access to all Specifications windows bydouble-clicking the appropriate ID number in the far left column of thetable. You may want to try this right now.


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Customize view

General data

Pipe data

Junction data

Figure 2.12 The Model Data window shows the input data in textform

Figure 2.13 The Transient Data tab in the Model Data Junction dataarea shows all transient data entered


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Step 6. Complete the last two checklist requirements

A. Specify pipe sectioningThe fifth checklist item is Section Pipes. This window cannot be openeduntil sufficient data is entered previously. First, the fluid must beselected (we did this Step 4C when we entered System Properties). Andsecond, all pipes must have a length and wavespeed entered. After thisdata has been entered, the pipes can be sectioned. The Section Pipeswindow helps divide the pipes into computation sections in a mannerwhich is consistent with the Method of Characteristics (MOC).

Select Section Pipes on the Analysis menu to display the Section PipesWindow (Figure 2.14). For this model the controlling pipe is P8. This isthe pipe with the shortest end-to-end communication time (i.e., L/a – thelength divided by the wavespeed). To satisfy the MOC, the followingequation must be applied:

n La ti

i

i

= ∆

where n is the number of sections in pipe i, L is the length, and a is thewavespeed. The ∆t is the time step. Since all pipes in the network mustbe solved together, the same time step must be used for each pipe. Witha given length and wavespeed for each pipe, it can be seen from theabove equation that it is unlikely that the number of required sections, n,for each pipe will be a whole number.

To address this situation, it is helpful to recognize that the wavespeed, a,is the least certain input parameter. It is therefore acceptable to allow upto a 15% uncertainty in wavespeed. By adjusting the wavespeed for eachpipe within this tolerance the sectioning can be made to come out aswhole numbers for each pipe. The Section Pipes window automates thisprocess by searching for sectioning which satisfies the requiredtolerance.

You can specify the tolerance on wavespeed by entering it into the Max.Percentage Error. The minimum and maximum allowable sections in thecontrolling pipe narrows the search space. Also, the PercentageIncrement directs the routine in how fine to search the search space.


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This model has been specified by Karney in such a way that the pipesectioning falls closely along whole numbers.

Figure 2.14 The Section Pipes window automates the sectioningprocess and calculates the time step.

For Min. and Max. Sections in Controlling Pipe enter 1 section foreach. Limit the Max Error to 1% for the search (usually 5 or 10% isfine). Check the box for Sort Sectioning by Minimum Error. Then clickthe Search button.

A list of possible sectioning is displayed. Click the top line with 1.0000sections in the controlling pipe. The resulting time step will be displayedas 0.100492 seconds.

The sectioning and resulting errors in the remaining pipes are displayedin the table near the bottom. Click OK and the fifth checklist item shouldbe completed.


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Note: The error in the Section Pipes window relates only to sectioningroundoff and not to overall model accuracy.

B. Specify transient controlThe sixth and final checklist item is Transient Control. This windowallows you to specify the time at which the transient starts and ends, aswell as how much data to include in the output file.

Figure 2.15 The Transient Control window offers features to specifythe time span for the transient and what output data iswritten

Select Transient Control on the Analysis menu to display the TransientControl window (Figure 2.15). Enter zero for Start Time and 65 for StopTime.


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The Transient Control window lets you enable or disable transientcavitation modeling. It also offers control over how AFT Impulse shouldrespond to artificial transients. Artificial transients are a problem thatcan sometimes occur when steady-state and transient conditions areinconsistent.

Data can be saved for all pipe stations or only selected stations. Theselected stations are shown in the lower half of the window on the PipeStations tab (see Figure 2.15). With the list next to the “Change AllPipes To” button set to “All Stations”, click the “Change All Pipes To”button. This will save all pipe station data for all pipes, which will beuseful later for animation purposes.

At the bottom of the window the projected output file size is shown. Youshould pay attention to this number, as the output file size can grow verylarge. In this case the output file will be 1.0MB. If the output file doesbecome excessively large, you will want to limit the number of timesteps and pipe output written to disk. This is discussed in Chapter 5.

Click OK to accept the current settings. The last checklist item shouldbe completed. The model is ready to be solved.

Step 7. Run the Solver

Choose Run from the Analysis menu or click the arrow icon on thetoolbar. During execution, the Solution Progress window displays(Figure 2.16). You can use this window to pause or cancel the Solver'sactivity.

The Two SolversAFT Impulse has two Solvers. The first is called the Steady-State Solver,which as its name suggests obtains a steady-state solution to the pipenetwork. The second Solver is called the Transient Solver. This solvesthe waterhammer equations.

Before a transient simulation can be initiated, the initial conditions arerequired. These initial conditions are the steady-state solution to thesystem. After the steady-state solution is obtained by the Steady-State


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Solver, AFT Impulse uses the results to automatically initialize theTransient Solver and then run it.

When the solution is complete, click the View Results button todisplay the text-based Output window. The information in the Outputwindow can be reviewed visually on the screen, saved to file, exportedto a spreadsheet-ready format, copied to the clipboard, and printed outon the printer.

Figure 2.16 The Solution Progress window displays the state of thesimulation

The transient output fileWhen the Transient Solver runs, the transient output data is written to afile. This file is given the same name as the model itself with a numberappended to the name, and with an OUT extension appended to the end.For all transient data processing, graphing, etc., the data is extractedfrom this file. The number is appended because AFT Impulse allows the


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user to build different scenarios all within this model. Each scenario willhave its own output file, thus the files need to be distinguishable fromeach other.

The output file will remain on disk until it is erased by the user or theinput model is modified. This means that if you were to close yourmodel right now and then reopen it, you could proceed directly to theOutput window for data review without rerunning your model.

Step 8. Review the output

The Output window (Figure 2.17) is similar in structure to the ModelData window. Three areas are shown, and you can enlarge each area byselecting the options from the Show list box on the Toolbar or from theView menu. The items displayed in the tables are those items you chosein the Output Control window (checklist item # 2).

The Output window allows you to review both the steady-state andtransient results. A summary of the maximum and minimum transientresults for each pipe is given on the Transient Max/Min tab in the pipearea (Figure 2.18). You can also review the solutions for each time step(i.e., a time history) for which data was written to file. These two datasets are located on the Transient Output tab and Transient Max/Min tabin the pipe area of the Output window (see Figures 2.17 and 2.18).


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Customize view

General results

Pipe results

Junction results

Figure 2.17 The Output window displays output results in text form


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Figure 2.18 The Transient Output and Transient Max/Min resultstabs are in the Output window Pipe section area. Theseshow transient results for pipes.

A. Modify the output formatIf you selected the default AFT Impulse Output Control, the Pipes tableof steady-state results (the tab on the far left in the pipe area) will showvolumetric flow rate in the second column with units of m3/sec (cubicmeters per second).


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Select Output Control from the Analysis menu or Toolbar. On theright side of the Pipe tab is the list of selected output parameters. ClickVolumetric Flow Rate and change the units by selecting liter/sec fromthe list at the bottom.

Click OK to display changes to the current results. You should seethe volumetric flow rate results, still in the second column, in units ofliter/sec. Notice the Velocity results in the third column.

Select Output Control from the Analysis menu one more time. Selectthe Pipe tab. The Reorder scroll bar on the far right allows you to reorderparameters in the list.

Select the Velocity parameter and use the Reorder scroll bar to move itup to the top of the parameter list.

Click OK to display the changes to the current results. You will seein the Pipes table that the first column now contains velocity and thethird column contains the volumetric flow rate. The Output Controlwindow allows you to obtain the parameters, units and order you preferin your output. This flexibility will help you work with AFT Impulse inthe way that is most meaningful to you, reducing the possibility oferrors.

Lastly, double-click the column header Velocity in the Outputwindow Pipes Table. This will open a window in which you can changethe units once again if you prefer. These changes are extended to theOutput Control parameter data you have previously set.

B. Graph the results

Change to the Graph Results window by choosing it from theWindows menu, Toolbar, pressing Ctrl-G, clicking anywhere on thewindow if it’s been restored or, if minimized, clicking on the minimizedwindow at the bottom of the Workspace and restoring it. The GraphResults window offers full-featured Windows plot preparation.


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Choose Select Graph Data from the View menu or the Toolbar toopen the Select Graph Data window (see Figure 2.19). From the All PipeStations… list choose pipe 7. Select the Outlet of pipe 7. This is thestation connected to the exit valve. A graph of this station shows thepressure and flow histories at the valve.

Figure 2.19 The Select Graph Data window controls the GraphResults content

Click the Add >> button to add this station to the list on the right.

From Graph Parameters select Hydraulic Gradeline (HGL) andchoose meters as the Y-Axis units. Then click the Show button todisplay the graph (Figure 2.20).

This pressure/head history can be compared to that published in Karney(1992). This is done in the Verifications documentation provided withAFT Impulse.


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Figure 2.20 The Graph Results window offers full-featured plotgeneration.

C. Animate the ResultsThe animation features in AFT Impulse can greatly aid theunderstanding of complex transient behavior. Open the Select GraphData window from the View menu or toolbar again. Select the “ProfileAlong a Flow Path” tab. There select pipes 1, 6 and 7 from the Pipes List(this is the sequence of pipes from the Supply Tank A at J1 to thedischarge valve at J7), select HGL from the Graph Parameters List, andselect Animate From Output File option in the Parameter values area(see Figure 2.21). Select the X-Axis Units and HGL Units as meters.Click the Show button.


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Figure 2.21 The Select Graph Data window allows selection ofanimation parameters.

Additional animation control features will be displayed on the GraphResults window (see Figure 2.22). Push the Play button and AFTImpulse will run through all of the results in pipes 1, 6 and 7 for entiretransient which animates the results. The animation can be paused orstopped, and the time can be reset to any desired time using the timeslider. If the animation is to fast, slow it down using the animation speedcontrol at the right.


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Figure 2.22 The Graph Results window allows animation of outputto be displayed. This is the animation shown at 8.1seconds.

D. View the Visual Report

Change to the Visual Report window by choosing it from the Windowmenu, Toolbar, pressing Ctrl-I, clicking anywhere on the window if it’sbeen restored or, if minimized, clicking on the minimized window at thebottom of the Workspace and restoring it. This window allows you tointegrate your text results with the graphic layout of your pipe network.


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Figure 2.23 The Visual Report Control window selects content forthe Visual Report window

Click the Visual Report Control button on the Toolbar (or Viewmenu) and open the Visual Report Control window, shown in Figure2.23. Default parameters are already selected, but you can modify theseas desired. For now, select Max Pressure Stagnation and Min PressureStagnation in the Pipe Results area. Click the Show button. The VisualReport window graphic is generated (see Figure 2.24).

It is common for the text in the Visual Report window to overlap whenfirst generated. You can change this by selecting smaller fonts or bydragging the text to a new area to increase clarity (this has already beendone in Figure 2.24, as has the selection to show units in a legend). Thiswindow can be printed, copied to the clipboard for import into otherWindows graphics programs, or saved to file.


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Figure 2.24 The Visual Report window integrates results with themodel layout

Conclusion

You have now used AFT Impulse's five primary windows to build asimple model. Review the rest of this User’s Guide for more detailedinformation on each of the windows and functions.


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C H A P T E R 3

Fundamental Concepts

This chapter explains the fundamental concepts and conventions AFTImpulse employs in processing pipe system analysis data, as well as howto interpret the information AFT Impulse processes.

Pipes and junctions

AFT Impulse employs two fundamental pipe system constructs: pipesand junctions.

Pipes are conduits for one-dimensional liquid flow. Junctions areconnector points for pipes and are elements at which flow balances aremade and where transients are initiated. Some junction types can onlyconnect to one pipe; others can connect with up to twenty-five. AFTImpulse provides a total of nineteen junction types.

In addition to balancing flow, junctions also influence the flow orpressure behavior of the system. For example, a reservoir junctionapplies a constant pressure at a location, and the flow there is free toadjust in whatever manner is consistent with the governing equations. Incontrast, an assigned flow junction applies a known flow rate at itslocation, allowing the pressure to adjust to that level dictated by thegoverning equations.

The nineteen junction types allow you to specify special kinds oftransient behavior, irrecoverable pressure losses or fluid behavior.Among the nineteen junctions there is some overlap in features.


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Junctions communicate with each other through the pipes connectingthem. Each pipe must be connected to two junctions. There are noexceptions to this rule.

A pipe differs from a junction in that it has a reference positive flowdirection. To say a pipe has a flow rate of 1 lbm/sec is meaninglessunless the flow direction is specified.

In general, you do not need to specify the actual flow direction in a pipe,because AFT Impulse sorts out the true physical flow directions of thesystem you define. For example, if you specify the reference positiveflow direction from left to right, and the solution is in fact the oppositedirection, then that information will be presented in the output as anegative flow rate. Inlet and outlet conditions correspond to thereference positive flow direction, not the actual flow direction.

However, it is important to specify the proper flow direction for pipeswhich connect to junctions such as pumps or control valves. AFTImpulse uses the flow direction of the pipes to determine in whichdirection to increase or reduce pressure.

Convention for flow entering and exiting

Many of the junction types permit fluid flow to come into or pass out ofthe system in a prescribed manner. The most obvious of these is thereservoir junction. Reservoirs maintain a constant pressure head at anode, and the flow adjusts according to the governing equations. Whilein principle the fluid is stored or removed from the reservoir, AFTImpulse considers the fluid as coming into or going out of the system.The stored reservoir fluid is not part of the system.

The convention for defining inward or outward flow for the system orany AFT Impulse object is that flow in is positive (that is, the objectgains the flow) and flow out is negative (the object loses the flow).

Features for modeling irrecoverable losses

AFT Impulse works almost exclusively with the concept of the lossfactor, also known as K factor or resistance factor. No provision hasbeen made for the equivalent length approach to solving pressure dropproblems. In some Specifications windows, other popular methods for


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specifying loss information are provided where they are consistent withthe loss factor method. For example, you will find Cv for valves.

Incorporating equivalent length dataThe equivalent length method is a popular method for modeling losses infittings and equipment. There are two ways you can incorporateequivalent loss data into AFT Impulse – one way which is acceptableand one which isn't.

First we will discuss the unacceptable way. You can account for lossesby adding to the actual pipe length, thus creating a fictitious length. Forsteady flow calculations this is acceptable. However, in modelingwaterhammer transients the true length is a critical aspect of reflection ofpressure waves. If the length is artificially increased, this will change thedynamic characteristics of the pipe and result in incorrect results. Anyapproximations which result in artificially changing pipe length areunacceptable.

The second way to incorporate equivalent length data is to apply adesign factor to the pipe. The design factor is specified on the PipeSpecifications window Optional tab, and acts as a multiplier on thestandard pipe friction. Thus if the pipe is physically 100 feet long, andhas fittings with 20 feet of equivalent length, a design factor of 1.2 canbe used to account for this. This maintains the true physical length andthus dynamic behavior.

Convention for specifying junction base areaLoss factors are area dependent. When specifying a loss factor at ajunction, the base area is of critical importance. By default AFT Impulseadheres to the following convention: whenever it makes sensephysically, the base area is always the upstream pipe flow area.However, this can be modified by the user. In most junctionSpecifications windows, the base area can be specified as the upstreampipe, downstream pipe, user specified area or diameter.

However, some junction types, especially those that allow three or moreconnecting pipes, specify loss factors based on each connecting pipeflow area, whether upstream or downstream. An example of this is a wyein which each of the connecting pipes has a different diameter. The ideaof specifying a loss factor based on upstream flow area loses meaningbecause there may be more than one upstream pipe. In these cases the


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loss factor is referenced to the base area of the actual downstreampipe(s) where it connects to the junction. The types of junctions that donot always follow the upstream pipe area convention are Tee/Wye,Branch, Reservoir, Assigned Pressure, and Assigned Flow. Otherexceptions are Valves and Relief Valves specified as exit valves.

Specifying lossesLocal losses can be specified in two ways. The most flexible way ofincluding a loss factor in an AFT Impulse model is to associate it with ajunction specifically defined for that type of loss.

Secondly, you can include loss factors with pipes. When you specifyAdditional Losses in the Pipe Specifications window, the loss isassumed to be distributed evenly along the pipe length, much likefriction losses. For this reason a pipe-associated loss factor is referred toas a distributed loss. The losses at junctions, on the other hand, are pointlosses.

When modeling for waterhammer, it is preferred that losses for all staticcomponents be included in pipes as Additional Losses. This reduces thenumber of pipes and model run time.

Modeling valve closures with K factorsIn AFT Impulse 1.0, if you wanted to close a valve when using K factordata, you had to specify extremely large values (10^10 or something likethat). In AFT Impulse 3.0, you can close a valve by specifying the Kfactor as a –1 (negative one).

Specifying frictional losses in pipesAFT Impulse also offers a flexible approach to selecting friction modelsfor pipes. For each pipe in the model you may specify an absoluteroughness, a roughness that is relative to the pipe diameter, a Hazen-Williams factor, a hydraulically smooth roughness, an explicit frictionfactor, or a frictionless pipe. There are some restrictions to wherefrictionless pipes can be located in a model. These are discussed inChapter 8.

There are also two special loss models. The MIT equation is applicablefor petroleum. And the Miller turbulent is applicable for lighthydrocarbons.


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Comment on importance of hydraulic losses duringtransientsGenerally speaking, hydraulic losses (also called system resistance) frompipes, fittings or equipment, has a second order effect on waterhammertransients. The system resistance can be important in determining localsteady-state pressures, and thus initial pressures in the system. It isworthwhile to account for all system resistances in some manner.However, component resistances are frequently lumped together forsimplicity. Lumping resistances together also results in models that runfaster.

Static vs. dynamic lossesPipe system fittings and equipment can be static or dynamic. Wheneverpossible, static elements should be lumped together with other elementsor, better, into pipes. Examples of static elements that should be lumpedare elbows, orifices, filters, and valves whose position remains constantover time. To discourage users from using static elements as junctions intheir model, such junctions are not offered in AFT Impulse. There are nojunction types for elbows, orifices or filters. However, valves can beeither static (i.e., no position changes), or dynamic (i.e., they can open orclose during transient modeling). The user can thus model a static valveif he/she chooses. However, it is better to only use valves for dynamiccases, as this will improve model run time.

Static losses (such as elbows, orifices, filters, and valves) can beincluded as Additional Losses in pipes.

Variable resistanceWhen experiencing waterhammer transients, the flowrate in a pipe willvary for each pipe computing station at an instant in time, and all pipestations will vary over time. As the flowrate varies, so does the Reynoldsnumber. This means that, strictly speaking, the pipe friction factor willchange as the flowrate changes, and will also change from station tostation.

This change in friction factor is typically neglected for two reasons.First, it complicates the computation method. And second, it is usuallynot important.

AFT Impulse allows the user to model either constant friction factor orvariable. The variable friction factor model results in model runtimes


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roughly three times as long. The choice is available in the TransientControl window in the Use Variable Pipe Resistance field.

Introduction to wavespeed and waterhammer phenomenon

Description of waterhammerWaterhammer is a phenomenon that occurs in all liquid piping systemswhenever some event disturbs the steady state. Rapid disturbances cancause large transient pressures that in extreme cases can cause acatastrophic failure of the piping system and/or damage to associatedequipment.

For more slowly occurring disturbances, the transient pressures may besmall or even unnoticeable. In such cases it is sometimes said thatwaterhammer doesn’t exist. What is meant by such statements is thatlarge, potentially destructive waterhammer doesn’t exist. Regardless ofthe magnitude, waterhammer always exists when the liquid velocity in apiping system is changed. In cases of small velocity changeswaterhammer transients are small and are typically and justifiablyignored by engineers.

The disturbances consist of coupled pressure and velocity waves thatpropagate throughout a pipe system. Across these waves a discontinuityin magnitude exists. An example is given later in this section to clarify.

To picture what happens during a waterhammer event, consider a 4000foot (1220 meter) long pipeline flowing water in a steady state condition.Any disturbance in the fluid will propagate near the acoustic speed ofthe liquid in both the upstream and downstream directions.

It turns out that the propagation speed is influenced by the pipingmaterial, wall thickness, diameter and structural support method. Theseother factors combine into a parameter called the wavespeed, which istraditionally referred to as a. The wavespeed is thus analogous to theacoustic speed, but combines the effect of pipe interaction. Although thewavespeed varies from case to case, a nominal value of 4000 feet/sec(1220 m/sec) is reasonable for water.

Now picture a transient event in this pipe, specifically that the valve issuddenly closed at the downstream end (see Wylie, et al., 1993, pp. 7).


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Assume the upstream supply is a reservoir, which acts as a fixedpressure.

When the valve closes this fact is communicated up the pipe to thesupply, telling it to stop supplying fluid because fluid is no longerneeded. This communication takes a finite time, and this time is in factrelated to the wavespeed. In our system it will take one full second forthe disturbance downstream to be communicated to the upstream endbecause the wavespeed is 4000 feet/sec and the pipe is 4000 feet long.During this second, the flow continues because it has not yet receivedthe message to stop.

Once the message reaches the upstream supply, however, it takes anotherfull second for the supply’s response to reach the valve. There is thus afull two second period in which the pipe disturbance communicatesupstream and the response comes back.

Instantaneous waterhammerWith few exceptions, it is safe to calculate the maximum possiblewaterhammer pressure surge by using the instantaneous waterhammerequation. The instantaneous waterhammer equation assumes that thetransient event occurs either instantaneously or rapidly enough such thatit is in effect instantaneous.

In such a case, it can be shown (Wylie, et al., 1993, pp. 4) by use of themass and momentum equation that the pressure transient is given by thefollowing equation:

VaP ∆−=∆ ρ (3.1)

where:

∆P = pressure surge

ρ = density

a = wavespeed

∆V = velocity change

By adding the pressure surge to the existing static pressure, one canobtain the maximum theoretical pressure in the pipe. However, in somecases the pressure can exceed the instantaneous prediction. These arediscussed in Chapter 9.


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Here’s an example. Assume flow in a pipe filled with water isinstantaneously stopped by a valve closure. Assume the density is 62lbm/ft3, the wavespeed is 4000 feet per second, and the initial velocity is10 feet per second. Also assume that before the transient the staticpressure is 50 psig.

psig 58550535

psig 50ft-lbm 2.23

slbf*in 144

ft 1*sft10*

sft4000*

ftlbm62

max

max

2

2

2

3max

=+=

+−

=

PP

P

WavespeedWhen a transient event is initiated in a pipe system, the remainder of thesystem must adjust to the new conditions. In order to adjust, theexistence of the event must be communicated to the rest of the system.This communication takes place at the wavespeed of the fluid. Thewavespeed is somewhat analogous to the sonic speed of the liquid.However, the wavespeed is affected by the pipe structure.

See Chapter 9 for a mathematical description.

Communication time in pipesTransient events communicate their effect through the pipe at thewavespeed of the pipe and fluid. When transient events occur, they arecommunicated upstream (or downstream) so the system can adjust to thenew conditions. No response to the changing conditions can be obtaineduntil the communication wave travels to the other end of the pipe andback. This communication time is thus given by the following:

aLt 2=∆ (3.2)

where L is the pipe length and a is the wavespeed.

Conceptual exampleLet's consider a simple example of a valve closure. A frictionless pipe isconnected to an upstream reservoir and a downstream valve. Fluid is


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flowing at a steady state. At time zero the valve is instantaneouslyclosed. We will look at four phases of the transient response.

Phase A: 0 < t < L/aThe first phase occurs after the valve has closed but before the resultingwave has traveled the pipe length and reached the upstream reservoir(see Figure 3.1). The wave, moving at speed a, travels from the valve tothe reservoir (see Figure 3.1, Phase A – the wave initially travels fromright to left). The fluid upstream of the wave front (on the left) does notyet know of the closed valve, and thus remains at the steady-statepressure and velocity. Thus before the wave reaches the inlet, fluidcontinues to flow into the pipe at velocity Vsteady even though the valve isclosed.

The portion of the pipe to the right, where the wave has already passed,knows of the closed valve and thus the motion has stopped (the velocityis zero). The kinetic energy of the fluid has been converted into potentialenergy by increasing the fluid pressure and expanding the pipecircumference. Figure 3.1, Phase A, shows the pipe expanded to the right(exaggerated for effect), but at the original diameter on the left. It furthershows a plot of the pressure and velocity in the pipe.


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a

V = Vsteady

V = 0

P∆PinstantaneousPsteady

xV

Vsteady

x

a

V = Vsteady

V = 0

P∆PinstantaneousPsteady

xV

-Vsteadyx

a

V = Vsteady

V = 0

V

-Vsteadyx

x

P

∆Pinstantaneous

Psteady

VVsteady

x

a

V = Vsteady

V = 0

x

P

∆Pinstantaneous

Psteady

Phase A: 0 < t < L / a Phase B: L / a < t < 2L / a

Phase D: 3L / a < t < 4L / aPhase C: 2L / a < t < 3L / a

Figure 3.1 Pressure and velocity in a pipe after instantaneousvalve closure. Pipe diameter changes with pressureexaggerated for effect


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Since the valve was closed instantaneously, the magnitude of thepressure wave can be obtained from the instantaneous waterhammerequation (Equation 3.1).

Phase B: L/a < t < 2L/aThe second phase occurs after the wave has reflected from the inletreservoir but before it has come back to the valve. The wave, stillmoving at speed a, travels from the reservoir to the valve (see Figure 3.1,Phase B). The fluid upstream of the wave front (on the left) now isaware of the closed valve. Further, the high pressure that built up duringPhase A means there is excess fluid in the pipe. During Phase B thisfluid is expelled to the only place that can receive it – the upstreamreservoir. Thus backflow occurs as the pressure returns to the steady-state level.

The fluid downstream of the wave front (on the right) does not yet knowthe reservoir is available to relieve the pressure, so it remains at the highpressure and zero velocity.

Phase C: 2L/a < t < 3L/aThe third phase occurs after the wave has reflected from the inletreservoir, traveled back to the valve and reflected from the valve, and isnow moving back towards the reservoir (see Figure 3.1, Phase C). Thewave is still moving at speed a.

Due to the pipe's attempt to establish equilibrium by flowing backtowards the reservoir in Phase B to relieve the high pressure, the systemhas overcompensated. During Phase C the pressure drops below thesteady value. The pressure drop is the same magnitude as the pressurerise in Phase A, but opposite in sign.

As the wave passes each location during Phase C, the fluid motion stopsand the pressure is reduced.

Phase D: 3L/a < t < 4L/aThe fourth phase occurs after the wave has reflected from the inletreservoir for the second time and is moving back towards the valve (seeFigure 3.1, Phase D). The fluid upstream of the wave front (on the left)has returned to the original steady values of pressure and velocity. Thefluid downstream of the wave front (on the right) is still at a reducedpressure.


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The fourth phase occurs after the wave has reflected from the inletreservoir for the second time , traveled back to the valve and reflectedfrom the valve, and is now moving back towards the reservoir (seeFigure 3.1, Phase C). The wave is still moving at speed a.

Once the wave reaches the valve for the second time, the four phases arerepeated. If the pipe is frictionless, the wave motion will continueindefinitely. If friction effects are included the wave will decay overtime and motion would eventually stop altogether.


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C H A P T E R 4

The Five Primary Windows

This chapter explores the features found in AFT Impulse's five primarywindows and explains the role each plays in the analysis process.

Overview

AFT Impulse has five primary windows. The primary windows aresubordinate to the AFT Impulse window and can be maximized orminimized within the boundaries of the AFT Impulse window. Theprimary windows are permanent in that you work in one of thesewindows at all times.

The primary windows work together to provide tools for entering modelinput, analyzing results for accuracy, and preparing results fordocumentation. Figure 4.1 summarizes the workflow using the primarywindows.

• The Workspace window allows you to build the model visually andsee the model layout.

• The Model Data window is the text-based complement to theWorkspace window. The Model Data window shows input data intext form. This window works hand-in-hand with the Workspacewindow to provide exceptional flexibility in manipulating data.

• The Output window displays the results of the analysis in text formand lets you produce attractive, effectively organized printed output.

• The Visual Report window merges the output data with the pipesystem layout from the Workspace to present a unique perspective of


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the results. This window also allows customized layout of theinformation for documentation and presentation purposes. Analternate use for the Visual Report window is to show textual inputdata combined with the input schematic.

• The Graph Results window provides full-featured Windows plottingcapability. Here results can be viewed in a variety of ways forevaluating system performance or identifying important trends. TheGraph Results window also allows graph customization.

You can change between the five primary windows by selecting themfrom the Window menu, the Toolbar, or by clicking the appropriateicons within the AFT Impulse window.

Graph Results

Visual Report

Workspace Output

Model Data

Figure 4.1 Primary window workflow in AFT Impulse

The Workspace window

The Workspace window is the sole area where you specify the modellayout and connectivity. The Workspace window has two separatefunctional areas: the Toolbox on the far left and the Workspace itselfencompassing the rest of the screen (see Figure 4.2).

At the top of the Toolbox (Figure 4.3), four tool icons are slightly offsetfrom the rest of the junction icons. These are the Selection DrawingTool, Pipe Drawing Tool, Zoom Select Tool, and the Annotation Tool.


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The Selection Drawing ToolThe drawing tool in the upper left corner of the Toolbox is the SelectionDrawing Tool (see Figure 4.3). This tool is one means of selectinggroups of objects on the Workspace for group operations such asdragging or duplication.

To select a group of objects, click the Selection Drawing Tool and drawa box around the desired objects on the Workspace.

If you Double-click the Selection Drawing Tool it remains active untilyou click it again a single time. This allows you to make a series ofselections.

Three keyboard modifiers extend the Selection Drawing Tool'sfunctionality. If you hold down the CTRL key when completing theselection drawing (just before releasing the mouse button), the SelectionDrawing Tool remains active. The CTRL key thus performs the samefunction as double-clicking the tool.

If you first double-click the Selection Drawing Tool or hold down theCTRL key, you can also hold down the SHIFT key while drawing theselection box and the enclosed pipes and junctions will be deselected.

Finally, if you first double-click the Selection Drawing Tool or holddown the CTRL key, you can also hold down the ALT key whiledrawing the selection box and the enclosed pipes and junctions willtoggle. That is, any pipes or junctions that were selected will becomedeselected, and vice versa.

Selecting objects completely or partially inside the boxIf you select the objects by dragging left to right, you will select allobjects completely inside the box.

If you select the objects by dragging right to left, you will select allobjects completely or partially inside the box (if this feature is enabledin the Workspace Preferences window).


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Toolbars

Toolbox

Workspace

Status bar

Figure 4.2 The Workspace window is where you assemble yourmodel

The Pipe Drawing ToolThe tool in the upper right corner of the Toolbox is the Pipe DrawingTool. With this tool you can draw new pipes on the Workspace, startingand ending at any desired location.

To draw a pipe, click the Pipe Drawing Tool. The mouse pointer willchange to a crosshair when you move it over the Workspace. Positionthe cursor at the pipe’s starting point, hold down the left mouse buttonwhile dragging the cursor to the pipe endpoint, then release the mousebutton.

Two keyboard modifiers extend the Pipe Drawing Tool's functionality. Ifyou hold down the SHIFT key down while drawing on the Workspace,you can draw pipes that are perfectly horizontal or vertical. This mayimprove the aesthetics of your model.

If you hold down the CTRL key when completing the pipe drawing (justbefore releasing the mouse button), the Pipe Drawing Tool remains


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active, and you can draw a series of pipes without returning to theToolbox each time.

Selection Drawing Tool

Branch

Assigned Flow

Area Change

Tee/Wye

Control Valve

Dead End

Surge Tank

Vacuum Breaker Valve

Pump

General Component

Zoom Select Tool

Pipe Drawing Tool

Reservoir

Assigned Pressure

Relief Valve

Valve

Check Valve

Gas Accumulator

Liquid Accumulator

Spray Discharge

Volume Balance

Annotation Tool

Figure 4.3 Tools on the Workspace window Toolbox

If you Double-click the Pipe Drawing Tool it remains active until youclick it again a single time. This allows you to draw a series of pipeswithout returning to the Toolbox each time.

Pipe handles and segmenting a pipeWhen a single pipe is selected, two black squares (or more if the pipe issegmented), called handles, appear. When the cursor is placed over apipe handle it changes to a crosshair and the pipe end can be moved orstretched by dragging with the left mouse button.

A pipe can be segmented or “bent” in up to ten segments (See Figure4.4). This does not affect the model or the results, only the visual


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representation. It does not add an “elbow” to the model or the resultingpressure drop effects of an elbow.

Figure 4.4 Pipe with five segments

This feature is useful in graphically bending the pipe around another partof the model or graphically showing that there are embedded elbows asAdditional Losses.

You can add a segment to a pipe in two ways. The easiest way to add asingle segment is to select Pipe Segments and Add from the Arrangemenu. If there are no segments, then a single segment can also be addedby selecting the Add/Remove Segment from the Toolbar. If the pipealready has one or more segments, this toolbar selection will remove allsegments.

A more flexible way of adding or removing segments is to use the PipeSegment Manager. To open the Pipe Segment Manager, choose PipeSegments and Manager from the Arrange menu. A window will display(Figure 4.5) where you can add, remove and merge segments. A“handle” will appear in the middle of the pipe for each new segment. Asthe cursor passes over the handle the cursor will change to a crosshair.Press and hold down the left mouse button and drag the middle of the


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pipe to the desired location. By holding the SHIFT key the pipe segmentwill move only at right angles.

Figure 4.5 The Pipe Segments window allows you to create, mergeand remove pipe segments.

The Zoom Select ToolThe second tool down from the upper left corner of the Toolbox is theZoom Select Tool. With this tool you can draw a box that will cause theWorkspace to zoom to that box. The maximum zoom size is 200%.

One keyboard modifier alters the Zoom Select Tool's behavior. If youhold down the SHIFT key while selecting the zoom area, the zoom statewill always be 100%.


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The Annotation ToolThe second tool down from the upper right corner of the Toolbox is theAnnotation Tool. With this tool you can draw a box that will contain anAnnotation displayed on the Workspace.

As shown in Figure 4.6a, the Annotation Tool allows you to create a textmessage and, by clicking on the appropriate tab, show an outline aroundthe message and draw lines with pointers to certain areas of the model.Different style, color and font options are provided.

Figure 4.6a The Workspace Annotation tool dialog box.

Annotations can be “attached” to the Workspace, a pipe or a junction.When attached to the Workspace, the annotation always remains in thesame location until you move it. When attached to a pipe or junction, theannotation will move when the pipe or junction is moved. The ScenarioOptions tab allows you to specify which scenarios in which theannotation appears.

Annotations print out with the model. Figure 4.6b shows the Workspacewith an Annotation displayed.

The Annotation Manager, available on the View menu, allows you tospecify whether an annotation will be shown or hidden, and in whichscenarios it should appear.


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Figure 4.6b Annotation added to the workspace

Junction iconsBelow the four drawing tools are nineteen junction icons. The junctionsallow you to model a large variety of pipe system components importantin waterhammer. To add a junction to a model, drag the desired iconfrom the Toolbox and drop it anywhere on the Workspace.

As you move the mouse pointer over the Toolbox, a Tool Tip identifiesthe type of junction under the mouse pointer.

The types and order of junction icons on the Toolbox can be modifiedthrough the Toolbox Preferences window. For more details on how tocustomize the Toolbox, refer to Chapter 7.

Editing featuresJunction and pipe objects on the Workspace can be manipulated asindividual items or as groups. This applies to cutting, copying, pasting,deleting, duplicating, dragging, locking, and undoing. Pipe objects on the


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Workspace can be stretched by selecting the pipe at one end anddragging it to a new location.

Selecting groups of objectsA group of objects can be selected for manipulation in five ways:

1. By clicking the Selection Drawing Tool and drawing a box aroundthe desired objects

2. By holding down the SHIFT key while clicking each object in thegroup

3. By selecting a pipe or junction, then choosing Select Flow Path fromthe Edit menu

4. By choosing Select All from the Edit menu

5. By choosing Select Special from the Edit menu

When the SHIFT key is depressed, you can click repeatedly on the sameobject to select and deselect it. Selected junctions are outlined in red;selected pipes are also shown in red. The selection color is configurablein the Workspace Preferences.

The Copy and Paste operations allow you to duplicate a group on theWorkspace. When you copy an item, it is placed on the Windowsclipboard; from there you can paste it into another Windows graphicsprogram as a picture.

New objects added to the Workspace from the Toolbox are undefined(refer to Chapter 5 for details on defining objects). Objects derivedthrough duplication or copying retain all relevant Specifications windowinformation. If a group of fully defined objects is duplicated or copied,AFT Impulse retains the object status and connectivity within the newgroup.

Selecting objects in a flow pathThe pipes and/or junctions in a flow path may all be easily selectedusing the Select Flow Path item on the Edit menu, Toolbar or the Popupmenu (shown when the right mouse button is pressed while the cursor isover an empty area on the Workspace). A flow path is a sequence ofpipes and junctions where there is no branching or flow sources or sinks.The endpoint junctions can be included in the flow path if the AllObjects Including Endpoints option is chosen.


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If a junction is selected which has multiple flow paths attached to it(e.g., a branch with 3 or more pipes) then objects in each of theconnected flow paths will be selected. This feature is very useful induplicating or deleting sequences of pipes and junctions. It can also beused to quickly define groups for graphing or for selected Output orModel Data listings.

Aligning objectsTools to align components in your model are located on the Arrangemenu. Also on this menu is the Adjust Pipes Automatically option,which causes all pipe endpoints to adjust to the center of connectedjunction icons.

Creating and Managing Saved GroupsGroups of pipes and junctions can be named for later recall. This is doneusing the Group Tools on the Edit menu. The easiest way to create anamed Group is as follows:

1. First select on the Workspace the pipes and junctions you want inthe group

2. Choose Groups -> Create from the Edit menu or toolbar.

3. You will be prompted for a name for the group. Enter a meaningfulname and click OK.

4. The Group Manager then opens. The selection of pipes andjunctions in the Group will match those currently selected on theWorkspace. If you want to modify the selections once more, you cando it here.

You can include Subgroups inside of your Groups. Subgroups areexisting Groups. If, for example, you included a Subgroup in yourGroup, and later change the contents of the Subgroup, the Group whichcontains the Subgroup would also be changed.

After you have created a Group, you can then quickly reselect thatGroup of pipes or Junctions on the Workspace by choosing Group ->Select from the Edit menu or Toolbar and choosing the name of theGroup.


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The Group Manager can be opened by choosing Group -> Manager fromthe Edit menu or Toolbar. The Group Manager allows you to modify,create, delete or rename Groups.

Another place Groups are available is in the Select Special window.

Bookmarks on the WorkspaceBookmarks can be added on the Workspace in relation to individualpipes or junctions. You can quickly navigate around the Workspace bymoving from Bookmark to Bookmark.

Use the Bookmark tools on the View menu to set Bookmarks and todisplay different bookmarked pipes or junctions.

Last ViewThe last view of the Workspace is always saved in memory and can bedisplayed by selecting Last View from the View Menu or Toolbar.

Panning the WorkspaceIf the Workspace has been enlarged beyond the default one screen size(Workspace scrollbars will be visible), then the Workspace can bepanned. To do this, hold down the CTRL key, press the mouse down onthe Workspace, and then drag in the direction you want to move theWorkspace. The cursor will change to any open hand while you arepanning.

Customizing features

Workspace PreferencesThe Workspace Preferences on the Options menu lets you customize theappearance and behavior of the Workspace. New defaults can be savedso AFT Impulse will always appear and behave according to yourpreferences.

See Chapter 7 for more information on Workspace Preferences.


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Adjusting the Workspace sizeShould the model expand beyond a single screen (as it often will), theWorkspace size can be expanded by choosing Workspace Size from theView menu. By clicking the boxes in this dialog window you can expandor contract the Workspace area. Each box represents the area of theoriginal Workspace screen. When the Workspace is enlarged, scroll barsautomatically appear to simplify navigation of the enlarged Workspace.The View menu also allows you to zoom in and out at ratios of 25%,50%, and 75% of the normal size. The Custom Zoom feature allows youto specify the exact zoom value you desire.

Selective display of pipes and junctionsPipes and junctions can be shown or not shown on the Workspace bychoosing Display Pipes and Junctions on the View Menu (see Figure4.7).

Figure 4.7 Pipes and junctions can be selectively displayed on theWorkspace using the Display Pipes and Junctionswindow on the View menu.


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Additionally, junctions may be alternatively displayed as an outline or asolid gray box. By selecting the pipe(s) or junction(s) in the lists andpressing the << Shift or Shift >> buttons, an “x” will move left or rightindicating how the objects will be displayed on the Workspace. Ajunction which is shown as an outline or a solid box may still be openedfor editing.

AFT Impulse does not limit the number of junction icons you can showat a time.

Scale/flip workspaceThe Workspace can be scaled horizontally and/or vertically by choosingScale/Flip Workspace on the Arrange menu (see Figure 4.8). Scalingwill move the junctions and will stretch or shrink the pipes so that thearea the model covers is changed by the percentage specified.

Flipping the Workspace horizontally and/or vertically will move thepipes and junctions to their mirror image locations. This is useful whenyou are building a system which has symmetrical or mirrored segments.For example, you could select the segment, duplicate it, and then flip ithorizontally, vertically, or both, with Scale/Flip Selected Objects Onlychecked.

Figure 4.8 The Workspace can be scaled or flipped

Renumbering the Workspace objectsObjects on the Workspace can be renumbered by choosing Renumberfrom the Edit menu (see Figure 4.9). Selected pipes and/or junctions canbe renumbered starting at the lowest selected number or at a specifiednumber. If a pipe or junction which is not selected to be renumbered


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already has the next number in sequence, it will be changed only if theForce Sequential checkbox is checked.

Impulse will start at the beginning of the flow path containing the lowestnumbered selected pipe and will attempt to march down the pipes in thedirection of flow. It is recommended that renumbering be performedafter the model has been completed so that the junctions and pipes arecompletely connected.

If you want certain parts of the model to be numbered in sequence, thenuse the Renumber feature on the selected parts only. It is also preferablein the case of large or complicated models to renumber small sections ata time.

Figure 4.9 Pipes and junctions can be renumbered using theRenumber window opened from the Edit menu

Using the Renumber WizardThe Renumber Wizard allows you to renumber pipes and/or junctions bysimply clicking on them on the Workspace. Normal Workspacefunctions are suspended when the Renumber Wizard is active. TheRenumber Wizard is activated on the Edit menu.

Background GraphicYou can load a graphic into the background of the Workspace and buildyour model on top of that graphic. The graphic could be, for instance, atopographic map or a facility drawing. Such graphics are loaded from the


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File menu by choosing Load Background Picture. At your option thebackground graphic can be included in Workspace printouts.

Specifications windowsEach object on the Workspace has an associated Specifications window.The Specifications window lists all the input data for the selected object.There are three ways to open an object’s Specifications window from theWorkspace:

1. Double-click the object

2. Select the object and click the Open Pipe/Jct Window icon on theToolbar

3. Select the object and press ENTER

The Specifications windows can also be accessed through the ModelData window.

The Model Data window

The Model Data window offers a text-based perspective on allengineering information in the pipe flow model (see Figure 4.10). Thiswindow is immensely useful for obtaining a global view of the modeland rapidly checking the input.


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Open ModelData Control

General datasection

Customize view

Pipe datasection

Junction datasection

Figure 4.10 The Model Data window provides a convenientsummary of model element specifications

The Model Data window does not offer any tools to build or add to amodel; all model assembly must occur in the Workspace.

The Model Data window allows you to manipulate existing elementsfrom the Workspace. Therefore, a complete model can be assembled inthe Workspace window without ever opening the Model Data window.In fact, for small models this may be the preferred approach.

The Pipes table allows display of all input data for all pipes in the model.Details of additional losses are displayed in the Pipe Loss Data table,accessed with an adjacent tab. This table displays all fittings with Kfactors grouped into 27 columns.

The other display tab in the pipe area is the Pipe Detail Summary, whichassembles a list of details about each pipe in another format useful tosome engineers.

The junction data area is below the pipe data area. Here the junction datais separated into tables for each junction type. Click the folder tabs tosee the data for any type of junction.


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The transient data for each junction is shown on the Transient Data tab(see Figure 4.10).

To edit information in the Model Data window, you can open theSpecifications window for the pipe or junction. This can be done bydouble-clicking the far left column where the pipe or junction IDnumbers are shown. Alternatively, you can use the Global Edit windowsopened from the Edit menu.

Display and printing featuresThe Show Data options on the View menu (and Toolbar) lets youspecify the combination of pipe, junction, and general information youwant to see in the Model Data window.

Frequently, there are empty columns in the Model Data window tables.These can be hidden by selecting Hide Empty Columns from the Optionsmenu.

The Print Format window lets you specify the content to include in theprinted Model Data report and select the font to be used for printing.This window is opened from the View menu (or Toolbar).

The Model Data Control windowThe Model Data Control allows you to customize what data to show forthe pipes and for each junction type and also which pipes and junctionsto show (see Figure 4.11). It can be opened from the View menu (orToolbar).

Pipe data displayThe data to be shown for pipes is selected on the left side of the ModelData Control window. Only the items with check marks will be shown inthe Model Data window. All pipes shown will report the selected items.

Junction data displayThe data to be shown for the junctions is selected on the right side of theModel Data Control window. The data is grouped by junction type.However, some data is general and applied to each of the junction types.Select the junction type from the list and then select the data for thatjunction type. Each type also includes the general data which may be


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selected/deselected. You can select/deselect the general data for alljunction types simultaneously by clicking Set Common.

Figure 4.11 The Model Data Control window allows selection of theparameters to show in the Model Data window

Show selected pipes and junctionsSpecific pipes and junctions can be shown in Model Data. Click theShow Selected Pipes/Junctions tab, then click the items in the lists toselect the pipe(s) and junction(s) to be shown.

Database connectionsIn the lower left of the Model Data Control window there is a Databasecheckbox. If this box is checked, your Model Data Control parametersare set up as determined by the database to which you are connected.This is referred to as an active database. To make it inactive, uncheckthe box or change one of the Model Data Control settings controlled bythe database.


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If the checkbox is unchecked, but enabled, you are connected to adatabase but the settings are not being passed to the Model Data Controlwindow. The database is thus inactive. To make it active, check the boxthen click the OK button.

If the checkbox is disabled, there is no connected database.

See Chapter 7 for an in-depth discussion of how databases areconfigured and administered through your local or wide area network.

The Output window

The Output window is the primary vehicle for communicating the resultsof an analysis in text form (see Figure 4.12). The Output windowfollows the same general window organization as the Model Datawindow.

When a model is submitted to the Solver, the Solution Progress windowappears. After processing is complete, you can view the results in theOutput window by clicking the View Results button.

If the information in the Output window is not in the desired format, youcan choose Output Control from the Analysis menu (or Toolbar) tomodify the format. The Output Control window (see Figure 4.13) letsyou specify the parameters, their units, and the order of their display inthe Output window (Figure 4.12).

Data in the Output window can be copied to the clipboard, saved to atext file, exported to a file in a format for direct importing into aWindows spreadsheet package, or sent to the printer. The printed outputfollows the same flexible, organized, and accessible format as the Outputwindow itself.

The Show options on the View menu (or Toolbar) lets you enlarge eachof the output sections for greater clarity. The Print Format window,available on the View menu (or Toolbar), lets you specify the content toinclude in the printed Output report and select the font to be used. TheSort window, opened from the Arrange menu (or Toolbar), offers theability to sort the Pipe Results or Junction Results tables according to thevalues in any of the columns. The Transfer Results to Initial Guessfeature on the Edit menu (and Toolbar) sets all pipe and junction initialflows and pressures equal to the steady-state results, which causes futuresteady-state runs to converge much faster.


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You can view the input data for individual pipes or junctions directlyfrom the Output window in one of two ways. First, if you press down theright mouse button on the pipe or junction number in the far left columnof the table you will see the Inspection window. Second, if you double-click the pipe or junction number you will open the Specificationswindow in read-only form where you can review input in detail.

Open Output Control

Customize view

General section

Pipe output section

Junction output section

Figure 4.12 The Output window displays output in text form

Transient output filesThe transient output is saved to an external file with the extension ".out".The file name will be the same as the input filename with a numberappended. The filename number allows AFT Impulse to distinguishamong output files for different scenarios. If you want to know whichspecific output file is used for a scenario, the name is given in theGeneral section of the Output window.

The content of the Output file is specified in the Transient Controlwindow. Details are given in Chapter 5, and summarized here. As theTransient Solver progresses, solutions are obtained for all pipe stations.


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Each pipe is divided into multiple computing stations. In order toperform the simulation, solutions must be obtained for each station foreach time step. When all pipe stations and time steps are considered, theamount of output data can be enormous. Saving all of this data can thuscreate a huge output file. The Transient Control window allows you toonly save selected pipe stations at selected time intervals to the outputfile. This allows more reasonable output file sizes, while still retainingthe data of most interest.

The output file saves the fundamental pressure and flow data that allowsall important transient parameters to be determined. However, the usercan only review output data that exists in the output file. If data for apipe station is not saved, the user cannot review data in the Outputwindow or Graph Results window at that location because the data wasnot saved.

The Output Control windowThe Output Control window (see Figure 4.13) allows you to customizethe parameters shown on the Output window. The control features aregrouped on seven tabs.

Pipe and Junction parametersThe first two tabs control the parameters to be shown for the pipe andjunction steady-state results. The list on the left side indicates theparameters not shown but available. This list can be viewed inalphabetized or categorized mode, which makes it easier to find aparameter. Choose a parameter to be shown by selecting it on the left-hand list and clicking the Add >> button. The selected item will move tothe list on the right which shows the order of the parameters to bedisplayed in the output and the units (if any) in which to display theresults.


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Change output orderusing this tool

These parameters areshown in the output

These parameters are notshown but may be added

Available parameters maybe shown alphabetically

or by category

Description ofoutput terms

Change units here

Database connection

Figure 4.13 The Output Control window selects parameters for theOutput window. Here the Pipe parameters (for steady-flow results) are shown.

You can change the order of the output by selecting a parameter on theright-hand list, then clicking the Reorder scroll bar on the far right eitherup or down. The list will be reordered accordingly. The output ispresented in the order defined in this list.

The units used for a parameter can be changed by selecting theparameter then choosing the desired units from the dropdown list belowthe list of parameters. By selecting the units of interest, you can obtainyour results in whatever units you find most convenient and meaningful.This means that you can enter all your input parameters in one set ofunits (or a variety of units, for that matter), and have all your outputparameters in a completely different set of units. This feature hasobvious benefits for international applications.

To set selected units for both pipes, junctions and summary reports tothe preferred units (set in Parameter and Unit Preferences), click the UsePreferred Units button.


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You will note that some output parameters do not offer the ability tospecify output units. These parameters either do not have unitsassociated with them (such as Reynolds Number or friction factor), orare in fact echoes of input parameters (such as Pipe Nominal Size).

Pipe and Junction Transient parametersThe third and fourth tabs control the parameters to be shown for the pipeand junction transient results. The list on the left side indicates theparameters not shown but available. Choose a parameter to be shown byselecting it on the left-hand list and clicking the Add >> button. Theselected item will move to the list on the right which shows the order ofthe parameters to be displayed in the output and the units (if any) inwhich to display the results.

Reordering the parameters and changing the units works similarly to thesteady-state methods described previously.

General outputThe General section is the text area located at the top of the Outputwindow. The project title, reference information and other settingsaffecting the General Output are defined here.

You can enter a descriptive title which will be used in the Model Data,Output and Visual Report windows. A title is required and can have upto 100 characters.

You can keep a lengthy explanation or any other documentation aboutyour model in the Reference Information section. Names of projects,individuals, and assumptions can all be kept with the model. Thisinformation can then be included in your reports to improve tracking.

The checkboxes allow you to enable certain special reports to be shownin the General Section.

SummariesSeveral Special Summary Reports can be displayed in the Outputwindow. These include a Pump Summary and Valve Summary. Thecontents of these summary reports and the engineering units can bespecified in the Summaries list at the right.


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Format and actionThe results can be automatically sent to the printer or saved to a filewhen the model has reached convergence. Additionally, the results canbe automatically saved as initial conditions by checking the TransferResults to Initial When Done option. The model can also beautomatically saved after the initial conditions are transferred. The valvestate (open, closed or failed) can also be saved along with the initialconditions.

When enabled, the Wrap Table Column headers option makes the outputtables wrap the column headers an extra line if the header is wider thanusual. There are four formatting methods for the pipe and junctionoutput data:

1. Align on the decimal point

2. Show only the specified number of digits

3. Align only when the differences in magnitudes are less than a user-set limit

4. Use exponential notation with the specified number of digits whenthe differences in the magnitudes are greater than a user-set limit

The first method allows the magnitude of the data to be easily comparedas the data is read down the column. However, a number whosemagnitude is small may force larger numbers to show many digits. Forexample, assume that the minimum number of digits is set to four, andthere are two output values: 1234.5 and .09876. These two values will bedisplayed in the output as:

1234.50000

0.09876

The second method will force the data to use exactly the number ofdigits specified, which may result in the decimal point location changingfrom value to value. Using this method with the above example andspecifying four digits, the output will be displayed as:

1235.

0.09876

The third method will align the data on the decimal point only if themaximum value in the column divided by the minimum value is less thana limiting order of magnitude that you set. If this ratio is greater than thelimit then each value in the column will be exactly the number of digits


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specified. Using the above example with a limit set to 4 then1234.5/.09876 = 12500 or an order of magnitude of 5. This is greaterthan the limit. The entire column will not be aligned and will bedisplayed as:

1235.

0.09876

The fourth method is similar to the third except that if the ratio is greaterthan the limit then the entire column will be shown in exponentialformat. Using the above example, the column would be shown as:

1.235E+03

9.876E-02

These formatting options are for the entire Output window; however, thelast two are calculated and applied on a column basis. In other words, ifyou choose the third option, one column may be aligned but another maynot be.

The output can be forced to always display in exponential notation bychoosing the checkbox.

For transient results, the maximum and minimum values for all selectedparameters are shown in the Output window. The results can show themax/min values for all pipe stations, or only the max/min values foranywhere in the pipe. You can choose which type of report you want byselecting Detailed or Summary in the Max/Min Transient TableContents area. For models with a large number of pipe sections, theSummary report is generally preferable because of the time required todisplay the Detailed table.

Show selected pipes/junctionsYou can choose which pipes and junctions are to be displayed in theOutput window. Clicking a selected item in the list on the Show SelectedPipes/Junctions tab will deselect it. Clicking a deselected item will selectit. This feature is especially useful when you are interested only in acertain area of the model.

You can quickly define the output you want by doing the following:

1. Select the pipes and junctions on the Workspace

2. Open the Output Control window and select the Show SelectedPipes/Junctions tab


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3. Click the None button for both pipes and junctions

4. Click the Select Special button for both pipes and choose the“Selected on Workspace” option

5. Click the Select Pipes button then the OK button

6. Do the same thing for the junctions

7. Click the Output Control window Show button

Database connectionsIn the lower left of the Output Control window there is a Databasecheckbox. If this box is checked, your Output Control parameters are setup as determined by the database to which you are connected. This isreferred to as an active database. To make it inactive, uncheck the box orchange one of the Output Control settings controlled by the database.

If the checkbox is unchecked, but enabled, you are connected to adatabase but the settings are not being passed to the Output Controlwindow. The database is thus inactive. To make it active, check the boxthen click the OK button.


See Chapter 7 for an in-depth discussion of how databases areconfigured and administered through your local or wide area network.

Command buttonsThere are eight buttons at the bottom of the Output Control window.Impulse has built-in default parameters, units and settings which you canchoose by clicking the Impulse Default button. You can also developyour own settings, tailored to your project or industry, and have theseused by default (instead of Impulse’s defaults). To make your owndefault, first select the output parameters, units and settings you wouldlike to use then click the Set As Default button. Your settings will besaved and will be used each time any new project is initiated. If youmake changes to the settings, and want to get back to your defaults, clickthe User Default button. The saved default settings are updated onlywhen you click Set As Default.

You can save the output settings (except for the selected pipes/junctionsand junction deltas) to a file by pressing the Save Control Format buttonand entering a file name. These settings are loaded again by pressing the


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Recall Control Format button and choosing the file name. For example,you may have a final report format that is always desired. You can recallthis format before generating final results. If you have another formatyou use for reviewing model accuracy, you may want a larger number ofparameters in the output.

The format files you create can be placed on a network for sharingamong a group or company, or incorporated into a company-widedatabase, allowing standardized reporting.

If you have made changes you don’t want to keep, click the Cancelbutton. Click OK to use the settings you have specified.

Output parameter descriptionsAt the bottom of the Pipe, Junction, Pipe Transient and JunctionTransient tabs a description is given for the selected parameter. Thereare three lines shown.

First is the Output Window table header. This is the text that is shown inthe headers of the pipe and junction tables on the Output window.

Second is the Visual Report Abbreviation. This is the text shown on theVisual Report window when you display an output parameter. This textis very short so as to save space.

Third is the description of the term.

Steady flow resultsFigure 4.14 shows the steady flow results in tables for pipes andjunctions as specified in the Output Control window.


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Figure 4.14 The Output window shows steady flow results in thePipes table and the various junction tables. Transientdata is shown on the transient tabs.

Transient flow results for pipesYou can review the transient results for any pipe station for any timewhich was saved to file as specified in the Transient Control window.These results are shown in the Output window Transient Output table(Figure 4.15)

Figure 4.15 shows the transient flow results at the initial time step(which is equal to zero here). The columns of data shown in Figure 4.15are specified in the Output Control window.

Note that the pipe stations shown are those saved to the output file.


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Time step forcurrently viewed

results

Time stepselection toolslides up and

down

Buttons toincrement or

decrementcurrently

displayed time

Figure 4.15 The Transient Output table in the Output windowshows output results for pipe stations and at specifiedtimes.

Transient Max/Min results for pipesYou can review the maximum and minimum transient results for all pipestations for all times. The data shown is not dependent on which pipestations or time steps were saved to file. Figure 4.16 shows the TransientMax/Min table in the Output window. The parameters that are shown arethe same as those specified in Output Control. However, there are fourcolumns for each parameter. The four columns show the maximum andminimum values for the parameter, and the times for each of those.


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Figure 4.16 The Transient Max/Min table details the maximum andminimum values for the entire simulation.

Summary Max/Min resultsThere is a summary version of the Transient Max/Min table. The versionshown in Figure 4.16 is what is called the Detailed version. You canswitch between detailed and summary views by selecting the option onthe Output Control "Format & Action" tab (see Figure 4.17).

The summary version shows only maximum and minimum values for theentire pipe, as opposed to each pipe station. The summary also shows thepipe station that is the overall maximum or minimum for the pipe.Therefore, for each Output Control transient parameter there are sixcolumns in the summary version of the Transient Max/Min table. SeeFigure 4.18 for an example.


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Figure 4.17 Detailed and Summary versions are available for theTransient Max/Min table

Figure 4.18 Summary version of the Transient Max/Min table


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Transient flow results for junctionsThe Transient Data table in the Output window Junction area showsselected parameter results for the entire simulation (Figure 4.19). Theparameters shown are selected in the Output Control window on theJunction Transient tab.

Figure 4.19 The Transient Data table shows junction transientresults.

Transient event messagesWhen events occur at junctions during the transient simulation, a log ofall events is displayed in the Event Messages list. Events are firstdisplayed grouped by junction, and then displayed again in temporalorder. The log is shown in the General area of the Output window on theEvent Messages tab. A list of when check valves close (and re-open),relief valves open (and re-close), control valves lose control (and regainit), as well as all standard event transients (see Chapter 10).


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Output window updatesWhenever you change the input model, all output windows and areerased, as is the output file. This prevents you from changing your inputmodel, forgetting to rerun it, and mistaking the previous Output windowfor the current model's results.

The Graph Results window

The Graph Results window, shown in Figure 4.20, allows you togenerate high quality printed graphs.

The Select Graph Data window, opened from the View menu (andToolbar), lets you control the content of the graph and offers severaltypes of standard graphs. The Customize Graph window, opened fromthe Options menu (and Toolbar), offers you control of the graph'sgeneral style. Fonts and colors can be selected.

Figure 4.20 The Graph Results window offers full-featured plotgeneration


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The graphical data can be exported to a file for later import as a crossplot against results for a different model. For example, let's say youwould like to see the effect on the system pipe pressure drop when youchange a valve loss factor. You could run the Solver for one value of Kand export the results to a file. Then you could rerun the Solver for anew value of K and import the results from the previous case for crossplotting. These features are accessed by choosing Save Graph Data andImport Graph Data from the File menu.

Graph “Sets” can be created and saved by the user using the tools in theSelect Graph Data window.

The Graph Results window is always erased when you make changes tomodel input. Rerun the model to graph the new results.

The Select Graph Data windowThe Select Graph Data window defines what type of graph you wantcreated and with what parameters and units (see Figure 4.21). You cancreate four types of graphs. Two types use time as the independentvariable, and the other two use pipe length.

Figure 4.21 The Select Graph Data window controls the GraphResults window


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Transient pipe resultsTransient pipe results can be plotted vs. time for any pipe station that issaved to the output file. As summarized earlier in this chapter anddiscussed in detail in Chapter 5, the Transient Control window allowsthe user to specify that data for selected pipe stations be saved. Asshown in Figure 4.21, each pipe is shown in an expandable list. When apipe is expanded, the pipe stations for which data was saved are shown.These pipe stations can be added to the list on the right.

The user can plot the data for the entire simulation, or only for somespecified time frame. Further, if Design Alerts are specified for the pipe(e.g., max/min operating pressures), these can be cross-plotted againstthe transient data.

Transient junction resultsSimilar to transient pipe results, transient junction results can be plottedvs. time for any junction that is saved to the output file. Junction dataincludes items such as pump speed and accumulator volume. Similar topipe output, these are specified in the Transient Control window.

Profile Along a Flow Path and EGL, HGL and Elevation ProfileThese two types of graphs will plot the selected parameter along a flowpath. The pipes that comprise the path must form a single, continuouspath. The independent variable is the length along the pipe(s).

A path of pipes can be specified in one of two ways. First, the user canselect the pipes in the displayed list. This permits only a single path to bespecified.

Second, one or more flow paths can be specified using groups (seeFigure 4.22). Only groups which consist of a pipe sequence will bedisplayed

The results can be the values at a particular point in time, or the overallmaximum and/or minimum values. The Profile Along a Flow Path graphtype also allows cross-plotting of Design Alerts such as max/minallowed pressures.

Both of these graph types support animation, discussed in the nextsection.


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Figure 4.22 Multiple paths can be cross-plotted using predefinedgroups.

Animating profile resultsThere are two animation options: Animate Using Output File, andAnimate Using Solver.

The option to Animate Using Output File uses data saved to the transientoutput file (discussed earlier in this chapter). This option can only beused with paths of pipes for which transient data was saved for allstations. The reason being that to show how a parameter such as pressurechanges along a pipe, the data at all computation stations must beavailable. Saving selected pipe stations is specified in the TransientControl window, and is discussed in Chapter 5.


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As its name implies, the Animate Using Output File option obtains thedata for animation by reading in data saved to the transient output fileand then displaying it in the graph.

The option to Animate Using Solver does not use the transient outputfile at all. Instead, it actually reruns the Transient Solver in thebackground to regenerate the transient data. This rerun does not writeany output to file. Because the model is being rerun, all data for all pipestations is available for each time step.

Figure 4.23 shows the Graph Results window when using animation. Asshown, additional controls become visible to allow the user to adjustanimation. At the left is the Play button, which starts the animation. Nextto it are the Pause and Stop buttons. If the Pause button is pushed, theanimation stops at that time step. One can print the graph or copy thedata if desired or, if using the Animate Using Output File option, use thetime slider controls (to the right of the Stop button) to set the timeforward or backwards. If using the Animate Using Solver, the solver canbe made to march one step at a time when one clicks the “+” button. TheStop button stops the transient and resets the time to zero.

The current time (in seconds) and time step are shown to the right of thetime controls. At the far right are the speed controls. Slowing theanimation involves adding time delays to the data display. Increasing thespeed involves skipping time steps. The default speed, right in themiddle, has no delays or skips.

If plotting a single path using the list of pipes in Select Graph Data, onecan cross-plot on the animation design alerts (in Profile Along a FlowPath) or steady-state results (in EGL, HGL and Elevation Profile).


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Figure 4.23 The animation options display additional controls

Each of these options has advantages and disadvantages. The AnimateUsing Output File option is usually a little faster. It also allows you touse the time slider to reset the time to any time step forwards orbackwards. The reason being that the transient output file has all of therequired data over time, and thus obtaining data for any other time is justa matter of reading the data from the proper place in the output file. TheAnimate Using Solver does not have this flexibility, since it can onlyobtain the data by marching through all of the time steps.

Another advantage of the Animate Using Output File option is that it canbe made to run much faster. When the animation speed control is set toincrease speed, intermediate time steps are skipped and not plotted. Ifthe speed is increased too much, the quality of the animation deterioratesbecause too much missing time data makes it difficult to follow what ishappening. When the speed is increased, the Animate Using Output Fileoption just skips time steps and reads in the data for each required timestep.


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On the other hand, the Animate Using Solver option cannot skipcalculation steps because it is running the Transient Solver. To obtainthe results for a future time step, it must calculate all of the intermediatetime steps.

The biggest advantage of the Animate Using Solver option is that it doesnot require one to save all of the required pipe station data to disk inorder run an animation. Because it is obtaining data from the TransientSolver, it can always run any animation desired. If one uses the AnimateUsing Output File option and has not saved all of the required data, thedata must be specified in Transient Control (as discussed in Chapter 5)and then the entire transient rerun. The Animate Using Solver optionallows one to run animations while keeping the output file sizes to aminimum.

Other graph controlsWhen you change graph types or parameters you can choose to keep thelabels and/or color styles by clicking the appropriate checkboxes on thelower right.

The Customize Graph windowThe Customize Graph window allows you to customize the plot bygiving you control over the many aspects including:

1. Fonts for titles, numbers, and legend

2. Colors for the graph background, foreground, and data sets

3. Line types and thicknesses

4. Symbol shapes

5. Normal, semi-log, or log-log plots

6. Borders and gridlines

7. Printing preferences (color or monochrome)

You can save the settings you set up by clicking the Save button on theSystem tab. If you make changes to the settings, and want to get back toprevious settings, click the Load button on the System tab and load thefile.


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The Auxiliary Graph Formatting windowThe Auxiliary Graph Formatting window allows you to more easilychange graph colors and curve thickness. These preferred settings can besaved as the default for future use. In addition, these preferences areused in all graphs throughout AFT Impulse. This window is opened fromthe Options menu or toolbar.

The Visual Report window

The Visual Report will to help you visualize the model data and steady-state and transient solutions in relationship to the model layout. TheVisual Report window displays the pipe system layout from theWorkspace window with information in text form (see Figure 4.24).Additionally, magnitudes of steady-state and transient parameters mayalso be shown with color variations.

The values to be shown may come from the input data (when in inputmode), Steady-State Output results, or Transient Output results. You canchange between these by selecting the option at the top of the VisualReport Control window.

In the Visual Report window, the positions of the junctions and pipes arefixed based on their locations on the Workspace. While the pipe systemlayout cannot be edited in the Visual Report window, the integrated textresults for each pipe or junction can be rearranged by dragging anddropping to improve the final appearance of the window.

The contents of the Visual Report window can be copied to theclipboard for transfer into another Windows graphics program or sent tothe printer.

The View menu allows you to zoom in and out at ratios of 25%, 50%,and 75% of the normal size, as well as choose custom sizes. This featurecompresses more results onto a single page. The printed size isconsistent with the zoom state. That is, if you zoom out to 50%, theVisual Report screen prints at 50% of its normal size. The Custom Zoomfeature allows you to specify the exact zoom value you desire.

The Visual Report window is always erased when you make changes tomodel input. If viewing output results, rerun the model to view the newresults.


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Figure 4.24 Visual Report window integrates input data and outputresults with model layout

The Visual Report Control windowThe Visual Report Control lets you control the content and appearanceof the Visual Report window (see Figure 4.25). You can also savecustom Visual Report window layouts to disk for later recall, so youdon't have to reformat this window repeatedly.

When the Visual Report window is opened and no output results arecurrently displayed, Visual Report Control will be in the Input DisplayMode. If output results are displayed, it will be opened in the SteadyOutput or Transient Output Display Mode. Parameters available fordisplay vary depending upon the mode, i.e. input, steady output ortransient output. If output results exist, the display mode may be changedat any time.


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Figure 4.25 The Visual Report Control window allows you toconfigure the Visual Report display

Display parametersThe parameters to be shown in the Visual Report window are selectedfrom the Display Parameters tab. The left list selects the pipe parametersand the right list selects the junction parameters. In these lists you canshow only the parameters you have selected in the Output Controlwindow, or all output parameters. You can select more than oneparameter; however, the Visual Report window may get crowded if toomany are selected.

General displayThe General Display tab allows you to configure several other items.


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The title, defined in the Output Control window, and filename can beshown or not shown depending on your preference. The scenario namecan also be shown.

Figure 4.26 The Visual Report with the units and Color Map in theLegend

The units for all parameters can be shown in a legend instead of next tothe parameter value (see Figure 4.24 or 4.26). This will reduce theamount of text and crowding shown in the Visual Report window. Thelegend can be moved to any location in the Visual Report window. If thisoption is chosen while in input-only mode, all values will be convertedto a consistent set of units.

You can change the font by clicking the Fonts button and choosing thefont desired or by using the font size change buttons on the Toolbar.

The number of digits to show in the numerical display can be configuredusing the provided drop-down list.

Normally on the Workspace, closed objects are displayed using specialgraphics (dashed lines, X’s, etc.). The number of a junction which hasspecial conditions set, by default, has an X preceding the J (e.g. XJ56).


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If you want these symbols to be used in the Visual Report window also,choose the Show Closed Symbols checkbox. Normally, pipes in closedsections are shown as dashed lines, and junctions outlined with a dashedline. If you want these symbols to be used in the Visual Report windowalso, choose the Use Closed Object Style checkbox.

If a background picture exists on the Workspace, it can be shown in theVisual Report by selecting the checkbox provided. Similarly, if a grid isshown on the Workspace it can be shown on the Visual Report.

Show selected pipes and junctionsYou can choose, on an individual pipe and junction basis, what youwould like to show in the Visual Report Control window (see Figure4.27). This display customization feature is useful for large modelswhere only some of the data points are of interest. For simplicity, thisdiscussion will focus on just the pipes; however, the junctions behave inthe same fashion.

Each pipe is listed with a set of x’s to the left of the name. The x’sindicate what will be shown. To change the settings, first choose thepipe(s) to change. Then use the checkboxes on top to set or clear the x’s.If more than one pipe is selected and the current options are different,the checkbox will look grayed and the first click will clear the checkboxand the second click will set it. You can have any combination ofsettings, including no settings at all. To restore the settings to what isshown in the Workspace and the data, click the Workspace Settingsbutton.

Color mapThe Color Map feature allows you to visualize magnitudes of variousparameters by coloring the pipes (see Figures 4.26 and 4.28). There isone Color Map for the steady output mode, one for transient outputmode, and another for the input-only mode. A Color Map is useful toshow graphically where, for example, high velocities or reverse flowsare located or to visualize the pipe diameters or lengths in a model.

To create a Color Map, first choose a parameter and units for which allpipes will be categorized. Then choose a color, select the operator (i.e. >,>=, =, <=, <) and enter a value, then press Add to Map. Values can becategorized on an absolute value basis by selecting this option.


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Figure 4.27 The Visual Report Control window allows you toindividually select the object, name, number and data tobe shown in the Visual Report window.

The order of entry is not important. Impulse will sort the entries first byvalue and then by the operator, with the > being above the <. The pipeswill be colored using the first match found in the map, starting from thetop of the map. If no match is found, the pipe will be the Workspace pipecolor. Be careful to avoid overlapping operators and values. If, forexample, the map had the following four colors,


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1) >=60

2) <60

3) >=20

4) <20

then there is no way colors 3 and 4 will be used because any pipe lessthan 20 is also less than 60. Since <60 is before <20 in the map, the pipewill be colored using 2.

Figure 4.28 The Visual Report Control Color Map allows you tospecify the display colors for pipes according to theiroutput values.

To avoid overlapping colors, only use less than (<) as the last entry. Abetter map would be to use

1) >=60

2) >=20


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3) <20.

You can choose to categorize the pipes based on their absolute values bychoosing the Display Absolute Value checkbox. To delete a map value,click the color you want to delete then click the Remove Setting button.

Instead of coloring the pipes, you could choose to color just the pipenumbers. This is done by choosing the Color Pipe Labels and clearingthe Color Pipes checkboxes in the Color Options area. You could alsoapply the colorization to both. Choose the Print Black & Whitecheckbox if you don’t have a color printer. The pipes will be printed inblack.

The color map values can be displayed in a legend by checking theDisplay in Legend checkbox (see Figure 4.28). The color map willappear in the legend after the pipe and junction units, if that option wasselected. The legend may be dragged and dropped to any location in theVisual Report window.

Command buttonsYou can save the output settings (except for the selected pipes/junctions)to a file by clicking the Save Options button and entering a file name.These setting are loaded again by pressing the Recall Options andchoosing the file name. This file may be shared among engineers orincorporated into a company-wide database.

Impulse will initially try to place the text for pipes and junctions basedon their positions on the Workspace. You can drag the text to newlocations which will give the best visual presentation. These positionsare saved with the model. You can reset the locations back to whereImpulse initially placed the text by clicking the Reset Locations buttonand then confirming the operation.

If you have made changes which you don’t want to keep, click theCancel button. Click Show to apply your settings.

Visual Report annotations

Annotations for the WorkspaceAnnotations created on the Workspace can be displayed on the VisualReport by opening the Workspace Annotation window (double-click the


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annotation) and selecting the Also Show on Visual Report on theScenario Options tab.

Workspace annotations displayed on the Visual Report can be moved onthe Visual Report but not edited.

Creating Visual Report annotationsAn annotation can be created specifically for the Visual Report bychoosing Create Annotation from the Edit menu or Visual ReportToolbar. When the Create Annotation tool is activated, you can draw anannotation on the Visual Report just like on the Workspace.

Visual Report annotation have all the properties of a Workspaceannotation, except they can be displayed only on the Visual Report.

The Toolbars

The Toolbars offer quick access to the features used most frequently inAFT Impulse. Figure 4.29 shows the Toolbars. There are actually sixtoolbars in all. One is the Common Toolbar, and it is always shown. Itsfeatures are common to all five Primary windows. In addition, there isone Toolbar for each Primary window, offering features appropriate forwork in that window. Each of the functions on the Toolbar is alsoavailable through the menu system.


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Figure 4.29 The Toolbars allow you to quickly access importantfeatures. There is one common Toolbar and a specialToolbar for each of the five Primary windows. You seeonly two toolbars at a time in AFT Impulse. The functionof each tool is indicated in the ToolTips that appearwhen you hold the cursor over the button.


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C H A P T E R 5

Building and Running Models

This chapter discusses the basics of building and running a waterhammermodel with AFT Impulse. AFT Impulse assumes that the user has a goodgeneral understanding of engineering pipe hydraulics and a basic idea ofwhat waterhammer is. Users who have not been exposed to pipe flowanalysis software may have difficulty grasping the network modelingconcepts initially.

Creating objects

The pipe flow model is assembled on the Workspace by arranginggraphical objects (pipes and junctions) that represent parts of a physicalpipe flow system. In addition to pipe and junction objects, annotationobjects can also be added.

PipesTo create a pipe, select the Pipe Drawing Tool from the Toolbox anddraw a line on the Workspace to represent the pipe. To draw multiplepipes more quickly, hold down the CTRL key while drawing each pipe.This keeps the Pipe Drawing Tool active, so you don't have to select itfor each pipe. Alternatively, you can double-click the Pipe Drawing Toolto “lock” it down. You can then continue to draw pipes until you clickthe Pipe Drawing Tool a second time.

To draw a vertical or horizontal pipe, hold down the SHIFT key whiledrawing the pipe.


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The length of the line on the Workspace has no relationship to thephysical length of the pipe. The Workspace pipe is merely an abstractrepresentation of a pipe.

If a pipe is drawn as an extremely short line, AFT Impulse eliminates theline immediately after it is drawn.

Another way to create new pipes is to duplicate existing pipes with theDuplicate or the Copy and Paste features found on the Edit menu. Whena new pipe is derived from a previous pipe, the new pipe retains theengineering information associated with the original pipe.

When a new pipe is created it is assigned a default ID number. When thepipe is displayed on the Workspace, the ID number is preceded by theletter “P” to indicate that it is for a pipe. An arrow on the pipe indicatesthe reference positive flow direction for the pipe. From the PipeSpecifications window you can change the ID number to any valuegreater than zero but less than 10,000. The reference positive flowdirection can be reversed by choosing Reverse Direction from the Editmenu or by selecting the reverse direction button on the Toolbar.

Tip: The pipe or junction ID number text can be dragged to a newlocation to improve visibility. The new location is saved with the model.If you want the text returned to its default location, double-click it. Ifyou want all pipe and junction text returned to the default locations,choose Reset Label Locations from the Arrange menu.

Each pipe must be connected to two junctions. There are no exceptionsto this rule.

A pipe can be segmented or “bent” in up to ten segments (see Figure5.1). This does not affect the model or the results, only the visualrepresentation. It does not add an “elbow” to the model or the resultingpressure drop effects of an elbow.


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Figure 5.1 Pipe with five segments

This feature is useful in graphically bending the pipe around another partof the model or graphically showing that there are embedded elbows asAdditional Losses.

You can add a segment to a pipe in two ways. The easiest way to add asingle segment is to select Pipe Segments and Add from the Arrangemenu. If there are no segments, then a single segment can also be addedby selecting the Add/Remove Segment from the Toolbar. If the pipealready has one or more segments, this toolbar selection will remove allsegments.

The other way to segment a pipe is to select the pipe on the Workspacethen choose the Pipe Segments and Manager from the Arrange menu. Awindow will display where you can add, remove and merge segments. A“handle” will appear in the middle of the pipe for each new segment. Asthe cursor passes over the handle, the cursor will change to a crosshair.Press and hold down the left mouse button and drag the middle of thepipe to the desired location. By holding the SHIFT key the pipe segmentwill move only at right angles.


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JunctionsTo place a junction, drag the icon from the Toolbox onto the Workspace.The nineteen available junctions represent various pipe systemcomponents.

When you duplicate a junction using the Duplicate or Copy and Pastefeatures on the Edit menu, the new junction retains the engineeringinformation associated with the original junction.

Junctions are shown on the Workspace with default ID numbers, whichyou can change in the Junction Specifications window. The letter “J”precedes the ID number to signify that it is for a junction. From thejunction’s Specifications window you can change the ID number to anydesired value greater than zero but less than 10,000. Identical IDnumbers can be assigned to both a pipe and a junction because the “P” or“J” will distinguish the two.

Unlike pipes, junctions do not have reference positive flow directions.Junctions for which the flow direction is important (such as pumps andcontrol valves) derive the flow direction from the flow direction of theconnecting pipes.

Morphing junctionsA junction can be “morphed” from one type to another. To morph ajunction, hold down the CTRL key and select a junction from theToolbox and drop it onto an existing Workspace junction. The junctiontype will change to the new junction type, and data that can be kept willbe copied into the new junction.

Splitting pipesIf you hold down the SHIFT key while selecting a junction from theToolbox and then drop the junction onto an existing pipe, the pipe willsplit into two pipes. The physical length of the original pipe will beautomatically halved, and the new pipe will be assigned the balance.Thus the sum of the two pipe lengths will equal the length of the originalpipe. Any additional losses in the original pipe will be left in the originalpipe, and the new pipe will have no additional losses.


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AnnotationsTo create an annotation, select the annotation tool on the Toolbox anddraw the outline of the annotation. When you release the mouse button,the Workspace Annotation window will display. Here you can enter textfor the annotation, draw lines and arrows, change colors, and drawoutlines. You also can change the colors of the annotation.

Annotations can be cut, copied, pasted, duplicated, and moved like pipeand junction objects. When you select an existing annotation, resizinghandles appear.

The Annotation Manager assists you in specifying which annotationswill display in which scenarios.

Moving objects

Objects on the Workspace can be moved individually or as groups.Clicking a Workspace object selects it. Pipe objects change color whenselected. Junction and annotation objects are outlined in red whenselected. This color is configurable in the Workspace Preferences.

To move an object, select it, drag it within the Workspace, and drop it inthe desired location. When an object is dragged beyond the existingWorkspace area, the Workspace is expanded accordingly.

A pipe object can be stretched by grabbing the handles (small blacksquares) at the pipe endpoints and moving an endpoint to a new location.

To prevent accidental movement, lock the objects on the Workspace.The Lock feature is accessed from the Edit menu or the lock button onthe Toolbar.

To group multiple objects for movement or other operations, hold downthe SHIFT key when selecting the objects. Objects can also be groupedby drawing a box around them using the Selection Drawing Tool on theToolbox. You can use the Select Special on the Edit menu to groupobjects based on specified criteria.

You can also select all objects in a flow path as follows:

• Select one object in the flow path

• Choose Select Flow Path from the Edit menu or Toolbar


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• Choose what type of objects you want to select

The endpoints of a flow path are the junctions which connect more thanone flow path, like a branch junction connected to three or more pipes.You can choose to include or exclude these end point junctions.

Tip: You can undo all pipe and junction movements using Undo on theEdit menu or Toolbar. You also can press the escape key during themovement to cancel the move.

Keyboard modifiersWhen you drag a junction, the endpoints of any connected pipes aremoved with the junction, thus maintaining the graphical connection.This functionality may be changed through the Workspace Preferenceswindow.

Editing objects

The objects you place on the Workspace can be edited with the editingcommands from the Edit menu or the Toolbar. Objects can be cut,copied, pasted, duplicated, and deleted. These operations can beperformed on individual objects or on groups. The Edit menu providesone level of undo for each editing operation. Alternately, you can use theUndo button on the Toolbar.

The Copy Graphics feature on the Edit menu will copy the specifiedWorkspace objects to the Windows clipboard. This image can then bepasted into other Windows applications.

Connecting objects

During construction of a new model, objects can be placed anywhere onthe Workspace. In order to assemble a model that is ready to submit tothe Solver, you must connect the objects properly.

Remember that connectivity only exists between junctions and pipes.There are no junctions that connect to junctions, and no pipes thatconnect to pipes.


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AFT Impulse will attempt to automatically connect pipes and junctionswhen they are moved. This eliminates unnecessary opening of the pipeand junction Specifications windows. If Impulse cannot make theconnection automatically, then the following three steps may be requiredto establish a connection between a junction and a pipe:

1. Graphically connect the objects on the Workspace. The pipeendpoint must terminate within the boundaries of a junction icon.Figure 5.2 shows a graphical connection between a pipe and ajunction.

2. After the pipe is graphically connected to two junctions, Double-click the pipe to open its Specifications window, then click OK toaccept the connected junctions as determined by AFT Impulse. It ismost efficient to do this when you are entering the pipe's data in itsSpecifications window.

3. Open the Specifications window of the corresponding junction andaccept the pipe connectivity determination. You can do this as youenter the junction's data in its Specifications window.

Figure 5.2 A pipe and junction graphical connection


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If the model seems to not be connected properly, you can also use theExtended Model Check feature on the View menu.

When you accept AFT Impulse's connectivity interpretation, theconnection advances from a graphical connection to a model connection(For more information on Specifications windows, refer to Chapter 6.)

The model connectivity you establish on the Workspace is retained onlyas long as you maintain the graphical objects in their current visualrelationship to each other. If you move a pipe or a junction and break thegraphical connection, AFT Impulse disconnects the two objects.However, if you move a group of connected objects, their modelconnectivity is maintained.

Defining objects

Each Workspace object in a model must be defined before AFT Impulsecan obtain a solution. AFT Impulse examines your model for properdefinition before it gives you access to the Solver.

To define an object, you must specify all the required property data forthe object and satisfy its connectivity requirements as described in thefollowing sections.

The Show Object Status feature identifies pipe and junction objects inthe model that are not completely defined.

Specifying required property dataTo specify the required property data for an object, open itsSpecifications window. This can be done in six ways:

1. Double-click the object on the Workspace

2. Select the object (by clicking on it once, or using the SelectionDrawing Tool) and press ENTER

3. Select the object and click the Open Pipe/Junction Window icon onthe Toolbar

4. Double-click the left column of the appropriate table in the ModelData window (where the object ID number is shown)

5. Jump from another Specifications window


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6. Double-click the connected pipe or junction number in aSpecifications window

When editing in a Pipe Specifications window, pressing the F5 functionkey jumps automatically to the next higher numbered pipe. Pressing F5while holding the CTRL key jumps to the next lower number. Junctionwindows behave similarly.

Parameters that functionally describe the pipe or junction are entered inthe Specifications window. Some parameters in the Specificationswindows are required while some are optional. The optional parametersoffer you added control over your model and the ability to obtain specialinformation.

Highlighting required informationYou can toggle the highlight feature on and off in any of the followingways:

• Double-click anywhere in the specifications window

• Press the F2 function key when a specifications window is open

• Choose “Highlight in Pipe and Jct Windows” from the Optionsmenu

The highlighting feature may be especially useful when you are firstlearning to use AFT Impulse or when you are having difficulty obtaininga defined object status.

A more detailed discussion of the contents of pipe and junctionSpecifications windows is given in Chapter 6.

Use status featureEach pipe and junction specifications window has a Status tab. TheStatus tab shows what required information for the pipe or junction ismissing.

Satisfying connectivity requirementsIn addition to entering the required information for each object in themodel, you must satisfy these connectivity requirements:


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• You must accept the connectivity information shown in each object'sSpecifications window (refer to “Connecting objects” earlier in thischapter).

• Each junction must have the appropriate number of pipes connectedto it. For example, a venturi must have exactly two connecting pipes,whereas a reservoir may have one to twenty-five connecting pipes.(Details on each of the junctions are given in Chapter 6 andsummarized in Table 6.1.)

• Pipes must always be connected to two junctions, one at each pipeendpoint.

Should you ever delete any of the required information in theSpecifications window or move a connected object from its connection,the object's status reverts to undefined.

For example, say you place a pump on the Workspace. A pump requiresinlet and outlet piping (unless it is a submerged pump). AFT Impulsereflects this by requiring each pump to be assigned an upstream pipe anda downstream pipe. Entering only the pump constants and setting anelevation is necessary but not sufficient information to define the pumpaccording to AFT Impulse's requirements. Once you have entered thepump data and properly connected it, the pump is defined.

Inspecting objects

The Inspection feature is a time saving feature for reviewing the dataassociated with a Workspace pipe or junction object. Inspection displaysthe data for an object in read-only format (see Figure 5.3).


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Figure 5.3 Inspecting from the Workspace with right mouse button

To inspect a pipe or junction object, position the mouse pointer on theobject and hold down the right mouse button. Inspecting is quicker thanopening the Specifications window if you want to look at an object'sinput data but not edit it.

If you press the SHIFT key when you perform the inspection, only theitems not yet specified are displayed. If you press the CTRL key whenyou perform the inspection, the output data will be displayed (if outputexists).

The Inspection feature is also available in each Specifications window.This is useful if you want to check the input data of objects connected tothe one on which you are working.

For example, when working in an Valve Specifications window, youmay wish to quickly check the upstream and downstream pipe diameterspreviously defined. Holding down the right mouse button on eachconnected pipe ID number will show you that pipe's primary inputparameters.

The Inspection feature works similarly in Pipe Specifications windowswhen you inspect connected junctions. Judicious inspecting can save youtime when assembling or troubleshooting a model.


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Using the Checklist

Assembling a model that is ready to submit to the Solver is not, at firstglance, an obvious process. The new user will wonder where to begin,and even the experienced user will at times lose track of whatinformation has already been entered and what information remains tobe entered. To help you in this process, the Checklist window tracks thestatus of your model and communicates to you where you lack completeinformation.

The Checklist shows six items (see Figure 5.4). The first four are alwaysrequired, and the last two are required if the it is a transient model.When a requirement has been met, you will see a check mark next to theitem in the Checklist window. You can view the Checklist by selectingChecklist from the View menu or clicking the check button on theToolbar. As a guide to the user, the Checklist items are also shown onthe Status Bar at the bottom of the AFT Impulse window (Figure 5.5).

When you have entered the proper information for all six Checklistrequirements your model is complete and ready to submit to the Solver.As long as the Checklist remains uncompleted the Model Status Light inthe lower left corner is red. If you try to run a model with an incompleteChecklist, you are informed what Checklist items are not completed. TheModel Status Light will turn green when the checklist is complete.

Figure 5.4 The six Checklist items help you track the status ofyour model


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Status bar

Status light

Figure 5.5 The Status Bar shows the status of each Checklist item.The Model Status Light turns green when the model iscomplete.

Tip: If you double click the checkbox on the Checklist window or StatusBar, the appropriate window will be opened. For example, double-clicking Specify Output Control will open the Output Control window.

Analysis TypeThe first item on the Analysis menu is Analysis Type. This drops downto two menu choices: Transient and Steady Only. By default theTransient option is selected when a new model is opened. A transientmodel always performs a steady-state analysis first, and then proceeds tothe transient solution. The Transient menu selection requires all sixChecklist items to be completed. The Steady Only option requires onlythe first four.


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Specify Steady Solution ControlFor a new model, the Checklist shows two items checked and fourunchecked. The first item is Steady Solution Control; this is checkedbecause the solution control parameters have been set to default valueswhich do not generally require modification.

AFT Impulse attempts to shield less experienced analysts from thedetails of numerical convergence by providing robust defaults. Moreexperienced users of numerical modeling software, however, will desireaccess to the parameters that control the behavior of the Steady-StateSolver. The Steady Solution Control parameters can be modified byopening the Steady Solution Control window from the Analysis menu.More information on Steady Solution Control is given later in thischapter and in Chapter 8.

Specify Output ControlThe second Checklist item, Output Control, is also checked initiallybecause Output Control default parameters have been specified.However, you will probably want to change the title of your analysisfrom the default (on the General tab).

The Output Control window allows you to choose the parameters youwant included in your output and the units for those parameters. You canalso rearrange the output parameters to appear in the order you find mostproductive. These features are discussed in more detail later in thischapter.

Specify System PropertiesThe third Checklist item can be addressed by changing to the SystemProperties window. Here the working fluid(s), gravitational level, andatmospheric pressure are entered. Defaults are assigned for gravity andatmospheric pressure. You can choose to use an Unspecified Fluid, anAFT Standard fluid or, if you have licensed the Chempak add-on, theChempak fluids. Chempak fluids permit user specified mixtures.

Define All Pipes and JunctionsThe fourth Checklist item, Define All Pipes and Junctions, is moreabstract and cannot be addressed like the first three items by simplyopening a window and entering the proper information. This item


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requires that the proper specifications have been entered andconnectivity requirements have been met for all objects in the model.When you have properly defined all Workspace objects, the fourthChecklist item will be completed.

Section PipesThe fifth Checklist item, Section Pipes, is only used when the AnalysisType is Transient. The details of pipe sectioning are related to thetransient solution method, the Method of Characteristics (MOC)discussed in Chapter 9. In summary, the pipe must be broken down intocomputing sections so the MOC can be used. There are strict criteria onhow this is done, and the Section Pipes window automates this process.

The Section Pipes window can only be opened after required pipe inputand fluid properties has been entered, as these data impact how thesectioning is performed.

Transient ControlThe sixth Checklist item, Transient Control, is only used when theAnalysis Type is Transient. Here the model start and stop times aredefined, as well as what data to save to the output file and estimates ofmodel run time and output file size.

Steady Solution Control

The solution control parameters you can specify are tolerances forpressure and flow rate balance, two numerical relaxation parameters, andthe number of iterations to perform before stopping (see Figure 5.6).


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Figure 5.6 The Steady Solution Control window allows controlover parameters that affect the Steady-State Solver

To open the Steady Solution Control window, select Steady SolutionControl from the Analysis menu. The parameters that control thebehavior of the Steady-State Solver are closely linked to the solutionmethodology employed by AFT Impulse. Therefore, a detaileddiscussion of the features in the Steady Solution Control window ispresented in Chapter 8.

The default Steady Solution Control settings have been chosen toprovide the most robust solution settings applicable to most models. Ingeneral, you do not need to modify these parameters. Change them onlywhen you are comfortable with the effect the changes will have on youranalysis or if you have a model that does not converge with the defaultsettings.

Output Control

Most engineering analyses involve processing a very large amount ofinformation, some of which is critical but most of which is of lesser


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importance. The key to accurately and efficiently analyzing engineeringsystems is the proper identification of the critical information that affectsthe system; when this is accomplished, resources and attention can befocused where they will have the most benefit. Too much secondaryinformation can obscure the critical information and result in lost timeor, worse, erroneous conclusions.

AFT Impulse recognizes this dilemma and provides the Output Controlwindow for customizing your output. The Output Control window isavailable on the Analysis menu and is shown in Figure 5.7. This windowis described in detail in Chapter 4 and will be described only brieflyhere. The Output Control window allows you to specify the following:

• Pipe and junction steady and transient output parameters to beincluded in the output

• Engineering units in which the output parameters will be expressed

• The order in which the output parameters will appear

• Content and order of summary reports

• Which pipes and junctions to show in the output

• Changes in parameter values between junctions

• Title appearing on the output report

• Model reference information for detailed documentation purposes

• Formatting preference of the numbers which appear in the outputparameters

• Where to direct the output once it has been obtained


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Change output orderusing this tool

These parameters areshown in the output

These parameters are notshown but may be added

Available parameters maybe shown alphabetically

or by category

Description ofoutput terms

Change units here

Database connection

Figure 5.7 The Output Control window selects parameters forOutput. The list of available parameters (on the left) canbe viewed alphabetically or by category.

System Properties

The System Properties window gives you control over the fluid andenvironmental properties that influence your pipe system analysis.Figure 5.8 shows the System Properties window, which is accessedthrough the Analysis menu.

The three fluid properties that are required for all analyses are density,dynamic viscosity and liquid bulk modulus of elasticity. If only a steady-state analysis is being performed, the liquid bulk modulus of elasticitywill not be used although input is still required. Fluid vapor pressure isan optional specification.

These properties can be entered by hand when the fluid is selected asUnspecified, or they can be obtained from the database list of AFTStandard or Chempak fluids. When selected from the list, density,


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viscosity and bulk modulus cannot be edited as they depend on thespecified temperature (and pressure if Chempak).

Figure 5.8 The System Properties window is where you choosethe fluid(s) for your model. The case above uses an AFTImpulse Standard fluid.

System fluid property variationThe System Properties window allows you to choose from one of twofluid property variation models. The first fluid property variation modelis for constant fluid properties. When this model is chosen, all pipes inthe system are assigned the fluid properties you specify in the SystemProperties window.

The second model is for variable properties. In this model, the fluidproperties assigned in the System Properties window are used as thedefault fluid properties for each pipe. You can then assign different fluidproperties within each Pipe Specifications window. This allows you tomodel systems that may be subject to large temperature variations orsystems that contain more than one fluid.


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If you choose a variable fluid property method, you can also constrainthe density to remain constant by selecting the Always Use ConstantDensity checkbox. By maintaining a constant density, the hydraulicgrade line concept retains meaning, even though the viscosity may vary.

Density and dynamic viscosityThe fluid density and dynamic viscosity can be obtained fromhandbooks, special databases or literature, vendors, or from your ownmeasurements. AFT Impulse allows these parameters to be entered inwhatever units you find most convenient.

Bulk modulus of elasticityThe fluid bulk modulus of elasticity can be difficult to find, although itcan be calculated with density and pressure data. The definition of bulkmodulus is:

ρρ

∆∆

=PK B

Vapor pressure and cavitationThe vapor pressure does not affect the steady-state solution, and thetransient solution can be obtained without vapor pressure. However,certain important phenomena occur when the fluid static pressure dropsto the vapor pressure. These phenomena all result from cavitation.

Cavitation occurs when the fluid static pressure drops to the fluid vaporpressure. In steady flow this is usually a result of flow through arestricted area of some kind. In transient flow it frequently occursbecause of reflected pressure spikes that are negative in magnitude.

Small bubbles or “cavities” of vapor tend to occur and persist until thefluid moves to a location where the static pressure is increasedsufficiently above the vapor pressure to cause them to collapse. Inaddition to causing undesired noise, cavities that collapse in proximity toa structural element can cause severe damage to the structure.

Pumps generally have a required NPSH (Net Positive Suction Head) orNPSP (Net Positive Suction Pressure) that must be provided by the pipesystem to prevent cavitation in the pump. Pump cavitation may lead todegraded pump performance, or, in the worst case, it may damage or


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even destroy the pump. In addition to pump cavitation, cavitation mayoccur downstream of restrictions such as valves and orifices.

The fluid databasesThe AFT Standard Database offers to the user 10 common fluids. Theoptional Chempak Database offers approximately 700 fluids, and alsooffers liquid and gas mixture capabilities.

The AFT Standard DatabaseThe AFT Standard Database offers property data based on temperatureor, if entered by the user, solids concentration.

To add a fluid into your model, select it from the list at the top andchoose the Add to Model button. The fluid will appear in the lower listof Fluids in Current Model. Enter a temperature and click the CalculateProperties button to display the density, viscosity, bulk modulus andvapor pressure.

The AFT Standard Database supports two different independentvariables. Most commonly, the independent variable will be temperature.All fluids provided by AFT are temperature dependent. User createdfluids can depend on either temperature or solids concentration. Fluidproperties dependent on solids concentration are useful for modelingsludge in pipe systems. The choice of which independent variable to useis offered in the Add New Fluid window opened from the FluidDatabase window.

More information on the custom fluids is given in Chapter 7.

The Chempak databaseThe optional Chempak database offers the user approximately 700fluids. In addition, it offers non-reacting mixture calculations.

A Chempak fluid is added the same way as a Standard fluid, by selectingit in the list at the top and choosing the Add to Model button (see Figure5.9). You also have the option of creating a mixture of Chempak fluidsby choosing the Create New Mixture and Add button. This will open theCreate Mixture window shown in Figure 5.10. Here you can specify amixture of fluids in any composition ratio you desire.


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Figure 5.9 The System Properties window is where you choosethe fluid for your model. The case above usesmethanol, a Chempak fluid.

The basis of all mixtures is either mass or mole, and is specified on theSystem Properties window itself. There are no limits to the number ofcomponents in a particular mixture.

The mixtures you create here are referred to as pre-mixtures because youare specifying the composition before the model is run.

Accuracy optionThere is one accuracy option for Chempak fluids and mixtures. TheState Property Accuracy option influences the number of elements usedin curve fits for calculating property data. Typically you should use thehigh accuracy option.


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Figure 5.10 The Create Mixture window (opened from the SystemProperties window) allows you to create predefinedmixtures for the model.

Viscosity modelsThe default viscosity model is Newtonian, which applies to a widevariety of important liquids including water. Some liquids exhibit adependence of viscosity on the fluid dynamics, and these fluids arecalled non-Newtonian. When one of these model is selected, additionalinput parameters are required (see Figure 5.11).

AFT Impulse offers four non-Newtonian viscosity models. Two of themodels apply to pulp and paper. These are the Duffy model and theBrecht & Heller model. Also available are models for Power Law fluidsand Bingham Plastics. See Tilton, 1997 for more information on theseviscosity models. More extensive discussion of these viscosity models isgiven in Chapter 8.


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Non-Newtonian flow in non-pipe elementsThe non-Newtonian viscosity models are designed for calculatingpressure loss in pipes. But what about other elements such as valves?

There does not seem to be any standard calculation methods for suchelements. AFT Impulse therefore offers an approximate method. At youroption, standard K values and/or loss data will be modified by ratioingthe provided loss values (assumed to be Newtonian) by the non-Newtonian and Newtonian friction factors. See Chapter 8 for morediscussion.

If you wish to use these correction factors, select the appropriate optionin the provided fields.

Figure 5.11 Non-Newtonian fluid data on System Properties.

Atmospheric pressureAFT Impulse uses a default atmospheric pressure of 1 standard earthatmosphere, or 14.696 psia. You are free to change this value. Thispressure is also used to convert absolute pressures to gauge pressures.Keep in mind that atmospheric pressure varies with altitude. If you aredoing one project at Sea Level and another at 8500 feet you should


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adjust the Atmospheric Pressure. See Figure 5.12 for the System Datatab.

Gravitational accelerationThe default for gravitational acceleration is 1 standard earthgravitational acceleration, or 32.174 ft/s2. You can change this to amultiple of standard earth accelerations (that is, number of g's) accordingto the design environment for your system. In principle, the body forceon a fluid system does not have to be due to gravity. See Figure 5.12 forthe System Data tab.

Figure 5.12 System Properties window System Data tab

Transition Reynolds NumbersAFT Impulse allows you to modify the transition Reynolds Numbers.The default values are Reynolds Number less than 2300 is laminar andgreater than 4000 is turbulent. In the transition zone a linearinterpolation is assumed. This applies for friction factors. See Figure5.12 for the System Data tab.

You can modify these transition Reynolds Numbers if you choose.


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Editing the AFT Standard fluid databaseAFT Impulse provides a fluid database for numerous common fluids. Ifthe working fluid is not in the database, you may elect to enter the fluidproperties in a custom database for later recall.

To enter or modify fluid properties in the fluid list database, click theEdit Fluid List button or select Fluid Database from the Database menu.

The fluids you define are incorporated into the fluid list as if they werenative to AFT Impulse. The fluid data is saved to disk and read in whenAFT Impulse is loaded. More information on building custom fluiddatabases into AFT Impulse is given in Chapter 7.

If you want to change the data for one of the AFT Standard fluids youcan select the fluid, click Add To Database, then enter a new name forthe fluid. You can now edit this copy of the original fluid data (theoriginal will still exist).

Finding object definition status

In order to complete the fourth checklist item you must properly defineall objects on the Workspace.

The Show Object Status feature helps you identify which objects on theWorkspace are defined and which are not. This feature can be toggled onand off by selecting it from the View menu or by clicking the flood lightbutton on the Toolbar.

Turning Show Object Status on displays the undefined junction and pipeID numbers in the Undefined Color, which is red by default. The colorcan be changed through the Workspace Preferences window. ShowObject Status helps you rapidly assess where your model requiresadditional information.

Leaving Show Object Status always toggled on is not recommended forlarge models, as it will slow the Workspace graphics response. Whenthis feature is on, every Workspace graphical operation you performmust be accompanied by a check of all pipe and junction connections.This check takes time, and could degrade your graphics performance.

The most efficient way to use Show Object Status is to turn it on onlywhen you have finished adding and arranging the Workspace objects. Ifyou need to graphically edit your model, it is best to turn Show Object


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Status off, perform your graphical editing, then turn it back on when youneed it.

Section Pipes

In a transient simulation, a common time step must be used for all pipes.The Method of Characteristics (MOC) requires that each pipe sectionsatisfies the following:

txa

∆∆

= (5.1)

where a is the wavespeed in the pipe, ∆x is the length of the section and∆t is the time step. The length of a pipe section is just the length of thepipe divided by the number of sections in that pipe:

nLx =∆ (5.2)

The time step can be obtained by combining Equations 5.1 and 5.2:

naLt =∆ (5.3)

The maximum time is obtained from the pipe with the smallest value ofL/a. This pipe will have one section (n = 1) and thus the time step willbe:

minmax a

Lt =∆ (5.4)

In any pipe system there will be one pipe that is the controlling pipe.The controlling pipe is that pipe which has the least number of sections,frequently only one. Once the controlling pipe is identified, the time stepis determined by solving Equation 5.4 for ∆t. Then the number ofsections in the remaining pipes is obtained from:

n La ti

i

i

= ∆

(5.5)


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The Section Pipes window automatically determines which pipe is thecontrolling pipe. It also provides searching tools that allow the user toidentify a sectioning strategy that has acceptable error levels.

Error is introduced at this point because of an approximation that isusually required in multi-pipe systems. Because the number of sectionsin pipes is derived from Equation 5.5, typically the number of sections,n, will not be a whole number. Since partial sections in a pipe cannot bemodeled, an alternative must be found.

The wavespeed for each pipe is the least certain parameter. Therefore,the wavespeed in each pipe is allowed to depart slightly from its originalcalculated value in order to cause n to be a whole number. Introducingthis error term modifies Equation 5.5 to the following:

( )n L

a tii

i

=±1 ψ ∆

where ψ is the accepted uncertainty in wavespeed up to ±15% (Wylie,et al., 1993, pp. 54).

Figure 5.13 The Section Pipes window automates the process ofobtaining appropriate sectioning criteria


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Figure 5.13 shows the Section Pipes window. The controlling pipe (#8)is shown at the right. The sectioning criteria is shown at the upper left,where the average and maximum sectioning errors are shown. The tableat the bottom shows the selected sectioning strategy and resultingmodified wavespeed. The resulting time step size is shown at the right.

Numbering conventionOnce a pipe is broken into sections, computation takes place where thesections join together. These are called pipe stations. The total numberof stations in a pipe is equal to the number of sections plus 1. The stationat the beginning of the pipe is numbered zero. See Figure 5.14.

Computingstations

1 2 3 4 5 6

0 1 2 3 4 5

Pipe sections

6

Figure 5.14 The computing stations are at the pipe sectionboundaries. The number of stations is one more thanthe pipe sections.

To obtain the pressure difference across a junction, compare the outletcomputing station of the junction’s upstream pipe to the first computingstation of the junction’s downstream pipe (see Figure 5.15).

J1 J2 J3P1 P2

J2P1

1,0 1,1 1,2 1,3 1,4

P2

2,0 2,1 2,2 2,3

Figure 5.15 The pressure drop across the valve at J2 in the AFTImpulse model at the top is given by the differencebetween the solutions at Pipe 1, station 4 and Pipe 2,station 0.

For junctions with more than one connecting pipe, the principle is thesame (see Figure 5.16).


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J1

J2

J3

P1 P2

J4

P3

P11,01,1

1,21,3

P2

P3

2,02,1

2,22,3

2,43,0

3,13,2

3,33,4

J2

Figure 5.16 Pipe station convention for AFT Impulse model. Pipes1, 2 and 3 have 3, 4 and 4 pipe sections, respectively.Because there is no loss at the J2 branch, the pressuresolution for Pipe 1, computing station 3 (i.e., 1,3) is thesame as both Pipe 2, computing station 4 (i.e., 2,4) andPipe 3 computing station 0 (i.e., 3,0). The flow solutionsat these three station will sum to zero at all times (in theabsence of vapor cavitation).


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Transient Control

Figure 5.17 shows the Transient Control window. At the top of thewindow is an area called the Transient Solver Control area. This areaoffers numerous input fields detailed in the following descriptions.

Figure 5.17 The Transient Control window configures the behaviorof the transient solver and output file data

Start and stop timeThe Start Time is the time at which you want the simulation to begin.Usually this will be zero, but you can define the starting time as any timethat is convenient for you.

Note the junction transient data that is time-based is referenced to thestart time. That is, if you want the simulation to begin at -10 seconds, theinitial transient data point should be at -10 seconds. The Stop Time is ofcourse when you want the simulation to end.


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Shown right below is the Time Step Size. This is selected in the SectionPipes window, and is statically shown here for reference. Along with theStart and Stop Times, the Total Time Steps can be determined. This isshown statically below the Time Step Size.

Save output to file…When AFT Impulse runs a simulation, the transient output data that isgenerated is written to an output file. This is a file with a .out extension.

There are two options in the lower left that allow you to specify howfrequently data is written to the output file. In the Figure 5.17 example,there are 20112 time steps that will be calculated in the simulation. Thefirst option, "Save Output to File Every Time Step", will write 20112time steps of data to file if chosen.

Output files can become extremely large if the data is saved for everytime step, and in such cases the second option (in Figure 5.17) "SaveIntermediate Output to File…" is a better choice. With this option AFTImpulse will skip over writing output to file at certain time steps. It willwrite it on increments of time step as specified in the input box. It isrecommended this value never be set above 25. In Figure 5.17, resultsfor every tenth time step will be written, for a total of 2011 time steps.

When output data is skipped, it will not appear in the output results orgraphs. The justification for skipping over data is that over small timeincrements the transient behavior is sufficiently linear that the graphedresults, for example, will appear the same as when all data is graphed.

It should be noted that AFT Impulse tracks the maximum and minimumvalues for pressure and flowrate, and does this for all computationswhether saved to the output file or not. Thus a review of the maximumand minimum results in the Output window will alert you to whether anysignificant data was skipped by using the intermediate output to filefeature.

Use variable pipe resistanceTraditional waterhammer modeling has ignored the effect of varyingpipe resistance (i.e., friction) with flowrate. This is for several reasons.First, for highly turbulent flow (i.e., large Reynolds numbers) the piperesistance does not depend on flowrate at all. On Moody charts this issometimes referred to as the "fully rough zone". Second, for turbulent


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flows that are not in the fully rough zone, the dependence of piperesistance is weak. Third, the waterhammer phenomenon is not affectedvery much by relatively small changes in pipe resistance. And finally,introducing varying pipe resistance complicates the calculation process.

Therefore, accounting for varying pipe resistance is not important inmany waterhammer applications. One application where it is importantis when the initial steady conditions have no flow in some pipes. Forinstance, when the transient being modeled is a pipe with a closed valvewhich is being opened. In such cases, it is difficult to obtain the steady-state pipe resistance because the flow is zero and thus no Reynoldsnumber exists. Thus it is probably best to use variable resistance.

If you choose to not use variable resistance, the pipe frictional resistanceobtained in the steady-state part of the simulation will be used as aconstant in the transient part of the simulation. On the other hand, if youallow the resistance to vary, the pipe friction will differ for each pipesection because the flowrate (and hence Reynolds number) is varyingalong the pipe due to the waterhammer transient. Further, the resistancewill vary with time as the flowrate changes.

If variable resistance is modeled, the required run time increases.Experience suggests the run time increase by about a factor of three.

Model transient cavitationWhen the transient pressure drops below the vapor pressure, transientcavitation will occur. You can model this using the cavitation modelsprovided. In AFT Impulse 3.0 there is only one model and it is called theDiscrete Vapor Cavity Model. If you do not model cavitation, then thefluid pressure can go below the vapor pressure and, additionally, belowabsolute zero pressure.

If the steady-state results indicate cavitation is occurring, this will causea problem for the transient simulation and an artificial transient may begenerated. If so, a warning will be given.

Stop run if artificial transient detectedArtificial transients occur when there is a mismatch between the steady-state results and the initial transient time step calculation. Many stepshave been taken to avoid artificial transients, but it is always wise toensure they do not exist. If you want the transient simulation to


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immediately stop if artificial transients are detected, then select theoption to have the simulation stop. If you choose to not stop thesimulation, the artificial transients will be displayed in the warningssection when the simulation is complete.

Include in transient output fileEarlier in this chapter it was discussed how one can skip saving outputfor certain time steps in order to minimize output file size. One can alsochoose to skip saving output for certain pipe stations. By default, all pipestations are saved but no junction data is saved. Here we will discusshow to specify what data is saved and the advantages and disadvantagesassociated with these choices.

Saving pipe outputFor pipes you have six choices for each pipe:

1. All Stations

2. Inlet and Outlet

3. Inlet Only

4. Outlet Only

5. None

6. User Specified (Up to Five)

As mentioned, the default is to save data for all pipe stations. Frequentlythe user is only interested in what happens at the inlet and outlet of thepipe, and, if so, a good choice is to only save the inlet and outlet. Thelast choice, User Specified, allows one to specify up to five stations thatwill be saved to the output file.

By selecting in the table your choices, you can save all stations for somepipes and no stations for the others. If you want to change settings for allpipes, use the "Change All Pipes To" button.

Disadvantages to saving all stationsWhen saving all stations, the output file size may be increaseddramatically. In addition, saving output to file takes a lot of time, andrun times can be increased significantly if all stations are saved. In suchcases, a good compromise is to save only the inlet and outlet stations.


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Advantages to saving all stationsIf one wants to see the pressure profile along a pipeline at certain timesduring the simulation, then one must save all the pipe stations. However,if one is interested only in the maximum and minimum pressures, theseare always saved to the file regardless of how many stations are saved.The maximum and minimum profile can be graphed in the Graph Resultswindow.

Saving junction outputAs mentioned, by default no junction output is saved. In general, mostdata for junctions can be obtained from the pipes. For instance, theflowrate through a valve over time can be obtained by looking at theupstream pipe flow rate. However, certain types of junction data is notobtainable in this way. In such cases, the data can be saved separately tofile and reviewed in the output or graphed.

Here are some examples of junction data that can be saved:

• Surge tank liquid surface height, inflow and total liquid volume

• Gas accumulator volume and pressure

• Spray discharge outflow

• Vacuum breaker valve inflow and total volume

• Pump speed

You can select which junction data you want to save by selecting theJunctions tab (Figure 5.17) and then selecting the junctions in the list.

Estimated file size and run timeAt the bottom of the Transient Control window (see Figure 5.17) is anestimate of the output file size and runtime.

Defining pressures for the system

AFT Impulse needs a pressure specified in the system, to be used as thereference point upon which all other pressures are based. In certaincases, however, more than one pressure must be specified. These cases


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include systems which contain pressure control valves, flow controlvalves, internal relief valves, or flow-specified pumps.

Chapter 12 contains a comprehensive discussion on this topic. See theRole of pressure junctions.

Using scenario manager

Scenario Manager is a powerful tool for managing variations of a model,referred to as scenarios. Scenario Manager allows you to:

• Create, name and organize scenarios

• Select the scenario to appear in the Workspace (the ‘current’scenario)

• Delete, copy and rename scenarios

• Duplicate scenarios and save them as separate models

• Review the source of a scenario’s specifications

• Pass changes from a scenario to its variants

All scenarios are saved within the model file.

The Scenario Manager windowIllustrated in Figure 5.18, the left side of the Scenario Manager windowcontains the scenario tree, displaying the names of the currently definedscenarios, their relationship to other scenarios and which is the currentWorkspace scenario (the current scenario is also identified in AFTImpulse’s Status Bar). On the right, the Summary tab displays asummary of the selected scenario’s specifications and a place where onemay record notes related to that scenario. The Details tab displays theselected scenario’s object tree, illustrating the scenario’s pipes andjunctions by number and name, the values for the various object’sattributes and the source of those values (see Figure 5.19).

Those attributes whose value comes from a different scenario are linked,while those values coming from the selected scenario are not linked.Linked attributes will inherit changes made in the same object andattribute of the linked ancestor. This inheritance will occur over as manygenerations as the link exists (see Figure 5.20).


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Creating, organizing and editing scenariosThe “All Scenarios Defined in Model” portion of the Scenario Managerwindow shows the name and relationship of all currently definedscenarios. Scenarios are referred to as either a parent or child, plus theBase Scenario, which may be thought of as the root scenario. Uponopening a new model file, only a Base Scenario will be present. Tocreate a new scenario below the Base Scenario, select the Base Scenarioby clicking on it, then click the Create Child button and enter a name forthe new scenario.

Figure 5.18 illustrates a model with a Base Scenario with three children,‘3 in PVC Pipe’, ‘3.5 in PVC Pipe’ and ‘4 in PVC Pipe’, representingthree different size pipe runs. Each of these, in turn, has two children,‘North hydrant open’ and ‘South hydrant open’, for the two operatingcases.

CurrentWorkspace

scenario

Scenario tree

Summaryinformation for

selectedscenario

Rename,delete, clone,

promote & savescenarios byclicking here

Create a newscenario by

clicking here

Figure 5.18 Scenario Manager Window

‘3 in PVC Pipe’, ‘3.5 in PVC Pipe’ and ‘4 in PVC Pipe’ were created byfirst selecting the Base Scenario then clicking on Create Child. ‘Northhydrant open’ and ‘South hydrant open’ were, in turn, created byselecting each of the above children scenarios for the three different pipe


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run sizes, then clicking on Create Child (Note: alternatively, one couldhave made the Base Scenario current, created the ‘3 in PVC Pipe’ child,then selected it and created the ‘North hydrant open’ and ‘South hydrantopen’ scenarios below it, then, with ‘3 in PVC Pipe’ still selected, clickon Other Actions / Clone With Children twice, then rename the newclones ‘3.5 in PVC Pipe’ and ‘4 in PVC Pipe’). The ‘North hydrant’ and‘South hydrant’ scenarios may be thought of as grand children of theBase Scenario. This concept of scenario ancestry is helpful in keepingtrack of the differences and similarities between scenarios discussedlater.

A box with a plus sign next to a scenario indicates it may be expanded toreveal children scenarios while a box with a minus sign indicates thelevel is fully expanded with all children scenarios shown, if any. No boxindicates there are no children. Levels may be expanded by clicking onthe plus sign and condensed by clicking on the minus sign.

Clicking on the Other Actions button reveals a menu providing thefollowing choices:

• Rename – renames the selected scenario

• Delete – deletes the selected scenario

• Clone Without Children – creates a copy of the selected scenario atthe same level (a sibling)

• Clone With Children – creates a copy of the selected scenario alongwith its children

• Promote – raises a scenario one level, i.e. from a child to asibling…active only for scenarios more than one level below thebase scenario

• Save Scenario to File Without Children – saves the selected scenarioto a new file without its children

• Save Scenario to File With Children – saves the selected scenarioand its children to a new file

It is important to remember that AFT Impulse’s Edit menu Undo doesnot apply to Scenario Manager operations. To undo a Rename scenariooperation, you can rename the scenario again to the old name. To undo aClone or Save Scenario operation, you can delete the clone or filecreated. Once a scenario is deleted or promoted, there is no way to un-delete or un-promote the scenario.


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Attributes of objectselected in object tree

along with currentValue, Units and

Scenario providing thesource of the value

The attributes list maybe filtered based on

the source of theattribute values

Object tree for the ‘3 inPVC Pipe’ scenario

Figure 5.19 Object tree and object attributes in Scenario Manager

Modifying individual scenariosWhen first created, a scenario is identical to its parent. To modify or runa scenario, it must first be made the current Workspace scenario byselecting it within the Scenario Manager window, then clicking on theLoad As Current Scenario button. (The name of the current scenario isdisplayed in both the Scenario Manager window and in AFT Impulse’sStatus Bar.) Once a scenario is the current scenario, changes may bemade to differentiate it from its parent using any of AFT Impulse’sediting tools and functions. When an attribute is changed within ascenario it breaks the link for this attribute. Subsequent changes in thescenario’s ancestors will no longer affect this attribute.

Properties of a scenario that may differ from its ancestors include:

• Presence and location of objects – one may add or delete pipes andjunctions within a scenario and the workspace locations may varyfrom scenario to scenario

• Pipe and junction specifications – all values set in the specificationwindows


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• System properties – fluid selected, temperature of fluid, viscositymodel, system data

• Solution control – solution method, tolerances, relaxation

• Output Control

• Transient Control

• Database Manager

• Visual Report Control

Workspace Preferences, Parameter and Unit Preferences, GeneralPreferences, Toolbox Preferences and Model Data Control apply to allscenarios.

Passing changes to child scenariosScenario Manager will pass changes made in a scenario to linkedattributes of its descendants, allowing multiple scenarios to be modifiedin one operation and maintain similarity between scenarios wheredesired.

Any attribute whose value is the same as that of the scenario’s parent islinked to the parent. In turn, if the parent’s attribute value is the same asthat of its parent, then the child’s attribute is linked to its grandparent.This ancestral linking of attributes can exist across any number ofgenerations. A change made to a scenario will be passed downward to itschildren, grandchildren, etc., as far as the link exists. Changes are NOTpassed upward from child to parent.

Once an attribute is changed within a scenario, the link is broken.Subsequent changes in the scenario’s ancestors, parent, grandparent,etc., will no longer affect that attribute. The Details tab of the ScenarioManager provides the information needed to determine attribute linking.See Figure 5.20.


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Attributes for a pipe in the‘3 in PVC Pipe’ scenario

These attributes willinherit changes made in

the ‘Base Scenario’

This attribute has beenchanged in the ‘3 in PVC

Pipe’ scenario and will notinherit changes made in the

‘Base Scenario’

Figure 5.20 Attribute values and sources for the 3 in PVC Pipescenario in Scenario Manager

Re-establishing broken linksA link may be re-established by returning the attribute to the same valueas that of its parent. This can be done manually be entering the value orselecting the Same As Parent option from within a pipe or junctionspecifications window, Steady Solution Control, Output Control, SystemProperties, Transient Control, Database Manager or Visual ReportControl.

Since links are identified by comparing attribute values of pipes orjunctions with the same Workspace ID number, renumbering a scenariowill break the links of all pipes and junctions renumbered. Sincenumbers must be unique, once a link has been broken by renumbering, itmay not be re-established.

Fast Scenario ChangesAFT Impulse remembers the most recent scenario you had open, andallows you to quickly change back to that scenario by using the LastScenario feature. Last Scenario is found on the View menu and Toolbar.


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Special modeling features

The modeling utilities described in this section help you build yourmodel more rapidly and keep track of what you are doing and whatassumptions you have made.

Waterhammer AssistantThe Waterhammer Assistant can be opened from the Help menu orToolbar. The Waterhammer Assistant reviews your model and offershelpful advice (see Figure 5.21).

Figure 5.21 The Waterhammer Assistant offers advice on modelingissues

By default, whenever a model is run AFT Impulse checks with theWaterhammer Assistant to see if it has any advice. If it does, theWaterhammer Assistant is displayed. The user then has the option of


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canceling the run or ignoring the advice and continuing. If you do notwant the Waterhammer Assistant to be consulted before each run, youcan specify this choice in the General Preferences window opened fromthe Options menu.

Workspace FindThe Find window utility shown in Figure 5.22 helps you find a particularpipe or junction by its ID number and name. The Find utility becomesmore useful for models that expand over multiple Workspace pages.

You can access the Find utility by selecting Find on the View menu orby clicking the button on the Toolbar. The Workspace is moved so thatthe pipe or junction is located near the center of the screen.

Figure 5.22 The Find feature for the Workspace is helpful forlocating a specific pipe or junction. It is opened fromthe View menu or Toolbar.


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Reverse DirectionEach pipe you create has a reference positive flow direction as indicatedby the arrow on the pipe. If you want to reverse the reference positiveflow direction, you can do so by manually moving the pipe endpoints.However, this procedure is tedious. The Reverse Direction utility togglesthe reference positive flow direction for the selected pipe or pipes.

The Reverse Direction utility is accessed from the Arrange menu or fromthe reverse direction button on the Toolbar.

Select SpecialSelect Special is a tool for selecting or deselecting objects based oncertain criteria (see Figure 5.23) and allows you to quickly select a groupof objects.

Figure 5.23 You can customize your selections using SelectSpecial. It is accessed from multiple locations withinAFT Impulse to assist with selecting groups of pipesand/or junctions.


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When first opened, Select Special will select all the objects currentlyselected on the Workspace. You may alternatively select objects basedon several other criteria available from the Selection Type drop-downmenu. Objects in the Pipe Selections or Junction Selections list may beselected or deselected based on the criteria by clicking the Select Pipesor Select Junctions button or the Deselect Pipes or Deselect Junctionsbutton.

Invert will deselect all selected objects and select all unselected objects.Clicking on the Select All or None buttons will result in all of theobjects to be selected or deselected respectively.

Several select/deselect operations may be performed in succession.When finished, click on OK button.

This tool is available from several windows and will select the items onthe window from which it is opened. For example, it could be used todisplay only the closed valves in the Output window when it is accessedfrom the Show Selected Pipes/Junctions tab in the Output Controlwindow.

Tip: To quickly show only a part of the model in the Output window,first select the objects on the Workspace for which you want to seeoutput. Then, on the Show Selected Pipes/Junctions tab in the OutputControl window, clear all pipe and junction selections by clicking theNone button. Then click on Select Special and, with Selected onWorkspace showing in the Selection Type, click Select Pipes, then clickOK. Repeat this for junctions and, finally, click OK in the OutputControl window and only the pipes and junctions you selected on theWorkspace will be shown in the Output.

Special ConditionsSome junctions have Special Conditions that alter the normal state of thejunction. For example, a valve can be closed, a pump turned off, or arelief valve opened.

The special conditions are set in one of three ways:

1. By selecting the junction(s) on the Workspace and choosing SpecialConditions from the Edit Menu


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2. By selecting the junction(s) on the Workspace and clicking theSpecial Condition icon on the Toolbar

3. By opening the junction’s Specifications window and clicking theappropriate condition on the Optional tab

The junctions that have special conditions set are shown using a specialsymbol before the ID number (an “X” by default). This symbol can becustomized in the Workspace Preferences window.

As shown in Figure 5.24, when the special condition results in a sectionof the model being closed or “turned off”, the pipes which will have zerosteady-state flow are displayed as dotted lines. This indicates visuallythat they are cut off from flow; the junctions are outlined with a dottedline. The default dotted line can be changed in the WorkspacePreferences window.

An X next to thejunction number

indicates a specialcondition has been set

Dashed lines for pipesand around junctions

indicate closed modelsections

Figure 5.24 Workspace display when a valve is closed using theSpecial Condition. Notice the “X” next to the valvenumber (J14), which is shown in red by default. Alsonotice the connecting pipes shown with dashed linesand junctions with dashed outlines which indicatenonflowing sections of the model.


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The Special Conditions for Control Valves are dependent on the ActionIf Control Is Lost setting for each valve. If the Action is to Fail Openthen the Special Condition for the control valve is to be failed open withthe loss equal to the Loss When Fully Open value. If, on the other hand,the Action is to Always Control or Fail Closed then the SpecialCondition for the control valve is to be closed.

Special conditions with no transient dataIf a Special Condition is applied to a junction without any transient data,then the transient solution will retain the Special Condition. Forinstance, if a Special Condition is set for a valve junction, the steady-state solution will solve with the valve closed. In addition, the transientsolution will run the entire simulation with the valve closed as wellbecause there is no transient data entered which implies the valve state isnot intended to change.

Special conditions with transient dataIf a Special Condition is applied to a junction that has transient data,then the transient solution will ignore the Special Condition. It is thenincumbent upon the engineer to relate the Special Condition to the initialtransient data.

For instance, assume the user wants to simulate the transient that occursduring a valve opening. The steady-state solution will have the valveclosed, and this is modeled by using a Special Condition for the valvejunction. In such a case, the Cv data entered on the valve junction's LossModel tab will be ignored.

On the Transient tab, a transient can be entered. The first data in thetable should then be Cv = 0, which corresponds to a closed valve. Fromthere the Cv can be increased above zero as desired to open the valve.

If the user were to keep the Cv equal to zero, then that would beequivalent to the previous case where the user set the Special Conditionbut did not enter any transient data.

Pump special conditionsPump Special Conditions are slightly more complicated than for valves.In fact, pump junctions have two types of Special Conditions. The first isto turn the pump off and have no flow through it. This is called "PumpOff No Flow". The second type of Special Condition turns the pump off


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but allows flow to go through the pump. This is called "Pump Off WithFlow Through".

Why would one want to use one Special Condition rather than the other?The simple answer is that the first Special Condition is more appropriatefor positive displacement type pumps and the second for centrifugalpumps.

Here's why. When a positive displacement pump is turned off and has apressure difference across it, it will usually act like a closed valve andnot allow flow to go through it. Thus the first Special Condition wouldbe most appropriate. For instance, assume one wants to model thetransient that occurs during the startup of a positive displacement pump.The best way to do this would be to use the first Special Condition withno flow, and then input a flowrate transient in which the first data pointis zero flow.

On the other hand, when a centrifugal pump is turned off and has apressure difference across it, in the absence of other valving whichprevents flow the pump will usually allow flow to go through it. Thusthe Special Condition that allows through flow would be mostappropriate. For example, assume one wants to model the transientduring the startup of a centrifugal pump. One would use the secondSpecial Condition, and, in conjunction with a pump curve entered at100% speed, input a speed transient with the initial speed as zero.

Special conditions only applied to junctionsSpecial Conditions in AFT Impulse can only be used for junctions.Unlike AFT Fathom, Special Conditions cannot be applied to pipes. Thereason is fairly simple. In AFT Fathom a pipe Special Condition closesthe pipe. But in reality one does not close a pipe. One instead closes avalve that is in the pipe. For user convenience AFT Fathom overlooksthis fact.

In contrast, AFT Impulse does not have as much freedom in such cases.When transients occur, there are reflections off system components suchas valves. If one were to "close a pipe" in AFT Impulse, it would not beknown exactly where the closure exists and thus the transient reflectionscould not be properly modeled.


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Transient special conditionsJunctions which accept transient data also support Transient SpecialConditions. The default for all junctions is None. The other choice isIgnore Transient Data. This allows one to have no transient initiation atthat junction without having to delete the transient data.

When a Transient Special Condition is set to Ignore, a # symbol isdisplayed adjacent to the junction number on the Workspace. Thissymbol can be customized in the Workspace Preferences window.

No reflections – infinite pipeThe Assigned Pressure and Assigned Flow junctions support a third typeof Transient Special Condition. It is called No Reflections (InfinitePipe). This condition is appropriate for the entrance to a long pipe linefrom which no reflections will occur during the simulation period.

When this Transient Special Condition is specified, a * symbol isdisplayed adjacent to the junction number on the Workspace. Thissymbol can be customized in the Workspace Preferences window. Adiscussion of infinite pipe behavior is given in Chapter 12.

Special Condition IgnoreThe Vacuum Breaker Valve, Gas Accumulator, Liquid Accumulator,Surge Tank, and Relief Valve junctions have a unique kind of SpecialCondition called Ignore. This makes the steady-state and transientsolvers ignore the presence of the valve. Thus not only is the dataignored, the junction itself is ignored. This is convenient when locatingone of these devices by trail and error. Rather than having to delete thejunction, it can be ignored.

Here is an example of when this feature might be useful. Assume thatone is trying to find the best location for a gas accumulator. If one doesthis with a single gas accumulator junction, it will be required to run acase, view the results, then relocated the junction by changing the pipelengths and rerunning. If one uses the Special Condition Ignore, one canplace multiple gas accumulator junctions in the model and then set all ofthem except one to Ignore so that only one of the junctions is used at atime.


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Merging modelsTwo models can be merged together using the Merge command on theFile menu. Here’s how:

• Open the first model

• Choose Merge on the File menu

• From the file list, select the model to be merged

If there are duplicate numbers, the objects in the second model will bechanged. The second model will be selected (highlighted) so that youcan move it into position (be careful not to click off the selected portionsor the second model may be deselected).

Tip: To move a selected group of objects, use a junction icon, instead ofa pipe, to drag the group. This will avoid missing the pipe and clickingon an empty area of the Workspace which will deselect your group.

Merging models with multiple scenariosIf the model you are merging from has multiple scenarios, only the basescenario can be merged. If you want to merge one of the children of thatmodel, do the following:

1. Open the model which you want to merge.

2. Open the Scenario Manager.

3. Select the child scenario you want to merge.

4. On the Other Actions button, choose the Save Scenario to FileWithout Children.

5. Enter a name and click OK. This process creates a new Impulsemodel with the desired scenario as the base scenario.

6. Open the original model and merge in the new model file created inStep 5.

Print Preview/SpecialThe Print Preview/Special window shown in Figure 5.25 allows you toaccess special print features such as Print Preview and Fit to One Page.It is accessed from the File menu.


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The Print Preview/Special window also offers you a preview of the printcontent and format for all primary windows. By clicking the PrintPreview button you can review your printed material at several zoomstates. Individual pages can be printed.

The Fit On One Page feature causes the graphic image generated fromthe Workspace or Visual Report windows to be compressed onto a singlepage. This is a convenient feature for creating reports of your model.AFT Impulse supports whatever printer page sizes are supported by yourprinter, so use of legal paper or larger may allow you to get more visibledetails. The graphics can also be centered vertically and/or horizontally.You also can fit your content onto multiple pages.

Figure 5.25 The Print Preview/Special window is opened from theFile menu or Toolbar

The Print Preview/Special window allows you to set the pageorientation, which can also be changed once you are in the Print Previewwindow itself.

Several other special printing features affect the Workspace and VisualReport. The Only Print Selected Pipes and Junctions works withWorkspace printing by selectively printing the pipes and junctionscurrently selected on the Workspace.

The Include Hidden Pipes and Junctions feature includes hidden pipesand/or junctions on the Workspace or Visual Report in the printedcontent. By default this box is not selected, and hidden pipes andjunctions are not included in Workspace and Visual Report printouts.


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By default, AFT Impulse prints the junction icons for the Workspace andVisual Report with the same colors as displayed on the screen. It alsoprints the background color as the same color as your Workspace orVisual Report background. You can also specify that the junction iconsbe printed in black and white (still with the color background) or that theentire printout be black and white.

If you added a Workspace Picture into the background, you can includethis picture in the printout.

Finally, if you have a grid displayed on the Workspace or Visual Report,you can include the grid in the printout.

Transfer Results to Initial GuessesAFT Impulse’s iterative Steady-State Solver discussed in Chapter 8takes initial guesses (also called initial conditions) and applies theprinciples of mass and momentum balance to improve the guesses. Thisis called iteration. It continues to refine the solutions until agreementwith the governing equations is obtained.

After a converged answer is obtained, you can choose to take theseresults and use them as the initial guesses for a subsequent run. Thiscauses much faster convergence in subsequent runs because you arestarting the Steady-State Solver at or very close to the actual solution.

This feature is called Transfer Results to Initial Guess, and it is on theOutput window Edit menu and Toolbar.

Batch modeAFT Impulse models can be run in batch mode, a feature that isespecially useful for running a group of models during lunch orovernight.

To run models in batch mode, follow these steps:

1. Create a text file that lists the file names of all models to be run.This is called the batch list file.

2. Either copy the input model files to the same directory as the batchlist file, or specify the complete path name for the input model files.Include the disk drive letter in the full path name. If the models are


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copied to a common directory, then only the file names are requiredin the batch list file.

3. Choose the Start Batch Run option from the AFT Impulse File menu.

4. Using the Print Format window, select the print content and formatyou desire, including font style. All reports will use the specifiedfont.

5. After closing the Print Format window, choose the batch list file youwant to run.

6. Assign the destination of the output results and start the run.

AFT Impulse then opens each model in sequence, automatically runseach model that has a completed Checklist, and sends results to thespecified destination. A special file called BATCH.LOG keeps track ofall pertinent information about the batch run. This file will automaticallyprint at the end of the batch run.

The BATCH.LOG file also logs any error messages that are displayed inthe process of running the models. The normal error messages youreceive when running AFT Impulse in interactive mode are not displayedon the screen, because they would interrupt the batch run.

The entire batch run can be canceled at any time by clicking the Cancelbutton in the Solution Progress window.

Running Scenarios in Batch ModeIf you save a model in any scenario that is not the base scenario, uponopening the model AFT Impulse will prompt you whether you want toopen into the scenario which was active when the model was saved, orinto the base scenario.

However, if you are running in batch mode the scenario active when themodel was saved is automatically opened without prompting.

Thus to run scenarios in batch mode, just save the model with thescenario active which you want to run. If you want to run multiplescenarios from the same model, for each scenario you want to run copythe model file as a different file name and then save the desired scenarioin each file.


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Print FormatWith either the Model Data or Output window active, you can open PrintFormat from the View menu (see Figure 5.26), allowing you to specifythe content and format of Model Data and Output reports.

You can include Model Reference Information (entered in the OutputControl window) to document your model or make important commentsyou want kept with the model.

The various tables in the General section, Pipe section, and Junctionsection can be selectively printed. All Pipe and Junction Notes areshown in the Model Data General section and can be printed at youroption.

Empty columns in the three Model Data tables can be ignored in theprinted report. You can also have each of the Print Content sections starton a new page. Both of these are formatting options in the Print Formatwindow.

Figure 5.26 Print Format window specifies the print content of theModel Data or Output window. It is opened from theView menu or Toolbar.


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The printer font and style of your choice can be selected for the printedreport. Large reports can be condensed by selecting small fonts. A goodchoice for large reports is Arial True Type Regular Style at a 6 or8-point size.

Extended Model CheckThe connections and settings of each pipe and junction are exhaustivelychecked using the Extended Model Check item on the View menu. Thisis also helpful if the model seems to not be connected correctly.

Math calculatorFor convenience, AFT Impulse offers you quick access to the WindowsCalculator; simply select Calculator from the View menu or pressCTRL+K in any specifications window.

Working with the steady and transient solvers

The term Solver conveys a mathematical equation solution methodwhose purpose is to model some physical phenomenon. Theimplementation is in the form of algorithms and subroutines which solvethe equations. AFT Impulse 3.0 has two Solvers because it is solving twoproblems.

Steady-state solverThe Steady-State Solver obtains a balanced steady-state solution to thesystem being modeled. It employs a Newton-Raphson matrix methodthat is iterative nature. An initial guess to the steady-state solution ismade, and the Newton-Raphson method progresses the guess towards afinal solution that agrees with the governing equations within sometolerance. Details are given in Chapter 8.

Control of the Steady-State Solver is given in the Steady SolutionControl window opened from the Analysis menu. Here the user canspecify items such as tolerances, relaxation and matrix methods.

The Steady-State Solver is based on that in AFT Fathom™, acommercial steady-state modeling software developed by Applied FlowTechnology. AFT Impulse contains most of the capability found in AFT


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Fathom for solving systems with known density, and can optionally beused for steady-state modeling without reference to transient modeling.

If you want to run a model in steady-state mode only, on the Analysismenu specify the Analysis Type as "Steady Only".

When you specify the model as Steady Only, certain input data normallyrequired for transient models are no longer required. Some examples arewavespeed for pipes, liquid bulk modulus, and input for the SectionPipes window and Transient Control on the Analysis menu. In fact,when modeling Steady Only the Checklist adjusts to only require fouritems to be completed.

Recommendations on Steady SolverIt is recommended that before running a transient simulation, the userfirst run the model in Steady Only mode to help ensure the model isassembled properly. Once confidence is established in the steady-stateresults, the Transient feature can be enabled and run.

Transient solverThe Transient Solver models the surge transients in a pipe system usingthe Method of Characteristics (MOC). Unlike the iterative Newton-Raphson matrix method used for steady-state solutions, the MOC is adirect solution method that marches in time. Details are given in Chapter9.

To function properly, the MOC requires an accurate steady-state solutionto provide the initial conditions. Thus when one runs a transientsimulation in AFT Impulse 3.0, the Steady-State Solver is always runfirst. The steady-state solutions are then automatically passed on to theTransient Solver, which uses them as the initial conditions for the MOC.This will reduce artificial transient problems to a bare minimum.

Artificial transientsThe fundamental cause of artificial transients is mismatched steady-stateconditions. One would hope that artificial transients would never occurin AFT Impulse 3.0, but that is not the case. If they do occur, there is oneof two causes. First, it may be an indication of a problem with the steadysolution. This could be a problem in the Steady-State Solver, or an inputproblem that could not be identified when the model was built.


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Secondly, a user could potentially specify mismatched steady-state andtransient behavior of a junction. For example, for a valve junction theuser inputs a steady-state valve Cv on the Loss Model tab. In addition,the user can enter a transient valve Cv on the Transient tab. The initialtransient Cv should match the steady-state Cv on the loss model tab. Ifthey do not, then an artificial transient would occur.

AFT Impulse attempts to identify such inconsistencies when the data isfirst input and then again before the model is run. If for some reasonAFT Impulse does not catch this problem and bring it to the user'sattention, then an artificial transient will occur.


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C H A P T E R 6

Pipe and Junction SpecificationsWindows

This chapter discusses the Pipe and Junction Specifications windows,the features they offer, and how you use these features to obtain a propermodel of your system.

Each pipe and junction on the Workspace has an associatedSpecifications window. In the Specifications window, you enter theengineering data that represents the pipe or junction you are modeling.The Specifications window also contains AFT Impulse's interpretation ofthe connectivity within your model.

Each of the nineteen junction types has a separate Specificationswindow. The Specifications window is the primary vehicle for enteringinput data for the components of your model. The Global Edit windows,discussed at the end of this chapter, can be used to change data formultiple pipes and junctions all at the same time.

You can access an object's Specifications window in six ways:

1. Double-clicking the object on the Workspace

2. Selecting the object on the Workspace and pressing ENTER

3. Selecting the object on the Workspace and clicking the OpenPipe/Jct Window icon on the Toolbar

4. Double-clicking the left column of the appropriate table in theModel Data window (where the object ID number is shown)

5. Clicking the Jump button in another Specifications window


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6. Double-clicking the connected pipe or junction number in anotherSpecifications window

Highlight featureThe highlight feature identifies required input data in the Specificationswindow. You can toggle the highlight feature on and off in any of thefollowing ways:

• Double-click within the specifications window (outside of thetabbed area)

• Press the F2 function key

• Choose Highlight in Pipe and junction Windows from the Optionsmenu

The highlighting feature may be especially useful when you are firstlearning to use AFT Impulse or when you are having difficulty obtaininga defined object status.

Jump featureBy clicking the Jump button in a Specifications window, you can movedirectly to the Specifications window of a selected pipe or junction.

Pipe Specifications window

Each pipe on the Workspace has engineering information associatedwith it. Each pipe must have an ID number, a length, a diameter, afriction model specification, and two connecting junctions. For transientanalyses, it must also have a wavespeed. Figures 6.1a-e show the PipeSpecifications window tabs.


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Figure 6.1a Pipe Specifications window Pipe Model tab.

Figure 6.1b Pipe Specifications window Additional Loss tab.


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Figure 6.1c Pipe Specifications window Design Alerts tab.

Figure 6.1d Pipe Specifications window Optional tab.


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Common input parametersThe common input parameters are located above the tabbed folders(Figure 6.1a-e) and are discussed below.

Pipe numberEvery pipe must have an ID number. When you create a new pipe, AFTImpulse assigns a default pipe number. The pipe number can bechanged; however, duplicate pipe ID numbers are not accepted, and IDnumbers must be greater than zero and less than 10,000. The pipenumbers you choose have no bearing on the model connectivity,direction, or layout. They are merely convenient identifiers.

Pipe nameA name can be assigned to each pipe for reference purposes. The defaultname is simply “Pipe.” Names do not need to be unique. The name canbe shown on the Workspace, in the Visual Report and in the Outputwindow. By using names, attention can be called to important or criticalpipes.

Figure 6.1e Pipe Specifications window Fluid Properties tab(available only when Variable Fluid Properties areselected in System Properties).


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Same As PipeThe Same As Pipe dropdown list allows you to conveniently set the pipeproperties to be the same as any existing pipe. You can copy all or partof the parameters from an existing pipe in the model to the current pipeby choosing one of the pipes in the list and then specifying whichparameters to copy. Only the parameters you choose will be copied tothe current pipe.

The Same As Previous Pipe button functions the same way as the SameAs Pipe except that the last pipe edited is used. This is convenient whenthere are several pipes that have the same or similar parameters.

Scenario same as parentIf the current model is a child scenario, in the Same As Pipe list there isan option to set the pipe to Parent Pipe Data. This will change theselected pipe to be the same as its parent.

Connected junctionsThe connected junctions area shows you AFT Impulse's interpretation ofthe junctions that are connected to the particular pipe. Every pipe mustbe connected to two junctions in order to be completely defined. When ajunction does not yet exist at one of the pipe endpoints, “None” appearsin the area where the junction ID number would normally be displayed.

All pipes on the Workspace have a reference positive flow direction. Thereference positive direction is indicated on the Workspace by an arrowon the pipe. Based on the reference direction, AFT Impulse identifies anupstream junction and a downstream junction.

To determine the properties of the connected junctions, you can use theInspection utility. To inspect, position the mouse pointer over theconnected junction ID number and hold down the right mouse button.

Pipe ModelThe Pipe Model tab (Figure 6.1a) allows you to input geometric data forthe pipe.


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Pipe lengthThe Length entry represents the length of pipe between the twoconnecting junctions. The length entered in the Pipe Specificationswindow has no relationship to the length of pipe you draw on theWorkspace. The Workspace pipe is merely an abstract used forconceptual modeling purposes. The pipe drawn on the Workspace issimilar to the conceptual pipe drawn on an engineer's calculation pad.

Every pipe must be assigned a length. This length can be any positivevalue. To the right of the Length field, a dropdown list allows you tochoose the units of the pipe length.

Pipe materialThe Pipe Specifications window allows you to leave the pipe materialunspecified or to choose a pipe material from a list.

AFT Impulse provides default pipe material data for several differentkinds of pipe. In addition to these pipe materials, you may add your own.Access to the Pipe Material Database is available on the Database menu.

By entering your own custom pipe materials, you can build a pipematerial database that is saved to disk and read in during startup. Onceyou enter materials data, AFT Impulse treats the pipe materials as if theywere native to the program. More detail on building a custom pipematerial database is given in Chapter 7.

When you select a pipe material from the materials list, two thingshappen:

1. In the Size area, the dropdown lists for nominal pipe Size and Typeare enabled

2. In the Friction Model area, a default friction data set andaccompanying handbook pipe roughness value is entered

The Type entry allows you to select the specific type, class, or scheduleof material for the nominal pipe size. Once the type is selected, the pipediameter is shown in the diameter field. This diameter cannot be edited.

In addition, if transient modeling is enabled and the user has selectedCalculated Wavespeed, the pipe wall thickness, modulus of elasticity,and Poisson ratio are shown. All of these parameters come from the pipematerial database, and cannot be edited. These three parameters areneeded for wavespeed calculations.


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If you select the Pipe Material as Unspecified, these three fields areeditable and you will need to enter data in each for wavespeedcalculations.

If User Specified Wavespeed is selected, these three fields will bedisabled no matter what Pipe Material is specified, as they are notneeded when the user supplies the wavespeed. Finally, if the model is forsteady-flow only, the three fields will always be disabled as they have nobearing on steady-state models.

In the Friction Model area, the value shown is not to be construed as thecorrect pipe roughness value for your application. The pipe roughnessdepends on many factors, most importantly the age of the pipe. AFTImpulse offers the pipe material roughness as a guide. You will want toenter the roughness value that is most representative of your application.

Multiple friction data sets can be compiled for each pipe. For example,friction data for pipes with different ages can be set up. These differentdata sets are then made available in the Data Set dropdown list. Moreinformation on creating multiple friction data sets is given in Chapter 7in the section on custom pipe materials.

Pipe diameterThe inner diameter of the pipe is entered in the Inner Diameter field inthe Pipe Specifications window. For pipes whose material is unspecified,the diameter and units may be entered directly.

If the material is specified from the Pipe Material list, the diametercannot be edited; the value is derived from the database for the selectedmaterial, size, and type.

The reduction in diameter due to scaling can be accounted for byentering a percent reduction in ID Reduction - Scaling field. Zeropercent would represent no reduction.

Pipe wall thicknessThe wall thickness is entered in the Size area of the Pipe Specificationswindow. Wall thickness is required for pipes in which the wavespeed iscalculated.

If the material is specified from the Pipe Material list, the wall thicknesscannot be edited; the value is derived from the database for the selected


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material, size, and type. This field is disabled if only a steady-statemodel is being run or if the User Specified Wavespeed is selected.

Pipe modulus of elasticityThe modulus of elasticity is entered in the Size area of the PipeSpecifications window. Modulus of elasticity is required for pipes inwhich the wavespeed is calculated.

If the material is specified from the Pipe Material list, the modulus ofelasticity cannot be edited; the value is derived from the database for theselected material. This field is disabled if only a steady-state model isbeing run or if the User Specified Wavespeed is selected.

Pipe Poisson ratioThe Poisson Ratio is entered in the Size area of the Pipe Specificationswindow. Poisson Ratio is required for pipes in which the wavespeed iscalculated.

If the material is specified from the Pipe Material list, the Poisson ratiocannot be edited; the value is derived from the database for the selectedmaterial. This field is disabled if only a steady-state model is being runor if the User Specified Wavespeed is selected.

Friction ModelA common problem in specifying pipe roughness is that long termoperation of the pipe often results in deposits forming on the inside ofthe pipe. This is commonly referred to as fouling. Fouling increasesroughness and, in severe instances, decreases the area available for fluidtransport (also causing greater pressure loss in the pipe). You areencouraged to seek out applicable pipe design data when specifying piperoughness in your model.

The Friction Model area of the Pipe Specifications window offers eightmethods for specifying frictional models. Methods 1, 2, 4 and 5 all relyon the Darcy-Weisbach friction factor method of calculating pipepressure drop.

1. Absolute roughness – AFT Impulse's default method is to specifythe roughness as an absolute average roughness height. Values ofpipe roughness can be found in many pipe handbooks or frommanufacturer's data.


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2. Relative roughness – Some pipe roughness specifications are givenas a relative roughness. In this case, the roughness height is dividedby the pipe diameter. Relative roughness has no units, so when youenter a relative roughness, the Units dropdown list is disabled.

3. Hazen-Williams – The Hazen-Williams method uses an empiricalfactor (CHW) to relate the flow rate to the pressure drop in the pipe.This method is still in common use in the field of water distribution.You can specify the Hazen-Williams factor for any pipe.

4. Explicit Friction factor – If you already know the friction factor forthe pipe, you can enter the value explicitly.

5. Hydraulically smooth – You can also specify a pipe ashydraulically smooth. Modeling a pipe as hydraulically smoothimplies that its roughness is negligible. However, having a smallroughness is not the same as being frictionless. Rather, the pipefriction factor follows the hydraulically smooth curve in theturbulent region of a standard Moody diagram. You are encouragedto consult a standard fluid mechanics textbook for additionaldiscussion of this subject.

6. Frictionless – For modeling purposes, it is occasionally useful tomodel a pipe as having no friction. There are limitations to wheresuch a pipe can be located in your model. Basically, a frictionlesspipe cannot be the sole connection between two branching junctions.

7. MIT Equation – The MIT Equation is appropriate for crude oil. SeeChapter 8 for a discussion of how this method is implemented.

8. Miller Turbulent – The Miller Turbulent method is appropriate forlight hydrocarbons. See Chapter 8 for a discussion of how thismethod is implemented.

Because you are free to specify the friction model for each pipeindividually, you can mix and match the friction models throughout yourpipe system if you choose.

If you find it more convenient to use a certain type of frictionspecification, like the Relative Roughness method for example, you maywant to change the defaults in the Parameter and Unit Preferenceswindow to reflect this. When you change the default method, the othermethods are still accessible, but the Pipe Specifications window alwayscomes up with your preferred method selected.


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The preceding friction models all apply to Newtonian viscosity models.The viscosity model is specified in System Properties window. If theuser chooses one of the non-Newtonian models, the preceding frictiondata may not be used in the calculation.

A more detailed theoretical discussion of the different friction models isgiven in Chapter 8.

Pipe SupportThe Pipe Support selection is made from the provided drop down list toobtain the constant c1. This constant is required for pipes in which thewavespeed is calculated. AFT Impulse provides seven common pipesupport types.

The meaning of the c1 constant and its relationship to the seven supporttypes is given in Chapter 9.

Pipe WavespeedThe wavespeed is the speed at which transient events propagate throughthe pipe. Typical values for the wavespeed are 2000-5000 ft/sec (700-1600 m/sec).

This parameter depends on the liquid acoustic velocity and the pipematerial and support. AFT Impulse will calculate the wavespeed for you,or you can provide your own user specified wavespeed.

Additional LossesThe additional loss factor is one way to enter loss factors in your model.The additional loss factor is added to the friction loss calculated by theSteady-State Solver. This is useful when a pipe section contains elbowsand other fittings.

You can use any of the losses supplied in the Impulse database or youcan enter a specific loss factor. To enter losses, click the AdditionalLosses tab (Figure 6.1b) and then click the Additional Losses button toopen the Additional Pipe Losses window (Figure 6.2). This windowallows you to specify losses from a table of standard losses.

Because many of the additional losses are dependent on pipe diameter,you must specify a diameter before you can open the Additional PipeLosses window.


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The types of pipe losses are divided into several categories representedby tabs. To add a loss to the pipe, click the tab which contains the typeof loss, then select the loss from the list at the top of the window. Youcan add up to 99 of each loss by entering the quantity in the box belowthe selection list. Press the Add to Pipe button and you will see the lossadded to the list of losses currently in the pipe. When you are doneentering all the losses in each of the categories, press OK and you willsee a summary list in the Pipe Specifications window.

Figure 6.2 Additional Pipe Losses window opened from the PipeSpecifications window

If you want to change the quantity of a loss which is currently in thepipe, select the loss you want to change from the bottom list and thenedit the quantity in the box below the list. To delete a loss, select it fromthe bottom list and click the Delete button.

The Area Changes loss does not change the size of the pipe, it only putsin a loss factor based on the pipe diameter and the area ratio of the loss.When this option is selected, notice the illustration of how the loss isdefined, where Apipe is the area of the pipe and As is the area of the loss.These area changes are useful to model fittings between a pipe andanother junction like a pump or control valve.


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The abbreviations used in the Additional Loss window are listed at thebottom of the window. You can double-click the image to see thereference information for the loss.

You can set a specific loss as the default so when that particular type ischosen the default loss will be already selected. To set a default, firstchoose a loss then click the Set As Default button. You can have adefault for each loss type except Area Changes.

Design AlertsA design alert (Figure 6.1c) allows you to specify certain maximumand/or minimum parameters for the pipe. After running the Steady-Stateor Transient Solvers, any exceeded value will be identified in theWarnings section of the Output window and the specific cell containingthe value in the Pipes section will be highlighted. Design Alerts can becross-plotted in the Graph Results window vs. the actual results. Onegood example where this is useful is for cross-plots of maximum andminimum allowed operating pressures.

Optional inputThere are several optional input parameters (Figure 6.1d). These aredescribed in the following sections.

Initial Flow Rate GuessThe Initial Flow Rate Guess is the initial guess at the mass flow solutionthrough the pipe. Specifying a good initial guess of the flow solutiondecreases the number of iterations the Steady-State Solver must executein order to arrive at a converged solution. However, specifying initialguesses takes time and good judgment.

If you do not specify an initial flow guess, AFT Impulse implements aproprietary method of generating first guesses. This method is usuallysufficient to get the Steady-State Solver going in the right direction so aconverged solution results. However, the method of generating firstguesses is not 100% effective, and there may be times when you willhave to specify the first guess in order to obtain a solution. Morediscussion on this subject is given in Chapter 8.


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Workspace displayYou have the option of specifying what to display on the Workspace.You can choose to display either the pipe ID number, the pipe name,both, or neither. This is helpful in large models to reduce the text on theWorkspace and to focus on what you think is important. The defaultsetting for this can be changed in the Workspace Preferences window.

Design FactorYou can specify two design factors for each pipe – one for the pipefriction and one for additional losses that you add to the pipe.

Design factors allow you to specify multipliers on the pipe frictioncalculations and additional losses that are applied by the Steady-Stateand Transient Solvers during the solution process. Design factors arehelpful for adding safety margins to your design calculations.

Pipe Line Size and ColorThe default thickness of the line shown on the Workspace and defaultpipe color is defined in the Workspace Preferences window. You canchange the displayed line thickness and color for each pipe individuallyif you desire.

Parallel PipesHere you can specify that the pipe is one of a number of parallel pipes.The pipe properties thus represent a single pipe. This is useful, forexample, to represent a bundle of tubes in a heat exchanger.

Support for Partially Full PipeThere is checkbox which indicates the pipe supports partially full pipecalculations. This means partially full along the axial direction, asopposed to partially full along the radial direction. This function onlyworks for pipes that have a slope, and when the pipe is partially full itdrains from the end with the higher elevation.

If this feature is not selected, the pipe will always be assumed to beliquid full. Selecting this feature implies that one end of the pipe isconnected to a junction at its higher elevation endpoint which allows gasto flow in, thus allowing the pipe to drain.


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If the option is selected, and the entry is 100%, it means the pipe isinitially 100% full, but can drain if backflow occurs at the high elevationendpoint. The percent full can also be set to something less than 100%.In this case, the pipe is assumed to be partially full initially. This is onlyvalid when the steady-state condition for the pipe is such that there is noflow. Because if there were flow in that pipe during steady-state, thepipe would not be partially full.

This feature only works with pipes which connect to a vacuum breakervalve, spray discharge, assigned pressure or exit valve junction at thehigh elevation end.

When selected, the predicted level of the liquid surface and other relatedparameters can be graphed in the Graph Results window. When pipescan be partially full, more accurate predictions result when the pipe ismodeled with more sections. A single section is not recommended. Atleast two sections should be used, and more if possible.

Intermediate Pipe ElevationsPipe inlet and exit elevations are obtained from the upstream anddownstream junction elevation data. The pipe elevations in between areassumed to vary linearly.

In general, the pipe elevation profile will have no effect on the hydraulicresults, and the linear assumption is suitable. However, even though thehydraulics are predicted accurately, the intermediate pipe pressures arenot (unless the elevation is in fact linear). As long as these pressures arewithin the allowables for the pipe, and do not drop below vapor pressure,then no problems arise. However, if the pressures do exceed theseconditions, it is important to find this out.

One way to account for this is to break the pipe up into smallerindividual pipes, with additional junctions (usually branch junctions)connecting them. These junctions can be set at the proper elevations, andthe actual elevation profile can thus be modeled. However, this too hasramifications, especially for transient models. With the increasednumber of pipes, this can force a shorter time step selection, which candrastically increase model runtime.

The solution to the just described dilemmas is to use intermediate pipeelevations. Here an elevation profile can be entered directly for the pipe,and only a single pipe is required as long as the diameter is constant.


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To use intermediate pipe elevations, tick the check box for UseIntermediate Pipe Elevations, and enter the elevation profile. To do this,the pipe length entered on the Pipe Model tab (Figure 6.1a) is required.AFT Impulse will ensure the total length in the intermediate elevationarea matches the length on the Pipe Model tab. It does so by fixing thelength of the final section to the difference between the sum of thelengths in the table and the actual length entered by the user. The endelevation is also fixed to that of the downstream junction. If the userlater modifies the downstream junction elevation, the pipe’s finalelevation section will be automatically updated.

Fluid PropertiesThis tab is only visible when variable properties are being modeled(Figure 6.1e). Variable property modeling is enabled in the SystemProperties window.

For models with variable fluid properties, you can assign different fluidproperties to each pipe in the model. This is convenient for modelingsystems that are subject to temperature variations and systems that havemultiple fluids. The pipe fluid properties are displayed in the PipeSpecifications window, and can be modified by selecting the FluidProperties button. Besides varying the fluid properties, the viscositymodel can varied from pipe to pipe. Thus some pipes in the model canbe modeled as Newtonian, while others as non-Newtonian.

NotesEach pipe can have notes associated with it. This is useful for listingassumptions, reference documents, drawings, measurements, etc.

StatusThe Status tab shows which required input data has not been entered.

Junction Specifications windows

The nineteen different types of junctions on the Toolbox offer a greatdegree of freedom in assembling a pipe network model. While there issome redundancy in capabilities between the junctions, each of thenineteen types of junctions offers specialized features to allow you to


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prepare a good conceptual model of the physical system of interest.Table 6.1 compares the features of the junction types.

There are nineteen junction Specifications windows, one for eachjunction type. Each junction Specifications window falls into one of twobasic window formats, depending on the number of connecting pipesallowed for that junction type.


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Table 6.1 Comparison of junction capabilities

Junction Type ConnectingPipes

SpecialConditions

CustomDatabase

WindowFormat

Area Change 2 √ 1

Assigned Flow 1 √ √ 1

Assigned Pressure* 1/1-25 √ 2

Branch 2-10 √ 2

Check Valve 2 √ 1

Control Valve 2 √ √ 1

Dead End 1 √ 1

Gas Accumulator 1-25 √ √ 2

General Component 2 √ 1

Liquid Accumulator 1-25 √ √ 2

Pump 1-2 √ √ 1

Relief Valve 1-2 √ √ 1

Reservoir 1-25 √ 2

Spray Discharge 1-4 √ √ 2

Surge Tank 1-25 √ √ 2

Tee/Wye** 3 √ 2

Vacuum Breaker Valve 1-2 √ √ 1

Valve 1-2 √ √ 1

Volume Balance 2 1

*Assigned Pressure junction connects to only 1 pipe for static pressure,

and up to 25 for stagnation

**Tee/Wye junction has a unique format closer to Format #2.


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Note: AFT Impulse does not prevent negative flow through junctions.Should this occur, AFT Impulse will use the same loss factor or pressuredrop data referenced to the defined upstream pipe, just as in forwardflow. Because it is possible that the loss factor or pressure drop datawould not be the same for both flow directions, the flow solution couldbe misleading. For this reason it is important that you either properlydefine the pipe flow directions for junctions or pay close attention to theflow directions that result. Should negative flow occur in any junction,AFT Impulse will give a warning in the output.

Format #1:Junctions with one or two connecting pipesThe first basic window format is for junction types that allow only oneor two connecting pipes (see Figure 6.3). While junctions do not have anexplicitly defined flow direction like pipes do, typically those with twopipes or less adopt a direction from the connecting pipes. For thesejunction types, upstream and downstream pipes are recognized by AFTImpulse based on the reference positive flow direction of the connectingpipes. The upstream and downstream pipes are displayed separately nearthe top of the window.

In this first basic window type, it is generally important to have thepipes' reference positive flow directions specified in the physicallycorrect directions. A good example of this is a Pump junction whichwill add pressure to the system in the direction of positive flow throughthe connecting pipes. The Pump junction interprets where to add thepressure based on the directions of the connecting pipes.

To inspect the connected pipe information in this window format,position the mouse pointer over the connected pipe ID and hold downthe right mouse button.


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Fixed display ofconnecting pipes

Figure 6.3 Specifications window format #1 (fixed connecting pipedisplay).

Format #2:Junctions with more than two connecting pipesThe second basic window format for junction Specifications windows isfor junctions that allow more than two connecting pipes (see Figure 6.4).These junctions typically allow up to twenty-five connecting pipes; theexception is the Tee/Wye junction, which allows only three.

Because the number of pipes connected to a junction may vary, thesecond basic format uses a table sized according to the number ofconnecting pipes.

An example of this second window format is the Branch Specificationswindow, shown in Figure 6.4, which has three connecting pipes. Todetermine AFT Impulse's interpretation of the model connectivity forthis second window format, you can review the contents of the tableareas. There are separate loss factors for flow going into the pipe fromthe junction and flow going out of the pipe into the junction. The


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direction of the pipe in reference to the junction is also shown in the pipetable.

To inspect connected pipe information in this window format, positionthe mouse pointer over the pipe ID number in the left column of the pipetable and hold down the right mouse button.

Parameters common to all junction Specifications windows

There are several junction parameters common to most junctionsdescribed in this section.

Junction NumberEach junction has an ID number (see Figures 6.3 and 6.4) that you arefree to change. However, no two junction numbers can be the same, andall junction numbers must be greater than zero and less than 10,000.Duplicate junction numbers are not accepted. The junction numbers youchoose are arbitrary and have no bearing on model connectivity,direction, or layout. They are merely convenient identifiers.

Junction NameEach junction can be assigned a name for reference purposes (seeFigures 6.3 and 6.4). The default name is the junction type. The namedoes not need to be unique. The name can be displayed on theWorkspace, Visual Report, Model Data and Output windows.


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Figure 6.4 Specifications window format #2 (pipes displayed in atable).

Junction ElevationEach junction has an entry for the elevation of the junction (see Figures6.3 and 6.4). Next to each elevation entry is a set of units. For junctionsthat use Format #1, the default inlet elevation is entered by the user, andthe outlet elevation is assumed to the same. A different outlet elevationcan be entered by clearing the “Same as Inlet” checkbox. Different inletand outlet elevations will have no effect on the flow solution, but willoffset the local static pressure at the connecting pipe by the hydrostaticpressure difference.

Junctions that follow Format #2 have a single elevation and offsets fromthat elevation (if they exist) for each individual connecting pipe enteredin the connecting pipe table.

Junction elevations are used to account for pressure changes due togravity and thus to allow calculation of absolute pressures in pipes. In a


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sense, junction elevations are arbitrarily defined. The magnitude of theelevation is not important from a solution standpoint, but the relativeelevation difference between junctions is of critical importance becauseof its effect on the pressure results.

Elevation is required input for every junction. AFT Impulse uses thisinput to determine the relative height changes between one end of a pipeand the other.

In the Parameter and Unit Preferences window you can set a defaultelevation for your system. When a default elevation is specified, eachtime you open a new junction Specifications window the defaultelevation is automatically entered. For systems that reside primarily atthe same elevation, this can save time in entering data.

Database ListThe Database List (see Figures 6.3 and 6.4) is a list of all customequipment you have entered into the Component database. It displayscustom equipment from both the local and network database sources.Selection of an item automatically retrieves the data and enters it into thejunction. Changes to the custom input data are not accepted. To stopusing a database selection, choose (None) from the database list.

Same As Junction listThe Same As Jct list (see Figures 6.3 and 6.4) is a list of all the junctionsof the same type in the model. This displays a list of parameters fromwhich you can choose to apply to the current junction.

Scenario same as parentIf the current model is a child scenario, in the Same As Jct list there is anoption to set the pipe to Parent Junction Data. This will change theselected pipe to be the same as its parent.

Pipe connectivityIn each junction Specifications window (see Figures 6.3 and 6.4), youcan see AFT Impulse's interpretation of your system's connectivity. Alljunctions must have the proper number of connecting pipes in order tobe completely defined (refer to Table 6.1).


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Jumping to another junctionYou can jump to another junction from within the Specificationswindow (see Figures 6.3 and 6.4). This will save the current junctioninformation before jumping to the other junction. This method avoidshaving to close the current junction Specifications window beforeopening another Specifications window.

Transient DataMany junction types have an area where active transient data can bespecified. Table 6.2 lists the junction types.

Table 6.2 Junctions that support transient data

Junction Type Type of Transient

Assigned Flow Flowrate

Assigned Pressure Pressure

Branch Flowrate source or sink

Check Valve Opening/Closing profiles

Control Valve Opening/Closing profiles, pressure or flow

Pump Speed, flowrate, trip or startup parameters

Relief Valve Opening/Closing profile

Reservoir Surface level changes

Spray Discharge Discharge opening/closing profiles

Surge Tank Surface Pressure

Valve K or Cv profile for opening and closing in part or full

The active transient data is entered on the Transient Data tab (Figure6.5). Table 6.2 lists the type of data that can be entered for each junctiontype. If a junction does not have any active transient behavior, there is noneed to enter any data on the Transient Data tab.

In most cases, transient data can be entered as absolute values or as apercentage of steady-state. The distinction is made when choosingbetween Absolute Values and Relative to Steady-State Value (Figure6.5).


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The first data point always needs to match the steady-state value whichis usually entered on the tab at the far left. If, for example in Figure 6.5,the steady-state data for Cv of 1000 is changed to 750, the transient datawill also need to be changed. However, if in Figure 6.5 the Relativeoption is specified, the first data point would be 100%, and when the Cvis steady-state is changed from 1000 to 750 there is no need to changethe transient data since the first data point is still 100%. But it is nowrelative to a different steady-state value.

Figure 6.5 Example of junction transient data. A here a valve isbeing closed over five seconds.

Repeat TransientIf the transient data is periodic, you can enter the data for one cycle ofthe period and then tick the check box for Repeat Transient. This willcause the one cycle of transient data to be repeated once it has reachedthe end. The repetition will continue until the end of the simulation.


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Transient Special ConditionsThere may be occasions where, once having entered the transient data,you do not wish the transient to activate for a particular case. One optionis to just delete the transient data. A second option is to specify theTransient Special Condition as Ignore Transient Data. In such a case, thedata can be left in the window for future use, but will not activate duringthe run.

A third type of Transient Special Condition is available for AssignedFlow and Assigned Pressure junctions. This is the No Reflections(Infinite Pipe) option. In such a case, the flow or pressure is used onlyfor steady-state calculations. During the transient the flow or pressure isnot fixed, but varies in accordance with the concept that no transientreflections occur. All transient data at this junction will be ignored.

The reasons for using this feature are important, but somewhat difficultto grasp. A lengthy discussion is given in Chapter 12.

Time based and event based transientsIn Figure 6.5, there is an area called Initiation of Transient, and theselection is Time. This means that the specified transient is linked to thetime of the simulation. Specifically, the valve transient data is activatedas soon as the simulation is started. Because the transient data isconstant for one second, the valve does not start to close until thesimulation time reaches one second.

Transients can also be initiated based on events. For instance, we canchange the valve in Figure 6.5 to start closing when the pressure at someother system location reaches a certain level. Or maybe when a tank hasfilled up and thus reached a certain level. In such a case, the time zerodata in Figure 6.5 does not refer to the start of the simulation, but isrelative to when the event occurs. If, for instance, the pressure neverreaches the specified level, the valve never closes.

Chapter 10 discusses in much more detail how event transient modelingcan be used.

Initial Pressure/Head GuessOn the Optional tab of each junction Specifications window (seeexample in Figure 6.6), except for Reservoir and Assigned Pressurejunctions, is an area for entering an initial guess at the pressure (or head)


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at the junction. This is only the initial guess and the solution will varyfrom this value; it does not specify a fixed pressure at the junction. Thisguess is used in setting up the steady flow solver.

Figure 6.6 The Optional tab offers several optional data inputs andis very similar for each junction type

Specifying a good initial guess decreases the number of iterations theSteady-State Solver must execute in order to arrive at a convergedsolution. However, similar to specifying Initial Flow guesses in pipes,specifying initial guesses takes time and good judgment.

The Reservoir and Assigned Pressure junctions do not allow you theoption of specifying an initial guess. Because the pressure is a knownquantity, no guess is needed. The pressure solution is already available.

If you do not specify an initial guess, AFT Impulse implements aproprietary method of generating first guesses. This method is usuallysufficient to get the Steady-State Solver going in the right direction so aconverged solution results. However, AFT Impulse's method of


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generating first guesses is not 100% effective, and there may be timeswhen you will have to specify first guesses in order to obtain a solution.

Displaying junction names and numbersAlso on the Optional tab of each junction Specifications window (seeexample in Figure 6.6) is the option to display either the junction IDnumber, the junction name, both, or neither. This is helpful in largemodels for reducing the text on the Workspace and focusing on what youthink is important. The default setting for this can be changed in theWorkspace Preferences window.

Special ConditionsSome junctions allow you to set Special Conditions (see example inFigure 6.6). This feature allows you to override the default behavior ofthe junction. For example, the special condition for a Valve junction isto close the valve. The special condition for a Relief Valve is for it to beopen no matter what the pressure. Not all junction types support specialcondition settings.

The junction for which the Special Condition is set will have an ‘X’placed before the junction ID number. If the special condition is to closethe junction, by default it will be outlined with a dashed line. Thesesettings may be changed in the Workspace Preferences window.

Chapter 5 discusses how to apply transient data to junctions with steady-state Special Conditions.

The Vacuum Breaker Valve, Gas Accumulator, Liquid Accumulator,Surge Tank, and Relief Valve junctions have a unique kind of SpecialCondition called Ignore. This makes the steady-state and transientsolvers ignore the presence of the valve. Thus not only is the dataignored, the junction itself is ignored. This is convenient when locatingone of these devices by trail and error. Rather than having to delete thejunction, it can be ignored.

Design FactorYou can specify a design factor for each junction (see example in Figure6.6). Design factors allow you to specify multipliers on the junctionpressure loss. These multipliers are applied by the Steady-State andTransient Solvers during the solution process. In the case of a pump, the


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design factor multiplies the pressure rise. Design factors are helpful foradding safety margins to your design calculations.

Note: If you are designing for some minimum condition, you may wantto use the design factors differently for pumps than for other junctiontypes. If you use 1.1 for all junctions with a pressure loss, you will get anextra 10% pressure drop. Using the same 1.1 on your pump will give anextra 10% pressure rise. This will work against the 10% margin on yourpressure losses. If you want to use a pump design factor in this case, itmight be best to use 0.9.

Changing the icon graphicYou may want to change the icon to better represent a specific junctiontype. This is accomplished by clicking the Change Icon button on theOptional tab (see example in Figure 6.6). The Change Icon windowappears, allowing you to choose a new icon from a list of availableicons.

Changing the icon sizeYou may want to change the size of the junction to distinguish it fromother junctions. This is accomplished by clearing the use WorkspaceDefault for Size checkbox and selecting the desired size in the dropdown list (see example in Figure 6.6).

Specifying Base AreaSome junction windows that follow Format #1 allow selection of thebase area for loss models. By default the base area will be the upstreampipe area. The user has the option of using the downstream pipe area ordirectly specifying an area or diameter.

NotesEach junction can have associated notes. This is useful for listingassumptions, reference documents, drawings, measurements, etc. Notesare entered in the provided area on the Notes tab.


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StatusThe Status tab shows which required input data has not been entered.

Area Change Specifications window

The Area Change Specifications window is shown in Figure 6.7. TheArea Change junction type must always have two connecting pipes. Thisjunction type allows you to model the irrecoverable loss that occurs atthe area transition between two pipes, whether expansion or contraction.

The Area Change Specifications window follows the first of the twobasic Specifications window formats, displaying the connecting pipes ina fixed format. A flow direction through the junction is adopted from thedefined directions of the two connecting pipes. Consistent with AFTImpulse's convention, the loss factor base area is referenced to theupstream flow area shown as the Base Area.

Figure 6.7 The Area Change Specifications window

The Area Change Specifications window offers two standard types ofarea change losses: the Conical Transition and the Abrupt Transition.The window shows a schematic of the selected geometry.


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The critical parameter that influences the magnitude of the area changeloss is the area ratio between upstream and downstream pipes. Until youhave input the information for the upstream and downstream pipes thatallow for flow area determination, the standard Area Change junctionloss factor is not calculated. For conical transition geometries you alsoneed to specify the conical angle.

If the standard loss factors provided are inadequate for your application,you may specify your own custom loss factor referenced to the upstreampipe flow area.

More information on the standard loss factor types for Area Changejunctions is given in Chapter 8.

Assigned Flow Specifications window

The Assigned Flow Specifications window is shown in Figure 6.8. TheAssigned Flow junction type allows you to connect one pipe and to inputa known steady-state and/or transient flowrate entering or leaving at aparticular location in your system. You also can include an irrecoverableloss with this junction type.


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Figure 6.8 The Assigned Flow Specifications window

The Assigned Flow Specifications window follows the first of the twobasic Specifications window formats, displaying the connecting pipeinformation in a fixed format. Depending on whether you specify thejunction as an inflow or outflow type, the required connecting pipe(upstream/inlet or downstream/outlet) will be enabled while the otherpipe is disabled. Note that if the flow is specified as Inflow (to thesystem) then the reference positive flow direction of the connected pipemust be away from the junction (i.e. the junction is upstream of thepipe).

Tip: If you see that the required pipe in the Specifications window states“None” but a pipe is connected to the junction graphically, then checkthe Inflow/Outflow setting on the junction or the reference positive flowdirection of the pipe.


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The Assigned Flow junction type allows you to specify positive flowrates as mass flow rates. Negative flow rates are not accepted for steadyflow, although are for transient flow. If, for example, you have anoutflow type junction and your system is physically flowing in, youcannot assign a negative flow rate to the junction. You must reverse theconnecting pipe flow direction to be consistent with the actual directionand change from an outflow type to an inflow type.

Transient DataOn the transient data tab a flowrate transient can be entered. A specialfeature is the ability to model a sinusoidal flowrate transient. Input forthe amplitude and frequency is required. This transient is summed to thatspecified in the transient data table. A chopped sine wave takes theabsolute value of the sine function.

More information on transient data, including event transients, is givenearlier in this chapter and in Chapter 10.

Special ConditionsYou can set a Special Condition for an Assigned Flow junction, whichwill turn the flow off and make it act like a Dead End during steady-state.

Special features for steady flowIf the steady-state flow is zero at the junction, there may be occasionswhere you want the Assigned Flow junction to act like a pressurejunction during the steady-state calculation. This would occur, forinstance, if a known flowrate profile, initially zero, exists at the junction,and there are no other suitable pressure junctions in the system. In such acase, the pressure is assigned on the Optional tab (see Figure 6.9).

Usually this feature would be used in conjunction with a SpecialCondition on the junction where it’s flow was turned off during steady-state.


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Figure 6.9 Optional tab allows junction to act like a fixed pressureduring steady-state if initial flow is zero.

Assigned Pressure Specifications window

The Assigned Pressure Specifications window is shown in Figure 6.10.The Assigned Pressure junction type allows you to connect up to twenty-five pipes. The Assigned Pressure junction is convenient for specifying aknown steady-state and/or transient pressure in your system. It alsoallows you to specify irrecoverable losses due to the fluid dynamiceffects of the connecting pipe geometry.


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Figure 6.10 The Assigned Pressure Specifications window

The Assigned Pressure junction type has much in common with theReservoir junction. In each case you specify parameters in order toachieve a known pressure. However, the Assigned Pressure junctionallows you to specify either stagnation or static conditions. This is usefulif, for example, you are modeling a system where the pressure in a pipeis known and the location is being used as a boundary in the model. Ifthe measured conditions are for a location with a velocity, then theyrepresent static conditions and the choice of static properties should beselected. In this case, the choice of a Reservoir junction would beinappropriate. Similar to the Reservoir, an Assigned Pressure junctioncauses the rest of the system to distribute the flow in a manner consistentwith the defined pressure. You can specify the units for pressure byselecting from the adjacent drop-down list.

The distance of each connecting pipe (if any difference exists) from theelevation can be entered in the pipe table.


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If the static pressure option is selected, only one connecting pipe isaccepted. The reason for this restriction is that multiple connecting pipescould have different flows and, hence, velocities. Different velocitieswill change the static pressure, thus a unique static pressure cannot bespecified.

This limitation does not affect specified stagnation pressure becausestagnation pressure does not change with velocity. See Chapter 8 formore information on this subject.

Transient DataOn the transient data tab a pressure transient can be entered. A specialfeature is the ability to model a sinusoidal pressure transient. Input forthe amplitude and frequency is required. This transient is summed to thatspecified in the transient data table. A chopped sine wave takes theabsolute value of the sine function.

More information on transient data, including event transients, is givenearlier in this chapter and in Chapter 10.

Branch Specifications window

The Branch Specifications window is shown in Figure 6.11a-b. TheBranch junction allows up to twenty-five connecting pipes. The Branchjunction is the most flexible junction type for building network modelsthat have flow splits. Other significant features are the ability to imposea steady-state and/or transient flow source or sink at the junction and theability to specify separate loss factors for each connecting pipe. TheBranch junction must have at least two connecting pipes.


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Figure 6.11a The Branch Specifications window Loss Coefficientstable.

The Branch Specifications window follows the second of the two basicSpecifications window formats. A table on the Loss Coefficients tabdisplays the connecting pipe information. This table grows in size toaccommodate up to twenty-five connecting pipes. After you add a fifthpipe a scroll bar appears, allowing you to review and enter loss factorsfor all pipes in the table.

For each connecting pipe, the pipe table lists the reference flow directionand up to two loss factors. The first loss factor is for physical flow out ofthe pipe and into the branch. The second is for physical flow into thepipe and out of the branch. The loss factors can be specifiedindependently or left as zero. To edit within the pipe table, simply clickin the appropriate column and row.

The distance of each connecting pipe (if any difference exists) from theelevation can be entered in the pipe table.


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The Branch junction also offers you the option of specifying a flowsource or sink at the junction. According to AFT Impulse's convention, asource of flow into the system is defined as positive, while aconsumption of flow with flow out of the system (a sink) is defined asnegative.

Figure 6.11b The Branch Specifications window Optional taballows you to specify a steady-flow source or sink.

Transient DataOn the transient data tab a transient source or sink flowrate can beentered. More information on transient data, including event transients,is given earlier in this chapter and in Chapter 10.


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Check Valve Specifications window

The Check Valve Specifications window is shown in Figure 6.12. TheCheck Valve junction type requires two connecting pipes. This junctiontype allows you to model the effect of a check valve which closes toprevent back flow.

The Check Valve Specifications window follows the first of the twobasic Specifications window formats, displaying the connecting pipes ina fixed format. The Check Valve junction does not have an explicit flowdirection, but adopts a flow direction from the connecting pipes.Consistent with AFT Impulse's convention, the loss factor base area isreferenced to the upstream flow area.

Figure 6.12 The Check Valve Specifications window

A check valve is a device that allows flow in only one direction. AFTImpulse assumes that the Check Valve is initially open. If the flowsolution indicates that forward flow will not occur, AFT Impulse closes


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the valve. The valve can close during the steady-state or transient portionof the simulation.

The Special Condition for a check valve is to fail the valve open so thatflow will be allowed to pass through it even if the flow is reversed.

Specifying lossesYou specify the losses for the Check Valve on the Valve Model tab inthe Specifications window. For convenience, you can specify theconstant loss characteristics of a Check Valve as a valve coefficient (CV)or as a loss factor (K). Chapter 8 details the relationship between K andCV.

Forward Velocity to Close ValveThis is the velocity at which the valve starts to close. By default thevelocity is zero. However, some valves are designed to start closing atsome negative velocity. You can specify that here.

Delta Pressure to Re-OpenFrequently after a check valve closes, there is pressure across the valveat which it will open again. You can specify that delta pressure here.

Transient DataThe closing of a check valve is a complicated transient process becausethe mechanical movement is coupled to the hydraulics of the pipe.Sophisticated models exist in the literature which attempt to model thiscomplicated process, but these require extensive data about the valveand typically more simplified models are employed.

AFT Impulse offers a more simplified model, whereby the user canspecify a closing profile transient of Cv or K vs. time. Remember thatwhen using the K loss model, a value of –1 (negative one) means thevalve is closed. The closing will occur as soon as the closing velocitycriteria is met (discussed above). If no closing transient profile isentered, the valve will close instantaneously.

You can also specify an opening profile of Cv or K vs. time, which is theprofile the valve will follow if it reopens. If no opening transient profileis entered, the valve will open instantaneously.


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The zero time for the closing transient is relative to when the valve startsto close because of backflow. For the opening transient, it is relative towhen the valve starts to open.

Control Valve Specifications window

The Control Valve Specifications window is shown in Figure 6.13.Control Valve junctions are always internal to the system, with twoconnecting pipes. This junction type allows modeling of valves that offerspecial pressure or flow control characteristics at a location in the pipesystem. In addition, the controlled pressure or flow can be changed withtime.

The Control Valve Specifications window follows the first of the twobasic Specifications window formats, displaying the connecting pipes ina fixed format. The Control Valve junction does not have an explicitflow direction, but adopts a flow direction from the connecting pipes.

Figure 6.13 The Control Valve Specifications window


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Control Valve typesYou can model four types of control valves:

• Pressure Reducing Valves (PRV’s)

• Pressure Sustaining Valves (PSV’s)

• Flow Control Valves (FCV’s)

• Pressure Drop Control Valves (PDCV’s)

Loss information for a control valve is not required, because controlvalves are dynamic devices that change their geometry in response to thepipe system behavior. The loss that results is that required to maintainthe control parameter. You can, however, specify the full-open loss. Thisis the loss that will occur should the valve fail to a full-open state.

A PRV is a device that controls the pressure in a pipe system. The PRVmaintains a constant control pressure downstream of the junction as longas the upstream pressure exceeds the control pressure. If the upstreampressure is lower than the control pressure, the ability to control pressureis lost.

A PSV is similar to a PRV in that it controls pressure in a pipe system.While the PRV maintains a constant downstream pressure, the PSVmaintains a constant upstream pressure. If the downstream pressure riseshigher than the control pressure, the ability to control pressure is lost.

An FCV is a device that maintains a constant flow rate in a pipe system.By setting the junction to an FCV type and entering a flow rate, thejunction will limit the flow through the connecting pipes to be equal tothe control flow rate. The FCV can lose its ability to control flow whenthe pressure drop across it becomes zero.

A PDCV is a device that maintains a constant (stagnation) pressure drop.For this option, valve failure cannot be modeled. An indicator that anunrealistic pressure drop has been demanded is a failure to obtain aconverged solution.

PRV/PSV static vs. stagnation pressureThe control pressure for a PRV or PSV can be either static or stagnation.The default selection is static, as this is the most frequent application inindustry. See Chapter 8 for more information.


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Action if control is lostIt is possible that the control valve cannot maintain the specified controlsetting during steady-state and/or transient conditions. There are fouractions the valve can take. The default action is Always Control (NeverFail). This option is physically unrealistic but useful for steady-statemodeling purposes. The reason this option is physically unrealistic isthat the valve must add pressure to maintain control, thus acting like apump. If this occurs, the user is clearly warned in the output. TheAlways Control (Never Fail) option is useful because it gives a clearindication to the user of the how far away the valve is from being able tocontrol.

For transient analyses, the Always Control (Never Fail) should not beused. If it is, and the valve fails, the transient analysis will terminate.

Other options for valve failure are Fail Closed or Fail Open.Additionally, if it fails open you can choose to have it close if the flowreverses. Once the control valve fails, open or closed, Impulse will solvethe model again. These are the appropriate choices for transientanalyses.

AFT Impulse will determine if the valve can maintain control based onthe Loss When Fully Open value. If this fully-open pressure drop isgreater than the pressure drop which balances the system, the controlvalve fails. In other words, the control valve can’t open wide enough(becoming less restrictive) to maintain control, so it fails. If you enterdata in the Open Percentage table on the Optional tab, you can select tohave the data taken directly from the table for the 100% open case. Youcan also choose to not have any losses associated with the valve in thefull-open state by selecting None. Note that when a control valve failsopen this fully-open loss will be used for the valve.

It is possible for the valve to control properly during the steady-state,and then lose control during the transient part of the run. In fact, it canlose control and regain it repeatedly.

Special ConditionsThe Special Condition for a control valve is dependent on the Action IfControl Is Lost setting. If the setting is Always Control (Never Fail) orFail Closed, then the Special Condition for the control valve is to close.If the setting is to Fail Open, then the Special Condition will be to open.


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In this case the valve will have a pressure drop equal to the Loss WhenFully Open value.

Open Percentage TableThe Optional tab allows data to be entered for special control valvecharacteristics. Specifically, the valve Cv and Flow Area can beSpecified vs. the Open Percentage of the valve. This data does not affectAFT Impulse’s flow solution. However, engineers frequently desire toknow the valve’s open percentage during operation to ensure it meetsdesign requirements. The Valve Summary (in the Output window)always displays the valve Cv, and if data is specified for open percentageand flow area it will also display open percentage and flow area at theoperating point.

Transient DataThe Control Valve junction is the only one with two transient tabs. Thefirst is the valve reaction to a failure, and the second is to the controlpoint. More information on transient data, including event transients, isgiven earlier in this chapter and in Chapter 10.

Failure TransientControl valves have what might be called inherent event logic. That is,rather than respond to events at some other location, the failuretransients respond to events of the valve itself. Specifically, the eventsare the valve's loss of control and possibly regaining of control.

The failure transient is the valve action when it loses control. If the valveis set to close upon failure, the closure transient will be used. This is a Kor Cv table vs. time. If the valve were to reopen, the opening transientwould be used. The time scale for the closing or opening of a valve isrelative to when the failure occurs. That is called time zero in a relativesense.

You can elect to not enter data for the closing or opening transient. If thevalve loses control, the valve action is assumed to be instantaneous. Ifthe failure action is to fully open, then the closing transient data willhave no meaning and not be used. In a sense the opening transient willhave meaning, but in reality when the valve loses control it is already


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fully open by that point, so no transient will occur. The valve will juststay fully open.

The closing and opening transients are thus intended for use when thevalve is set to close upon failure to control.

Control TransientThe other type of transient data supported by the Control Valve junctionis the Control Transient. This is an optional transient you can apply tothe control point. For instance, a change in control pressure for aregulator valve can be modeled.

Control transients support events, although only a single event can bemodeled. Thus the regulator pressure could be modified when someevent occurred in the system.

Dead End Specifications window

The Dead End Specifications window is shown in Figure 6.14. The DeadEnd junction must always have one connecting pipe. This junction typeis one way to define flow in a pipe as truly zero.

In steady flow, dead ends have no flow and thus play no role in thehydraulic solution to the system. However, in transient flow pipes thatterminate in dead ends can have flow. Thus dead ended lines can and doinfluence the transient response of the system. Unless you knowotherwise for a specific system, dead ends should always be included inthe model.


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Figure 6.14 The Dead End Specifications window

The Dead End Specifications window follows the first of the two basicSpecifications window formats, displaying the connecting pipes in afixed format. The only required information is the connecting pipe andthe elevation.

Gas Accumulator Specifications window

The Gas Accumulator Specifications window is shown in Figure 6.15.The Gas Accumulator junction allows up to twenty-five connectingpipes. This junction type is useful for modeling a gas volume designedas a surge protection device (Figure 6.16). The Gas Accumulator may beintegral with the pipe system, separated by an orifice, or by a shortconnector pipe. The waterhammer theory for a gas accumulator is givenin Chapter 9. During steady flow the gas accumulator typically acts alossless branch junction, although it can be made to act like a pressurejunction. This is discussed later in the gas accumulator section.


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The Gas Accumulator Specifications window follows the second of thetwo basic Specifications window formats. A table displays theconnecting pipe information. The table grows in size to accommodate upto twenty-five connecting pipes. After you add a fifth pipe a scroll barappears, allowing you to review all the pipes in the table.

Initial conditions and polytropic processAFT Impulse needs to know the initial volume of gas. This can bedetermined in one of two ways. First, the Actual Initial Volume can beentered. If the accumulator is passive, the initial volume depends on theinitial pressure at the accumulator. Thus the initial pressure must beknown in order specify the Actual Initial Volume. One way to do this isto first run the model in Steady Only mode to determine the pressure.

Some gas accumulators have an active design (with compressors) whichmaintain a specified volume. In such a case, the initial volume is knownand independent of the gas pressure, and can thus be entered directly.

The second way is to enter the gas volume at some user specifiedpressure. This is called “Volume at Specified Pressure”. When selected,the user must specify a Reference Gas Volume and Reference GasPressure. Frequently this will be the gas volume at atmospheric pressure.When the steady-state is run, the gas volume will be determined from thesteady-state pressure, assuming the gas volume ratio is inverselyproportional to the gas pressure ratio.

The gas volume will change during the transient according to thethermodynamic law. The law requires a polytropic constant to relate thepressure to the gas volume. An isentropic process should use apolytropic constant equal to the specific heat ratio of the gas (for air it is1.4). An isothermal process should use a polytropic constant of 1.Typically the process falls in between the two and an average polytropicconstant can be assumed (for air the average is 1.2).

The gas in the accumulator is assumed to behave as an ideal gas.


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Figure 6.15 Gas Accumulator Specifications window

Maximum and minimum volumesIf the gas is constrained in some way to a maximum or minimumvolume, this can be entered in the optional fields. If a maximum orminimum exists, AFT Impulse will not allow the gas volume to exceedthis value. If no data is entered, the volume can grow as large or small asnecessary.

Interface with pipe systemThe Gas Accumulator can connect into the pipe system in three ways: itcan be integral with the pipe, it can be separated by an orifice, or it canbe connected by a short pipe.


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These options are provided in the Flow Restrictor and Connector Pipeareas. If neither option is used, the accumulator is assumed to beattached directly to the pipe with no hydraulic loss.

If the Flow Restrictor is used, the accumulator is assumed to beseparated from the pipe system by an orifice, which causes a hydraulicloss as the liquid flows in and out of the accumulator. The loss isspecified by entering a flow area and discharge coefficient. Further, thisloss can be different for inflow vs. outflow.

LC

ConnectorPipe

Orifice

PA

Am&

Figure 6.16 Schematic of a gas accumulator

Lastly, the accumulator can be connected by a short pipe which isconsidered to be short in relation to the other pipes in the system afterthey are sectioned. If the pipe is short, it can be assumed to reactinstantaneously with the accumulator. The short pipe accounts for liquidinertia and friction. If a connector pipe exists which cannot beconsidered short, it should be modeled as an actual AFT Impulse pipe.

The Flow Restrictor and Connector Pipe can both exist in the sameaccumulator. That is, a flow restrictor can be modeled at the top of aconnector pipe, and the effect of both will be included.


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Special features for steady flowAs mentioned previously, the gas accumulator acts like a branch junctionduring steady flow. However, the user can specify that it behave like apressure junction. Why would one want to do this? The most obviouscase is where the gas accumulator is modeling a pressurizer type ofcomponent in a closed system. A pressurizer is sometimes called anexpansion tank or accumulator. The main purpose is to provide thermalexpansion volume when the temperature of the process fluid changes.Sometimes these tanks are open to the atmosphere, in which case aSurge Tank would be a better choice than a gas accumulator.

When a pressurizer is modeled as a gas accumulator, there will typicallybe no other pressure type junctions in the system. In fact, the pressurizeris the component which controls the static pressure on the system duringsteady-state operations. However, during transients the pressure willchange. Thus there is a need to allow a gas accumulator to act as apressure junction during the steady-state, and then like a normal gasaccumulator during the transient. This is the function of the InitialPressure for Zero Steady-State Flow on the Optional tab (see Figure6.17).

When the accumulator is a pressurizer during steady-state, it has zero netflow. The pressure entered will be the steady-state static systempressure. If the gas accumulator is not in fact arranged as a pressurizerand data is entered in the field for Initial Pressure for Zero Steady-StateFlow, AFT Impulse will warn the user before running the transientsimulation.


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Figure 6.17 Gas Accumulator Specifications window

Graphing Gas Accumulator dataYou can track the gas volume, liquid flowrate, and pressure in theaccumulator by selecting these options in the Junction section of theTransient Control window. These parameters can be graphed orreviewed in the Output window.

General Component Specifications window

The General Component Specifications window is shown in Figure 6.18.The General Component junction type is internal to the system andrequires two connecting pipes. This junction type allows you to modelthe irrecoverable loss that occurs through equipment that may not beexplicitly represented on the Toolbox and to specify loss factors as afunction of a flow.

The General Component Specifications window follows the first of thetwo basic Specifications window formats, displaying the connecting


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pipes in a fixed format. The flow direction through the junction isdetermined by the defined directions of the connecting pipes. Consistentwith AFT Impulse's convention, the loss factor base area is referenced tothe upstream flow area.

Figure 6.18 The General Component Specifications window

When you select Variable K Factor or Resistance Curve, the GeneralComponent Specifications window makes additional features available.Using these new features you can input loss factors or pressure dropsthat vary with flow. To enter these factors, you can specify polynomialconstants, fit a curve to available data, or use interpolated x-y data.

When a variable loss is specified, AFT Impulse modifies the loss factorin the Solver to agree with the solution. You can choose any of theoptional flow and pressure parameters provided, and you can specify themost convenient units.

No transient data can be entered for a General Component. From atransient point of view, the junction is a static element. For this reasonthe user should use General Components with caution. Where possibleall static element hydraulic losses should be entered into pipes and notjunctions.


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Liquid Accumulator Specifications window

The Liquid Accumulator Specifications window is shown in Figure 6.19.The Liquid Accumulator junction allows up to twenty-five connectingpipes. This junction type is useful for modeling a liquid chamber that hasmore elasticity than the pipe itself. This may be a liquid-solid tank or acondenser box, for example.

Figure 6.19 Liquid Accumulator Specifications window

The Liquid Accumulator Specifications window follows the second ofthe two basic Specifications window formats. A table displays theconnecting pipe information. The table grows in size to accommodate upto twenty-five connecting pipes. After you add a fifth pipe a scroll barappears, allowing you to review all the pipes in the table.

Initial conditions and elastic processThe relationship between liquid accumulator volume and pressure isassumed to be linear. The amount of volume change for a given amountof pressure change is given by the elasticity of the accumulator. Thisinput will need to be obtained from test data or approximated based onmechanical correlations.


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The Initial Volume of the tank is a required input. This volume willchange during the transient according to the linear law.

Pump Specifications window

The Pump Specifications window is shown in Figure 6.20. The Pumpjunction type requires one or two connecting pipes. This junction typeallows you to model the pressure (head) that is added to the system by apump, and the pump response to a trip or startup. The pump can be ofeither the centrifugal or positive displacement kind. It also providesaccess to curve fitting tools. Data can also be interpolated between theprovided data.

The Pump Specifications window follows the first of the two basicSpecifications window formats, displaying the connecting pipes in afixed format. A flow direction through the junction is adopted from thedefined directions of the connecting pipes.

Because all pumps are different, AFT Impulse does not provide anypump types. You must obtain the proper pressure and flow data thatdescribes the pump from the manufacturer or from test data. You willwant to save frequently-used pumps into the Component database.

In performing curve fitting or specifying pump constants, you can workwith pressure or head and with volumetric or mass flow rate. A varietyof units are provided in dropdown lists. These features allow you toenter your pump information in the most convenient and meaningfulformat.

Pump ModelThe basic definition of the pump is performed on the Pump Model tab.You can model a pump with a pump curve or an assigned flow. Whenassigning a flow, the total pressure/head developed by the pump iscalculated. This is useful for modeling positive displacement pumps.

The pump operating speed can be entered as a percent of full speed. It isassumed that the pump curve is entered at full speed. You can use aspeed that is lower or higher than full speed. Impulse will adjust the fullspeed pump curve using standard affinity laws relating speed ratios topressure rise ratios and flow rate ratios.


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Submerged pumpIf a pump is submerged, there is no upstream pipe. Thus only oneconnecting pipe is needed for the discharge. This is modeled by tickingthe check box and entering the pump suction conditions.

Figure 6.20 The Pump Specifications window.


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Do not allow backwards rotationFor pumps modeled using four quadrant methods, the model allows thepump to spin backwards. If there is an anti-reverse spin device on thepump, then one can model this by selecting the “Do Not AllowBackwards Rotation” option.

Check valve at pump dischargeAFT Impulse offers a dedicated check valve junction, but frequently thecheck valve is close to the pump discharge and the pipe length betweenthem is very small. This can result in models with unacceptable runtimes if a check valve junction is used for transient models.

You can model a check valve at the pump discharge assuming negligiblepipe length between them. The valve model is relatively simple, withinstantaneous closure once backflow occurs. There is also an input areato account for valve closure starting at some other velocity than zero.

Pump configurationsPump data can be entered for multiple configurations. This is availableonly for pumps modeled as pump curves. The default is a singleconfiguration.

A pump configuration is a pump with a specific impeller trim andoperating speed. Multiple impeller trims and operating speeds can bespecified as part of the pump, then a particular combination can bechosen.

Figure 6.21 shows the Pump Configuration window. Raw data can beentered, imported from file, or pasted from the clipboard. Then apolynomial curve can be fitted to the data. Data for head, NPSH, andefficiency or power can be entered. Also data for the pump’s end ofcurve and NPSH constant for variable speed can be entered.

If multiple configurations are entered, they are displayed on the PumpSpecifications window in dropdown lists for selection (see Figure 6.22).

Controlled discharge pressure or flow (variable speed)You can optionally model a controlled pump. The pump will then varythe speed to deliver the flow rate or discharge pressure you set. Theresulting speed will be reported in the Pump Summary in the General


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Results section on the Output window. Alternatively, a dischargepressure, it can be controlled only if it exceeds some level, otherwise thepump will operate on its regular pump curve.

These parameters are set in the Auxiliary Input tab.

During transients, the pump speed will vary as the transient conditionsaffect the pump. The resulting pump speed can be graphed in the GraphResults. The response of the controller during the transient is assumed tobe instantaneous.


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Figure 6.21 Pump Configuration window Pump Data tab abovewhere raw data is entered and curve fit, CompositeGraph tab below.


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Multiple configurationselections displayed

Figure 6.22 Pump Specifications window with multipleconfigurations available.

NPSHR and NPSPR (optional in pump configuration)The net positive suction head required (NPSHR) or net positive suctionpressure required (NPSPR) can be expressed as a function of mass orvolumetric flow rate. This data is optional and is not required tocompletely define the pump. Both the required and available values aregiven in the Pump Summary in the General Output section.

For variable speed pumps, you can supply an affinity exponent to adjustthe full speed NPSHR/NPSPR to that at the alternate speed. Accordingto the affinity laws, the head ratio and speed ratio are related by thesquare law. However, the NPSHR ratio does not necessarily obey thisaffinity law. This exponent allows you to account for this.

Note that in the Elevation area a data field is provided for the referenceelevation for NPSH calculations. This may be the pump suctionelevation or pump centerline.


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Efficiency and power usage (optional in pumpconfiguration)The pump efficiency or power usage can be specified. This does notaffect the pump performance during steady-flow, but allows for quickassessment of power used at the operating point and the proximity toBEP (Best Efficiency Point). If efficiency or power data is supplied, thepower, efficiency and percent of BEP at the operating point is given inthe Output Pump Summary.

During transient flow, the efficiency/power usage is not optional if fourquadrant pump modeling is used. In this case, the efficiency/power datawill affect the results.

Transient DataAs discussed previously, pumps can be modeled as a pump curve or asan assigned flow. Using assigned flow pumps in steady flow modeling isuseful for sizing a centrifugal pump. In AFT Impulse, however, that isnot appropriate during transient modeling. When modeling a centrifugalpump transient, a pump with a pump curve is essential. Positivedisplacement pumps act as constant flow devices, and the assignedflowrate feature allows one to adequately model such pumps.

Pumps with curvesOnce a pump curve is entered, several options are available for transientmodeling. Modeling a pump with a curve is usually associated with acentrifugal pump.

No transient from pump, no back flowIf a pump remains at 100% speed during a transient, and the flow isalways positive, then no transient model is required. The appropriateTransient Model selection is therefore “None”. In such a case, the pumpperformance will be based on the provided head curve which onlyapplies to positive flow. Of course, if there is a check valve after thepump which prevents backwards flow, this is the best choice.

No transient from pump, but back flow possibleIf a pump remains at 100% speed during a transient, but backwards flowis possible, the head curve data is insufficient. Therefore four quadrant


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data can be used. As in other cases, the appropriate four quadrant dataselection is the one with the closest specific speed. This Transient Modeloption is called “No Transient – Four Quadrant”. This case is onlypossible when there is no check valve after the pump.

Known speed transients, no back flowAn estimate of the pump speed change with time can be entered in thetransient data area (see Figure 6.23). The speed change can be a time-based or event-based transient. The appropriate Transient Model is“Without Inertia”. This model uses the provided speed data and thepump homologous laws to determine the pump performance. Since theonly pump performance data is for forward flow, only forward flow orzero flow can be modeled. As before, if there is a check valve after thepump which prevents backwards flow, this is the best choice when thepump speed is known.

Figure 6.23 A speed transient can be entered for a pump with acurve


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Pump startup transient with possible reverse flowIf the speed for startup is known or can be approximated, and reverseflow is possible (no check valve exists), then a four quadrant model canbe used. With the speed known, we use the four quadrant data FH only.Other data for moment of inertia and initial synchronous speed in rpm isnot required.

Chapter 9 offers more information on the parameters and equations, andthe user is encouraged to consult one of the excellent texts for morebackground on this method (Chaudhry, 1987, Chapter 4, Swaffield, etal., 1993, pp. 154-166, Wylie et al., 1993, Chapter 7).

Pump transients with inertiaThere are four pump models in AFT Impulse that account for pumpinertial effects and predict the speed over time for either. Two allowreverse flow and/or speed, while two only work with forward or zeroflow. These are:

• Trip With Inertia and No Back Flow or Reverse Speed

• Startup With Inertia and No Back Flow or Reverse Speed

• Trip With Inertia – Four Quadrant

• Startup With Inertia – Four Quadrant, Known Motor Torque/Speed

Predicting the response of a pump during a trip or startup requiresadditional data for the pump. Use of the homologous pump laws isessential. If backflow is possible, "four quadrant" pump curve data isrequired. The first model mentioned above assumes the presence of acheck valve which prevents backflow. The second model also assumesthere is no backflow, although no check valve is assumed. In such cases,the pump will operate in the first quadrant. Thus four quadrant data isnot required.

The third and fourth models allow backflow and possibly reverse speed,and thus require four quadrant data. All four models will be discussed inthe following sections.

Trip with inertia and no back flow or reverse speedWhen no backflow or reverse speed occurs through the pump, thenormal pump curve data for flow vs. head can be used to model thepump response during a trip. Pump power data is also required as a


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function of flow. Data for the pump rotating moment of inertia and theinitial synchronous speed in rpm is required.

This model will use the pump head curve and power curve to determinethe pump torque, and use the moment of inertia to predict the pumpspeed decay over time.

See Chapter 9 for more information on the methodology.

Startup with inertia and no back flow or reverse speedWhen no backflow occurs through the pump, and the pump motor torquevs. speed is known, the normal pump curve data for head vs. flow andpower vs. flow can be used to model the pump response during a startup.Data for the pump rotating moment of inertia and the initial synchronousspeed in rpm is required.

This model will use the pump head curve and power curve to determinethe torque from the fluid, balance this with the torque from the motor,and use the moment of inertia to predict the pump speed rise over time.


Trip with inertia – four quadrantThis model allows the pump to flow and/or spin backwards, and requirespump data on the head and torque characteristics for reverse flow andspeed. Data for the pump rotating moment of inertia and the initialsynchronous speed in rpm is required.

In addition, dimensionless curves of FH and FB are required. Theparameters FH and FB are properties of the pump, and it is typicallyassumed that for pumps of similar specific speed they are invariant.Figure 6.24 shows data for a pump with a specific speed of 0.46 as givenby Wylie, et al., (1993, pp. 147-148).


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Figure 6.24 Pump with four quadrant data for pump trip modeling

AFT Impulse provides data of FH and FB vs. θ for pumps of twenty-onespecific speeds. The references come from Donsky (1961) as given byWylie, et al. (1993), Brown & Rogers (1980), and Thorley (1996). SeeChapter 9 for a list of the available selections. The conventionalassumption is that you should use four quadrant data for a pump with theclosest specific speed. At lower specific speeds, Brown & Rogers (1980)question this assumption.

If you should need to construct data for other sets, please consult thethree textbooks referenced below for additional information. Note thatthe textbooks do not give a method for constructing these data sets, butthey contain references that you can pursue.

Chapter 9 offers more information on the parameters and equations, andthe user is encouraged to consult one of the excellent texts for morebackground on this method (Chaudhry, 1987, Chapter 4, Swaffield, etal., 1993, pp. 154-166, Wylie et al., 1993, Chapter 7).


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Startup with inertia - four quadrant, known motortorque/speedWhen backflow can occur through the pump, and the pump motor torquevs. speed is known, four quadrant modeling is required to model astartup. The options for four quadrant data are the same as just discussedfor pump trip cases. Data for the pump rotating moment of inertia andthe initial synchronous speed in rpm is required.

This model will use the four quadrant data to determine the torque fromthe fluid, balance this with the torque from the motor, and use themoment of inertia to predict the pump speed rise over time.


Estimating the pump inertiaChapter 9 offers a method described by Wylie et al. (1993) to estimatethe moment of inertia, I, for a pump and the entrained liquid.

Event modeling – including trips followed by restarts or viceversaAll pump inertial modeling is a result of events. The Time option isdisabled. By default, a single event is selected and the event parameter isitself time. The default criteria is zero time, which means that the pumptrips or starts on the first step of the simulation. If you want the pump totrip or start at some later time which is known, you can just change thetime criteria for the event. Pumps can also be tripped or started based onany other of the available event criteria such as pressure or flow.

Pumps also support dual events, both cyclic and sequential. When thefirst event is a pump trip, the second event will always be a pump start,and vice versa. This allows a pump to be tripped and then restarted atsome later time, for example. The restart can occur while the pump isstill spinning down.

If the first event is a pump trip using "Trip With Inertia and No BackFlow or Reverse Speed", then the restart model will be "Startup WithInertia and No Back Flow or Reverse Speed", and vice versa. This meansthat motor torque vs. speed data is required to model the restart.

On the other hand, if the first event is a pump trip using "Trip WithInertia - Four Quadrant", then the restart model will be "Startup WithInertia - Four Quadrant, Known Motor Torque/Speed", and vice versa.


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As before, this means that motor torque vs. speed data is required tomodel the restart.

Controller transientIf the pump has a variable speed controller (see Figure 6.25), a controlflow or pressure transient can be specified. Again, this transient can betime-based or event-based (see Figure 6.26).

The Figure 6.26 transient essentially increases the speed of the pump toachieve 150 gpm of flow.

Note that the resulting pump speed transient with time can be saved tothe output file (see Transient Control window in Chapter 5). If saved, thespeed can be plotted vs. time.


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Figure 6.25 A pump with a variable speed controller can have atransient on the control point.


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Figure 6.26 A control point transient can be entered for a pump.

Pumps as assigned flowWhen a pump is modeled as an assigned flow, then the flowrate can bevaried over time. This flow transient can be time-based or event-based.The transient data might look like that in Figure 6.26.

Modeling pumps as an assigned flow will be most appropriate forpositive displacement type pumps. The transient can represent thestarting or stopping of a pump, or it can represent the pulsating transientflow that can occur in reciprocating pumps. More information ontransient data, including event transients, is given earlier in this chapterand in Chapter 10.

Special ConditionsPump junctions in AFT Impulse have two types of Special Conditions.The first is to turn the pump off and have no flow through it. This iscalled "Pump Off No Flow". The second type of Special Condition turnsthe pump off but allows flow to go through the pump. This is called"Pump Off With Flow Through".


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Why would one want to use one Special Condition rather than the other?The simple answer is that the first Special Condition is more appropriatefor positive displacement type pumps and the second for centrifugalpumps.

Here's why. When a positive displacement pump is turned off and has apressure difference across it, it will usually act like a closed valve andnot allow flow to go through it. Thus the first Special Condition wouldbe most appropriate. For instance, assume one wants to model thetransient that occurs during the startup of a positive displacement pump.The best way to do this would be to use the first Special Condition withno flow, and then input a flowrate transient in which the first data pointis zero flow.

On the other hand, when a centrifugal pump is turned off and has apressure difference across it, in the absence of other valving whichprevents flow the pump will usually allow flow to go through it. Thusthe Special Condition that allows through flow would be mostappropriate. For example, assume one wants to model the transientduring the startup of a centrifugal pump. One would use the secondSpecial Condition, and, in conjunction with a pump curve entered at100% speed, input a speed transient with the initial speed as zero.

Representing Multiple Pumps With One JunctionIf multiple identical pumps exist in parallel or closely spaced in series,rather than modeling each as a separate junction they can all becollapsed into a single junction. When doing so, intermediate piping isignored. This is often advantageous in that short pipes around the pumpwhich can cause long run times can be ignored.

To use this feature, specify the number of pumps at the location andwhether they are in parallel or in series.

Viscosity correctionsViscosity corrections to standard pump curves have been published inthe Hydraulic Institute Standards. The method, based on the 14thedition, 1983, is employed. The method itself consists of a semi-logarithmic diagram relating flow rate, head rise and viscosity. Thedetails of this diagram have been digitized into numerical relationshipsinside AFT Impulse 3.0. The corrections to head rise, flow rate and


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efficiency, referred to as CH, CQ and CE, respectively, are given in thePump Summary output.

The viscosity corrections are assumed to be valid during transients.

Graphing pump dataYou can track the pump speed by selecting this option in the Junctionsection of the Transient Control window. This parameter can be graphedor reviewed in the Output window.

Relief Valve Specifications window

The Relief Valve junction type can have one or two connecting pipes.This valve stays closed until it cracks open, at which time it relieves toan external pressure or to another pipe system.

The Relief Valve Specifications window, shown in Figure 6.27, followsthe first of the two basic Specifications window formats, displaying theconnecting pipes in a fixed format.

Figure 6.27 Relief Valve Specifications window


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Three relief valve configurations: internal, exit and inlineThe Relief Valve junction can be an internal relief valve, an exit reliefvalve, or an inline relief valve. If internal, the valve relieves into anotherpipe which was previously isolated. In this case the valve can only havetwo connecting pipes. Before cracking open the internal relief valve actslike a regular valve which is closed.

A Relief Valve junction that is at an exit relieves to an external ambientpressure such as the atmosphere. An exit relief valve only has oneconnecting pipe. Relief valves that are located at an exit must also besupplied with an associated back pressure.

An inline Relief Valve junction discharges to an external ambientpressure, and is located between two pipes. When closed, the valve is alossless connection and allows flow between the pipes.

Specifying lossesYou specify the losses for the Relief Valve on the Valve Model tab inthe Specifications window. For convenience, you can specify theconstant loss characteristics of a Relief Valve as a valve coefficient (CV),as a loss factor (K), or as a variable Cv where Cv depends on pressure.Chapter 8 details the relationship between K and CV.

Transient valve crackingTransient data for a relief valve is only used for the constant Cv andconstant K models. The variable Cv model does not use pre-definedtransient data.

The transient data for a relief valve is somewhat different from otherjunctions. The transient data does not refer to the absolute time in themodel. Time zero is the moment of cracking, not the start of the model.

You specify the transient relief valve data by clicking the Transient Databutton. A dialog window will open where you can enter the data. Onceentered, the data will display in the Transient Input Data table in thewindow.

A zero value of Cv indicates a closed valve. For a lossless valve, enter avery large Cv thus approximating infinity.

To simulate a closed valve with one of the K factor methods, use a Kfactor of –1 (negative one).


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Transient DataAFT Impulse allows the user to specify an opening profile transient ofCv or K vs. time. Remember that when using the K loss model, a valueof –1 (negative one) means the valve is closed. The opening will occuras soon as the cracking pressure is exceeded. If no opening transientprofile is entered, the valve will open instantaneously.

You can also specify a closing profile of Cv or K vs. time, which is theprofile the valve will follow if it closes. If no closing transient profile isentered, the valve will close instantaneously.

The zero time for the closing transient is relative to when the valve startsto close because of backflow. For the opening transient, it is relative towhen the valve starts to open.

More information on transient data is given earlier in this chapter and inChapter 10.

Transients with variable CvFor typical passive relief valves, it is not possible to pre-specify thetransient valve behavior. The valve Cv depends on the pressuredifference across the valve, as the pressure difference is what causes thevalve to open. When the pressure difference is below the crackingpressure, the valve remains closed. When it rises above the crackingpressure, it opens. As the pressure difference increases, it opens furtheruntil it is fully open. Any further pressure increases to not increase theCv, and so the fully open Cv is used for all higher pressures.

Reservoir Specifications window

The Reservoir Specifications window is shown in Figure 6.28a-b. TheReservoir junction type allows you to connect up to twenty-five pipes.One connecting pipe is required. The Reservoir junction is convenientfor specifying a fixed pressure in your system. During a transient, youcan vary the liquid surface elevation in a prescribed way.

A Reservoir junction applies a defined pressure at the junction locationin the model. When solving a pipe flow system, a Reservoir causes therest of the system to distribute the flow in a manner consistent with thedefined pressure.


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Figure 6.28a The Reservoir Specifications window ReservoirModel tab.

The Reservoir Specifications window follows the second of the twobasic Specifications window formats. A table on the Loss Coefficientstab displays the connecting pipe information. This table grows toaccommodate up to twenty-five pipes. After you add a fifth connectingpipe, a scroll bar appears, allowing you to review and enter loss factorsfor all pipes in the table.

The pipe table displays the reference positive flow direction of eachconnecting pipe. To enter loss factors, select the cell in the table and editthe value. Each pipe can use a different loss model or custom value.

The depth or elevation for each connecting pipe can be entered in thetable. The depth is the distance below the liquid surface level.


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Figure 6.28b Individual pipe depths (or elevations) and losscoefficients may be specified in the ReservoirSpecifications window

If a pipe returns to the reservoir above the liquid surface, the depth isentered as a negative number, or the elevation is specified higher thanthat of the liquid surface. Pipes that discharge above the liquid surfaceare assumed to have liquid free fall to the liquid surface. AFT Impulseapplies the proper boundary condition for pipes with negative depths. Ifthe pipe is above the liquid surface, the only appropriate condition is forthe fluid to be flowing from the pipe into the reservoir. Fluid cannot flowfrom the reservoir into the pipe as the pipe is above the liquid surface. Ifthis occurs, AFT Impulse will assume the fluid flowing into the pipe isthe same as the reservoir fluid, solve the system, and then give a warningin the output.


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Transient DataOn the transient data tab the surface elevation can be changed with time.More information on transient data, including event transients, is givenearlier in this chapter and in Chapter 10.

Spray Discharge Specifications window

The Spray Discharge junction must have at least one connecting pipe,and it allows up to four. This junction offers a flexible way of modelinga flow exiting the system through a nozzle or spray device. The flowoccurs across a user specified hydraulic loss such as a dischargecoefficient. This can be varied with time to open or close the SprayDischarge junction.

The Spray Discharge Specifications window, shown in Figure 6.29,follows the second of the two basic Specifications window formats. Atable displays the connecting pipe information. The pipe table grows insize to accommodate up to four connecting pipes.


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Figure 6.29 The Spray Specifications window

The exit pressure to which the Spray Discharge junction discharges mustbe entered.

Two geometries can be modeled. The first is a normal, one hole spray,where the discharge flow area of the hole is entered. The second is asparger which, in principle, works in the same way as a spray. For thesparger, the flow area of a single hole and the number of holes areentered. This assumes that the holes are hydraulically close in proximityand are the same area.

Loss ModelThe discharge coefficient for the junction is entered in the appropriatebox. The physical area of the exit flow can also be entered. These twoparameters are used to calculate the discharge flow rate, which dependson the difference between the internal pressure in the system and thespecified exit pressure.


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The K Fire Sprinkler model uses the K value as defined by the firesprinkler industry. Whereas the standard K loss value is dimensionless,the K Fire Sprinkler has units associated with it. See Chapter 8 for moredetails.

Transient DataOn the transient data tab the discharge flow area or K data can be variedwith time to simulate the opening or closing of the junction. Moreinformation on transient data, including event transients, is given earlierin this chapter and in Chapter 10.

Graphing Spray Discharge dataYou can track the discharge flowrate and total mass or volumedischarged at the spray discharge junction by selecting this options in theJunction section of the Transient Control window. This can be graphedor reviewed in the Output window.

Surge Tank Specifications window

The Surge Tank Specifications window is shown in Figure 6.30. TheSurge Tank junction allows up to twenty-five connecting pipes. Thisjunction type is useful for modeling a small vessel designed as a surgeprotection device (see Figure 6.31). The Surge Tank may be integralwith the pipe system, separated by an orifice, or by a short connectorpipe.

The Surge Tank Specifications window follows the second of the twobasic Specifications window formats. A table displays the connectingpipe information. The table grows in size to accommodate up to twenty-five connecting pipes. After you add a fifth pipe a scroll bar appears,allowing you to review all the pipes in the table.


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Figure 6.30 Surge Tank Specifications window, with a variableheight and flow restriction orifice

Tank geometryThe Tank Height is the physical top of the tank in relation to theelevation. If there is a Connector Pipe, then the Tank Height is relativeto the top of the Connector Pipe. This height is used in the case ofoverflow. If the surge tank overflows, spillage will occur and the liquidwill exit the pipe system not to be recovered. If you do not want tomodel the overflow, specify a very tall tank or do not enter a TankHeight.

If the Tank Cross-Sectional Area is specified as Constant, the surge tankis assumed to be a vertical tank with constant cross-sectional area. If thetank cross-sectional area changes with height, this can be entered byselecting the variable option and entering the cross-sectional variation inthe table.

The Tank Cross-Sectional Area is used for the purpose of determiningthe liquid volume. It will determine how high the liquid will rise or drop,since a given volume of liquid will flow into the tank during a time step.


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Interface with pipe systemThe Surge Tank can connect into the pipe system in three ways: it can beintegral with the pipe, it can be separated by an orifice, or it can beconnected by a short pipe.

These options are provided in the Flow Restrictor and Connector Pipeareas. If neither option is used, the surge tank is assumed to be attacheddirectly to the pipe with no hydraulic loss.

If the Flow Restrictor is used, the surge tank is assumed to be separatedfrom the pipe system by an orifice, which causes an hydraulic loss as theliquid flows in and out of the surge tank. The loss is specified byentering a flow area and discharge coefficient. Further, this loss can bedifferent for inflow vs. outflow.

LL

LC

ConnectorPipe

Orifice

Tm&

Figure 6.31 Surge tank schematic

Lastly, the surge tank can be connected by a short pipe which isconsidered to be short in relation to the other pipes in the system afterthey are sectioned. If the pipe is short, it can be assumed to reactinstantaneously with the surge tank. The short pipe accounts for liquidinertia and friction. If a connector pipe exists which cannot beconsidered short, it should be modeled as an actual AFT Impulse pipe.

The Flow Restrictor and Connector Pipe can both exist in the same surgetank. That is, a flow restrictor can be modeled at the top of a connectorpipe, and the effect of both will be included.


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One-Way Surge TankOne drawback of surge tanks is that they must be located in low pressureparts of the system because high pressure locations require a very tallsurge tank (a water surge tank located where the pipe system is at 100psig must have a liquid surface at least 230 feet higher than the pipe).Unless there is a check valve connecting the surge tank to the pipe. Sincethe tank can only flow out, it is only useful to limit low transientpressures by flowing liquid into the system when the pressure drops.However, when the pressure rises, the check valve closes and the tankcannot accept any fluid and thus decrease the system pressure.

Such a design is called a One-Way Surge Tank, and it can be modeled bydefining the check valve characteristics.

Transient DataOn the transient data tab the surface pressure can be varied with time tosimulate a tank pressurization. More information on transient data,including event transients, is given earlier in this chapter and in Chapter10.

Special features for steady flowAs mentioned previously, the surge tank acts like a branch junctionduring steady flow. In such cases the steady-state liquid height in thesurge tank is calculated (it is the steady-state EGL at the junction).However, the user can specify that it behave like a pressure/reservoirjunction. Why would one want to do this? One important case is wherethe surge tank is modeling a pressurizer type of component in a closedsystem. When open to the atmosphere, a pressurizer is sometimes calledan expansion tank or head tank. The main purpose is to provide thermalexpansion volume when the temperature of the process fluid changes.Sometimes these tanks are closed, in which case a Gas Accumulatorwould be a better choice than a surge tank.

On other occasions, one may want to model a fixed steady-state liquidlevel at a location, but is interested in the liquid level changes in a finitesize tank.

These two possibilities have different implications which will beexplored here, along with the standard use where the surge tank acts like


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a branch. The both cases the input is on the Optional tab (see Figure6.32).

Figure 6.32 Surge Tank junctions allow the user to specify an liquidlevel for special steady-state behavior.

Surge tank as a branch during steady-stateWhen a Surge Tank is used as a surge suppression device as in pumpedsystems, it will act like a branch during steady flow. There will be noflow in or out of the tank, and the surface level will equal the localgradeline of the system. Thus the surface level is not an input, but iscalculated. This is the default behavior of the Surge tank junction.

During the transient, flow will go in and out of the tank, and the liquidlevel will change.

Surge tank as a pressurizer during steady-stateWhen a pressurizer is modeled as a surge tank, there will typically be noother pressure type junctions in the system. In fact, the pressurizer is thecomponent which controls the static pressure on the system duringsteady-state operations by virtue of its liquid surface level. However,


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during transients the liquid surface level will change. Thus there is aneed to allow a surge tank to act as a pressure junction during the steady-state (in which the liquid level is fixed), and then like a normal surgetank during the transient (in which the liquid level varies). This is thefunction of the Initial Liquid Height for Steady-State Flow on theOptional tab (see Figure 6.32).

When the surge tank is a pressurizer during steady-state, it has zero netflow. The liquid level entered will be the steady-state static systempressure. A flow balance exists at the junction because the net flow iszero.

During the transient, flow will go in and out of the tank, and the liquidlevel will change.

Surge Tank as a finite size reservoir during steady-stateWhen the surge tank is not a pressurizer, but acts like a reservoir, it willhave a net flow imbalance during steady-state just like Reservoirjunctions do. This is how pressure junctions work (see Chapter 12).Once the transient begins, the liquid surface level will vary as for anysurge tank.

Difference between a Reservoir and Surge TankA Reservoir junction is assumed to be sufficiently massive in volumethat surface level changes do not occur during the simulation. A goodphysical example of this is when the liquid source is a large lake.

However, sometimes the liquid source comes from a finite size tank, andthe liquid surface level will in fact change during the simulation. Totrack the change, the geometry of the source tank must be known. Thiscan be entered for a Surge Tank, but not a Reservoir.

Model as dipping tube vesselA dipping tube vessel is a combination surge tank and gas accumulator(see Figure 6.33). As long as the liquid level remains below LA in Figure6.33, it acts like a conventional surge tank. If the liquid level rises aboveLA , gas gets trapped in the upper region and the device acts like a gasaccumulator.

The dipper tube capability is entered on the Optional tab (see Figure6.32).


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LL

LC

ConnectorPipe

Orifice

Tm&

LA

Figure 6.33 Dipping tube vessel geometric representation. Whenthe liquid level rises above LA, it acts like a gasaccumulator.

Graphing Surge Tank dataYou can track the surge tank flowrate, total mass or volume, and liquidsurface level pressure by selecting these options in the Junction sectionof the Transient Control window. These parameters can be graphed orreviewed in the Output window.

Tee/Wye Specifications window

The Tee/Wye Specifications window is shown in Figure 6.34. TheTee/Wye junction type requires three connecting pipes. To increaseclarity, the window allows you to arrange the pipes with respect to animage. Four different tee/wye models are available.

The Tee/Wye Specifications window does not strictly follow either ofthe two basic Specifications window formats. Instead, the threeconnecting pipes are arranged on the Arrangement tab using providedlists and images.

Loss factorsThere are two Loss Models available for Tee/Wye junctions: Simple andDetailed. If you choose the Simple type of Tee/Wye, no loss factors will


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be calculated. This is equivalent to a Branch with no loss factors enteredfor the connecting pipes. In some systems the losses associated with aTee/Wye are small compared to the overall loss in the system. To avoidunnecessary calculations you should choose the Simple type of Tee/Wyeunless you need the actual losses.

Figure 6.34 The Tee/Wye Specifications window

The Detailed type of Tee/Wye will calculate the loss factors as theydepend on the flow split as well as the geometry. Because of thedependence on the flow split, loss factors usually cannot be calculatedbefore performing an analysis. AFT Impulse incorporates the bestavailable models to calculate tee and wye loss factors during the actualsolution. These calculations account for the percentage of the flow splitand the angle of connection. This offers a significant advantage overhand calculated losses.

If you have specific loss factors that you want to use for a tee/wyejunction, you should use a Branch junction instead.

The Tee/Wye junction functions the same during the steady-state andtransient.


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Valve Specifications window

The Valve Specifications window is shown in Figure 6.35. The Valvejunction type requires two connecting pipes, unless you specify it as anexit valve, in which case only one connecting pipe is allowed. Thisjunction type allows you to model the irrecoverable loss that occursthrough a flow control component. You also have the ability to changethe loss with time to open or close the valve in full or in part.

Figure 6.35 The Valve Specifications window

The Valve Specifications window follows the first of the two basicSpecifications window formats, displaying the connecting pipes in afixed format. A flow direction through the junction is adopted from thedefined directions of the connecting pipes. Consistent with AFTImpulse's convention, the loss factor base area is referenced to theupstream flow area shown as the Base Area. Valve junctions aretypically internal to the system, with two connecting pipes.

Valves that are located as exit flow control mechanisms are specified asexit valves. These valves require an associated back pressure definition.Exit valves can only be connected to a single upstream pipe. To specify


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an exit valve, tick the Exit Valve checkbox. Then specify the exitpressure.

Numerous handbook loss factors for generic valve configurations aregiven in the Valve Data Source area by selecting Handbook Data.

For more information on the loss models for these valves, refer toChapter 8.

Transient DataOn the transient data tab a transient Cv or K table can be entered tosimulate the opening or closing of the valve. For a closed valve, the Kfactor is infinite. To account for this, enter a K factor of –1 (negativeone) in the transient data table. More information on transient data,including event transients, is given earlier in this chapter and in Chapter10.

Special ConditionsThe Special Condition for a valve always closes it.

Vacuum Breaker Valve Specifications window

The Vacuum Breaker Valve Specifications window is shown in Figure6.36. The Vacuum Breaker Valve junction type (also called an air inletvalve) requires two connecting pipes.

The Vacuum Breaker Valve Specifications window follows the first ofthe two basic Specifications window formats, displaying the connectingpipes in a fixed format. The flow direction through the junction isdetermined by the defined directions of the connecting pipes.


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Figure 6.36 Vacuum Breaker Valve Specifications window

The Vacuum Breaker Valve offers protection for low pressureconditions. When the internal pressure drops below the crackingpressure (usually atmospheric), it opens to admit air (or whatever gasyou specify). This keeps the pressure from dropping much below theatmospheric pressure. When the internal pressure rises above theatmospheric pressure, the air is discharged. To prevent large pressurespikes, the discharge flowrate is frequently kept lower than the incomingflowrate by using a smaller discharge flow area for outflow than inflow.

A vacuum breaker junction can connect with one pipe or two. Whenconnected to one pipe, the valve is located at the end of the pipe. Whenconnected to two pipes, the valve is “inline” and allows air to flow inand out at that location.

During steady-state a vacuum breaker valve with one connecting pipeacts as a dead end. If connected to two pipes, it acts as a lossless valveallowing liquid to flow between the two pipes.

By default, the vacuum breaker junctions allows air to flow in and out.Alternatively, it also model only air inflow or outflow.


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Graphing Vacuum Breaker Valve dataYou can track the amount of air in the system and the flowrate byselecting these options in the Junction section of the Transient Controlwindow. These parameters can be graphed or reviewed in the Outputwindow.

Volume Balance Specifications window

The Volume Balance Specifications window is shown in Figure 6.37.The Volume Balance junction type is internal to the system and requirestwo connecting pipes.

The Volume Balance Specifications window follows the first of the twobasic Specifications window formats, displaying the connecting pipes ina fixed format. The flow direction through the junction is determined bythe defined directions of the connecting pipes.

Figure 6.37 The Volume Balance Specifications window


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The Volume Balance junction is useful for modeling pipelines with amoving interface between fluids of different density. Such a scenario isin reality a transient application, because the mass in the system is notsteady with time. AFT Impulse assumes the interface does not moveduring the transient.

Volume Balance junctions should be used in conjunction with thevariable physical property model (see System Properties window).

Global Edit windows

The Global Pipe Edit and Global Junction Edit windows, accessiblethrough the Edit menu, assist you in making large scale changes to yourmodel.

Global Pipe editingThe Global Pipe Edit window, shown in Figure 6.38, allows you to editmost of the pipe data in the model at the same time. In the Global PipeEdit window, select the pipes you want to edit, then click the Select PipeData button. This will open the Pipe Specifications window (see Figure6.1) in which you can enter the data you want to change on each of theselected pipes.

When you are done entering the data to be changed, click the OK buttonin the Pipe Specifications window. The Global Pipe Edit window nowlists the parameters to change in a list on the right. Select the parametersyou want to change and then click the Apply Selections button.

Note that you cannot change some data, like the pipe ID number, usingthis global method.


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Figure 6.38 The Global Pipe Edit window allows you to change datafor many pipes at the same time

Global Junction editingThe Global Junction Edit window allows you to edit elevation,name/number display option, initial pressure, design factor, andWorkspace icon size for all junctions (see Figure 6.39a). This windowalso offers the ability to select specific junction types for specific datachanges (see Figure 6.39b). For example, with a couple mouse clicks allvalve junctions can be turned off or the pump curves in your model canbe changed to a new pump curve.

To edit the common junction data, select Common Data option enter thedata in the Common Parameters area. Then check the boxes on the rightto select the data to be changed and click the Apply Selections button.


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Figure 6.39a The Global Junction Edit window allows you tochange data that is common among different typesof junctions, as shown above.

Editing specific junction dataTo edit specific junction data do the following:

1. Select Specific Data For … option

2. Select the junction type in the dropdown list

3. In the Junction List, choose the junctions you want to edit

4. Click the Select Specific Junction Data button. The Specificationswindow for the junction type you selected will be opened.

5. Enter the data you want to change for each of the selected junctions.

The Specifications window for the junction type you selected will beopened. Enter the data you want to change for each of the selectedjunctions.

When you are done entering the data to be changed, click the OK buttonin the Junction Specifications window. The Global Junction Edit


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window now itemizes the parameters to change in a list on the right.Select the parameters you want to change then click the ApplySelections button.

Figure 6.39b The Global Junction Edit window allows you tochange data for many junctions at the same time.You can change detailed data for junctions of thesame type, as shown above.


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C H A P T E R 7

Customizing AFT Impulse

This chapter explains how to customize AFT Impulse in ways thatincrease your productivity. To accelerate repetitive operations you cancustomize the way the Workspace looks and behaves and the way outputis formatted. Changes to the program's appearance and functionality aresaved to the IMPULSE3.INI file in your Windows directory.

You can also build custom databases that are read in during startup andused as if they were native to the program. These databases are saved tothe IMP_USER3.DAT file in your Windows directory.

If you pursue extensive customizing of AFT Impulse, you are stronglyadvised to make regular backup copies of the two files (IMPULSE3.INIand IMP_USER3.DAT). If the files ever become corrupted you couldlose all the custom data.

Tools are also provided to allow creation of network databases to sharefrequently used data and report formats across a local or wide areanetwork.

Parameter and Unit Preferences

The Parameter and Unit Preferences window, shown in Figure 7.1, isaccessed from the Options menu. The features in this window allow youto specify the default parameters with which you would like to work. Bychanging any of the parameters in this window you do not lock yourselfout from certain features. Rather, when you encounter lists or optionsduring assembly of your model, AFT Impulse offers your default


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parameters as a first choice. This way you do not have to repeatedlychange the parameter in the Specifications windows.

Parameter PreferencesThe first parameter you can set is the Use Most Recent Pipe Size Datafeature. When this is selected, AFT Impulse remembers your most recentpipe material size setting and sets new or unspecified pipes to this pipematerial and size.

You can also set a default pipe material and default friction model. Youcan change the material and loss model of each individual pipe at anytime, but this will be the first choice.

Figure 7.1 Parameter and Unit Preferences window sets defaultvalues to use.

There are Default Design Factors. The function of design factors isdiscussed in more depth in Chapter 8. If you want to set up defaults thatare applied to your pipes and junctions, here is where you do it.


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If you want to change the design factors for existing pipes or junctions,see the Global Pipe Edit and Global Junction Edit topics in Chapter 6.

Another parameter on this tab is the Default Elevation. The DefaultElevation is especially useful when the majority of your junctions allreside at the same elevation. When you specify a default elevation, allthe junctions you create will show the default elevation, saving you fromhaving to enter the same elevation repeatedly. AFT Impulse defaults toworking without a default elevation specified.

Unit PreferencesYou can set the units you prefer to use in AFT Impulse on the UnitPreferences tab (see Figure 7.2). These units are selected as a first choicein the unit selection boxes. You can choose any of the units available. Toset a new default, choose the unit type from the list on the right and clickthe Set As Preferred Unit button.


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Figure 7.2 Using the Unit Preference tab in the Parameter and UnitPreferences window, you can set the default units youwant to work with and choose to show only the unitsyou want to work with.

You also can customize the units used by AFT Impulse. For eachparameter, AFT Impulse can use many different units. However, if onlya few are ever typically used, then the others can be removed. This willdecrease the number of items in the unit selection boxes, making it easierto choose the one desired. If a preferred unit is removed from thoseavailable to Impulse, a new one will have to be chosen before leavingthis area. You also must have one set of units available for eachparameter.

To customize the units, first choose the parameter to modify on the liston the left. Then, among the unit types on the right, check or uncheck theboxes for the units you want or do not want to see.


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You can select or unselect all displayed units by clicking the appropriatebutton. Also, English and SI units can be selected as a group forparameters by clicking the appropriate button.

Tip: To choose only English or SI units for all parameters, go to the UnitPreferences tab, choose the Default Unit System then click the ApplyDefault Units button. This will use AFT Impulse’s default units. Thiswill also change the preferred units.

Setting preferred unitsTo set a particular unit as the preferred unit, choose it in the list on theright, and click the Set as Preferred Unit button.

Description of selected unitA more complete description of the selected unit is shown at the lowerright of the window in the Unit Description field.

Database connectionsIn the lower left of the Parameter and Unit Preferences window there aretwo Database checkboxes. If these boxes are checked, your Parameterand Unit Preferences parameters are set up as determined by thedatabase to which you are connected. This is referred to as an activedatabase. To make them inactive, uncheck the boxes or change one ofthe Parameter and Unit Preferences settings controlled by the database.You can specify database connections separately for parameters andunits.

If the checkboxes are unchecked, but enabled, you are connected to adatabase but the settings are not being passed to the Parameter and UnitPreferences window. The databases are thus inactive. To make themactive, check the boxes then click the OK button.

If the checkboxes are disabled, there are no connected databases.

Later in this chapter we’ll discuss how databases are configured andadministered through your local or wide area network.


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Command buttonsThere are eight buttons at the bottom of the Parameter and UnitPreferences window. Impulse has built-in default parameters, units andsettings which you can choose by clicking the Impulse Default button.You can also develop your own settings, tailored to your project orindustry, and have these used by default (instead of Impulse’s defaults).To make your own default, first select the parameters, units and settingsyou would like to use, then click the Set As Default button. Your settingswill be saved and will be used each time any new project is initiated. Ifyou make changes to the settings, and want to get back to your defaults,click the User Default button. The default settings are updated onlywhen you click Set As Default.

You can save the parameter and unit settings to a file by pressing theSave Preferences button and entering a file name. These setting areloaded again by pressing the Recall Preferences button and choosing thefile name. The format files you create can be placed on a network forsharing among a group or company, or incorporated into a company-wide database, allowing standardized reporting.

If you have made changes that you don’t want to keep, click the Cancelbutton. Click OK to use the settings you have defined.

Workspace Preferences

The Workspace Preferences window is accessed from the Options menu.The features offered in this window allow you to customize theappearance and behavior of the Workspace.

Pipes and JunctionsYou can customize the pipe thickness and style, and the junction size, onthe Pipes and Junctions tab, shown in Figure 7.3.


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Figure 7.3 The Pipes and Junctions tab in the WorkspacePreferences window.

Pipe Line OptionsNormally, all pipes on the Workspace are displayed using the same linethickness. This thickness value is set using the dropdown list at the topof the Pipe Line Options area. A sample of how the pipe will bedisplayed is shown immediately below the dropdown list.

There are two additional options that will vary the line thicknessaccording to the pipe diameter. If you choose the Vary Thickness WithPipe Diameter option, the pipe line thickness will be varied according tothe pipe physical diameter. The smallest diameter pipe in the model willbe displayed as a line 1 pixel thick and the largest diameter pipe will bedisplayed as a line the thickness you specify in the Pipe Line Thicknessdropdown list. All other pipes will be shown using a thicknesssomewhere in between, according to their diameter.


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Similarly, the Vary Thickness Using option will vary the pipe linethickness from the smallest diameter pipe in the model to the largest.However, in this option you can specify the range of pipe diameters andline thicknesses to use. If a pipe is smaller than the minimum diameterspecified it will be shown using the minimum thickness. Accordingly, ifa pipe is larger than the maximum diameter, the maximum thickness willbe used. This option is useful to keep a few very large or very smallpipes in the model from skewing the display.

By varying the pipe thicknesses you can easily see the feeding pipes, thelarge flow area pipes, or the small supporting flow pipes.

Each individual pipe allows you to modify its thickness, which overridesthe default size set here.

Special Conditions OptionsYou can change the pattern of a closed pipe by choosing the style (e.g.dots or dashes) from the dropdown list in the Closed Pipe/JunctionOptions area. All pipes, whether closed using the Special Conditionsoption, or closed as a result of another object, will be displayed usingthis style.

The junctions in a closed flow path are outlined using this style. Anexample of the closed pipe style is shown below the list box. If youchoose Solid, no distinction will be no distinction made between aclosed and open pipe.

Action When Drawing Selection Right To LeftBy default this option is turned ON, and when ON the SelectionDrawing tool on the Workspace will select all objects completely orpartially within the selection box when the selection box is drawn fromright to left.

Modifying icon sizeThe default size of the junction icons on the Workspace can be changedto 25%, 50%, 75%, or 100% of the full size. Click the option button nextto the desired size in the Junction Icon Size area; the sample junctionwill change, reflecting your choice. Reducing the junction size may freeup more Workspace area and present a more useful or appealingperspective of your models. Each individual junction allows you tomodify its size, which overrides the default size set here.


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Action when dragging junctionsNormally, when a junction is dragged around the Workspace, the pipesstay attached unless the CTRL key is pressed during the movement. Youcan reverse this action (pipe will detach while dragging unless the CTRLkey is pressed) by checking the Leave Connected Pipes in Place checkbox. This is useful when you are rearranging the model.

Auto increment labelsWhen new pipes or junctions are added to the Workspace, they areassigned a new number that is greater than the highest pipe or junctionsnumber by one. The increment can be changed from this default to ahigher value.

Icon sourceLimitations exist in Windows 95, 98 and Me that can affect AFTImpulse’s ability to display large models on the Workspace. Thelimitations relate to what are called GDI resources. The practical resultis that models that display more than 150-250 junctions may havetrouble. In such cases, it is recommended the Icon Source be changed tothe Large File Icon Method. This allows you to use all icons that aredisplayed on the Toolbox, plus it offers the ability to rotate icons. TheToolbox Icons Only option only allows display of icons that are shownon the Toolbox. When either of these options are chosen, the WorkspaceIcon area on each junction Specifications window is disabled. Usingdifferent color junctions is therefore not available.

Windows NT, 2000 and XP do not experience these limitations and theuser should thus always use the Complete Icon Set.

Display OptionsFrom the Display Options tab you can change the symbols used, textcontent, and location of the text on the Workspace (see Figure 7.4).

Workspace gridA grid can be displayed on the background for modeling convenience.The grid can be fine, medium or coarse. The grid lines can be solid ordashed. And finally, the pipes and junctions can be snapped to the gridyou specify.


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The grid can be included in printouts by specifying its inclusion in thePrint Preview/Special window.

Displaying source/sink indicatorThe Branch and Assigned Flow junctions specify flow into (source) orout of (sink) the system. These junctions can display a small symbolindicating whether the junction represents a flow source or sink. Youcan change these symbols, for example, to a + and - by entering thesymbol in the appropriate field in the Flow Junction Symbol area. To notshow any symbol on any of these junctions, uncheck the DisplaySource/Sink Indicator checkbox.

Figure 7.4 The Display Options tab in the Workspace Preferenceswindow.


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Special condition junction symbolTo better locate an object which has Special Conditions set (e.g., a pumpturned off), the object will have a Closed Symbol preceding the IDnumber. By default this is an ‘X’, but can be customized by entering thedesired symbol. If no symbol is entered no symbol will be displayed.This symbol is also used in the Junction Results tables on the Outputreport. To not use any symbol on any object, uncheck the Display ClosedIndicator checkbox. Additionally, junctions which are closed in any wayare outlined using the Closed Pipe Line Style. To not outline thejunctions, uncheck the Display Outline Around Closed Junctionscheckbox.

Pipe direction arrowsEach pipe has a reference direction, and this direction can be displayedin one of two ways. First, an arrow can be displayed on the Workspacepipe. Second, an arrow can be displayed next to the pipe number. If anarrow is shown on the pipe itself, one of several styles can be selected.

Displaying name and/or ID numbersYou can set the default number and/or name display setting for junctionsand pipes in the Pipe/Junction Display area. This setting will be used forall new pipes and junctions. You can change the setting on a pipe-by-pipe or junction-by-junction basis in the Specification window or byusing Global Edit.

Setting the default junction label positionThe default label position for all junctions on the Workspace can beadjusted in the Junction Label Location area. Click the option button atthe desired location around the sample junction icon. Note that thejunction (and pipe) labels can be moved individually

Allowable Workspace label movementsBy default, pipe and junction labels Workspace can be moved to newlocations. You can change this functionality to only allow movementswhen holding the shift key.


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Junction transient symbolsThere are special symbols displayed on the Workspace for transientrelated data. These symbols can be modified here. By default, a "T"symbol is displayed if a junction has transient data. A "#" symbol meansthat a Transient Special Condition has been specified. And a "*" symbolmeans that the junction is to have no reflections.

Colors and FontsThe color of pipes, junction labels, etc., can be set on the Colors andFonts tab (see Figure 7.5). To change the color of an item such as theWorkspace background, select the Workspace Background from the listand click the Set Color button. You also can just double-click theWorkspace Color itself. The color of individual pipes can be assigned ineach pipe window. This will override the default pipe color specifiedhere.

In addition to the color area, the junction and pipe font is displayed. Tochange fonts, click the Change Fonts button to open the Fonts windowthen, choose a new font or font style. A sample is displayed above theChange Font button.


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Figure 7.5 The Colors and Fonts tab in the Workspace Preferenceswindow.

Sample WorkspaceThe tab for Sample Workspace gives you sample of how the Workspacepipes and junctions will appear if you click the OK button.

Database connectionsIn the lower left of the Workspace Preferences window there is aDatabase checkbox. If this box is checked, your Workspace Preferencesparameters are set up as determined by the database to which you areconnected. This is referred to as an active database. To make it inactive,uncheck the box or change one of the Workspace Preferences settingscontrolled by the database.


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If the checkbox is unchecked, but enabled, you are connected to adatabase but the settings are not being passed to the WorkspacePreferences window. The database is thus inactive. To make it active,check the box then click the OK button.


Later in this chapter we’ll present a discussion of how databases areconfigured and administered through your local or wide area network.

Command buttonsThere are eight buttons at the bottom of the Workspace Preferenceswindow. Impulse has built-in default parameters, units and settingswhich you can choose by clicking the Impulse Default button. You alsocan develop your own settings, tailored to your project or industry, andhave these used by default (instead of Impulse’s defaults). To make yourown default, first select the settings you would like to use, then click theSet As Default button. Your settings will be saved and will be used eachtime any new project is initiated. If you make changes to the settings,and want to get back to your defaults, click the User Default button. Thesettings are updated only when you click Set As Default.

You can save the parameter and unit settings to a file by pressing theSave Preferences button and entering a file name. These setting areloaded again by pressing the Recall Preferences button and choosing thefile name. The format files you create can be placed on a network forsharing among a group or company, or incorporated into a company-wide database, allowing standardized reporting.

If you have made changes which you don’t want to keep, click theCancel button. Click OK to use the settings you have defined.

Toolbox Preferences

The Toolbox Preferences window is accessed through the Options menu.Figure 7.6 shows the layout of the Toolbox Preferences window. Thefeatures in this window allow you to customize the Toolbox in themanner that is most productive for you.

Once you have changed the appearance and functionality of the Toolbox,you can set your changes as the user default values by clicking the Set


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As Default button. From then on, AFT Impulse will always load withyour default Toolbox settings.

Figure 7.6 Toolbox Preferences window allows you to customizethe Workspace Toolbox.

Customizing Toolbox contentsWhen you open the Toolbox Preferences window you can see two largeareas labeled Current Toolbox and Unused Tools. By dragging thejunction type icons from the Current Toolbox to the Unused Tools areayou can remove these junction types from the Toolbox. This may beuseful if you use only a limited number of junction types and don't wantthe unused types displayed on the Toolbox.


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You can also reorder the icons on the Toolbox by dragging and droppingthem to the desired locations on the Current Toolbox.

Database connectionsIn the lower left of the Toolbox Preferences window there is a Databasecheckbox. If this box is checked, your Toolbox Preferences parametersare set up as determined by the database to which you are connected.This is referred to as an active database. To make it inactive, uncheckthe box or change one of the Toolbox Preferences settings controlled bythe database.

If the checkbox is unchecked, but enabled, you are connected to adatabase but the settings are not being passed to the Toolbox Preferenceswindow. The database is thus inactive. To make it active, check the boxthen click the OK button.


Later in Chapter 7 a lengthy discussion is given of how databases areconfigured and administered through your local or wide area network.

General Preferences

The General Preferences window (Figure 7.7) allows you to:

• Specify automatic model saving

• Options for opening models with scenarios

• Enable Tip of the Day feature

• Require a check of the Waterhammer Assistant before runningmodels

• Default locations for saving models

• Common database locations

• Backup folders where your IMP_USER3.DAT and IMPULSE3.INIfiles are copied (this is useful for keeping backups should youchange computers)

• Specify how AFT Impulse handles transient junction data that doesnot cover the entire time range of the simulation (either maintain the


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data as constant at the last data point or extrapolate). Maintain is thedefault option.

Figure 7.7 General Preference window

Customizing the Output

AFT Impulse allows you to save your output preferences as the userdefault. To do this, change the Output Control window to your preferredarrangement, then click the Set As Default button. From then on thisarrangement will be the default used when any new project is initiated.

If you make changes to the Output Control window and you want toreturn to your user default setup, click User Default. Clicking ImpulseDefault activates the original defaults used by AFT Impulse. No updatesto the defaults are made unless you click Set As Default.

You can save frequently-used output formats to file for later reuse. Forexample, for checking modeling errors you may wish to create an OutputControl file named “ALL.DAT” that has all parameters, which you canquickly access when needed. You may have another named“FINAL.DAT” for final reports. The Output Control format files can be


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located on a PC network for access by all engineers to allow for standardreporting between different projects and groups.

More extensive discussion on using Output Control to customize youroutput is given Chapter 4.

Specifying graph style preferences

When preparing graphs in the Graph Results window, you may prefer towork with certain colors and styles. The Customize Graph windowallows you to save the style preferences using the features on the Systemtab. You can later recall these preferences using the features in the samearea.

In addition, an Auxiliary Graph Formatting window is available to makeit easier to change graph colors and curve thickness. This window alsoallows one to specify default colors and curve thickness.

Specifying Visual Report preferences

The Visual Report window content is specified in the Visual ReportControl window. To open the Visual Report Control window, choose itfrom the View menu or Toolbar. Through the Visual Report Controlwindow you can choose the parameters you want to see in the VisualReport window.

The parameters and text locations are automatically saved with themodel. However, you may want to save the settings for use with adifferent model. To save your preferred layouts to an AFT Impulsevisual layout file, click the Save Options button and assign a file name.The default file extension is .ivs. Later you can recall your customizedlayouts for rapid preparation of the Visual Report window.

The many features in the Visual Report Control window are describedfurther in Chapter 4.

Customizing Additional Pipe Losses

The Additional Pipe Losses window presents organized lists of pipe lossfittings. This list is customizable. You can add your own fittings to the


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base list provided with AFT Impulse. The Additional Pipe Losseswindow is accessed through the Pipe Specifications window.

The standard losses provided with AFT Impulse are located in theIMPULSE3.DAT file in the same directory as the AFT Impulseexecutable file. You should not change anything in this file.

The custom user information loaded into the Additional Pipe Losseswindow is found in the ADDLOSS.DAT file in the directory where AFTImpulse is installed. Each data line represents a loss and is divided intothe nine fields listed in Table 7.1. The structure of this loss data isidentical to that in the IMPULSE3.DAT file, which you can open anduse as an example.

Table 7.1 Pipe loss data fields for additional losses

Field # Description Required

1 Reference number Yes

2 Name of component Yes

3 Source Yes

4 Diameter No

5 Angle No

6 Percent open No

7 Area ratio percentage No

8 K factor Yes

9 Page reference No

All lines in the ADDLOSS.DAT file must have a reference number(field 1), name (field 2), source (field 3), and K factor (field 8). Anycustom information you add must have a letter “U” in the source field toindicate it is a user-entered item. The “C”, “I”, and “M” characters in thesource field for other components are for references to Crane, Idelchik,and Miller, respectively.

The reference number must be unique between both theADDLOSS.DAT file and the IMPULSE3.DAT file.

Table 7.2 lists the reference numbers for Bends, Valves, Check Valves,Orifices, Entrances, Exits, and other components. Enter custom valvesusing reference numbers consistent with the subcategories.

No area change data is included in the ADDLOSS.DAT file. Areachange loss factors are all calculated based on standard correlations.


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As it is infrequently used, no Additional Pipe Losses data customizationfeatures have been built into the Impulse interface, relying instead ondirectly editing text files to effect changes.

Building custom databases

One of the more powerful features of AFT Impulse is that it providesextensive customization capabilities. AFT Impulse provides the tools tobuild custom databases for fluid properties, pipe materials, and alljunction types except Dead End and Tee/Wye. AFT Impulseincorporates these databases internally and offers them to you throughlists.


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Table 7.2 Additional loss component reference numbers

Component Reference number Description

Bends 100000-100999 Standard

101000-101999 Mitre

102000-102999 Smooth Flanged

Valves 200000-200999 Angle

201000-201999 Ball

202000-202999 Butterfly

203000-203999 Cylindrical

204000-204999 Gate

205000-205999 Globe

206000-206999 Plug

207000-207999 Poppet

208000-208999 Three-way

Check Valves 300000-300999 Swing

301000-301999 Lift

302000-302999 Tilting Disk

303000-303999 Stop-check

Orifices 400000-400999 Sharp-edged

401000-401999 Long

Entrances and Exits 500000-500999 Entrances

501000-501999 Exits

Miscellaneous 600000-600999 Differential Flow Meters

601000-601999 Honeycomb

602000-602999 Screen

603000-603999 Tee

There are two types of databases that may be customized: local andexternal (Note: the AFT Default Internal Database is not included heresince it may not be altered). The local database is located in theIMP_USER3.DAT file in the Windows directory.

In AFT Impulse you can create multiple external or local customdatabases. The Database Manager window on the Database menusimplifies this process.


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When the databases are placed on a network, the data managementfeatures described below can be implemented.

Specific, quality-checked data can be entered into databases located on alocal or wide area network of PC’s. This data is not accessible to the endusers through AFT Impulse itself, so it is guarded data.

Once placed on a network, users can connect to any database found inthe Database Manager. Users can connect to as many databases as theywant, and get access to all data in databases using Database Manager.

Adding custom fluid properties to the Fluid DatabaseYou can open the Fluid Database window from the Database menu orfrom within the System Properties window. Figure 7.8 shows the layoutof the Fluid Database window. This window allows you to reviewcurrently defined custom fluids, to change the parameters that define thefluids, and to enter new fluids. All fluid properties are assumed to be afunction of temperature only.

Figure 7.8 Fluid Database window


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Once you have entered a custom fluid, it will appear in the Fluid Listarea of the System Properties window whenever you use AFT Impulse.

You can add new fluids to the database by clicking the Add New Fluidbutton and opening a new window (see Figure 7.9). The constantsprovided allow you to describe the variation of density, dynamicviscosity, bulk modulus of elasticity, and vapor pressure as they dependon temperature or solids concentration for newtonian fluids. In addition,there is an option to describe the variation of density, viscosity, rigiditycoefficient and yield stress for a Bingham Plastic non-newtonian fluid asit depends on solids concentration. This window provides the tools toperform polynomial curve fits of fluid data; or, if you already haveappropriate constants, you can input them directly.

Figure 7.9 Add New Fluid window

Density, viscosity and bulk modulus are required for all fluids. Vaporpressure is required if cavitation modeling is desired.


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The Fluid Database window also allows you to open the Change FluidData window, which is essentially the same window as the Add NewFluid window. For easy editing, data for the fluid you wish to change isentered for you in the appropriate areas. The original curve fit data youmight have used is displayed if you elected to save the data in a raw datasource file.

The Add New Fluid/Change Fluid Data windows displays cross-plottingof your custom curves against the curve fit data points. You can alsoimport data from text files into these windows.

Adding custom pipe materials to the Pipe MaterialDatabaseYou can open the Pipe Material Database window from the Databasemenu. Figure 7.10 shows the Pipe Material Database window. Using thiswindow, you can easily add to the user database the geometricinformation for any pipe material in the length units of your choice.

Figure 7.10 Pipe Material Database window

After you have entered a custom material, each time you open a PipeSpecifications window the material will be available to you for quickselection. This minimizes the need to work with handbooks.


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Pipe materials typically have a range of nominal sizes and a specifictype, class, or schedule that describes the actual dimension. AFTImpulse works with the inner diameter of the pipe, since that is theparameter that directly affects the fluid mechanics.

You can also add multiple Friction Data Sets for each pipe material.This allows you to keep, for example, different friction values that mayapply to the same pipe under different conditions. These conditionscould be related to the age of the pipe or the type of fluid being carried.

To add custom pipe materials to the user database, choose the entrylocation you desire in the list on the left (Pipe Materials, Sizes andTypes). Then click the New button below. For example, to add anentirely new material, click the Pipe Materials selection in the list thenclick the New Material button. To add a new size for an existing material(say Steel), click Steel in the list then click the New Size button. To adda new type to the Steel 1 inch size, click Steel 1 inch in the list then clickthe New Type button.

When adding a new material, you also will be asked to specify a nominalsize, type of material, diameter, wall thickness, friction data set, andfriction value to define the first data. From then on you can enteradditional nominal sizes and types. The name descriptors you choose forthe material, size, and type are at your discretion. They can contain textmixed with relevant numbers if you desire.

In addition to manual entry of the custom pipe materials, you also canimport pipe material data from a file. When you click the Import FromFile button, a dialog box will appear that is self explanatory. Just followthe directions in the dialog box. By choosing to import from a file, youwill be able to have a separate copy of the data for later access.

Friction Data SetsA friction data set can be associated with the entire pipe material, orwith specific sizes or types. For instance, a friction data set associatedwith Steel (i.e., the top level) might be called “25 year usage” and have aroughness value of 0.005 inches. This data set is then available to allsizes and types of steel. However, if “25 year usage” is associated onlywith the 1 inch of Steel, it will apply only to the pipe schedules that are 1inch.


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You can also associate friction data sets with a specific type (i.e.,schedule). For instance, you can make a “25 year usage” and associate itonly with 1 inch schedule 40 Steel pipe and assign its roughness value as0.006 inches. Then all Steel pipe would use 0.005 inches for “25 yearusage” friction while the different value for 1 inch schedule 40 of 0.006inches would supercede the value of 0.005 inches associated with allSteel pipe.

Finally, among multiple friction data sets you can specify a default dataset.

Pipe Physical PropertiesThe Pipe Material Database window allows you to assign physicalproperties of the pipe. The density and Poisson ratio of the pipe can beentered as constants, and the modulus of elasticity as a function oftemperature.

Adding custom junction data to the ComponentDatabaseTo simplify modeling and eliminate repetitive data entry, AFT Impulseallows you to save frequently-used junction data to a custom database.Once saved, the custom equipment date can be immediately recalledfrom the Database List in the Specifications window. Using this tool,you can easily build your own database of pumps, valves, or othercomponents supplied by specific manufacturers. If transient data isentered, it too is saved to the database.

To save a Custom junction to the database, open the junctionSpecifications window and enter all required data. Close the window andchoose Add Component to Database from the Database menu. You willbe prompted to enter a name for the custom component. Enter adescriptive name and close the window. If you open the junction again,the custom name will be selected in the Database List.

AFT Impulse accepts custom data for all junction types except VolumeBalance. Volume Balances are not included because there is no custominformation for Volume Balance and thus nothing to store.


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Editing the Component DatabaseYou can delete or rename any existing database component by openingthe Component Database window from the Database menu (see Figure7.11). To edit an existing database entry, select the item in the list andclick the Edit Data button. This opens the data into a specificationswindow where the data can be modified.

Figure 7.11 Component Database window

Database Manager

Accessed from the Database menu, Database Manager allows you to –

• Connect to and disconnect from databases

• Create and delete databases

• Review the content of databases

• Move contents between databases


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Setting available databasesTo work with a database you must first make it available to AFTImpulse. Referring to Figure 7.12, clicking on the Add EngineeringDatabase button will open a standard file dialog box with which youmay browse to and select a database. A database may be removed fromthe list of available databases by selecting it and clicking on the RemoveDatabase button. Contents of a database may be reviewed by selecting adatabase from the list and clicking on the Review Content button.

Figure 7.12 Available Databases in the Database Manager

The AFT DEFAULT INTERNAL DATABASE and the AFT IMPULSELOCAL USER DATABASE may not be removed and will alwaysappear in the list of available databases. External shared databases willalso always appear in the available database list. These databases aredistinguished from other external databases by being listed in theDATABASE.LIB file residing in the AFT Impulse directory.

Connecting and disconnecting to a databaseTo use a database within a model it must first be made available, asdescribed above, and connected using the features in the Connect To


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Database tab. To connect a database, select it from the list of availabledatabases and click on Add to Connections (see Figure 7.13). Similarly,to disconnect a database, select it from the list of Connected Databasesand click on Disconnect Database. The sections contained in the selecteddatabase will appear in the list to the right, Database Selections, fromwhich the sections to be used in the model may be selected; i.e.components, fluids or pipe materials.

Figure 7.13 Connect To Database tab of the Database Managerallows you to connect to and disconnect fromdatabases, review their contents and select thesections of a database to be used.

The contents of a connected database may be reviewed by selecting itand clicking on Review Content.

Clicking on Set As Default will establish the list of connected databasesas the default list, while clicking on User Default will set theconnections to the default list.


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Editing databasesThe Edit Database tab of Database Manager allows you to create anddelete databases, and edit their contents by copying sections or itemsfrom one database to another. The tab is divided into a Source Databasearea on the left and a Changing Database area on the right (see Figure7.14).

To edit a database, first choose a source database by selecting one of thefour options:

• Currently available external

• Data From Current Model – components, fluids and materials madein the current model using the Component Database, Fluid Databaseor Pipe Material Database

• Local User Database – IMP_USER3.DAT located in your Windowsdirectory, where component, fluid and materials data created by theuser is stored by default

• Other External – by selecting this option and clicking on the SelectDatabase, you are able to browse and select a database file from anyavailable drive and directory


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Select the database tobe used as the source

Click here to select anexternal database as

the source

File name, title andavailable sections

within the source arelisted here

Selected items may bemoved or copied from

the source to thedatabase being

changed

Figure 7.14 Edit Database tab allows you to select sections from aspecified database and add them to an existing or newdatabase.

The file and descriptive name of the selected database are displayed aswell as the sections available in that database, i.e. fluids, components orpipe materials.

The database to be changed and what changes are to be made arespecified in the Changing Database portion of the Edit Database tab.

• Select Database to Edit… – browse to and select an existingdatabase file

• Create New Database… - specify a location, file name anddescriptive name for a new database. The file and descriptive nameof the selected or created database appears below (Figure 7.15).

• Delete database – deletes the database displayed

• Review content – displays the content of the database listed

• Delete section – deletes from the listed database the section selectedfrom the Database Sections list


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Figure 7.15 After specifying a database file name, you will beprompted for a description which will then appear inthe various lists of the Database Manager when thisdatabase is selected.

Benefits of shared databasesWhen the Impulse databases are placed on a network, powerful datamanagement procedures can be implemented. Specific, quality-checkeddata can be entered into databases located on a local or wide areanetwork of PC's. This data is not accessible to the end users throughAFT Impulse itself, so it is guarded data.

Once the data is placed on a network, users can connect to any databaselisted in DATABASE.LIB by selecting the database in the ConnectDatabase window on the Database menu. Users can connect to as manydatabases as they want, and get access to all data in databases listed inDATABASE.LIB.

In addition, special modeling control files can also be included in yournetwork databases. These include:

• Output Control format files

• Model Data Control format files

• Workspace Preference format files

• Parameter Preference format files

• Unit Preference format files

• Visual Report Control format files

• Toolbox Preference format files


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If the user connects to a database with these files and selects to beconnected to any of these control files, his model will inherit the controlformatting specified in the control files. This means that special reportlayouts, special graphical representations and colors, and other specialcontrols can be decided upon by the database constructor, and thosecontrol features can be selected into local models by end users. This ishighly useful for creating customized reports for certain customers orproject requirements.

By way of example, if the Output Control of Figure 4.13 is specified ascoming from the “New Mile High Stadium Project” database, it wouldappear as shown in Figure 7.16.

If you uncheck the Database checkbox or change certain data in theOutput Control window, the database will remain connected but changeto an inactive state. When the database is inactive, none of its data flowsthrough to the model. To bring the Output Control data back into syncwith the connected database, recheck the Database checkbox in OutputControl.

This entire custom database concept lends itself naturally to the idea ofspecific project databases. Each project has differing requirements thatdepend on the nature of the project and the end customer. Data for eachproject can be assembled into separate databases, allowing faster andmore accurate transmission of the controlled data and desired reportingformats to engineers.

Maintaining a network database allows commonly-used data to becontrolled, which may be helpful in instances where quality control isstrict.


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Databaseconnection

Figure 7.16 Output Control window with database connected andactive.

Creating an enterprise-wide, network database system

OverviewUsers of AFT Impulse on a network can access common databases tosimplify model verification, improve analysis and reporting consistency,and eliminate redundant work.

To accomplish this there is some initial setup that must be done. First,the data for components, materials, formats, etc., needs to be created andsaved to various data files. Secondly, these files must be made availableto the users of AFT Impulse by creating a file named DATABASE.LIBin the IMPULSE folder. Thirdly, the users need to connect to the newdatabases listed and select the sections to use. Each of these broad stepsis discussed below. To maintain control and integrity of the data, itwould be advised that one person is given the responsibility to create andmaintain the network databases. The discussion below will be givenfrom the viewpoint of this person.


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Figure 7.17 shows the relationship between Internal Databases andExternal Databases as well as the database file listing that AFT Impulseuses to find these databases. An External Database is any database filethat is created by the user and listed in the IMP_DBUSER.LIB residingin the users local Windows directory. The user can make these availableand unavailable, and connect and disconnect from these databases usingDatabase Manager. The database files themselves may be located on anylocal or mapped drive. These database files are also distinguished bybeing under the local user’s complete control. External SharedDatabases, on the other hand, are common to all AFT Impulse users onthe network, therefore the local user cannot remove them from the list ofavailable databases. However, the user can choose to connect ordisconnect from these databases using Database Manager. Finally, tocomplete the discussion, there are two special databases, the DefaultInternal Database and the Local User Database. These files are AFTImpulse Internal Databases which cannot be disconnected or madeunavailable.

Creating database filesThe first thing to do is create the files that contain the information to beshared. These files will be referred to as database files and can containinformation about pipe materials, components (e.g. pumps, valves, etc.),fluids, Output Control formats, Visual Report settings, etc. Thefollowing are several ways to create database files:

• Use the Edit Database feature in the Database Manager, as describedpreviously. Data can be taken from the current model or otherdatabases.

• Save the control formats to separate files. This can be done on theOutput Control window, Visual Report control window, etc.

• Copy and paste sections from one file into another file using a texteditor.


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Vail Resort Materials

Development Output Control Settings

Final Report Output Format

Visual Report Format

AFT Default Internal Database

AFT Impulse Local User Database

My Custom Visual Report Format

My Custom Output Settings

External Shared Databases(DATABASE.LIB)

VailMaterials.dat

Development Output.dat

Final Report Output.dat

Visual Report Format.dat

Impulse Internal Databases

IMPULSE3.DAT

IMP_USER3.DAT

Database Titles/DescriptionsDatabase Groups and Filenames

External Databases(IMP_DBUSER.LIB)

My Report Output.dat

My VR Format.dat

Figure 7.17 An example of the relationship between ExternalDatabases and External Shared Databases.

This discussion will focus on the first and second methods. Let’s assumethat the person responsible for the network database has already createda new pipe material. Also, assume that the goal is to create a network-wide database to contain this material, as well as two Output Controlsettings and a Visual Report format.

The information for the material is copied from the Local User Database(IMP_USER3.DAT) to a new file using the Database Editor in theDatabase Manager (see Figure 7.14). For the source database chooseLocal User Database and then select Pipe Materials. Then click CreateNew Database and enter a file name, VailMaterials.dat, for this example.Then enter a description of the database. This description is how AFTImpulse users will refer to the database and should concisely describewhat is in the database (for this example we’ll use “Vail ResortMaterials”). Then click Add to copy the pipe materials from the Local


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User Database to this new database file. The pipe material database filehas now been created and is ready to be used.

Now we need to create two Output Control settings, one with a widevariety of parameters for use during the project development phase andanother, more concise set, with the pertinent data for a final report.Again, there are many ways of creating these database files. For thedevelopment report format, let’s create a database file directly. Open theOutput Control window and select the pipe and junction outputparameters needed. Then click Save Control Format and enter the filename (Development Output.dat in this case). You may want to open thefile in Notepad or other text editor and change the file heading (first lineof the file) from the default to something more descriptive,“Development Output Control Settings”, for example.

Now, for the settings for a final report, let’s create the file using theDatabase Editor in the Database Manager (see Figure 7.14). Open theOutput Control window again, select the parameters needed for the finalreports and click OK. Then open the Database Manager and go to theEdit Database tab (see Figure 7.14). Click Data From Current Model andthen choose the Output Control section. Then click Create NewDatabase and enter a file name, Final Report Output.dat, for thisexample. Then enter a description of the database, something descriptivelike “Final Report Output Format”. Then click Add to copy the OutputControl format from the local model to the new database file.

To complete the database files, we need to create a Visual Reportformat. To do this a model must first be run; any model can be used inthis case. After the model is run and a converged answer is obtained, goto the Visual Report Control window and select the parameters to showon the Visual Report. Then click Save Options, then enter the name ofthe file, for example, Visual Report Format.ivs. Again, you may want toopen the file in Notepad or other text editor and change the file heading(first line of the file) from the default to something more descriptive,“Visual Report Format”, for example.

Sharing database files using DATABASE.LIBNow that the database files for Pipe Materials, Output Controls Settingsand a Visual Report Format have be created, they need to be madeavailable to the users of AFT Impulse. Depending on the scope of use,these database files can be placed into one of two groups. If the databaseis to be used locally by one user, it is put in the External Database group.


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On the other hand, if the database is to be common among many usersand shared over a network it is put into the External Shared Databasegroup.

Continuing the above example, the goal is to share these database files,so ultimately they need to be External Shared Databases and listed in theDATABASE.LIB file in the Impulse directory on the network. However,since Impulse does not write directly to the network files for securityreasons, it is easiest to temporarily treat these files as local ExternalDatabase files which will be listed in the IMP_DBUSER.LIB file locatedin the Window folder of the local machine.

It is recommended that External Shared Databases files be copied to thenetwork server and placed in a subfolder of the AFT Impulse folder (e.g.AFT Products\AFT Impulse\Databases). The files may, however, belocated anywhere on the network that is accessible to the users. For ourexample, we are going to put all four database files on the network in asubfolder called Databases. It is easier to copy the files to the Databasesfolder before they are made available in AFT Impulse. This will helpestablish the correct paths to the files as the files are made availableusing the Database Manager.

Once the database files are in their final locations, open the DatabaseManager and click the Add Engineering Database button on the Connectto Database tab (see Figure 7.12). Each of the new database files wehave created needs to be added to the Available Databases. Once added,the description of the database you just added appears in the AvailableDatabases list. When you have added all the new database files, clickClose and the External Database listing file (IMP_DBUSER.LIB) inyour Windows folder is automatically updated.

Now we need to discuss how the paths should be setup. When you add adatabase, AFT Impulse will write the full path of the database to theExternal Database listing (IMP_DBUSER.LIB). That means that if youbrowsed to the database file using a mapped drive letter then the pathwill include that drive letter. However, if other users try to access thedatabases but have the AFT Impulse folder mapped to a different driveletter they will not be able to locate the database files. Other optionsinclude using either relative paths or the computer name in the path (e.g.\\Network Server\Eng Apps\AFT Products\AFTImpulse\Databases\filename.dat). You can automatically have AFTImpulse use the computer name when you add the file by choosingNetwork Neighborhood, then the computer and then browse to the


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database file. If you would rather use relative paths, you can manuallyedit them in the database listing file. All database files must be relativeto the AFT Impulse folder where the executable is located.

When the four database files were added above, they became ExternalDatabases and were listed in IMP_DBUSER.LIB. Now we need to movethis listing from the local machine to the AFT Impulse folder on thenetwork so that all users can access the database files. Once the listingfile is moved to the network, it must be renamed DATABASE.LIB.Once this is done, all users can now access the database file we havecreated.

Connecting to the external shared databasesNow that the databases have been placed on the network and madeavailable to all AFT Impulse users, each user must now connect to thedatabases and make the sections active as needed. When a user opens theDatabase Manager, the External Shared Databases are listed in theAvailable Databases section on the Connect To Database tab (See Figure7.13). Choose a database from this list and click Add to Connections andthe database will now appear in the Connected Databases. Then choosethe section(s) from the list on the right-hand side to become active. Adatabase can be connected without having any sections selected andactive. Finally, to have these connections in place every time you startAFT Impulse, click Set As Default. To apply these settings to a modelmade previously, open the model, go to Database Manager, click UserDefault and, if you have set the default, the connections and setting willnow be applied to the model.


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C H A P T E R 8

Steady-State Hydraulic Theory and LossModels

AFT Impulse employs standard matrix solution techniques for solvingsteady-state pipe network problems. The solution techniques use thebasic engineering concepts of mass and momentum balance to determinethe steady-state flow distribution and pressure losses in the network.

This chapter presents an overview of the steady-state solution techniqueand the steady-state solution control parameters available to you. Thischapter also describes each of the junction types as it relates to thesolution scheme. Many junction types have standard, handbook lossmodels. The nature of the loss models that AFT Impulse uses andreferences for those loss models are also given.

The Steady-State Solver

AFT Impulse's Steady-State Solver is designed to achieve a balancedflow and pressure solution in a pipe network. Before using the Steady-State Solver in an application, you must have at least a basicunderstanding of pipe fluid mechanics. A review of basic pipe fluidmechanics can be obtained from standard undergraduate fluid mechanicstextbooks (Fox and McDonald 1985; White 1979).

AFT Impulse makes use of standard matrix solution techniques (Jeppson1976). The method is known as the H-Equation method, where H, thepiezometric head, is solved for at each junction by forcing continuity offlow through each connecting pipe. Simultaneously, the head loss across


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each pipe is updated based on the flow balance information. The flowrate and head are solved in an inner-outer loop algorithm, where the flowis guessed, the head loss is calculated consistent with that guess, and theflow is updated according to the new pressure drop information. TheNewton-Raphson method is employed to refine each successive solution,resulting in a sparse square matrix that is solved during each solutionpass.

The concepts of pressure and hydraulic grade line (HGL, also calledpiezometric head) are related but use different frameworks forconsidering pipe system behavior. The HGL includes both the static andelevational effects of pressure. The relationship between the two is givenby Equation 8.1:

zg

PHGL +=ρ

(8.1)

where z is the elevation. Each concept offers certain advantages.

The solution technique makes use of the continuity and one-dimensionalmomentum equations. AFT Impulse uses pressure and mass flow rate asthe unknown parameters because they are more fundamental in naturethan the head and volumetric flow rate used by Jeppson and thus extendmore naturally to mechanical and chemical systems.

In the following discussion, subscripts denote values at junctions. Thus,Pi represents the pressure at junction i. Double subscripts denote valuesalong pipes connecting two junctions. Thus, ijm& represents the mass

flow rate in the pipe connecting junctions i and j.

Application of the law of mass conservation to each junction yields:

01

=∑=

n

jijm& (8.2)

where n is the number of pipes connected to junction i. Equation 8.2states that the net mass flow rate into each junction must sum to zero.

The basic equation for pipe pressure drop due to friction can beexpressed with the Darcy-Weisbach equation:

=∆ 2

21 V

DLfPf ρ (8.3)


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where fP∆ is the frictional pressure loss. The total pressure changebetween junctions is given by the momentum equation in the form of theBernoulli equation:

fPgzVPgzVP ∆+++=++ 22

2212

11 21

21 ρρρρ (8.4)

The static and stagnation pressure are related as follows:

221 VPPo ρ+= (8.5)

Substituting Equation 8.5 into 8.4

foo PgzPgzP ∆++=+ 22,11, ρρ

Solving for the frictional pressure drop for a constant area pipes yields:

fjijoio PzzgPP ∆=−+− )(,, ρ (8.6)

where i and j denote upstream and downstream junction values.

The definition of mass flow rate is:

AVm ρ=& (8.7)

Combining Equations 8.3 and 8.6 and substituting for velocity, V, usingEquation 8.7 gives the mass flow for each pipe:

ijij

jijoio mR

zzgPP&=

′−+−

2/1,, )(ρ

(8.8)

where ′Rij is the effective flow resistance in the pipe and the subscript ijrefers to the pipe connecting junctions i and j.

221

ijij

ij

ijijij

AK

DLf

Rρ

+=′ (8.9)

Note that ′R is not the same as R, which is the resistance used in theOutput Control window and is expressed by the following equation:

2RQH=∆


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Substituting Equation 8.8 into Equation 8.2 results in the equation to beapplied to each junction i:

0)(

1

2/1,, =

′

−+−∑=

n

j ij

jijoioR

zzgPP ρ(8.10)

where n is the number of pipes connected to junction i.

To be completely general, Equation 8.10 should be written for junction i:

Appliedi

n

j ij

jijoio mR

zzgPP,

1

2/1,, )(

&=

′

−+−∑=

ρ(8.11)

to allow for application of boundary condition flow rates to a boundaryjunction node.

Equation 8.11 as applied to each junction in the network represents thesystem of equations that need to be solved to determine the stagnationpressure at each junction. To solve this system, the Newton-Raphsonmethod is employed. In the Newton-Raphson method, new values foreach unknown are calculated based on the previous value and acorrection that uses the first derivative of the function.

In this instance the function would be of the form:

Appliedi

n

j ij

jijoioi m

RzzgPP

F ,1

2/1,, )(

&−

′

−+−= ∑

=

ρ(8.12)

The method involves finding all the junction stagnation pressures, Po,i ,that cause all of the Fi to go to zero, thus satisfying Equation 8.11 at alljunctions.

When applied to a system of equations, the Jacobian matrix contains allthe required derivative information to employ the Newton-Raphsontechnique. The Jacobian, JF, is given by:


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=

no

n

o

n

o

n

nooo

nooo

F

PF

PF

PF

PF

PF

PF

PF

PF

PF

J

,2,1,

,

2

2,

2

1,

2

,

1

2,

1

1,

1

∂∂

∂∂

∂∂

∂∂

∂∂

∂∂

∂∂

∂∂

∂∂

L

M

L

L

The column matrix oPv

contains the stagnation pressure at each junction,

and column matrix vF contains the F values (Equation 8.12) at each

junction. The updated solutions for oPv

are obtained from the followingNewton-Raphson equation:

FJPP Foldonewovvv 1

,,−−= (8.13)

Traditional implementations of the Newton-Raphson method requireinitial guesses at the solution before the Steady-State Solver can begin.This often results in the situation where you have to know the solutionahead of time in order to solve for it. To avoid placing this burden on theuser, AFT Impulse uses a proprietary method for generating firstguesses. This method is usually sufficient to get the solution going in theright direction, especially for systems that don't have highly non-linearfeatures.

However, there are times when AFT Impulse's first guess does not leadto a converged solution. In these cases, AFT Impulse provides you withthe option of specifying your own initial guesses of flow and pressure.

Stagnation vs. static pressure boundariesWith two exceptions (to be discussed), all pressure-type boundaryconditions in AFT Impulse are stagnation. This works very well forthings such as storage tanks, cooling ponds, lakes, etc., where thevolume associated with a pressure is large and will not change(significantly) with time. These boundaries have no velocity associatedwith them, and using stagnation pressure is thus appropriate. Theseboundary conditions are most clearly rendered in AFT Impulse by use ofa Reservoir junction.


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1

42 3

Figure 8.1 Reservoir with one connecting pipe

Consider the reservoir in Figure 8.1. When the liquid in a reservoir flowsinto connecting pipes, the static pressure immediately drops due to theincrease in velocity (see Figure 8.2).

Distance (x)Location 2 Location 4

Po,2

P2

222

1 VρP4

Po,4

242

1 Vρ

Location 3

∆Ploss

Pres

sure

Figure 8.2 Steady-state pressure profile along horizontal pipeshown in Figure 8.1.


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It is tempting to say that because there is no velocity in the reservoir,then it does not matter whether the reservoir boundary pressure isconsidered as a static or stagnation pressure because they are equal. Thisis a misconception. The pressure boundary condition in an AFT Impulsemodel is applied at location 2 in Figure 8.1, not location 1. At location 1the static and stagnation pressures are the same, but they are not thesame at location 2. As shown in Figure 8.2, they differ by the amount ofthe dynamic pressure (1/2)ρV2

2.

The appropriateness of either stagnation or static pressure as a boundarycondition depends on whether there is a velocity associated with thepressure that is specified. Is the pressure in a vessel or large (lowervelocity) header, or is it in a pipe somewhere in the middle of a system?If the first, stagnation is more appropriate; if the second, static.

Pressure boundary conditions with no velocity are more common acrossall piping industries, and are usually appropriate. However, this is notalways the case.

When to use static pressureNow let's consider the application of a true static pressure boundarycondition. A static boundary condition has a unique velocity associatedwith it. From Equation 8.4 it is clear that application of the staticpressure is not sufficient to specify location 1. The velocity andelevation is also required.

In AFT Impulse, elevation data is entered for junctions. The pipes adoptthe elevation of the junction to which they are connected.

But where does the velocity information for Equation 8.4 come from?One might respond that it can be obtained from the flow rate. But thisraises another question: what if the flow rate isn’t known? Put anotherway, what if the flow rate is what we are solving for?

When the user supplies a stagnation pressure boundary condition, AFTImpulse can use Equation 8.6. Here no velocity information is required.A stagnation pressure boundary can thus be supplied to multiple pipesthat might have different velocities. That is why the same reservoirjunction can connect with multiple pipes.

However, if the user specifies a static pressure as the boundarycondition, to apply Equation 8.4 a unique velocity must be supplied.


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Thus it is only possible to connect a static pressure to a single pipe witha unique velocity.

Important: A static pressure boundary has a unique velocity associatedwith it, and can thus connect to only one pipe.

Where would one find a static pressure boundary? The best example iswhen the boundary condition is inside a pipe. A pipe system model canstart and end anywhere it is convenient for the user. It may be convenientto not start the model at the physical boundary (such as a tank) but at aparticular location in the pipe system. This could be, for example, at thelocation of a pressure measurement. Or it could be at the boundary of thepipe system for which your company is responsible, with anothercompany responsible for what is on the other side of that boundary.

If one needs to model such a situation, the Assigned Pressure junctionallows one to model either a static or stagnation pressure. The defaultstagnation pressure allows connection of up to 25 pipes. If staticpressure is chosen, only one connecting pipe is allowed.

Static pressure at pressure control valveAnother example of static pressure is at pressure control valves. Forpressure control valves (i.e., PRV’s and PSV’s) the default controlpressure is static pressure. The reason is that the measured pressure thatprovides feedback to the controller will typically be a static pressuremeasurement. You have the option of modeling pressure control valvesas either static or stagnation pressure.

Open vs. closed systems

In order to model a closed system, only one pressure junction is used inthe model. Typically this would be either a Reservoir or AssignedPressure. As elaborated in Chapter 12, pressure type junctions are aninfinite source of fluid and do not balance flow. How then can one beused to balance flow in a closed system?

To answer this question, it is worth considering how AFT Impulse viewsa closed system model. AFT Impulse does not directly model closedsystems, and in fact does not even realize a closed system is beingmodeled.


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Consider the system shown in Figure 8.3. This is an open system. Fluidis taken from J1 and delivered to J4 and J5. Because AFT Impulse’ssteady-state solution engine solves for a mass balance in the system, allflow out of J1 must be delivered to J4 and J5. Because the flow issteady-state, no fluid can be stored in the system; what goes in must goout.

P1 P2

P3

P4

J1 J2 J3

J4 J5

Figure 8.3 Open system - Flow out of J1 equals the sum of J4 andJ5

Now consider the systems in Figure 8.4. The first system appears to beclosed, while the second appears to be open. If the same boundarycondition (i.e., surface elevation and pressure) is used for J1, J11 andJ12 in the second system, to AFT Impulse it will appear as an identicalsystem to the first system. The reason is that AFT Impulse takes the firstsystem and applies the J1 reservoir pressure as a boundary condition topipes P4, P9 and P10. The second system uses three reservoirs to applyboundary conditions to P4, P9 and P10. But if the reservoirs all have thesame elevation and pressure, the boundary conditions are the same as J1in the first system. Thus the same boundary condition is used for P4, P9and P10 in both models, and they appear identical to AFT Impulse.


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P1 P2

P3P4

P10

P6

P7P8

P9

P11

J1

J2

J3

J4

J5

J6J9J8

J10

P1 P2

P3P4

P10

P6

P7P8P9

P11

J1

J2

J3

J4

J5

J6J9J8

J10

J11

J12

Figure 8.4 The first system is closed, and the second open. Inboth systems the flow into P10 is the sum of P4 and P9.If the second system has the same conditions at J1, J11and J12, the two system will appear identical to AFTImpulse.

But how is the flow balanced at J1 in the first system? Looking at thefirst system, one sees that to obtain a system balance, whatever flowsinto P10 must come back through P4 and P9. Because there is overallsystem balance by the Steady-State Solver, it will give the appearance ofa balanced flow at the pressure junction J1. If there is only one boundary(i.e., junction) where flow can enter or leave the pipe system, then no


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flow will enter or leave because there isn’t anywhere for it to go. Thusthe net flow rate will be zero at J1 (i.e., it will be balanced). Butrecognize that AFT Impulse is not applying a mass balance to J1directly. It is merely the result of an overall system balance.

Verifying network solutions

As discussed previously, AFT Impulse uses a Newton-Raphson matrixmethod to obtain a system level mass balance. When the Steady-StateSolver is finished, all branching sections are balanced for flow. Oncesolved, it is a simple matter to go back and sum up all mass flows at eachbranch to verify that these equations are satisfied. In fact, AFT Impulsedoes just that.

After the Steady-State Solver converges, AFT Impulse loops over eachjunction and adds up the inflow and outflow of mass and compares thesum to the solution tolerances. If it appears that any junction is out oftolerance, a warning is given in the output.

In the Solution Balance Summary table AFT Impulse provides additionalinformation on the balance at each junction. This table can be displayedby selecting the option in the Output Control window General Outputtab. Figure 8.5 shows an example of the Solution Balance Summarytable.


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Figure 8.5 Solution Balance Summary table shown in the Outputwindow offers a balance report of all junctions in themodel.

Pressure drop calculation methods

AFT Impulse can model both Newtonian and non-Newtonian fluidbehavior. A non-Newtonian fluid is one where the viscosity depends onthe fluid dynamics. For example, as the velocity changes a non-Newtonian fluid’s viscosity can change.


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On the other hand, a Newtonian fluid’s viscosity does not depend on thefluid dynamics, and is a function only of the thermodynamic state (i.e.,temperature and pressure). Most industrial fluids follow the Newtonianfluid behavior model, although there are important industrialapplications where the fluids are non-Newtonian.

Pressure drop calculation methods for Newtonian fluidsAFT Impulse offers the Darcy-Weisbach loss model approach as thedefault method for describing pipe frictional losses. Standard textbooksgive additional discussion on this method. AFT Impulse uses the Moodyapproach to calculating friction factors; note that the Moody approachdiffers from the Fanning friction factor by a factor of 4.

Except for laminar flow, there are no precise explicit friction factorcorrelations available. Like all the more precise methods, AFT Impulseuses an iterative formulation of friction factor. All methods of frictionfactor calculation must address the rather poorly understood area oflaminar to turbulent transition.

Roughness-based methodsRoughness-based methods use a pipe roughness value to calculate thepressure drop. This can be in the form of an absolute roughness (whichhas units of length) or a relative roughness that ratios the pipe roughnessto its diameter.

Laminar flowFor laminar flow AFT Impulse uses the standard laminar equation:

Ref 64

= Re < 2300 (default) (8.14)

Note that the laminar friction factor does not depend on the userroughness.

The System Properties window allows you to change the default laminartransition Reynolds Number (see Chapter 5).


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Turbulent flowThe Colebrook-White iterative friction factor equation is used to obtainfriction factors in the turbulent flow regime.

235.9log214.1

−

+−=

fReDf ε

Re > 4000 (default)

The System Properties window allows you to change the defaultturbulent transition Reynolds Number (see Chapter 5).

Transition flowFor Reynolds Numbers in the transition flow regime, a linearinterpolation is used between the laminar and turbulent values.

Hydraulically smoothThe friction factor in the turbulent range is calculated using Colebrook-White with roughness = 0 while in the laminar range it continues to becalculated using Equation 8.14 above.

Hazen-Williams methodAFT Impulse also offers the Hazen-Williams method of specifyingirrecoverable loss information. The Steady-State Solver converts theHazen-Williams factor to a Darcy-Weisbach friction factor (Walski1984, 37). This allows a consistent solution approach to be used for allpipe system models, while retaining the flexibility of the two approachesto account for losses.

( )Cf VDHW =

17 250 54 0 081

.. .

where V is in ft/s and D is in ft.

MIT Equation for crude oilThe MIT Equation is appropriate for crude oil and is given by thefollowing equation (Pipe Line Rules of Thumb Handbook):


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( )5

2212.139D

fQL

dP waterρρ=

( )

f =

−

1

4 0868591964 38215

2

. lnRe

. ln Re .

( )56.818059,10

2 −=

νν

dQRe

where:

dP = pressure drop (psid)

L = length (miles)

f = friction factor, MIT

Q = volumetric flow rate (barrels/hour)

ρ = density (lbm/ft3)

ρwater = density of water (62.3 lbm/ft3)

D = diameter (inches)

Re = Reynolds number, MIT

ν = kinematic viscosity (Seconds Saybolt Universal)

Miller Turbulent methodThe Miller Turbulent method is appropriate for light hydrocarbons andis given by the following equation (Pipe Line Rules of ThumbHandbook):

( )( )

( )( )Qd dP L d dP L

water

water=

+

2.5

0 5

3

25 91134 35

.log ..ρ ρ

ρ ρ

µ

where:

dP = pressure drop (psid)

L = length (miles)


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Q = volumetric flow rate (barrels/hour)

ρ = density (lbm/ft3)

ρwater = density of water (62.3 lbm/ft3)

d = diameter (inches)

µ = dynamic viscosity (centipoise)

Frictionless pipesFrictionless pipes are convenient for connecting junctions that may nothave a physical pipe between them. There are, however, somelimitations where a frictionless pipe may be located in a model. Anexample is a frictionless pipe that connects two Assigned Pressurejunctions. Such a pipe would have an infinite flow rate. In such casesAFT Impulse identifies inappropriate placement of frictionless pipes andinforms the user when a model is run.

Pressure drop calculation methods for non-Newtonian fluids

Duffy method for pulp and paper stockThe Duffy method includes 20 different stock selections to obtain theconstants necessary to apply the method (TAPPI 1988):

V K Cmax = ′ σ

V Cwater = 122 1 4. .

If V < Vmax, then

dHL

FKV C D= α β γ

where:

dH /L = head loss per length (m water / 100 m pipe)

C = pulp consistency (% dryness)

Vmax = max velocity (for Duffy Equation) in above equation(m/s)


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Vwater = water velocity in above equation (limit at which pulpeffects are negligible), (m/s)

D = inner pipe diameter (mm)

F = correction factor for temperature or pipe roughness(dimensionless)

K’, K, σ, α, β, γ = constants depending on pulp type(dimensionless)

If V > Vmax but less than Vwater, then use above head loss equation with V= Vmax. If V > Vwater then just use normal friction calculation for water.

The constants used for the specific type of paper stock are displayed inthe General Results section of the output. In addition, you can optionallydisplay in the output the Vmax and Vwater terms, the dryness coefficiententered, and the Safety Factor either entered or calculated.

Brecht & Heller method for pulp and paper stockThe Brecht & Heller method includes eight different stock selections forthe constants (Ingersoll-Dresser Pumps, 1995). The Brecht & Hellermethod is recognized to be more conservative than Duffy. That is, thepressure drop predictions are higher.

157.1

205.0

CVDeR ρ

=′

636.197.3

eRf

′=

dHfV LK

D=

2

where:

Re’ = pseudo Reynolds Number (dimensionless)

D = inner diameter (feet)

V = average velocity (ft/s)

ρ = stock density (lbm/ft3)

C = pulp consistency (% dryness)


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f = pseudo friction factor (dimensionless)

K = constant depending on pulp type (dimensionless)

L = length (feet)

The constants used for the specific type of paper stock are displayed inthe General Results section of the output. In addition, you can optionallydisplay in the output the special Reynolds Number and Friction Factorterms.

Power Law non-NewtonianThe Power Law model of pipe friction is given by the following (Tilton1997)

( ) nnn

rn

nKLPQ 31

1

312+

+

∆

= π (8.15)

where:

K = constant with units of dynamic viscosity

n = dimensionless constant

Power Law flow is laminar. Substituting Equation 8.15 into 8.14 yields:

nnn

nn

DVK

DmAf

+

=

−+ 312

13

&

This friction factor can then be used in Equation 8.3.

Bingham Plastic non-NewtonianThe Bingham Plastic model of pipe friction is given by the following(Carthew et al.):

∆HL

SgD

R VgDpipe

y c= +

32

6 2ρ ρ(8.16)

where:

Rc = coefficient of rigidity

Sy = yield stress


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The head loss and pressure loss are related as follows:

221 V

DLfpHg ρρ =∆=∆ (8.17)

Bingham Plastic flow is laminar. Substituting Equations 8.17 into 8.14yields:

+=

DR

VS

Vf cy

664ρ

This friction factor can then be used in Equation 8.3.

Non-Newtonian flow through non-pipesAt your option in the System Properties window, you may apply acorrection to hydraulic losses at elements other than pipes. This includesjunctions and pipe additional losses.

The correction is as follows:

newtonian

newtoniannonnewtoniannewtoniannon f

fKK −− =

where the friction factor is for the upstream pipe.

Design factors

In each pipe you can specify a Design Factor for the pipe friction. This isa multiplier that is applied to the friction factor calculated with thepreceding methods.

Steady Solution Control parameters

AFT Impulse provides you control over a number of parameters thatinfluence numerical convergence:

• Pressure solution tolerance

• Mass flow rate solution tolerance

• Flow relaxation


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• Pressure relaxation

• Maximum iterations

• Matrix method

The default values for the solution control parameters perform well inthe majority of cases. Modify the default parameters only whennecessary or when you are comfortable with numerical convergenceissues.

Solution tolerance specificationThe nature of tolerance specification for pressure and mass flow rate isthe same. A discussion of tolerance specification will therefore coverboth areas.

AFT Impulse uses an iterative approach to obtain pipe flow solutions; itcan obtain a converged solution to as many significant digits as you want(within the computer's numerical capability). However, for practicalengineering problems, each digit after about the fourth significant digitgenerally has low confidence associated with it because of theuncertainties in the input data. The more digits, the lower theconfidence. Further, obtaining those additional digits in your solutionrequires additional computer resources, which typically means longerrun times. At a certain point, obtaining additional significant digitsbecomes counterproductive.

Therefore, the Steady-State Solver needs to know when to stop iteratingand assume that a sufficient solution has been obtained. Because youknow the nature of your pipe system much better than the Steady-StateSolver, you are in a better position to specify the appropriate level ofconvergence. You do this by setting the tolerances in the Steady SolutionControl window.

AFT Impulse bases the solution tolerance specifications on absolutechange or relative change of the unknown parameter. As the Steady-State Solver iterates, AFT Impulse continuously updates the solution.Each time a solution update is obtained, AFT Impulse compares the newsolution to the old solution for each individual pipe and junction.Presumably, when the correct solution is being approached, thecorrections made during each iteration become smaller and smaller.When all corrections for an iteration are smaller than the tolerance youspecify, AFT Impulse considers the solution converged.


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The absolute change method is generally more reliable for iterativesolvers like AFT Impulse because it is less sensitive to the magnitude ofthe solution, whether close to zero or very large. However, in principle,specifying absolute tolerance requires some knowledge of the finalsolution. If you specify mass flow convergence as sufficient when theflow no longer changes by 1 lbm/sec each iteration, then your solutionwill be compromised if the magnitude is also near 1 lbm/sec. In this caseit would be better to set the tolerance 3 or 4 orders of magnitude belowthe solution.

In programming terms, the logic for absolute tolerance looks as follows:

If EndFalse eConvergenc

ElseTrue eConvergenc

Then )Junctions All(For If ,,

=

=

<− absoldjnewj TOLPP

Relative tolerances, on the other hand, check only the relative change ofthe number. That is, each successive change is divided by the numberitself. This removes the requirement to be knowledgeable about the finalsolution, but can give trouble for problems where one of the solutions istoo close to zero (because the change is divided by a number close tozero, making a very large number).

In programming terms, the logic for relative tolerance looks as follows:

If EndFalse eConvergenc

ElseTrue eConvergenc

Then )Junctions All(For If,

,,

=

=

<−

relnewj

oldjnewj TOLP

PP

Table 8.1 shows a tolerance example for pressures. Notice how relativechange does not have units, but absolute change has units (of psia).

The default in AFT Impulse is relative tolerance because experienceindicates it is the most robust.


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AFT Impulse 3.0 offers options for combining absolute and relativetolerance. You can tell AFT Impulse to assess convergence based onwhether either an absolute or relative criteria is satisfied, or whetherboth absolute and relative criteria are satisfied.

Table 8.1 Example of relative and absolute change for pressure ata junction. AFT Impulse looks at the maximum changefor all junctions and checks that it is less than thetolerance.

Iteration #Pressure

(psia)Relative Change

Absolute Change (psia)

1 100.0 — —2 60.0 0.6667 40.003 72.0 0.1667 12.004 66.0 0.0909 6.005 67.0 0.0149 1.006 66.9 0.0015 0.10

Figure 8.6 Tolerance input on Steady Solution Control window.


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Figure 8.6 shows the tolerance input area in the Steady Solution Controlwindow. Table 8.2 shows an example of how the four different criteriawould be applied to flow rate iterations.

Table 8.2 Example of four tolerance methods for a mass flow ratesolution. Absolute tolerance = 0.0005 lbm/s, Relativetolerance = 0.0001.

Iteration #

Mass Flow Rate (lbm/s)

Relative Change

Absolute Change

(lbm/sec)

Meets Abs. Tolerance ?

Meets Rel. Tolerance ?

Meets Abs. OR Rel.

Tolerance ?

Meets Abs. AND Rel.

Tolerance ?1 20.0000 — — — — — —2 12.0000 0.666667 8.000000 NO NO NO NO3 14.0000 0.142857 2.000000 NO NO NO NO4 13.5000 0.037037 0.500000 NO NO NO NO5 13.6000 0.007353 0.100000 NO NO NO NO6 13.5900 0.000736 0.010000 NO NO NO NO7 13.5913 0.000096 0.001300 NO YES YES NO8 13.5907 0.000044 0.000600 NO YES YES NO9 13.5909 0.000015 0.000200 YES YES YES YES

How tolerances relate to solution accuracyThe tolerances you set do not have very much to do with how close youare to the true solution. Do not make the mistake of concluding that thetolerance you specify means AFT Impulse’s solution is that close to thetrue solution. As in Table 8.2, the tolerance is how close the currentiteration values are to the previous iteration values. That is all it means.

If there is concern that the AFT Impulse results may not be close enoughto the true answer, you have the option of lowering the tolerances,rerunning the models, then comparing the results to the previous run.After performing such a comparison, if you are satisfied that the resultsare not changing in a significant way then you can feel confident in themodel predictions. This of course assumes that there are no input errors.

RelaxationAnother feature of iterative solvers is called relaxation. In short,relaxation slows the Steady-State Solver's progress toward a solution andin the process reduces the ability of poorly behaved (non-linear)components to destabilize the solution. AFT Impulse implements aproprietary adaptive relaxation scheme that reduces the relaxation


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dynamically in response to highly non-linear features of the problem.However, you are free to override AFT Impulse's adaptive approach.

When applied to the flow solution, the relaxation is used as follows:

( ) oldoldnewnew mmmrm &&&& +−=

where r is the relaxation. The solution after each iteration is modified byweighting it with the previous solution and applying the relaxationfactor. The relaxation must always be greater than zero and less than orequal to 1. The march toward a solution is thus relaxed, because theimpact of the new solution is minimized by the magnitude of therelaxation parameter. In essence, by specifying small relaxation values,you reduce the speed of the march while causing the march to go moresmoothly. This is sometimes the only practical approach for highlynon-linear problems.

A relaxation value of 1 is the same as specifying no relaxation at all.Each new solution is used directly for the next iteration step. Arelaxation of zero is not accepted because no updates would occur to theunknown parameters and no solution would be obtained. See Table 8.3for an example.

Table 8.3 Example of different relaxation values. Old Flow Rate isthat at the previous iteration. Ideal New Flow Rate is thatdetermined by the matrix solution. Actual New Flow Rateis what is actually accepted for the new iteration.

Relaxation Value

Old Flow Rate (lbm/s)

Ideal New Flow Rate (lbm/s)

Actual New Flow Rate (lbm/s)

1 10 20 200.5 10 20 150.2 10 20 120.1 10 20 11

0.05 10 20 10.50 10 20 10

There are two relaxation parameters in AFT Impulse 3.0 (See Figure8.7): Flow rate relaxation and pressure relaxation.

Flow Rate RelaxationFor poorly-behaved systems, a flow rate relaxation of 0.1 is advised.Experience shows that using flow rate relaxation factors between 0.5 and


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1 has limited usefulness. If a flow rate relaxation of 1 does not work, youshould generally try flow rate relaxation parameters between 0.1 and 0.5.If these still do not work, there are probably other areas where yourmodel requires modification. In extreme cases, a flow rate relaxationfactor between zero and 0.1 may lead to convergence.

Pressure RelaxationBecause of the nature of the matrix solver, experience indicates that ifpressure relaxation is used, it should always be set to 0.5 or 1.0.Anything else will usually destabilize the solution. A pressure relaxationvalue of 0.5 usually works best.

Avoiding false convergenceTo avoid a false convergence it is important to keep the tolerance at leastone order of magnitude smaller (two orders or more is recommended)than the relaxation. A false convergence can occur when the changefrom the old value to the new value is small enough to be within thetolerance because it is restricted by the relaxation.

Figure 8.7 Other tab on Steady Solution Control window.


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Maximum IterationsThe Maximum Iterations parameter in the Steady Solution Controlwindow determines when the Steady-State Solver considers the modelnonconvergent; it does not directly affect the solution. There may bemany reasons why a model does not converge. You want to be sure thatthe Steady-State Solver does not continue indefinitely searching for asolution that cannot be obtained because of an input error or ill-behavedsystem.

Most properly defined models will converge in 50,000 iterations. Forlarger models, more than 50,000 iterations may occasionally be required,especially when relaxation is being used. AFT Impulse's default of50,000 iterations is sufficient for most models.

Matrix MethodAFT Impulse’s network solution method requires solution of matricesthousands of times. The default method is Gaussian Elimination, whichis very robust and usually provides the fastest convergence.

For larger systems, Gaussian Elimination with Pivoting and the LUDecomposition methods can be better and are therefore provided.

Solution Progress window and iteration history

The Solution Progress window (Figure 8.8) shows you how the currentstate of the solution compares to the tolerances you specify. It also givesinformation on your Steady Solution Control settings and gives youhelpful feedback on what stage the Steady-State Solver is in.

When the Solution Progress window is displayed during execution, youhave the option of pausing or canceling the Steady-State Solver in themiddle of a run. If you pause, you can change solution controlparameters during a run and resume execution with the new parameters.You also can check the iteration history.


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Figure 8.8 The Solution Progress window shows you the status ofthe Steady-State and Transient Solvers. This includesshowing how far out of tolerance the Steady-StateSolver is, and when the steady solution has converged.

The Iteration History window shows you a complete history of alliterations, including the pipe or junction that was most out of tolerance.If a model is not converging, knowing the pipe or junction most out oftolerance can be helpful in troubleshooting the model.

Warnings in the solution

A number of warnings may be given in the output when you run AFTImpulse's Steady-State Solver. These warnings are intended to alert youabout potential problems with the model or solution. Some warningsmay prove to be harmless upon closer inspection, but you should neverignore them.

There are different warning levels depending on the importance of thewarning.


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1. WARNING – A warning that actually says WARNING offersimportant information about the converged solution or the modelitself. The information should always be reviewed.

2. CAUTION – A cautionary warning is of less importance than a fullwarning. The included information may or may not be of interest tothe user.

3. Informational – The informational warning is for information onlyand is not serious.

A complete list of the warnings you may receive, with a briefexplanation given of each, is given in the Help System. Whenever anywarnings occur that are WARNING level, the General Results section ofthe Output Window will be displayed with the text in red.

Junction types and the Steady-State Solver

The junction types fall into four broad categories: pressure specificationjunctions, flow specification junctions, pressure modifying junctions,and pipe joining junctions. With judicious junction type selection youcan model almost any pipe system.

Pressure specification junctionsThe Reservoir junction, Assigned Pressure junction, and the SprayDischarge junction always behave as pressure specification junctions.The Control Valve junction, when set as a pressure reducing orsustaining valve, also behaves as a pressure specifying valve.

In addition, the Valve junction and the Relief Valve junction (whencracked open) both allow valves to be specified as exit valves. Exitvalves have an associated back pressure, and therefore behave aspressure specification junctions as well. A submerged pump acts like apressure junction. These junctions offer several complementary ways ofinputting known pressure information to your model.

Three other junction types can function as pressure junctions as specialcases. The Assigned Flow junction, when it is closed directly or byanother junction, can act as a pressure junction. The Gas Accumulatorcan act as a pressure junction if it is in a location where it acts as atopping tank or pressurizer. In such a case it will have no flow. A SurgeTank can also act as a pressure junction. In each of these three cases, the


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junctions do not act as pressure junctions unless specifically specified bythe user. This specification is made on the Optional tab of each junctionin the area referring to steady-state flow behavior.

Pressure specification junctions do not balance flow. Instead, they applya known pressure at a specific location in the model. By applying aknown pressure, the flow at these junction types adjusts to the flowrequired to maintain the pressure you specify. The output for one ofthese junction types will show flow exiting or entering the systemthrough the junction. This is in contrast to other junction types where theflow is always balanced and no flow goes in or out of the system unlessexplicitly defined.

Pressure junctions are an infinite source of flow (i.e., mass). See Chapter12 for more discussion on how pressure junctions work.

Flow specification junctionsThe Assigned Flow junction and the Dead End junction always act asflow specification junctions. In the case of Dead End junctions, the flowis specified as zero. The Control Valve junction behaves as a flowspecification junction when it is specified as a Flow Control valve;similarly, the Pump junction behaves as a flow specification junctionwhen it is set to an assigned flow.

The Assigned Flow junction allows nonzero flow rate specification. Thisflow is defined as outflow or inflow, and the pressure adjusts at thejunction to that required to maintain the specified flow. For FCV’s andpumps with assigned flows, the pressure change across the junctionadjusts to achieve the specified flow.

How does a flow junction compare to a pressure junction? Besides theobvious difference of providing pressure vs. flow, the flow junction is aninfinite source of pressure. In contrast, the pressure junction is aninfinite source of flow (i.e., mass).

For example, let’s say you used an Assigned Flow junction to specify aninflow to your pipe system. In so doing you want a flow rate of a millionpounds per day. But you accidentally assign the units wrong and input amillion pounds per second. Because the assigned flow junction is aninfinite source of pressure, it can provide an infinitely high pressure –whatever is necessary to drive the specified flow.


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Pressure modifying junctionsPressure modifying junctions comprise the following junction types:Area Change, Valve, Check Valve, Relief Valve (when internal open),Pump, and General Component junctions. These junction types havespecial geometric or flow control features that reduce the pressure in thesystem. In the case of Pump junction, pressure is added to the system.

Pipe joining junctionsAFT Impulse has six pipe joining junctions: the Branch and theTee/Wye are two. The Tee/Wye junction joins three pipes together. TheBranch junction can join from two to twenty-five pipes. The GasAccumulator, Liquid Accumulator and Surge Tank accept from two totwenty-five pipes and by default act essentially as branches duringsteady flow. The Vacuum Breaker Valve junction accepts only two pipesand acts a branch junction under steady flow.

Special junctionThe Volume Balance junction is a special case. It acts like a pseudo-transient junction where the mass flow rate is abruptly changed tomaintain a constant volume flow of two fluids with different densities.

Modeling irrecoverable losses

Chapter 6 discusses in detail the features offered by AFT Impulse tomodel irrecoverable losses. Chapter 3 discusses the basic conventionsused by AFT Impulse for defining losses.

As discussed in Chapter 3, AFT Impulse allows you to specify the basearea for modeling losses at junctions. By default the upstream pipe areais used.

Junction pressure losses are modeled as point losses (and point additionsfor pumps). They are not smoothed out over a length. Pipe losses, bothfrictional losses and additional minor losses, are smoothed out over thepipe length and are more accurately referred to as distributed losses.


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Loss models and reference material

Table 8.4 lists the sources for the loss models used in AFT Impulse. Thelosses implemented directly in the code were chosen on the basis of easeof use. Many loss factor types are functions of the flow, and thus toogeneral to be easily incorporated.

Table 8.4 Loss model references

Junction Type ReferencesBend Crane 1988, A-29

Area Change Crane 1988, A-26 and Idelchik 1994, 208, 216

Orifice Idelchik 1994, 218, 220

Tee/Wye Idelchik 1994, chapter 7 and Miller

Valve Ingersoll-Rand 1970 Crane 1988, A-29

Crane (1988) offers good general purpose correlations for modelingirrecoverable losses in pipe systems. A well prepared pipe flow analystwould be advised to have copies of all of these references.

Another lesser known source of loss factor information is Idelchik'sHandbook of Hydraulic Resistance (1994). This reference isindispensable for the engineer who must make detailed hydraulicassessments of pipe systems in which so-called minor losses play asignificant roll. The reference is voluminous in scope and full of tables,charts and equations for calculating loss factors for almost any pipearrangement.

Miller's Internal Flow Systems (1990) is another reference offering goodgeneral purpose hydraulic data.

AFT Impulse models component losses according to the followingequation:

221 VKPloss ρ=∆ (8.18)

where K is commonly referred to as the loss factor. Other loss modelsand their relationship to K are described at the end of this chapter.


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Design factorsEach junction that allows modeling of pressure losses allows input of aDesign Factor. The design factor is multiplied by the K factordetermined by methods discussed in this chapter.

Many junction types allow modeling of K factors as well as otherpressure loss methods. The Design Factor is also applied to these othermethods as a multiplier on the pressure loss.

Area changeTwo standard area change loss geometries are available: the conicaltransition and the abrupt transition.

The conical expansion correlation (Crane 1988, A-26) is:

2

12

sin6.2

−

=

down

upup A

AK µ

(µ < 45 degrees)

2

1

−=

down

upup A

AK (µ > 45 degrees)

The conical contraction correlation (Crane 1988, A-26) is:

2

12

sin8.0

−

=

up

down

up

down

up

AA

AA

K

µ

(µ < 45 degrees)

2

2sin15.0

−

=

up

down

up

down

up

AA

AA

K

µ

(µ > 45 degrees)

The abrupt expansion correlation (Idelchik 1994, 208) is:


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2

1

−=

down

upup A

AK

The abrupt contraction correlation (Idelchik 1994, 216) is:

2

75.0

15.0

−

=

up

down

up

down

up

AA

AA

K

Bend

90 degree bendsThe three Bend loss correlations are all for turbulent Reynolds Numbers(Crane 1988, A-29).

The K values for a smooth, flanged bend are provided in Table 8.5.

Table 8.5 K values for 90 degree smooth, flanged bends

r/d K r/d K1 20 f T 8 24 f T

1.5 14 f T 10 30 f T

2 12 f T 12 34 f T

3 12 f T 14 38 f T

4 14 f T 16 42 f T

6 17 f T 20 50 f T


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Table 8.6 Pipe friction factors used for Crane formulas

Nominal Size

Friction Factor f T

Nominal Size

Friction Factor fT

Nominal Size

Friction Factor fT

½” 0.027 2” 0.019 8”-10” 0.014

¾” 0.025 2 ½”, 3” 0.018 12”-16” 0.013

1” 0.023 4” 0.017 18”-24” 0.012

1 ¼” 0.022 5” 0.0161 ½” 0.021 6” 0.015

The standard threaded elbow is given by:

TfK 30=

where fT is the turbulent friction factor given in Table 8.6.

The Mitre bend is given by:

TfK 60=

Non-90 degree bendsThe two non-90 degree bend loss correlations are for turbulent ReynoldsNumbers (Crane 1988, A-29).

A smooth, flanged bend is given by:

( ) KKDrfnK T +

+−= 5.025.01 π

where n is the number of 90 degree bends, K is the loss factor for one 90degree bend (Table 8.5), and fT is given by Table 8.6.

The Mitre bend is given by Table 8.7.


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Table 8.7 K values for Mitre bends

µ (deg.) K

0 2 f T

15 4 f T

30 8 f T

45 15 f T

60 25 f T

75 40 f T

90 60 f T

µ

ValveStandard valve loss models are used from Idelchik, Miller and Crane.

OrificeThe orifice loss factors are all for turbulent Reynolds Numbers. Thesharp-edged orifice shown in Figure 8.9, is given by the following(Idelchik 1994, 218):

2375.02

11707.0

−+

−

=

down

orifice

up

orifice

orifice

upup A

AA

AA

AK

Figure 8.9 Sharp-edged orifice

The round-edged orifice, shown in Figure 8.10, is given by thefollowing (Idelchik 1994, 220):


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−+

−′

=

275.02

11down

orifice

up

orifice

orifice

upup A

AA

AK

AA

K

−′

−+

375.0

112up

orifice

down

orificeA

AK

AA

where:

( )orificeDrK /7.71047.003.0 −•+=′

For other orifice configurations, see Chapter 4 of Idelchik's Handbook ofHydraulic Resistance (1994).

Figure 8.10 Round-edged orifice

Tee/WyeThe loss factors calculated for tee and wye junctions involvecomplicated correlations that depend on the flow split, the ratio of flowareas, and the angle of the connecting pipes. The models used by AFTImpulse are taken mostly from chapter 7 of Idelchik's Handbook ofHydraulic Resistance (1994). A few of the more important equationsthat AFT Impulse uses will be presented here, and references will begiven for the others.


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All tees and wyes have either diverging or converging flow streams.Two sets of equations are required for each configuration.

In the following discussion, the parameter Kc,s represents the loss factorbased on the combined flow stream flow area, and velocity for the sidebranch. Similarly, Kc,st is for the straight-through pipe (the run).

Figure 8.11 depicts the nomenclature. K factor subscripts refer to basearea reference and location in the Tee. For example, Kc,s represents the Kfactor for the side branch (s) referenced to the combined (c) pipe flowarea.

Ac Ast AcAst

As As

c = combined flow st = straight-through flow s = side branch

Figure 8.11 Idelchik’s nomenclature for diverging and convergingflow through Tee/Wye junction

In the AFT Impulse Tee/Wye window, a simpler and more generalnomenclature is used. AFT Impulse refers to the a, b and c pipes. Thisallows AFT Impulse to cover all the possibilities in Table 8.10 withoutspecifying exactly which pipe is the combined flow pipe. In some cases,you may not know all the flow directions in a system, and might not beable to specify which is the combined flow pipe.

AFT Impulse’s input method allows you to specify which pipe is thebranch pipe. Idelchik calls this the “s” pipe for “side branch”. AFTImpulse calls it the c pipe. If the tee angle is 90 degrees, it is notimportant which pipe you call the a pipe and which the b pipe. Thedistinction between a and b takes on more importance for non-90 degreepipes. In this case, AFT Impulse allows you to specify the angle of thebranch pipe.

Figure 8.12 shows the Tee/Wye Specification window Arrangement tab.The α angle itself is specified on the Tee/Wye Model tab, where thereare other options for how to model the tee.


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Figure 8.12 Tee/Wye Specifications window allows you to specifythe a, b and c pipes. The c pipe is always the branchpipe. This general method means you do not need toknow the flow direction ahead of time.

Diverging CaseSide branch (Idelchik 1994, 418):

−

+=

∆= αλ

ρcos21

2/

2

2,c

s

c

s

c

ssc V

VVV

VPK

where λ is found from Table 8.8.


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Table 8.8 Diverging case λ values

As / Ac Qs / Qc λ

≤ 0.35 ≤ 0.4 1.1 - 0.7 Qs / Qc

≤ 0.35 > 0.4 0.85

> 0.35 ≤ 0.6 1.0 - 0.6 Qs / Qc

> 0.35 > 0.6 0.6

Straight run, constant area (Idelchik 1994, 419):

2

2,2/

=

∆=

c

sst

c

ststc Q

QV

PK τ

ρ

where τst is given in Table 8.9:

Table 8.9 Diverging case τst values

As / Ac Qs / Qc τ st≤ 0.4 0 - 1.0 0.4

> 0.4 ≤ 0.5 2 (2 Qs / Qc - 1)

> 0.4 > 0.5 0.3 (2 Qs / Qc - 1)

Converging CaseSide branch (Idelchik 1994, 417):

2/2,c

ssc

VPK

ρ

∆=

−

−−

+′= αcos2121

222

c

s

s

c

c

s

st

c

sc

csQQ

AA

QQ

AA

AQAQA

Straight run, constant area (Idelchik 1994, 417):

αρ

cos2112/

22

2,

−

−−=

∆=

c

s

s

c

c

s

c

ststc Q

QAA

QQ

VPK


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References for all tee and wye junction correlations are given in Table8.10.

Table 8.10 References for Tee/Wye junction correlations

Geometry Type Direction Reference

Constant area, sharp Diverging tee Branch Idelchik 1994, 451

Diverging tee Run Idelchik 1994, 453

Converging tee Branch Idelchik 1994, 429-432

Converging tee Run Idelchik 1994, 429-432

Splitting tee From branch Miller 1990, 323*

Combining tee To branch Miller 1990, 316*

Constant area, r/D = 0.1 Diverging tee Branch Idelchik 1994, 459-461

Diverging tee Run Idelchik 1994, 453





Area sum, sharp Diverging tee Branch Idelchik 1994, 452

Diverging tee Run Idelchik 1994, 454-455





Wye Diverging wye Branch Idelchik 1994, 451

Converging wye Run Idelchik 1994, 473

*Estimates have been made on available data

K and CV (valve coefficient)The definition for CV is as follows (Lyons 1982, 133):

( )C

Q

PV

valve

=∆ 62 4. / ρ

where Q is in gpm, ∆P is in psid, and ρ is in lbm/ft3.


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It can be shown that K and Cv are related by the following approximateexpression (Lyons 1982):

KACvalve

v= 1460

2

2

where A is in in2 and CV2 is in its normal units (always gpm2/psi).

K for fire sprinklersThe definition of K value for fire sprinklers is:

PQKsprinkler ∆

=

where Q and ∆P can be in the units of your choice. The preference in theUSA is gpm for flowrate and psid for pressure. In Europe it is frequentlyliter/min for flow rate and bars for pressure. Note that Ksprinkler is notdimensionless, but has units associated with it. Also, note that sprinklervendor values for Ksprinkler implicitly assume that water is the fluid, sothere is a built-in density of 62.3 lbm/ft3 or 1000 kg/m3. If this sprinkleris used for something other than water, the Ksprinkler value will need to beadjusted.

Also note that if the spray is into the atmosphere, as is typical, the ∆P isthe same thing as the gage pressure. That is why the previous equation isstated by some vendors as just P in the denominator, rather than ∆P.When stated as just P, the pressure is gage.

Note: The fire sprinkler K is not the same as the dimensionless K valueas given by Equation 8.18 and used extensively throughout AFTImpulse.

The Ksprinkler is related to the discharge coefficient. It can be shown thatthe relationship is given by

sprinklerwater

D KAC2

ρ=


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Pumps

Pumps can operate at variable speeds. Affinity laws (also calledhomologous laws) allow an estimation of the pump performance atspeeds other than the design speed.

Before discussing the affinity laws, it should be noted that the affinitylaws are an approximation. In many cases pump manufacturers will haveperformance data for a number of speeds. These data should be used ifavailable. When not available, the affinity laws can be used with anunderstanding that an approximation is being made.

The affinity laws for pumps are as follows:

2

1

2

1

2

2

1

2

1

ss

QQ

ss

HH

=

=

∆∆

(8.19)

where s is the speed. If the pump data is input as a polynomial, the speedwill affect the curve as follows:

4

23

322

222

2

432

322

222222

2

41

231

221

21

221

22

41

31

2111

sQe

sQdcQsbQasH

sQes

sQds

sQcs

sQbsasH

eQsdQscQsbQsasHsH

eQdQcQbQaH

++++=∆

+

+

++=∆

++++=∆=∆

++++=∆

(8.20)

where ∆H1 is the input curve that typically represents 100% speed and∆H2 is the head rise at speed s. In this form, the speed s will be a decimaland not a percent. That is, 50% speed will result in the appropriate s of0.5. AFT Impulse determines this automatically in the Steady-State andTransient Solvers.

If you model the pump as a controlled flow or discharge pressure, AFTImpulse will backsolve Equation 8.20 for the speed s and include that inthe output Pump Summary.


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Chempak thermophysical property database

The optional Chempak Database offers approximately 700 fluids to theuser. In addition, it offers non-reacting mixture calculations. Informationon the theory and calculation methods used in Chempak is given inAppendix D.

Accuracy optionsWhen performing interpolations for properties, a 2 point or 4 pointinterpolation scheme can be used. The 2 point scheme implemented inAFT Impulse is the Standard Accuracy option. The 4 point scheme is thehigh accuracy option.

AFT Standard fluids

The AFT Standard Database offers about 10 common fluids to the user.Information on how to use these fluids within AFT Impulse are given inChapter 5.

ViscosityThe dynamic viscosity in AFT Impulse is assumed to be a function oftemperature only. In general this is a good assumption, although systemsat very high pressure usually start to show some pressure dependence.These properties are modeled as polynomial curve fits of temperature.


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C H A P T E R 9

Theory of Waterhammer and SolutionMethodology

This chapter discusses the mathematical models used to predictwaterhammer events.

Description of waterhammer

Please see Chapter 3 for a summary description of waterhammer in asimple one-pipe system.

Instantaneous waterhammer

With few exceptions (discussed at the end of the chapter), it is safe tocalculate the maximum possible waterhammer pressure surge by usingthe instantaneous waterhammer equation. The instantaneouswaterhammer equation assumes that the transient event occurs eitherinstantaneously or rapidly enough such that it is in effect instantaneous.

In such a case, it can be shown (Wylie, et al., 1993, pp. 4) by use of themass and momentum equation that the pressure transient is given by thefollowing equation:

VaP ∆−=∆ ρ (9.1)


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where:


ρ = density

a = wavespeed


By adding the pressure surge to the existing static pressure, one canobtain the maximum theoretical pressure in the pipe. However, in somecases, such as transient cavitation, the pressure can exceed theinstantaneous prediction. The engineer should therefore be cautious inits application.

Here’s an example. Assume flow in a pipe filled with water isinstantaneously stopped by a valve closure. Assume the density is 62lbm/ft3, the wavespeed is 4000 feet per second, and the initial velocity is10 feet per second. Also assume that before the transient the staticpressure is 50 psig.

psig 58550535

psig 50ft-lbm 2.23

slbf*in 144

ft 1*sft10*

sft4000*

ftlbm62

max

max

2

2

2

3max

=+=

+−

=

PP

P

Wavespeed

When a transient event is initiated in a pipe system, the remainder of thesystem must adjust to the new conditions. In order to adjust, theexistence of the event must be communicated to the rest of the system.This communication takes place at the wavespeed of the fluid. Thewavespeed is somewhat analogous to the sonic speed of the liquid.However, the wavespeed is affected by the pipe structure.

The relationship between these parameters is expressed in the followingequation (Wylie, et al., 1993, pp. 27):


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( )( )[ ]eDEKcKa

1

21+

=ρ

where:

a = wavespeed

K = fluid bulk modulus of elasticity

ρ = fluid density

E = pipe modulus of elasticity

D = pipe inner diameter

e = pipe wall thickness

c1 = correction factor

The sonic speed of the liquid is given by the (square root of the)numerator. The denominator contains the reduction from sonic speedcaused by the pipe properties.

The correction factor, c1, is a function of how the pipe is restrained.Some examples from Wylie, et al., 1993, page 27-28 are:

1. Thin-walled pipe anchored at upstream end only

211 µ−=c

2. Thin-walled pipe anchored throughout

21 1 µ−=c

3. Thin-walled pipe free to expand throughout

11 =c

4. Thick-walled pipe anchored at upstream end only

( ) ( )21121 µµ −

+++=

eDD

Dec

5. Thick-walled pipe anchored throughout

( ) ( )21 112 µµ −

+++=

eDD

Dec


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6. Thick-walled pipe free to expand throughout

( )eD

DDec

+++= µ12

1

7. Circular tunnel

( )µ+= 121 D

ec

Given the previous, it should be apparent that in any given pipe thewavespeed can only be approximated.

Communication time in pipes

Transient events communicate their effect through the pipe at thewavespeed of the pipe and fluid. When transient events occur, they arecommunicated upstream (or downstream) so conditions can adjust to thenew conditions. No response to the changing conditions can be obtaineduntil the communication wave travels to the other end of the pipe andback. This communication time is thus given by the following:

aLt 2=∆ (9.2)

Any event that occurs with a time frame shorter than this is equivalent toan instantaneous event.

Method of Characteristics

The analysis of waterhammer in liquid pipe systems is based on thetransient mass and momentum equations. For a complete development ofthe equations please see one of the many excellent references (Wylie, etal., 1993, Chaudhry, 1987, and Swaffield et al., 1993). Solving theequations based on a prescribed relationship between time step anddistance step is the essence of the Method of Characteristics which willbe discussed in depth in the following sections.


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Momentum equationFollowing from Wylie, et al., 1993, page 22, the momentum equationcan be expressed as

02

)sin(

1

=+++DVfV

gtV

xP α

∂∂

∂∂

ρ(9.3)

where:

P = pressure

V = velocity

ρ = density

x = distance along pipe

t = time

g = acceleration due to gravity

D = diameter

f = friction factor

α = angle of pipe slope

Mass continuity equationFollowing from Wylie, et al., 1993, page 27, the continuity equation canbe expressed as

0

2

=+tP

xV

ga

∂∂

∂∂

(9.4)

where:

a = wavespeed

Equations 9.3 and 9.4 are two partial differential equations with the twounknowns P and V and the two independent variables x and t.Application of the Method of Characteristics will convert these twopartial differential equations to four ordinary differential equations asfollows. To maintain flexibility with g-level, the development will leavethe parameter P in the equations rather than convert to head, H, as inWylie, et al., 1993.


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The equations are identified as L1 and L2 .

12)sin(

1 L

DVfV

gtV

xP

=+++ α∂∂

∂∂

ρ(9.5)

22 L

tP

xVa =+

∂∂

∂∂ρ (9.6)

Combine these two equation linearly using a parameter, λ, (Wylie, et al.,1993, pp. 27)

0 21 =+ LL λ

or,

++

+

xVa

tV

tP

xP

1 2

∂∂λρ

∂∂ρ

∂∂

∂∂

λλ

02

)sin( =++D

VfVg

ραρ (9.7)

If

2 1 adtdx λ

λ== (9.8)

then

a1

±=λ

and

adtdx

±=

From calculus

tP

dtdx

xP

dtdP

∂∂

∂∂

+= (9.9)

tV

dtdx

xV

dtdV

∂∂

∂∂

+= (9.10)

Substituting Equation 9.8 into 9.7


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++

+

dtdx

xV

tV

tP

dtdx

xP

∂∂ρ

∂∂ρ

∂∂

∂∂λ

02

)sin( =++D

VfVg

ραρ (9.11)

and substituting Equations 9.9 and 9.10 into 9.11

02

)sin( =++

+

DVfV

gdtdV

dtdP ρ

αρρλ

or

02

)sin( 1=+++±

DVfV

gdtdV

dtdP

aρ

αρρ (9.12)

Now multiply Equation 9.12 by (a dt), which also equals dx

02

)sin( =+++± dxD

VfVdxgadVdP

ραρρ (9.13)

The gravity term contains the term α, and the term can be converted asfollows

gdzdxdxdzgdxg )sin( ρραρ =

= (9.14)

where dz is the vertical change in pipe elevation. Therefore, substituting9.14 into 9.13

02

=+++± dxD

VfVgdzadVdP

ρρρ

Further substitution of the mass flow rate for velocity yields

02

2 =+++± mmdx

DAfgdzmd

AadP &&&

ρρ (9.15)

Integrating Equation 9.15 with the positive sign along the C+

characteristic from point A to P (Figure 9.1) yields


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02

2 =+++ ∫∫∫∫P

A

P

A

P

A

P

A

x

x

z

z

m

m

P

Pdxmm

DAfdzgmd

AadP &&&

&

& ρρ (9.16)

where the subscript P denotes the current solution point at x = i at time t= ∆t, and subscript A denote the values at point x = i -1 at the previoustime step, t = 0. A similar equation can be written for the C-

characteristic from point B to P.

The term on the right before the equal sign in Equation 9.16 can beintegrated assuming a second order accurate representation of m& toobtain

( ) ( ) ( )

02

2 =∆

+−+−+−

AP

APAPAP

mmDA

xf

zzgmmAaPP

&&

&&

ρ

ρ(9.17)

t = 0

t = ∆t

t = 2∆t

t = 3∆t

t = 4∆t

t = 5∆t

x = 0 x = LA B

P

x = i x = i+1x = i-1

C+ C-

Figure 9.1 Method of Characteristics grid

Similarly


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( ) ( ) ( )

02

2 =∆

+

−+−+−−

AP

APAPAP

mmDA

xf

zzgmmAaPP

&&

&&

ρ

ρ(9.18)

Introducing two convenient parameters:

Impedance:

AaB

=

Resistance:

22 DA

xfRρ

∆=

Equations 9.17 and 9.18 then become

( ) ( ) ( ) 0 =+−+−+− APAPAPAP mmRzzgmmBPP &&&& ρ

( ) ( ) ( ) 0 =+−+−+−− APAPAPAP mmRzzgmmBPP &&&& ρ

Substituting more general variables for A, B and P, and simplifying theequations yields

newiPPnewi mBCP ,, &−= (9.19)

newiMMnewi mBCP ,, &+= (9.20)

where:

( )1,1,1 −−− −−+= iioldioldiP zzgmBPC ρ&

( )1,1,1 +++ −+−= iioldioldiM zzgmBPC ρ&

oldiP mRBB ,1−+= &

oldiM mRBB ,1++= &

Equations 9.19 and 9.20 are referred to as the compatibility equations(Wylie, et al., 1993, pp. 42). Since all the parameters in CP, CM, BP, and


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BM are known, the only unknowns in 9.19 and 9.20 are Pi,new and newim ,& .Thus there are two equations and two unknowns.

For example, to solve for the pressure at an interior pipe point, the flowrate can be eliminated and the pressure solved for directly:

MP

PMMPnewi BB

BCBCP++

=, (9.21)

This value for pressure can be substituted back into equation 9.19 and9.20 to solve for flow rate, or the flow rate can be solved for directly:

MP

MPnewi BB

CCm+−

=,& (9.22)

The compatibility equations allow a solution of all interior points in apipe. However, the pipe endpoints are solved by applying specificboundary conditions. The boundary condition relationships build on theprevious methods, and offer the final needed information to generate acomplete solution.

Transient vapor cavitation

When the rarefaction pressure waves drop as low as the vapor pressureof the fluid, the fluid experiences local vaporization and vapor cavitiesform. There are several approaches to model this. AFT Impulseincorporates one of the more simple models called the Discrete VaporCavity Model, or DVC Model (Wylie, et al., 1993, pp. 66-69).

To apply the method, consider what happens when the fluid pressuredrops to the vapor pressure. When a solution for pressure at a node isbelow the vapor pressure, the pressure is then set to the vapor pressure.When this happens there is not enough flow to maintain continuity, so acavity forms. This results in an imbalance of flow into and out of thenode.

Assuming the cavity occurs discretely at the node location, the volume isgiven as follows:

satnew PP =


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oldvapornewvapor VV ,, =

( )ρ2,,,,tmmmm olddownnewdownoldupnewup

∆++−−+ &&&&

(9.23)

If no vapor existed previously, the old volume is zero. When the vaporvolume becomes negative, the cavity collapses and the pressure againrises above vapor pressure. In this case the conventional methods(Equations 9.19, 9.20) again are applicable.

The DVC method is well understood and documented for interior pipepoints. However, application of the method to general boundaryconditions in multi-pipe systems is limited. The theory for many of thejunctions used in AFT Impulse is developed in Walters, 1991.

Pipe sectioning

The relationship between pipe sectioning and time step is given by

tnLa∆

= (9.24)

where n is the number of sections and L/n is the length of each pipesection. If the transient event occurs within some particular time step,the number of sections, n, must be chosen such that it is consistent withthe time step in Equation 9.24.

Specifying the number of sections for multi-pipe systems is morecomplicated. Each pipe has its own wavespeed and length. For each pipethe following must be satisfied:

taLni

ii ∆

=

(9.25)

In Equation 9.25 as applied to each pipe in the system, the time step ∆tmust be the same for all pipes. The length, L, is a given for each pipe andthus cannot be changed. The wavespeed, a, is also a given for each pipe,although it is known with less certainty than is the length. The freeparameter is then the number of sections, n.


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In any pipe system there will be one pipe that is the controlling pipe.The controlling pipe is that pipe which has the least number of sections,sometimes only one. Once the controlling pipe is selected, the time stepis determined by solving Equation 9.24 for ∆t. Then the number ofsections in the remaining pipes is obtained from Equation 9.25.

However, this presents a dilemma. Because the number of sections inremaining pipes is derived from Equation 9.25, typically the number ofsections, n, will not be a whole number. Since partial sections in a pipecannot be modeled, an alternative must be found.

Because the wavespeed for each pipe is the least certain parameter, anuncertainty in wavespeed up to 15% is possible. Therefore, thewavespeed in each pipe is allowed to depart slightly from its originalcalculated value in order to cause n to be a whole number.

Therefore, Equation 9.25 becomes

( ) taLn

i

ii ∆±

= 1 ψ

where ψ is the accepted uncertainty in wavespeed up to ±0.15 (Wylie,et al., 1993, pp. 54).

Two other things should be noted about pipe sectioning. Because eachpipe section is explicitly solved along the characteristic lines, breakingpipes into continuously smaller sections offers no improvement inaccuracy. That is, using twice the number of sections in a pipe will notyield a more accurate prediction (see Chapter 12). This is in contrast toother kinds of finite difference methods where more increments offersimproved accuracy.

The other item of note is the effect of breaking the model into additionalincrements. According to Equation 9.24, when the number of sections isdoubled, the time step must cut in half. This results in increasing runtimeby a factor of 4 for each doubling of the number of sections.

The Section Pipes window, discussed in Chapter 5, automates theprocess of obtaining acceptable roundoff error when assigning sections.


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Effects of flow resistance

The resistance to flow inside the piping system is a result of friction andirrecoverable losses through valves and fittings. Without resistance thepressure waves would never damp out.

It is beneficial to include all of the resistance in the system. However,properly locating the resistance is not nearly as important inwaterhammer analysis as it is in conventional steady flow analyses.What this means is that it is usually more efficient to “lump” theresistances together into a single pipe, rather than use a separate junctionto model the resistance for each individual component.

For example, it is usually best to lump the valve resistance into theadjacent pipes unless there are valve position changes during thesimulation. This is true of all types of local resistances (i.e., valves andfittings).

The effect on the results of lumping a resistance is usually minor, whilethe benefit of faster computation time is major. The faster computationresults from the fact that the user can use less pipe sections to model thesame system.

Assigned Pressure theory

With an assigned pressure, the pressure is known at the pipe endpoint.The unknown is then the flow rate, which can be obtained by solving theappropriate compatibility equation.

If the pressure is known at the upstream end of the pipe, the negativecompatibility equation is used (Equation 9.20).

M

Mknownnewi B

CPm −=,&

Conversely, if the pressure is known at the downstream end, the positivecompatibility equation is used (Equation 9.19).

P

knownPnewi B

PCm −=,&

If the known pressure changes with time, then a current pressure isobtained for the current time step and used in the equations.


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Assigned Pressure vapor cavitation theoryAssigned Pressure junctions cannot reach vapor pressure because thepressure is specified and is therefore always above vapor pressure.

Reservoir theory

A reservoir is similar to an assigned pressure. With the known reservoirheight, the pressure can be obtained at the junction and the flow can besolved directly (Wylie, et al., 1993, pp. 43):

surfaceRnewiknown PgHPP +== ρ,

Then the flow rate is obtained by employing the compatibility equations.

If the pressure is known at the upstream end of the pipe, the negativecompatibility equation is used (Equation 9.20).

M

Mknownnewi B

CPm −=,&


P

knownPnewi B

PCm −=,&

If the reservoir height changes with time, then a current height isobtained for the current time step and used in the equations.

Reservoir vapor cavitation theoryReservoir junctions cannot reach vapor pressure because the pressure isspecified and is therefore always above vapor pressure.

Assigned Flow theory

With an assigned flow, the flow rate is known at the pipe inlet or outlet(Wylie, et al., 1993, pp. 43). The unknown is then the pressure, whichcan be obtained by solving the appropriate compatibility equation.


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If the flow rate is known at the upstream end of the pipe, the negativecompatibility equation is used (Equation 9.20).

knownMMnewi mBCP &+=,


knownPPnewi mBCP &−=,

If the known flow rate changes with time, then a current flow rate isobtained for the current time step and used in the equations.

Assigned Flow vapor cavitation theoryWhen the calculated pressure at an Assigned Flow junction drops belowvapor pressure, a vapor cavity forms at the junction.

Assuming the specified flow is an inflow, the vapor volume calculationis as follows:

oldvapornewvapor VV ,, =

( )ρ2,,,,tmmmm olddownnewdownoldupnewup

∆++−−+ &&&&

The upm& terms are known because they are specified terms for inflow

junctions. For outflow junctions the downm& terms are known. Similar toa pipe interior node, when the vapor volume is negative, the cavitycollapses and the fluid pressure then rises above the vapor pressure.

Dead End theory

A dead end is similar to an assigned flow, except the flow rate is zero(Wylie, et al., 1993, pp. 44). The unknown is then the pressure, whichcan be solved by calling the appropriate compatibility equation.

If the dead end is at the upstream end of the pipe, the negativecompatibility equation is used (Equation 9.20) with zero flow.


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Pnewi CP =,

Conversely, if the dead end is at the downstream end, the positivecompatibility equation is used (Equation 9.19) with zero flow.

Mnewi CP =,

Dead End vapor cavitation theoryWhen the calculated pressure at a Dead End junction drops below vaporpressure, a vapor cavity forms at the junction.

Assuming the dead end is at the end of a pipe, the vapor volumecalculation is as follows:

( )ρ2,,,,tmmVV oldupnewupoldvapornewvapor

∆−−+= &&

since the downm& terms are always zero. For dead ends at the beginningof a pipe, the upm& terms are zero. Similar to a pipe interior node, when

the vapor volume is negative, the cavity collapses and the fluid pressurethen rises above the vapor pressure.

Branch theory

At a branch, there can be multiple pipes (Wylie, et al., 1993, pp. 51-53).An additional relationship is needed, and that relationship is theconservation of mass. The total mass flow in and out of the junctionmust sum to zero.

In addition, the branch has a single pressure solution, Pj, and thissolution is common to all inflowing pipes.

The compatibility equation is written for each junction. For the pipesflowing into the branch, the positive equation is used (Equation 9.19)

P

jPnewi B

PCm

−=,&

where Pj is the branch junction pressure, as yet unknown.


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For the pipes flowing out of the branch, the negative equation is used(Equation 9.20)

M

Mjnewi B

CPm

−=,&

Now, sum all of the pipe flow rates into the junction

0, =−=∑ jBCnewi PSSm& (9.26)

where

M

M

P

PC B

CB

CS ∑∑ += (9.27)

∑∑ +=MP

B BBS 11

(9.28)

In general, there may be a known flow into the branch (a flow sink) orinto the branch (a flow source). Therefore rewrite Equation 9.26 as

AppliedjBCnewi mPSSm && −=−=∑ ,

Now solve for the junction pressure

B

AppliedCj S

mSP

&+= (9.29)

With the pressure known, substitute back into the compatibilityequations to obtain the flow rate into the branch coming from eachconnecting pipe.

The applied flow rate may vary with time.

Branch vapor cavitation theoryWhen the calculated pressure at a Branch junction drops below vaporpressure, a vapor cavity forms at the junction.

Assuming a flow source/sink is specified, the vapor volume calculationis as follows:


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( )( )

( ) ρ2,,

,,

,,

,,t

mmmm

mm

VV

oldBnewB

oldoutnewout

oldinnewin

oldvapornewvapor∆

+−

++

−−

+= ∑∑

&&

&&

&&

where Bm& is the branch source/sink term defined as positive flow intothe branch. The inm& terms are obtained from the positive compatibilityequation, while the outm& terms are obtained from the negative equation.

Similar to a pipe interior node, when the vapor volume is negative, thecavity collapses and the fluid pressure then rises above the vaporpressure.

Tee/Wye theory

The Tee/Wye junction is a static element, and its solution method isessentially the same as the Branch junction. The only difference is thatthe loss factors are updated dynamically similar to steady flow.

Valve theory

A valve junction has a pressure loss across the valve. When the valvecloses this loss becomes infinite (Wylie, et al., 1993, pp. 57). With twoconnecting pipes, the upstream and downstream pipe pressures areobtained from the compatibility equations (Equations 9.19, 9.20):

upnewiupPupPupnewi mBCP ,,,,,, &−=

downnewidownMdownMdownnewi mBCP ,,,,,, &+=

From a mass balance:

valvedownnewiupnewi mmm &&& == ,,,,

The pressure drop is related to the flow rate according to the following:

2,,,, valvevalvedownnewiupnewi mRPP &=−

where


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Rvalve = Valve Resistance

Substituting the compatibility equations (Equations 9.19, 9.20) in andeliminating the pressures

( )valveupPupP mBC &,, −

( ) 2,, valvevalvevalvedownMdownM mRmBC && =+−

Rearranging terms,

( )valveupPupPvalve

valve mBCR

m && ,,2 1

−−

( ) 01,, =++ valvedownMdownM

valvemBC

R&

Using the quadratic equation, the solution for positive flow through thevalve is

( )downMupPvalvevalve BBRm ,,1 +−=&

( ) ( )downMupPvalvedownMupPvalve CCRBBR ,,2

,,2 41 −+++

For negative flow, the solution is

( )downMupPvalvevalve BBRm ,,1 +=&

( ) ( )downMupPvalvedownMupPvalve CCRBBR ,,2

,,2 41 −−+−

After solving for flow rate, valvem& , the upstream and downstreampressures can be obtained from the compatibility equations.

If the valve closes then 1/Rvalve = 0 and

0=valvem&

Exit valvesIf the valve is an exit valve, then the downstream pressure is known and


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upPvalvevalve BRm ,1−=&

( )exitupPvalveupPvalve PCRBR −++ ,2

,2 41

Valve vapor cavitation theoryWhen the calculated pressure at the upstream or downstream side of avalve junction drops below vapor pressure, a vapor cavity forms on thatside of the junction.

Vapor cavities can form on either side of the valve or on both sides.

oldupvapornewupvapor VV ,,,, =

( )ρ2,,,,tmmmm oldvalvenewvalveoldupnewup

∆++−−+ &&&&

olddownvapornewdownvapor VV ,,,, =

( )ρ2,,,,tmmmm olddownnewdownoldvalvenewvalve

∆++−−+ &&&&

As an example, when a vapor cavity forms downstream of a valve, thedownstream pressure become fixed at the vapor pressure. Thecalculation then is similar to an exit valve, which also flows to a fixedpressure.

upPvalvevalve BRm ,1−=&

( )satupPvalveupPvalve PCRBR −++ ,2

,2 41

When a vapor cavity occurs on both sides of the valve, the pressure is atvapor pressure on both sides and thus there is no pressure drop acrossthe valve. Accordingly, the flow rate goes to zero.



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Infinite Pipe theory

An infinite pipe junction is a special junction that accepts pressurewaves but never reflects any waves (Wylie, et al., 1993, pp. 121-122).This junction is useful in modeling the part of a pipe system which is solong that its communication time results in no interaction with the rest ofthe system. A discussion of when this junction should be used is given inChapter 12.

Note: In steady flow, the infinite pipe junction is usually a known flow,but can also be a known pressure. Thus the Infinite Pipe capability isoffered through the Assigned Flow or Assigned Pressure junction. It isspecified by setting the transient special condition to No Reflections(Infinite Pipe).

The pressure and flow at an infinite pipe is calculated by extrapolatingthe known pressures and flows at nearby pipe stations at previous timesteps. Calculations are therefore performed at an “infinity point”, oneartificial station beyond the physical pipe end. The values at the infinitypoint are then used for each successive calculation.

tititi PPPP ∆−+∆−+∆− +−= 3,22,1,newinfinity, 33

tititi mmmm ∆−+∆−+∆− +−= 3,22,1,newinfinity, 33 &&&&

Assuming that the infinite pipe is at the downstream end, the currentpressure can be solved using the positive characteristic compatibilityequation (Equation 9.19),

newiPPnewi mBCP ,, &−=

and the negative characteristic compatibility equation (Equation 9.20)

newiMMnewi mBCP ,, &+= (9.30)

For Equation 9.30, the normal CM or BM cannot be calculated because thevalues past the end of the pipe are not known. This is where the infinityvalues are used. Instead, the following are used:

( )1oldinfinity,oldinfinity, +−−−= iiM zzgmBPC ρ& (9.31)


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oldinfinity,mRBBM &+= (9.32)

In Equation 9.31, use the following approximation which is perfectlyaccurate for straight pipes

iiii zzzz −=− −+ 11

Also, in equations 9.31 and 9.32 it is assumed that the pipe propertyvalues B and R remain valid for the infinity point, which is a goodassumption unless the pipe properties change.

Infinite Pipe vapor cavitation theoryThe Infinite Pipe junction is designed to only pass pressure waves outthrough the junction, but never to allow any reflections. If the junctiondrops to vapor pressure, a cavity would occur. But it would causereflections. Therefore, vapor cavities at these junctions are ignored andthe pressure is allowed to fall below the vapor pressure. This maintainsthe function of the junction.

In such cases, the user may want to change the model.

Spray Discharge theory

Spray discharge junctions (shown in Figure 9.2) model spray locations ina system. The spray junction discharges to a known pressure through anorifice or nozzle of some kind.

Pexit

spraym&

Figure 9.2 Spray discharge schematic

The relationship for pressure drop across the nozzle is as follows


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221 VP ρ=∆

( )2

2,2

1)sgn( spraysprayD

sprayexitnewj mAC

mPPP &&ρ

=−=∆

where, for clarity, spraym& is defined positive flowing out of the system.

In addition, the spray junction is similar to a branch junction, so thefollowing equation applies with the flowrate defined positive out.

B

sprayCnewj S

mSP

&−=,

where SC and SB are given by Equations 9.27 and 9.28.

Combining the preceding equations and eliminating flow rate,

( ) 012

1sgn 222 =−++

B

Cexit

BsprayD SSPm

Sm

ACm &&&

ρ

or

( )0

2

2)sgn(

22

222

=−

+

+

B

CexitBsprayD

B

sprayD

SSPSAC

mS

ACmm

ρ

ρ&&&

This is a quadratic equation, and the mass flowrate can be obtained byuse of the quadratic formula.

Note: If the spray junction pressure falls below the exit pressure, reverseflow will occur. If the exit pressure is the atmosphere, the reverse flowwill be air. However, Impulse assumes the inflow is at the pipe systemdensity. This can lead to erroneous results if ignored. Reverse flow canbe determined by graphing the flow at the spray junction or looking atthe summary of maximum and minimum flows in the Output window.


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Spray Discharge vapor cavitation theoryWhen the calculated pressure at a Spray Discharge junction drops belowvapor pressure, a vapor cavity forms at the junction. This is similar to aBranch junction.

The vapor volume calculation is as follows

( )( )

( ) ρ2,,

,,

,,

,,t

mmmm

mm

VV

oldspraynewspray

oldoutnewout

oldinnewin

oldvapornewvapor∆

++

++

−−

+= ∑∑

&&

&&

&&

where spraym& is the spray outflow term. The inm& terms are obtained

from the positive compatibility equation, while the outm& terms areobtained from the negative equation.


Pump theory

The pump junction can model the effects of a pump with a knownchange in pump speed (Wylie, et al., 1993, pp. 59-60), or with inertialdata where the pump speed is calculated.

Modeling pump as a known speedAssuming that the pump curve can be modeled by a quadratic, then

2pumppump cQbQaH ++=∆

where a, b and c are obtained from curve fits of vendor data. This curveassumes the pump operates at 100% speed. During the transient, thespeed will change.

The affinity law for pumps given by the following equations


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constant12 =∆

αH

constant2=αQ

where α is the pump speed. Applying these relationships the pump curveis transformed

2

2

2 αααpumppump Q

cQ

baH ′+′+′=∆

22pumppump QcQbaH ′+′+′=∆ αα

Converting to pressure rise

′+

′+′=∆ 2

22

pumppump mcmbagP &&ρρ

ααρ

22pumppump cQbQaP ++=∆ αα (9.33)

where the primes on the constants were dropped after they were dividedthrough by ρg.

By use of the two compatibility equations (Equation 9.19, 9.20),

pumpPPnewup mBCP &−=,

pumpMMnewdown mBCP &+=,

and substituting these equations into Equation 9.33

( ) ( )22

pumppump

pumpPPpumpMM

mcmba

mBCmBC

&&

&&

++=

−−+

αα

This is a quadratic equation for the pump flow rate which can be solvedby the quadratic formula


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−+=

cbBB

Q downMupPpump 2

,, α

( )( )

−+

−+−−×

21

2,,

,,24

11α

α

bBB

CCab

downMupP

downMupP

With the solution for the pump flow rate, the pressure upstream anddownstream of the pump can be obtained from Equations 9.19 and 9.20.

Pumps with higher order curve fitsIf the pump is modeled with a third or fourth order curve, the full speedpump curve is:

432pumppumppumppump eQdQcQbQaH ++++=∆

and with the speed included,

4

4

3

3

2

2

2 αααααpumppumppumppump Q

eQ

dQ

cQ

baH ′+′+′+′+′=∆

Combine this with the compatibility equation as in the previous sectionto obtain:

( ) ( ) pumppumpPPpumpMM mbamBCmBC &&& αα +=−−+ 2

42

32pumppumppump memdmc &&&

αα+++

This equation is then solved by Newton-Raphson iteration to obtain theflow rate, pumpm& .

Backflow through the pumpIf the pump cannot supply enough head to overcome the systemrequirements at some time during the transient, backflow will occurthrough the pump. When this occurs, the pump curve loses meaning.

To address this situation, AFT Impulse makes an approximation. In thecase of backflow it is assumed that the head rise for all negative flow


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rates is equivalent to the head rise at zero flow for the given curve at thegiven speed. With the head rise known, the negative flow rate throughthe pump is obtained by using both compatibility equations with theknown head rise. After the flow rate is known, the pump suction anddischarge pressures are calculated.

To avoid this approximation, a four quadrant model can be used.

Flow rates past the pump maximumIt is possible for the system transients to cause flow rates through thepump greater than the maximum flow from the curve. In such cases,AFT Impulse assumes the head rise is zero and calculates the flow ratefrom the compatibility equations.

To avoid this approximation, a four quadrant model can be used.

Submerged pumpsPumps can also be submerged in a reservoir. In such cases the upstreampressure is known, and the flow rate is obtained from the following:

−=

cbB

m downMpump 2

, α&

( )( )

−

−+−−×

21

2,

,24

11α

α

bB

CPab

downM

downMR

Where PR is the pressure at the pump suction.

Pump with known speed vapor cavitation theoryWhen the calculated pressure at the upstream side of a Pump junctiondrops below vapor pressure, a vapor cavity forms on that side of thejunction.

Usually vapor cavities form on the upstream side of the pump only, butin cases where there is backflow through the pump cavities can alsoform on the downstream side.


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oldupvapornewupvapor VV ,,,, =

( )ρ2,,,,tmmmm oldpumpnewpumpoldupnewup

∆++−−+ &&&&

olddownvapornewdownvapor VV ,,,, =

( )ρ2,,,,tmmmm olddownnewdownoldpumpnewpump

∆++−−+ &&&&

As an example, when a vapor cavity forms upstream of a pump, theupstream pressure becomes fixed at the vapor pressure. The calculationthen is similar to a submerged pump, which also flows from a fixedpressure.

−= 2

,

2 ρ

υα

c

bBm downM

pump&

( )( )

−

−+−−×

21

2,

,24

11ρα

α

bB

CPab

downM

downMsat

Similar to a pipe interior node, when the vapor volume is negative thecavity collapses and the fluid pressure then rises above the vaporpressure.

Modeling pump as with unknown speedThere are four pump models in AFT Impulse 3.0 that account for pumpinertial effects and predict the speed over time. These are:

• Trip With Inertia and No Back Flow or Reverse Speed

• Startup With Inertia and No Back Flow or Reverse Speed

• Trip With Inertia – Four Quadrant

• Startup With Inertia – Four Quadrant, Known Motor Torque/Speed

Predicting the response of a pump during a trip or startup requiresadditional data for the pump. Use of the affinity laws for pumps isessential. If backflow is possible, "four quadrant" pump curve data isrequired. The first two models mentioned above assume the presence of


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a check valve which prevents backflow, or just that backflow neveroccurs. In such cases, the pump will usually operate in the first quadrant.Thus four quadrant data is not required.

The third and fourth models allow backflow, and thus require fourquadrant data. All four models will be discussed in the followingsections.

Trip With Inertia and No Back Flow or Reverse SpeedWhen no backflow or reverse speed occurs through the pump, thenormal pump curve data for flow vs. head can be used to model thepump response. Pump efficiency data is also required as a function offlow.

The method is presented by Watters (1979, p. 169). The unbalancedtorque is given by:

dtdIT ω

−= (9.34)

where T is the torque on the pump, I is the moment of inertia of thepump and entrained liquid, and dω/dt is the change in angular speed withtime.

The pump power is related to the pump torque by:

ωTP = (9.35)

The pump power is related is a function of the flow as specified by theuser. By rearranging and integrating Equation 9.34 one obtains

∫∫ −=T

ddtI

ω1

Assuming an average value of torque over the time step one obtains:

ω∆−=∆

TIt 1

With the speed known at the previous time step, one can solve for thenew speed as

ItToldnew

∆−= ωω


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Adding in the definition of torque from Equation 9.35

ItTT oldnew

oldnew∆+

−=2

ωω (9.36)

The old torque is known from the previous step, and thus the new speedcan be solved for by iteration. The mass flow rate and head are obtainedfrom Equations 9.19 and 9.20 (with the pressure converted to head) andthe pump curve supplied by the user.

Startup With Inertia and No Back Flow or Reverse SpeedThis model uses a similar approach to that just discussed, but adds motortorque into the calculation. This is described, for example, in Brown &Rogers (1980).

dtdITT fluidmotor

ω=−

Integrating over a time step:

ItTTTT oldfluidnewfluidoldmotornewmotor

oldnew

∆

+−

+

+=

22,,,,

ωω

(9.37)

The “old” motor torque and fluid torque at the previous time step areknown, and the values at the new time step are both functions of the newspeed. The fluid torque is related to the power through Equation 9.35,and the power can be obtained from the new flow.

The motor torque is entered by the user as a function of speed (seeFigures 9.3 and 9.4), and is therefore known at the new speed. Thisprocess thus requires iteration.

This calculation uses the user provided pump data for head vs. flow andpower vs. flow, and thus only works for positive or zero flows. The massflow rate and head are obtained from Equations 9.19 and 9.20 (with thepressure converted to head) and the pump curve supplied by the user.

The typical shape of a motor torque vs. speed curve is shown in Karassik(2001, p. 8.25, Figure 26), which is reflected in the Figure 9.4 data.


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Figure 9.3 Motor torque vs. speed data is used for startupmodeling


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Figure 9.4 Example graph of motor torque vs. speed

Trip With Inertia – Four QuadrantThe method used for modeling a pump trip is described in mostwaterhammer textbooks (Chaudhry, 1987, Chapter 4, Swaffield, et al.,1993, pp. 154-166, Wylie et al., 1993, Chapter 7). From the conventionalmethod, four dimensionless parameters are defined:

R

RR

R

RR

NN

mm

QQv

TT

PP

HHh

=

==

=

∆∆

=∆∆

=

α

β

&

&(9.38)


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where the R subscript refers to rated conditions, usually those at the bestefficiency point. In summary, h is the ratio of the pump head to the ratedhead, β is the ratio of the pump torque to the rated torque, v is the ratioof the flowrate to the rated flowrate, and α is the ratio of thesynchronous speed to the rated synchronous speed.

Figure 9.5 Pump window with pump data entered for specificspeed of 0.46.

Two further dimensionless parameters are created to relate theseparameters:

22

22

)(

)(

vF

vhF

B

H

+=

+=

αβθ

αθ

(9.39a)

where


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απθ v1tan−+= (9.39b)

Some references choose to define θ slightly differently:

vαθ 1tan−=

AFT Impulse allows you to select either definition as the basis for the FHand FB data in Equation 9.39a.

The parameters FH and FB are properties of the pump, and it is typicallyassumed that for pumps of similar specific speed they are invariant (seeBrown & Rogers for a discussion questioning this assumption). Figures9.5 and 9.6 show data for a pump with a specific speed of 0.46 as givenby Wylie, et al., (1993, pp. 147-148).

Figure 9.6 Variation of FH and FB for a pump with specific speedof 0.46 (see Wylie, et al., 1993).


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AFT Impulse provides data of FH and FB vs. θ for pumps of twenty-onespecific speeds, as given by Wylie, et al., which was derived fromDonsky (1961). See Table 9.1 for a list of all specific speeds available.

If you should need to construct data for other sets, please consult thethree textbooks referenced above for additional information. Note thatthe textbooks do not give a method for constructing these data sets, butthey contain references that you can pursue.

Table 9.1 Four quadrant data available in AFT Impulse

-- Specific Speed -- Reference(-) (gpm units) (metric units)0.3 810 15.7 Brown & Rogers, Partial, From Nevada Test, 1980

0.39 1060 20.5 Kittredge, 19560.42 1140 22.1 Brown & Rogers, from Bureau of Reclamation, 19800.46 1270 24 Donsky, 19610.48 1320 25.5 Brown & Rogers, Partial, From Nevada Test, 19800.55 1490 28.8 Brown & Rogers, Partial, From Nevada Test, 19800.57 1565 30.3 Brown & Rogers, Partial, From Nevada Test, 19800.71 1935 37.4 Kittredge, 19560.79 2160 41.8 Thorley, 19961.21 3300 64 Thorley, 19961.36 3725 71.9 Thorley, 19961.44 3940 76.1 Thorley, 19961.61 4400 85.1 Brown & Rogers, from Bureau of Reclamation, 19801.83 5000 97 Thorley, 19961.9 5200 101 Thorley, 1996

2.32 6340 123 Thorley, 19962.48 6790 131 Thorley, 19962.54 6940 134 Thorley, 19962.78 7600 147 Donsky, 19613.2 8760 170 Thorley, 1996

4.94 13500 261 Donsky, 1961

With data for FH and FB, the compatibility equations (9.19, 9.20) areused as follows:

( ) pumpPMPMpump mBBCCP &++−=∆ (9.40)

But from Equations 9.38 and 9.39,

( ) HRpump FvPP 22 +=∆ α (9.41)


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Rpump mvm && =

Substituting Equations 9.41 into 9.40 obtains:

( ) ( ) 022 =+++−− HRMPRMP FvPBBmvCC α& (9.42)

Since the value of FH depends on θ, which depends on α and v (seeEquation 9.39b), iteration is required to find the appropriate value fromthe tabulated data supplied (e.g., Figure 9.5).

We need one more equation to solve for all parameters in Equation 9.38.We can get this from the pump torque. The change in rotational speedwith torque was given in Equation 9.34. Defining several more terms,

R

R

R

TTTT

N

=

=

=

β

β

απω

00

602

where NR is the rated speed in rpm, and the "0" subscript refers toconditions at the beginning of the time step.

We can then write,

( )0602

602 ααπαπω −== RR NdNd

Equation 9.34 can be written

dtdITTT ω

−=+

=2

0 (9.43)

Substituting these terms into Equation 9.43 obtains

( ) 015 0 =−

∆−+ ααπββ

tTNI

R

Ro (9.44)

Using Equations 9.38 and 9.39 in Equation 9.44 obtains


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( ) ( ) 015 0

22 =−∆

−++ ααπβαtT

NIFvR

RoB (9.45)

Equations 9.42 and 9.45 are solved simultaneously using a Newton-Raphson iterative method. Details are given in Wylie, et al., 1993, pp.149-152.

Startup With Inertia – Four Quadrant, Known MotorTorque/SpeedFor pump startup where backwards flow occurs, four quadrant data canbe used along with the known motor torque vs. speed to calculate thetime to reach full speed. This model is especially useful for cases wherea pump is being started with backflow already established.

The methodology is similar to that just discussed Trip With Inertia –Four Quadrant section. However, Equations 9.43, 9.44 and 9.45 must bemodified to include the motor torque. Including the motor torquechanges these equations as follows:

dtdI

TTTTTT ffmm

fmω

=+

−+

=−22

0,0,

where the m subscript refers to the motor torque and the f subscriptrefers to the fluid torque. Equation 9.44 becomes:

( ) 015 0,, =−

∆−−−+ ααπββββ

tTNI

R

Rommoff

and Equation 9.45 becomes:

( ) ( ) 015 0,,

22 =−∆

−−−++ ααπβββαtT

NIFvR

RommofB

Estimating the pump inertiaObtaining data for rotational moment of inertia is difficult andfrequently requires estimation. Wylie et al., 1993, reports a method forestimating the moment of inertia, I, for a pump and the entrained liquid.The moment of inertia for the pump impeller including entrained wateris estimated as:


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( ) 9556.0

37105.1

=

NPI p (9.46)

where P is the power in kW and N is the rotational speed in rpm.

An estimate of the motor rotational inertia is

48.1118

=

NPIm (9.47)

The total moment of inertia is the sum of the two.

Applied Flow Technology provides an Excel spreadsheet which usesthese correlations to predict the inertia.

Liquid Accumulator theory

A liquid accumulator, also called a lumped capacitance, models a liquid-filled container in a system (Wylie, et al., 1993, pp. 124-125). Theinteraction with the pipe system occurs as the container stretches andcontracts in response to transient pressures.

The liquid accumulator is similar to a branch, and thus the sameequations are involved.

B

newACnewj S

mSP ,

,&+

=

where SC and SB are given by Equations 9.27 and 9.28. The relationshipbetween accumulator pressure and volume is assumed to be linear,

VVPK

∆∆

=′

where K’ is the effective bulk modulus of the container, and has units ofpressure. The change in volume can be related to flow rate as follows:

( )ρ2,,tmmV oldAnewA

∆+−=∆ &&

Therefore,


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( )ρV

mmtKPP oldAnewA

oldjnewj 2,,

,,&& +∆′

−=

Substituting and eliminating flow rate

tKVS

mtK

VPSP

B

oldAoldjCnewj

∆′+

−∆′

+= ρ

ρ

2

2,,

,

&

Liquid Accumulator vapor cavitation theoryWhen the calculated pressure at a Liquid Accumulator junction dropsbelow vapor pressure, a vapor cavity forms at the junction.

The vapor volume calculation is as follows:

( )( )

( ) ρ2,,

,,

,,

,,t

mmmm

mm

VV

oldAnewA

oldoutnewout

oldinnewin

oldvapornewvapor∆

+−

++

−−

+= ∑∑

&&

&&

&&

where Am& is the accumulator flow out of the junction. The inm& termsare obtained from the positive compatibility equation, while the

outm& terms are obtained from the negative equation.


Relief Valve theory

Relief valve junctions stay closed until the cracking pressure is reached,at which point it will open and relieve the pressure. The valve canrelieve either to an outside ambient pressure or into another pipe in thesystem. There are therefore two models required to address each case.


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Internal relief valveAn internal relief valve is similar to a regular valve except that itremains closed until the cracking pressure is exceeded. One significantdifference is if the Variable Cv model is used. This is distinct to therelief valve junction.

Exit relief valveAn exit relief valve is like a regular exit valve. When the crackingpressure is exceeded it relieves to the exit ambient.

B

newexitCnewj S

mSP ,

,&−

=


The pressure drop across the exit is

221 VP ρ=∆

or,

2,2,

2newexit

exitexitnewj m

AKPPP &

ρ=−=∆

Substituting

2,2

,

2newexit

exitexit

B

newexitC mAKP

SmS

&&

ρ=−

−

This is a quadratic equation in which newexitm ,& can be obtained from thequadratic formula.

02212,2

2, =

−++

B

Cexit

exitnewexit

exitBnewexit S

SPA

KmA

KS

m ρ&&

If using the variable Cv model, the K factor above itself becomes afunction of ∆P, which requires iteration to solve for the mass flow rate.


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Inline Relief ValveWhen this valve opens, it is similar to a Spray Discharge junction withtwo pipes connected. If the Variable Cv model is used, the solutionrequires iteration.

Relief Valve vapor cavitation theory

Internal relief valvesInternal Relief Valve junction vapor cavitation is identical to that of aregular valve. Initially the valve is closed and acts like a regular valve. Ifit cracks open, then it acts like an open valve.

Exit relief valvesExit Relief Valve junction vapor cavitation acts like a dead end junction.If the valve cracks, it acts like a spray discharge junction.

Check Valve theory

A check valve is identical to a regular valve, except that it closes whenreverse flow occurs.

Check Valve vapor cavitation theoryThe check valve vapor cavitation model is identical to a regular valve,except that it closes when reverse flow occurs.

Control Valve theory

We can use the methods for known flow or pressure to calculate theconditions at a control valve. AFT Impulse flow and pressure controlvalves allow the control setting to be changed with time. Whetherconstant or changing, the flow or pressure is known and can be used tosolve the equations.


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Flow control valvesWith the known flowrate, one can substitute into Equations 9.19 and9.20 and solve for the upstream and downstream pressures, respectively,at the valve. These can then be compared to the failure settings to see ifloss of control occurs. For instance, if the upstream pressure was lessthan the downstream, that would indicate loss of control.

Once the valve loses control, it acts like a regular valve as discussedearlier in the chapter.

Pressure control valvesPressure control can be specified on the upstream or downstream side ofthe valve. A discussion of one these control locations will apply to theother. Let's consider the downstream pressure control case – a PressureReducing Valve (PRV).

With the known pressure downstream, one can substitute the knownpressure into Equation 9.20 and solve for the flowrate through the valve.This flowrate can then be substituted into Equation 9.19 to solve for theupstream pressure. The upstream pressure can then be compared to thecontrol pressure and failure settings to see if loss of control occurs. Forinstance, if the upstream pressure was less than the control pressure, thatwould indicate loss of control.

Once the valve loses control, it acts like a regular valve as discussedearlier in the chapter.

Pressure drop control valvesThe flow through a constant pressure drop is obtained by subtractingEquation 9.20 from 9.19 and solving for flowrate.

MP

MPnewi BB

PCCm+

∆−−=,&

Surge Tank theory

A surge tank is an open tank to the atmosphere (Figure 9.7) and is usedas a surge suppression device (Wylie, et al., 1993, pp. 128-129).


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The flow balance on the surge tank junction is similar to a branchjunction, with the deficit equaling the flow out of the surge tank, Tm& .

Similar to a branch, it can be shown that for all connecting pipes to thejunction

B

newTCnewj S

mSP ,

,&+

=


LL

LC

ConnectorPipe

Orifice

Tm&

Figure 9.7 Surge tank schematic

The optional connector pipe and orifice effect is incorporated by thelumped inertia relationship as follows:

newTnewLsurfacenewj mCCgLPP ,21,, &−−+= ρ

where C1 and C2 are defined in conjunction with lumped inertia. Referto Equations 9-50 and 9-51 later in this chapter.

The new tank liquid height can be related to the old height by thefollowing:

tA

mmLL

T

oldTnewToldLnewL ∆

+−=

ρ2,,

,,&&

Substitution yields


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∆+−+−

+

T

oldToldLsurface

B

newTCA

tmLgP

SmS

ρρ

2,

,, &&

02 ,21, =−−

∆+ newT

T

newT mCCA

tmg &&

Solving for newTm ,&

+∆+−

+

∆−−−

=

BT

T

oldToldLsurface

B

C

newT

SAtgC

CA

tmLgP

SS

m1

2

2

2

1,

,

,ρ

ρ&

&

This flow rate can be back substituted to obtain the junction pressure andsurge tank liquid height.

Surge Tank vapor cavitation theoryAny short-lived vapor cavities that develop at a Surge Tank junction areassumed to have a negligible effect compared to the surge tank itself.Conceptually, vapor bubbles would rise by buoyancy to the surge tanksurface and evaporate at the surface.

Therefore, vapor cavitation is ignored at surge tanks.

Gas Accumulator theory

A gas accumulator (shown in Figure 9.8) is an enclosed tank filled withgas and has a free surface. It is used as a surge suppressor (Wylie, et al.,1993, pp. 125-128).

The flow balance on the accumulator junction is similar to a branchjunction, with the deficit equaling the flow out of the accumulator, Am& .

Similar to a branch, it can be shown that for all connecting pipes to thejunction,

B

newACnewj S

mSP ,

,&+

=


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The potential connector pipe and orifice effect is incorporated by thelumped inertia relationship as follows

newAnewAnewj mCCPP ,21,, &−−=

where C1 and C2 are defined in conjunction with lumped inertia. Referto Equations 9-50 and 9-51 later in this chapter.

LC

ConnectorPipe

Orifice

PA

Am&

Figure 9.8 Gas accumulator schematic

The gas volume and pressure in the accumulator can be related by thethermodynamic law

An

AA CVP =

where n is the polytropic constant (1.4 for isentropic air, 1.0 forisothermal) and CA is a constant.

Therefore,

An

newAnewA CVP =,,

and

( )ρ2,,,,tmmVV oldAnewAoldAnewA

∆+−= &&


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By substitution

newAnewAB

newAC mCCPSmS

,21,, &

&−−=

+

or

newAB

newACnewA mCC

SmS

P ,21,

, &&

+++

=

Combining equations and simplifying,

( )( ) An

newAnewA CmCCmCC =++ ,65,43 &&

where

ρ

ρ

2

2

1

6

,,5

24

13

tC

tmVC

CS

C

CSSC

oldAoldA

B

B

C

∆=

∆+=

+=

+=

&

This is a non-linear equation that can be solved using Newton-Raphsoniteration. Newton-Raphson has the following form:

( )( )i

iii xF

xFxx′

−=+1

where xi is the current value of x, xi+1 is the new value, F is the functionof x to drive to zero, and F´ is the derivative.

In this case

( )( ) An

newAnewA CmCCmCCF −++= ,65,43 &&

When the value of newAm ,& has been found that causes F to equal zero,the equation will be satisfied. To solve by Newton-Raphson thederivative of F is given by,


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( )nnewAnewAnewA

mCCCmCC

CFmd

dF,654

,65

7

,&

&&++

+=′=

where

67 CnCC A−=

By iteration, newAm ,& is calculated. This value can then be back-substituted to obtain the new junction pressure and new gas volume.

Gas Accumulator vapor cavitation theoryAny short-lived vapor cavities that develop at a Gas Accumulatorjunction are assumed to have a negligible effect compared to theaccumulator itself. Conceptually, vapor bubbles would rise by buoyancyto the accumulator surface and evaporate into the accumulator gas. Theireffect on the system would thus be negligible compared to that of theaccumulator gas. Therefore, vapor cavitation is ignored at gasaccumulators.

Vacuum Breaker Valve theory

A vacuum breaker valve (sometimes called an air inlet valve) allows airto flow into the pipe during low pressure events, and expels the air whenthe pressure rises again. Typically the cracking pressure is atmospheric.The theory is summarized in Wylie, et al., 1993, pp. 130-131, anddetailed here.

There are four cases that can occur – two for inflow and two for outflow.The inflow can be subsonic or sonic, as can the outflow. These make upthe four cases. The flowrate equations are given as follows:

Sonic inflow( )( )γγγγ

−+

−

+=12)1(

211

o

oindRT

PACm& (9.48a)


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Subsonic inflow

( )

−

−

=+ γγγ

γγ

12

12

o

i

o

i

o

oindPP

PP

RTPACm& (9.48b)

Sonic outflow( )( )γγγγ

−+

−

+=12)1(

211

L

ioutdRT

PACm& (9.48c)

Subsonic outflow

( )

−

−

=+−− γγγ

γγ

12

12

o

i

o

i

L

ioutdPP

PP

RTPAC

m& (9.48d)

where:

Pi = pressure in the pipe at the junction node

Po = atmospheric pressure

To = atmospheric temperature

TL = liquid temperature

CdAin = flow area for inflow

CdAout = flow area for outflow

R = gas constant

γ = specific heat ratio cp/cv

Equation of stateBesides the compatibility equations (9.19 and 9.20), we need anotherequation to form a complete set. The ideal gas equation of state is used:

mRTVP =

Substituting terms one obtains


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( )

( )

+

∆+

=

−−+

∆+

oldgasnewgasold

oldupnewupolddownnewdownoldL

i

mmtm

QQQQtVRTP

,,

,,,,

2

2

&&

Eliminating the volumetric flowrate using the mass flowrate obtains

( )

++

∆

=

−−++

∆

oldgasnewgasold

oldupnewupolddownnewdownL

old

L

i

mmt

m

mmmmt

VRTP

,,

,,,,

2

12

&&

&&&&ρ

Using Equations 9.19 and 9.20 to eliminate mass flowrate yields:

++

∆

=

−

−−+

−+

∆

newgasoldgasold

oldupP

iPolddown

M

Mi

L

old

L

i

mmt

m

mB

PCmB

CPt

VRTP

,,

,,

2

12

&&

&&ρ

(9.49)

Now we can use Equations 9.48 to eliminate newgasm ,& as follows:

Sonic InflowThe pressure at the junction can be solved directly from Equation 9.49 asfollows:

02

2

112

=

++

∆−

∆+−+−−+

+

sonicgasold

L

old

L

u

L

d

PL

P

ML

Mi

PLMLi

mmt

mRT

tVmm

BC

BCP

BBP

old

oldold

&&

&&

ρρρρ

ρρ


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where sonicm& is the sonic inflow which can be calculated from Equation9.49a based completely on input data.

Subsonic InflowEquation 9.49 is modified using Equation 9.48b and solved iterativelyusing Newton-Raphson. Defining Pr = Pi/Po obtains:

( )( )( ) 0

12

2

2

11

12

22

=−

−

+

+

∆−

−+−−

∆+

+

+ γγγγ

γ

ρρρρ

ρρ

rro

Loind

gasold

L

L

u

L

d

PL

P

ML

Moldor

PLMLor

PPT

RTPAC

mt

mRT

mmB

CB

Ct

VPP

BBPP

old

oldold

&

&&

Defining terms for Newton-Raphson solution yields


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( ) FCPCPCPPC rrb

ra

r =+++− 432

25.0

1

( ) ( )r

rb

ra

rb

ra

r dPdFCPCbPaPPPC =++−− −−−

32115.0

1 25.0

( )

+

∆−=

−+−−

∆=

+=

−=

+=

=

old

oldold

gasold

L

L

u

L

d

PL

P

ML

Moldo

PLMLo

oLoind

mt

mRTC

mmB

CB

Ct

VPC

BBPC

TRTPACC

b

a

&

&&

2

2

11

12

1

2

4

3

22

1

ρρρρ

ρρ

γγ

γγγ

where the function F is solved for Pr.

Subsonic OutflowEquation 9.49 is modified using Equation 9.48d and solved iterativelyusing Newton-Raphson. Defining Pr = Pi/Po obtains:


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( ) ( )[ ] 01

2

2

2

11

12

22

=−−

+

+

∆−

−+−−

∆+

+

+−− γγγγγ

ρρρρ

ρρ

rrL

ooutDr

gasold

L

L

u

L

d

PL

P

ML

Moldor

PLMLor

PPRTPACP

mt

mRT

mmB

CB

Ct

VPP

BBPP

old

oldold

&

&&

Defining terms for Newton-Raphson solution yields

( ) FP

CCPCPPCr

rb

ra

r =+++− −− 1432

5.01

( ) ( )

rr

br

ar

br

arr

dPdF

PCC

bPaPPPPC

=−+

+−− −−−−−−−

242

115.01

15.0

+

∆−=

−+−−

∆=

+=

−=

+=

=

old

oldold

gasold

L

L

u

L

d

PL

P

ML

Moldo

PLMLo

Looutd

mt

mRTC

mmB

CB

Ct

VPC

BBPC

RTPACC

b

a

&

&&

2

2

11

12

1

2

4

3

22

1

ρρρρ

ρρ

γγ

γγγ

where the function F is solved for Pr.


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Sonic OutflowUsing Equation 9.48c, the pressure at the junction can be solved directlyfrom Equation 9.49 as follows:

( ) ( )( )

02

211

2

11

121

2

=

+

∆−

−

++

−+−−

∆+

+

−+

old

oldold

gasold

L

LoutDi

L

u

L

d

PL

P

ML

Moldi

PLMLi

mt

mRT

RTACP

mmB

CB

Ct

VP

BBP

&

&&

γγγγ

ρρρρ

ρρ

Lumped inertia pipe with orifice theory

AFT Impulse uses a pipe with lumped inertia (Wylie, et al., 1993, pp.122-124) as a connector for some elements, although it is not a completeelement by itself. Such an element may also include an orifice (or acheck valve, which will be treated as an orifice) as shown in Figure 9.9.

1

3

2

orifice

∆z

Figure 9.9 Lumped inertia with friction schematic


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Summing forces acting on the lumped section, 2,

dtdVLAFFFFF gorificefriction

22231 ρ=+−−−

or,

dtmdLFFFFF gorificefriction

2231

&=+−−−

Now,

zgAF

AAC

mF

AA

mDfLF

APFAPF

g

oDorifice

friction

∆=

=

=

==

2

222

22

222

22

2

2

233

211

2

2

ρρ

ρ

&

&

Substituting terms and dividing by area A2

dtmd

ALgzm

ACm

ADfLPP

oD

2

2

2222

222

22

231 22

12

1 &&& =−−−− ρ

ρρ

Taking average values over time, and denoting current time by a prime,

22222

233112

122

mmAD

fLPPPP&& ′−

+′−

+′

ρ( )

tmm

ALgzmm

AC oD ∆−′

=−′− 22

2

222222

1 &&&& ρ

ρ

Collecting terms and simplifying

22131 mCCPP ′+=′−′ &

where


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22

2131

22 mtA

LgzPPC &∆

−+−= ρ (9.50)

tALm

ACm

ADfLC

oD ∆++=

2

222222

22

22

211&&

ρρ(9.51)

If a connector pipe exists but no orifice then

tALm

ADfLC

mtA

LgzPPC

∆+=

∆−+−=

2

222

22

22

22

2131

21

22

&

&

ρ

ρ

If an orifice exists but no connector pipe

2222

1311 m

ACC

PPC

oD&

ρ=

−=

If neither exist then

00

2

1==

CC

Interpreting cavitation results

Transient cavitation is a highly complex phenomenon. AFT Impulse usesthe Discrete Vapor Cavity (DVC) Model, which is a fairly simplerepresentation of the phenomenon (Wylie, et al., 1993, pp. 67-69). Whenvapor pockets form in a system, the fluid is no longer continuous andthus the wavespeed decreases. Neglecting the wavespeed changecompromises the results to some degree.

In addition, the DVC Model assumes that the vapor cavities form at thecomputing stations, rather than being distributed along a pipe. This alsois an approximation. The DVC Model implicitly assumes that thecavities are small compared to the volume of the pipe section and thatthey do not move during the simulation. If the cavities grow as much as10% of the computing volume, the results are further compromised. If


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the pipe is at an angle or vertical, the vapor bubbles can move due tobuoyancy. The importance of this can be checked by comparing thepredicted bubble lifetime to the time it would take bubbles to movebetween computing stations (assuming a typical bubble velocity).

A result of the DVC Model is that when cavities form and collapse,numerous non-physical pressure spikes occur which complicates datainterpretation. The interpretation thus requires considerable engineeringjudgment.

Here are some general guidelines analysts can follow:

1. The first one or two pressure spikes after cavity collapse tend to bethe most reliable. Fortunately, these spikes are usually the worst andtherefore sufficient engineering data is available to draw aconclusion.

2. After the first one or two spikes, the non-physical pressure spikesfrequently begin. Sharp spikes that appear to occur for only a singletime step can usually be ignored. Rather, attention should be focusedon the major trends of the pressure results. These tend to be wheremore pressure solutions are grouped together, rather than isolatedspikes.

3. Pay attention to the maximum vapor volumes in relation to thecomputing volume. If the vapor volume percentage grows to morethan 10% then be more cautious in the interpretation.

4. The Method of Characteristics method without cavitation is areliable and accurate solution method. However, after cavitationoccurs the reliability of the predictions decreases significantly. Thereason for offering the DVC Model is that, even with itsdeficiencies, it offers significantly better information than would beobtained if cavitation were to be ignored. Used with the abovecautions, the DVC Model in AFT Impulse can be used successfullyto model many systems that experience cavitation.

Instantaneous waterhammer equation exceptions

The instantaneous waterhammer equation predicts the theoreticalmaximum pressure rise in a pipe. It was discussed in Chapter 3, and herewe will discuss cases where pressure surge can exceed that predicted bythe equation.


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Wylie, et al., (1993, pp. 4) gives the instantaneous waterhammerequation as:

VaP ∆−=∆ ρ (9.52)

where:


ρ = density

a = wavespeed


To apply this equation, one would typically determine the steady-statevelocity and assume the flow stops suddenly. The -∆V is thus equal tothe steady flow velocity.

Following is a description of cases where the pressure surge can exceedthat predicted by the instantaneous waterhammer equation.

CavitationWhen a positive pressure surge occurs, it is always followed by negativepressure surge that drops the pressure. If the drop in pressure causescavitation, then the pressure spike that occurs when the cavity collapsescan cause the local pressure to exceed that predicted by Equation 9.52.

The potential to cavitate can be assessed by comparing the pressure risein Equation 9.52 to the steady-state pressure. If the pressure surge isgreater than the steady-state pressure, then cavitation is possible. If it isless than the steady-state pressure and the fluid has a low vapor pressure(like water), then it will probably not cavitate. If the fluid has high vaporpressure, this will need to be subtracted from the steady-state pressureand then compared to the pressure surge.

Pipe size changesWhen using Equation 9.52, the case considered is usually a valveclosure. Equation 9.52 is frequently applied with the ∆V obtained basedon the pipe diameter at the valve. However, if smaller diameter pipesexist upstream that carry the same flowrate, their ∆V will be largerbecause they have less area. This will result in a larger pressure surge inthe smaller pipe.


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To be safe, Equation 9.52 needs to be applied to the pipe with the largest∆V potential in the entire system.

Multiple pressure surge sourcesIf there is potential for multiple elements to generate pressure surge atthe same time, the surge pressures can interact and add together, thusexceeding the individual surge pressures.

Line pack due frictionIn systems with high friction, pressure can increase above theinstantaneous values because of something called line pack. Evensystems with modest friction can experience this. In brief, the friction inthe system prevents the flow from coming to a complete stop in casessuch as a closing valve. With the closed valve, the fluid continues tomove towards the closed valve, albeit at a lower velocity, increasingpressures above the instantaneous prediction. See Wylie (1993) for moreinformation.


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C H A P T E R 1 0

Modeling Time and Event BasedTransients

In general, transients can be initiated in one of two ways. They can beinitiated according to a prescribed time schedule or according to certainevents that happen in the system. These two categories are referred to astime-based and event-based transients.

Event-based transients break down further into three categories: Singleevent, cyclic dual events and sequential dual events. The behavior oftime-based and the three event-based transients are described in thischapter.

Time-based transients

Time-based transients occur in accordance with the time of the transientsimulation. The start and stop times of the simulation are specified in theTransient Control window. The initiation of time-based transients arepre-specified before a model is run.

For example, consider a valve closure transient. Assume the valve startsto close at two seconds into the simulation, and the time it takes for thevalve to close is one second. After that, the valve stays closed. If theinitial valve Cv is 250, the transient would appear as in Figure 10.1.

In the "Initiation of Transient" area there are four options. For time-based transients, the option is specified as Time. In the Transient Dataarea the data is entered. Here the Cv profile of the valve will follow theprofile entered and start to close at 2 seconds.


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Figure 10.1 Time-based transient specifies the transient behaviorbefore simulation is run.

Event-based transients

Event transients are initiated when some user specified criteria is met.For example, a valve can start to close when a certain pressure is reachedat a point in the system. If the pressure is never reached, the transient isnever initiated.

Event transients are one of three types. The three types will be discussedin the following sections.

Single event transientsTo specify a Single Event transient, select Single Event in the Initiationof Transient area of the junction window (see Figure 10.2). An "Event"tab will appear where the event criteria is entered. In Figure 10.2, the


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criteria is such that the transient will be initiated when the outlet staticpressure in Pipe 2 exceeds 150 psig. If the pressure at this location neverreaches 150 psig, the transient specified in the Transient Data area willnot occur.

Figure 10.2 Single event transient specifies how a junction willrespond if and when some condition is satisfied.

The data in the Transient Data table has a slightly different meaning thandoes a time-based transient. Here time zero is relative to the time atwhich the event criteria is first met. For example, if Pipe 2 reaches 150psig at 3.65 seconds, the valve will start its transient at 3.65 seconds.The valve will close (i.e., Cv = 0) at 1 second after the event initiation,or 4.65 seconds.

Dual event transients: cyclic and sequentialDual event transients contain two different transients for the junction.


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Cyclic eventsCyclic dual events can repeatedly switch from one to the other based onthe behavior of the model. For example, these could be high and lowpressures at some location. When the low pressure setting is reached avalve transient is initiated which closes the valve. The pressure may thenbuild up to a point where the high pressure setting is reached. In thiscase a second valve transient is initiated which opens the valve. Shouldthe pressure drop to the low pressure setting again, the valve will againclose. The two settings will continue to cycle the valve on and off untilthe end of the simulation.

Cyclic dual events are specified by selecting Dual Event Cyclic in theInitiation of Transient area of the junction window (Figure 10.3). Whenselected, two event tabs appear. Here the first and second event can bespecified. In this case the first event is a pressure at Pipe 2 inlet less than50 psig. The transient that is initiated when this event occurs is in thetable of the First Transient tab in Figure 10.3. Here the valve closes onesecond after the event.

A second event, not shown, is specified on the Second Event tab, withthe accompanying transient specified on the Second Transient tab, alsonot shown. Typically, the first data of the second transient should matchthe final data of the first transient. In other words, if the valve is closedas in Figure 10.3 by setting the Cv value to 0, the first data point of thesecond transient should be zero and then proceed to some higher valuewhich represents the opening of the valve.

More often than not the two events will specify the same parameter atthe same location. For example, if the first event specified a pressure at acertain pipe, the second event would also specify pressure at the samepipe location. However, this is not required. You can specify the firstevent as pressure and the second as flowrate, for example, or you couldspecify the two events to be at different locations in the pipe system.


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Figure 10.3 Dual Event Cyclic selection for transient initiation

Sequential eventsSequential dual events progress from the first event to the second and nofurther. For example, these could be two high pressure settings. Whenthe first setting is reached the valve opens partially, and when the secondsetting is reached, which would likely be at a higher pressure than thefirst event, the valve opens the rest of the way. Similar to a single eventtransient, if the pressure drops and then rises again, the events would notbe initiated again.

Sequential dual events are specified by selecting Dual Event Sequentialin the Initiation of Transient area of the junction window (Figure 10.3).Similar to Cyclic Dual Events, two event tabs appear where the first andsecond event can be specified. The transients that are initiated when theevents occur are specified in the table of the First Transient and SecondTransient tab.


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Similar to Cyclic Dual Events, the two events do not have to be for thesame parameter (e.g., pressure) or at the same location.

Junctions with inherent event logic

There are three junction types in AFT Impulse which have built-in, orinherent, event logic. These are the check valve, relief valve and vacuumbreaker valve. The user does not need to specify the nature of the events,and in fact is not allowed to.

The inherent event logic is very similar to the Dual Event Cyclic logicdescribed previously.

Check valveThe check valve has two built-in events. The first is that it closes whenbackflow starts to occur. The second is that it reopens when sufficientpressure differential occurs. If the user does not enter a closing oropening transient in the check valve window, these transients areassumed to be instantaneous.

Relief valveThe relief valve has two built-in events. The first is that it opens whenthe cracking pressure is reached. The second is that it closes again whenthe pressure falls back below the cracking pressure. If the user does notenter an opening or closing transient in the relief valve window, thesetransients are assumed to be instantaneous.

Vacuum breaker valveThe vacuum breaker valve is very similar to the relief valve, and has twobuilt-in events. The first is that it opens when the set pressure is reached.The second is that it closes again when the pressure rises above the setpressure and all gas is expelled.


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Thought experiment to further clarify event transients

To further clarify the capabilities of event transient modeling, here wewill discuss how one could use a regular AFT Impulse Valve junction tosimulate a check valve.

As discussed previously, check valves have inherent event logic. Thatlogic assumes that the valve closing transient is initiated when backflowbegins. Once a certain pressure differential exists across the closedcheck valve, the valve reopens.

One could use a regular valve junction to simulate a check valve asfollows. First, the valve would need to be specified as Dual Event Cyclicin the Initiation of Transient area. The first event would be a flowratethat is less than or equal to zero at the inlet of the valve's discharge pipe.A Cv transient could then be entered to simulate how the check valvecloses.

The second event would be a differential pressure across the check valvethat is greater than or equal to the check valve's reopening pressuredifferential. A reopening Cv profile could be entered in the secondtransient data table.

With the above considerations, the valve junction and check valvejunction would respond identically.

Other transient features

Absolute vs. relative transient dataIn the Transient Data area (see Figure 10.1, 10.2 or 10.3) it can be seenthat there are two options: "Absolute Values" and "Relative To Steady-State Values". In the case of a valve junction, the first option allows youto put in actual Cv values (for example) vs. time. The second optionallows you to put in a percentage of the steady-state Cv value. While notshown here, the steady-state value is entered on the Loss Model tab.

For junctions other than valves, different parameters are specified as thetransient parameter. For example, the transient could be the steady-statepump speed, pump flowrate, spray discharge area, or boundary pressure.


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Repeat transientFor transients that are periodic, you can specify that the transient repeatsitself. This is specified by selecting the "Repeat Transient" checkbox(see Figure 10.1, 10.2 or 10.3). When Repeat Transient is selected, thefirst and last data points must match. After the transient reaches the finaldata point, it returns to the first data point and starts over.

Add time offsetThere may be times you want to shift the time data points in the transientdata table. For instance, assume you have a valve specified as a time-based transient where the valve starts to close after five seconds. Furtherassume that you wish to change the transient so that it starts to closeafter seven seconds. In such a case, you want to shift all time values bytwo seconds. If you select the Edit Table button (see Figure 10.1, 10.2 or10.3), there is menu choice for Add Time Offset. If you select this, youare prompted to enter an offset time, and all times in the table will shiftby the specified time.

Graphing the transient dataIt is helpful to see the transient data in graph form, and this is easilydone by selecting the Show Graph button (see Figure 10.1, 10.2 or 10.3).

Event messages

Events that are initiated during the simulation are displayed in the twoEvent Messages area of the Output window (see Figure 10.4). The firstis a list of event messages sorted by junction. The second is a list ofevent messages sorted by time. If no events occur, the Event Messagestab will be hidden.


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Figure 10.4 The Event Messages area shows a listing of all eventsthat occur during the simulation sorted by junction andtime.

Transient indicators on the Workspace

To help the user quickly see which junctions are specified with transientdata, a "T" is prepended to the junction number on the Workspace.Figure 10.5 shows an example where it is seen the junction J7 hastransient data.


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Figure 10.5 Junctions that have transient are shown with anadjacent "T" on the Workspace as J7 is shown here.

Transient data in Model Data

All transient junction data can be viewed in the Model Data windowjunction area on the Transient Data tab (see Figure 10.6). In addition, ifevents are specified the event data will also be displayed.


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Figure 10.6 All junction transient data is shown in the Model Datawindow on the Transient Data tab.


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C H A P T E R 1 1

Reducing Surge Pressures

This chapter provides information on different options for reducingsurge pressures in piping systems.

General considerations

Surge pressures that are excessively high can burst pipes and damageequipment. Surge pressures that reduce the local pressure can result inpipes being crushed (due to atmospheric pressure exceeding the internalliquid pressure), cavitation and liquid column separation (which can thenresult in large pressure spikes when the cavity collapses), andsubatmospheric pressures that are unacceptable for drinking waterpipelines.

The surge pressures result from changing conditions in the system,which change the fluid velocity. Options include slowing down thesystem changes and thus reducing velocity or providing locations whichcan deliver extra fluid when the pressure decreases or accept extra fluidwhen the pressure rises.

If using a surge suppression device, it is generally desirable to locate thedevice as close as possible to the cause of the transient. The practicalityof the different options depend highly on the particular case of interest.

Methods for surge reduction are discussed in detail in Swaffield et al.(1993, Chapter 6), Wylie et al. (1993, Chapter 10), and Chaudhry (1987,Chapter 10).


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Larger pipe sizes

If at the design stage, one can consider using larger pipes in the area ofconcern. This will reduce fluid velocity and hence pressure surge.

Flexible hose

Flexible hose can sometimes be used in small systems, which reduceswavespeed and hence pressure surge.

Slower or modified valve closure profiles

If the pressure surge is due to valve closure, the magnitude of the surgecan frequently be reduced by closing the valve more slowly. Anotheroption is to close the valve in a different manner. Swaffield et al. (1993,pp. 204-206) discuss closing the valve more quickly at first and thenmore slowly at the end. Examples are given where the first 80% ofclosure occurs over the first 20% of closure time, with the final 20%closure occurring over the remaining 80% of the time.

Parallel valves

Rather than use one large valve to stop the flow, perhaps two or morevalves can be used in parallel. The timing of the valve closures isstaggered, thus allowing a portion of the flow to be stopped with eachvalve.

Relief valves

Relief valves can be used to avoid high pressure situations. In addition,inward relief valves can be used for low pressure situations.


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Vacuum breaker valves for low pressures

Vacuum breaker valves, also called air inlet valves, open to permit airinto the system during low pressure conditions. These are usuallylocated at high elevation points along the pipeline.

Surge tank

Surge tanks can be an effective way to reduce surge pressures if thesteady-state system pressures are relatively low. High steady-statepressures result in high hydraulic gradelines. Since the liquid level in asurge tank rises to the local hydraulic gradeline, they would have to bevery tall if located in a high pressure region.

Gas accumulator

Gas accumulators can be effective in spreading out the wave front of atransient and thus reducing peak pressures. Gas accumulators change thefrequency response of the system and can in fact amplify pressure surgeif not sized and located properly.


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C H A P T E R 1 2

Special Topics: IncludingTroubleshooting Models

This chapter discusses techniques that can be implemented to generatemodels using AFT Impulse, and provides direction when you are havingproblems obtaining convergence in your model.

The philosophy of computer modeling

Some might say that a “good” computer model is one that accuratelypredicts the response of a system; but this is only partly true. Thepracticing engineer generally must weigh the accuracy of the modelagainst the risk, schedule, and/or budget involved. Through experience,an engineer learns where to focus resources and when an answer is goodenough to base important decisions on.

Although AFT Impulse is designed to speed up the modeling process, itshould never be used as a “black box.” Your model input and outputshould always be carefully reviewed.

To make the most effective use of AFT Impulse, you should follow theseguidelines:

1. Be aware of what AFT Impulse can and cannot do and theassumptions it makes.

2. Learn and use the tools in AFT Impulse that are appropriate for thespecific analysis you want to perform. Detailed and accurate answers


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demand detailed and accurate input. Quick and dirty answers requireless detailed input.

3. Learn and use the tools in AFT Impulse that allow you to processinformation in the formats that mean the most to you. Because pipeflow applications vary so much within different industries, AFTImpulse is customizable for the way you want to do your pipe flowwork. Understanding the tools will help you gain greater conceptualcontrol over your pipe flow analyses and reduce modeling errors.

4. Check your input data. Use the features available in AFT Impulse toreview your results for input errors. Pay attention to the informationin the output printout. Don't neglect warnings in the output. Performhand analysis checks of results.

5. Bound your problem. Instead of trying to calculate the exactresistance in a network, or the right inertia for a pump, use yourjudgment, guided by data, on how much variation might occur. Byrunning your model at the upper and lower limits, you can obtainanswers, for example, on the maximum and minimum possiblepressure surge in the system. If the results at the maximum andminimum are acceptable, your analysis is probably finished. If theyare not acceptable, you can further refine the input model.

Troubleshooting steady-state models

What should you do if your model will not run for steady-state, or doesrun and gives strange results? There are numerous possible causes ofsuch problems. This section identifies common problems and offersstrategies you can use to troubleshoot models yourself.

Use the Model Data windowBesides its usefulness for documentation purposes, the Model Datawindow is a powerful tool for finding input errors in your model.

Look at the model in Figure 12.1. If you suspected an input error in oneof the pipes, the task of looking for the error in each individual pipeinput window would be tedious.

Now look at the Model Data window for the pipes in Figure 12.2. Here itis much easier to scan down columns of data looking for input errors. In


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this case, one of the pipe lengths was accidentally entered in miles ratherthan feet.

Figure 12.1 Workspace layout of a pipe system with an input errorin one of the pipes (error not shown).

Poor pump curve fitsThe Pump Specification window allows curve fits of pump data up tofourth order. Depending on the number of data points entered, pumpcurve fits that look like the one in Figure 12.3a are possible.

AFT Impulse interprets flow rates past the inversion as acceptableoperating points, which can lead to strange results or non-convergence.You should always take care that your pump curves cross the flow rateaxis as shown in Figure 12.3b. If you attempt to generate a curve fit likethat shown in Figure 12.3b, AFT Impulse will warn you. In addition, itwill warn you when you try to run the model.


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Figure 12.2 Model Data window display for the model in Figure12.1. From this perspective it is much easier to see thatone of the pipes (#4) has length data entered in milesrather than feet. This is not automatically an input error,as AFT Impulse accepts pipe input with mixedengineering units. However, more frequently than notthe user intends to use consistent units, so thisprobably is an error.


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0

2

4

6

8

10

12

0 50 100 150 200

Flow (ft3/sec)

Pre

ssur

e (p

sid)

0

2

4

6

8

10

12

0 50 100 150 200

Flow (ft3/sec)

Pre

ssur

e (p

sid)

Figure 12.3 Top graph shows a third or fourth order pump curve fitwith an inversion at 150 ft3/sec. AFT Impulse interpretsflow rates to the right of this inversion as acceptablewhich can cause convergence problems or erroneousresults. All pump curve fits should include sufficientdata such that the curve crosses the zero pressure riseaxis as shown in the bottom curve.

a

b


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Use the Output window Sort featureThe Sort Output feature in the Output window is discussed in Chapter 5.If a model runs but the results look incorrect, use the Sort feature in theOutput Window to look for extremes in velocity, pressure drop or otherparameters. An input error (like incorrect diameter) may be easier to seeby looking at its effect on the results.

Tee/Wye junction complexityAs discussed in Chapter 8, tee/wye junctions contain powerful andsophisticated models for determining the pressure losses at flow splitsand converging flow streams. This power comes at the cost of additionalmodel complexity. The complexity can lead to convergence problems inmodels with numerous tees. Here are a couple of recommendations.

Tee/Wye junctions have two loss options: Simple and Detailed. TheSimple loss model ignores losses at the tee. The Detailed model uses thecorrelation summarized in Chapter 8. The potential convergenceproblems occur when using the Detailed loss model.

The potential problem is not with the Detailed tee model itself. Rather, itis due to multiple tees coupling together in such a way that AFT Impulsecannot converge on all of the flow splits.

You can quickly and easily check if a convergence problem is beingcaused by tees. Using the Global Junction Edit window, you can changeall tees to the Simple loss model and rerun the model. If it nowconverges, then the problem was due to the use of Detailed tee modeling.

Experience has shown that this problem can frequently be resolved byusing absolute tolerance criteria in the Steady Solution Control windowrather than relative tolerance. Tolerance criteria is discussed at length inChapter 8.

You also have the option of just using the Simple (lossless) tee modelfor all tees. This is how tees have been traditionally handled, so losslesstees do have some utility in engineering calculations.

Lower the flow rate relaxationThe Steady Solution Control window allows you to change thenumerical relaxation parameter for both flow rate and pressure. A fullexplanation of relaxation is given in Chapter 8. It is recommended that


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you use only 0.5 or 1.0 for pressure relaxation. But lower values of flowrate relaxation can be very helpful.

A good first step is to lower the flow rate relaxation to 0.05 or 0.1. Bylowering the flow rate relaxation you improve AFT Impulse's ability toobtain convergence. The cost is slower run times. In some cases,accepting the slower run times that come with flow rate relaxation is theonly means of obtaining convergence. You can lower the relaxationvalues as low as you want, but it is recommended it not be loweredbelow 0.01.

Try absolute toleranceMost models converge more reliably with relative tolerances, which isthe default in AFT Impulse’s Steady Solution Control window. But somemodels converge much better with absolute tolerance. What this usuallymeans is that there is at least one element of the model that is close tothe real answer in absolute terms but does not lock in on a relative(percentage) basis.

If a model will not converge with relative tolerance, you can trychanging to absolute tolerance. Even better, try the option to convergeon either absolute or relative, which will offer the most flexibility.

An explanation of absolute and relative tolerance is given in Chapter 8.

Make initial flow rate guesses for pipesIf you do not provide any initial guesses, AFT Impulse will make its ownguesses. On occasion, the AFT Impulse guesses are far away fromreality. For some models, providing a crude initial guess for all the pipeflow rates in the system can help the model converge.

The crude guesses can be as simple as selecting all the pipes andinitializing all of them to 1 lbm/sec, even though some operate at 0.1lbm/sec and others at 10 lbm/sec.

Review the Iteration HistoryThe Iteration History window is discussed in Chapter 8. You can use thisto see which pipes or junctions are farthest out of tolerance, which maydirect your troubleshooting efforts toward these elements.


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Change boundary conditionsChanging the boundary pressures or flow rates can sometimes give youinsight into what is giving the model trouble.

Turn off parts of the modelUsing the Special Conditions feature, you can turn off elements in yourmodel and, in effect, shut down entire sections. Then you can focus ongetting selected sections to run to convergence.

Break a large model into submodelsAFT Impulse provides convenient features for deleting parts of yourmodel and merging parts back in. A section of your model can bequickly deleted by selecting it on the Workspace and choosing Deletefrom the Edit menu. If the section is instead Cut, it can be Pasted into anew model of its own for testing or holding. At a later time, the removedsections can be added back into the current model by using the Mergefeature on the File menu.

The advantage of breaking your model into sub-models is that you caninvestigate the convergence characteristics of each part of your modelindependently. If you can get each sub-model to converge, you should beable to get the original model to converge also. If one of your sub-models will not converge, it is doubtful your original model willconverge either.

Control valve failure issues in steady-state

AFT Impulse offers modeling of PRV’s, PSV’s and FCV’s. Based on theinteraction with the pipe system, control valves can end up in a situationwhere they cannot control to the desired control points. When thisoccurs, it is an indication that the desired control point cannot beobtained unless the control valve acts like a pump and adds pressure tothe system. To diagnose which control valves (if any) need to addpressure to reach their control point, set their failure state to AlwaysControl (Never Fail).

It is very important to grasp that when a valve fails to control and thefailure action is set to Always Control (Never Fail), the resulting


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solution in the Output is only useful for diagnostics and does notrepresent a physically meaningful answer. Why is that? Because in orderto maintain the control pressure or flow the valve had to add pressure. Ifa control valve is operating in this mode, you will be clearly notified inthe Output window, both in the Warnings and in the Valve Summary.

Altogether, there are four failure options for a control valve:

1. Maintain control by adding pressure (useful for diagnostics only)

2. Go to the valve’s full open state

3. Close the valve

4. Go to the valve’s full open state but close if the flow reverses

In such cases where failure to control exists, one or more valves may failsimultaneously. Once failed, AFT Impulse will not return a valve to itscontrol capability during the steady-state part of the simulation.

Infinite pipe junctions

An infinite pipe junction is a special junction that accepts pressurewaves but never reflects any waves (Wylie, et al., 1993, pp. 121-122).This junction is useful in modeling the part of a pipe system which is solong that its communication time results in no interaction with the rest ofthe system.

This feature is offered as part of the Assigned Flow and AssignedPressure junctions. During steady-state a fixed flow or pressure exists atthe junction, but during transient flow both the pressure and flow changein a manner consistent with the fact that no waves are reflected. Thetheory for an infinite pipe is given in Chapter 9.

Judicious use of infinite pipe junctions can significantly reduce modelrun time. The reason is that if there is a pipe which is so long it will notreflect any waves during the simulation, the extra computation for thatpipe can take a lot of time. The infinite pipe junction is a way ofavoiding any computations for the pipe but still maintaining its effect onthe system.


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Application exampleA real life example is shown in Figure 12.4. The system represents partof a tank farm for refined petroleum products. Gasoline flows from arefinery some 60 miles away into the tank farm. The supply pipeline isrepresented by pipe P1. Pipes P2-P9 are relatively short local pipes at thetank farm with changing diameters. This model is for gasoline, and thegasoline comes into junction J10 and flows initially to the tank at J106.The transient of interest is when the J201 valve is opened and the J104valve is closed. This will divert the incoming gasoline from J106 toJ212. The total scale of the tank farm piping is less than a mile.

The transient lasts for about 30 seconds. During the opening and closingof the valves, the transient waves propagate out to J106 and J212, aswell as to the supply junction J1. Using the wavespeed of gasoline it canbe determined that any waves that propagate to pipe P1 will not reflectback to the tank farm within 30 seconds, and thus will not impact thetransient. However, during the transient procedure, gasoline still flowsfrom junction J1 into the system.

One approach is to model pipe P1 as the full 60 mile length. Thus all thepipe stations along 60 miles must be calculated for the entire 30 secondsimulation. A second approach is to specify pipe P1 as very short (13feet long in this case, chosen for modeling convenience based on otherconsiderations) and to specify J1 as an infinite pipe junction.

Both approaches will give the same transient results. However, the firstapproach would take over 100 times as long to run (a 1.9 hour run vs. a 2minute run when using an infinite pipe). Again, the difference in runtimeis that with an infinite pipe, no computation is required along the 60 milepipe except for two stations at the entrance.


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Figure 12.4 Example of where an infinite pipe junction can be used.Here J1 is specified as an infinite pipe. Use of aninfinite pipe reduces the model run time by a factor of100.

How pressure junctions work in steady-state

Pressure junctions (e.g., Reservoirs or Assigned Pressures) in AFTImpulse are an infinite source or sink of fluid. This means that they candraw or discharge as much fluid as is necessary to maintain the specifiedpressure. Some engineers have difficulty grasping this concept, andmisuse pressure junctions in AFT Impulse. This section offers a physical


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example of how pressure junctions work to clarify for those havingdifficulty with the concept.

Consider the physical pumping system shown in Figure 12.5. Thissystem pumps water from a tank near the shore of Lake Michigan to atank on a nearby hill.

Lake Michigan

ShorelineElevation

TransferPipeline

TransferPump (J3)

DischargeTank (J2)

SupplyTank (J1)

Figure 12.5 Physical pumping system to transfer water fromshoreline supply tank (J1) to discharge tank (J2) at topof hill

An engineer is tasked with sizing the pump for this system. The engineerhas an idea of the discharge head needed for the pump, and so builds anAFT Impulse model as shown in Figure 12.6.


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J1

J2

J3 J4P1> P2>

P3>

Figure 12.6 Model to size pump attempting to represent system inFigure 12.5, but actually represents the system inFigure 12.7.

The Figure 12.6 model is attempting to represent the physical system inFigure 12.5, but it in fact represents the physical system in Figure 12.7.

Lake Michigan (J4)

ShorelineElevation

TransferPipeline

TransferPump (J3)

DischargeTank (J2)

SupplyTank (J1)

Figure 12.7 Physical system represented by model in Figure 12.6.


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Why? Because the reservoir junction (J4), which is an infinite source offluid, has been located between the pump (J3) and the discharge tank(J2). The J4 reservoir isolates the supply tank and pump (J1 and J3) fromthe discharge tank (J2). The J4 reservoir is very much like placing LakeMichigan between the supply and discharge piping.

If the engineer changes the surface elevation in J4, it is like changing thesurface elevation of Lake Michigan. It will change the flow rate in thepipes, but no matter how high the water level of Lake Michigan, it doesnot change the fact that the pumping system is isolated from thedischarge tank. The flow rate in the piping from J4 to J2 is entirelydependent on the difference in surface elevations between J2 and J4, andis not influenced by the J3 pump in any way.

How then should the physical system in Figure 12.5 be modeled? Itshould be modeled as shown in Figure 12.8.

J1

J2

J3P1>

P2>

Figure 12.8 Proper model of physical system shown in Figure 12.5.

If the goal is to size the pump at J3, the model is Figure 12.8 can be usedwith the pump modeled as an assigned flow pump.

The role of pressure junctions in steady-state

In modeling a generalized pipe network, it is possible to constructmodels that do not have a unique solution. A common occurrence of thisis when a model contains one or more sections completely bounded byknown flow rates. A series of examples with accompanying explanationwill clarify situations where this occurs. The length of this topic isproportional to the frequency with which engineers get confused over


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the issue. It became apparent that an exhaustive discussion would bevery helpful for some engineers. Do not dismiss this topic too quicklybecause of its length.

Before beginning, it will be helpful to consider an aspect of thephilosophy of computer modeling. At the risk of stating the obvious, itmust be recognized by the user that a computer model cannot calculateanything that cannot, in principle, be calculated by hand. All thecomputer does is accelerate the calculation process. Some users expectthe model to generate independent information and become frustratedwhen the software requests additional information from them. But if theuser was doing the calculation by hand that same information would stillbe required. The difference is that in the case of the hand calculation, theengineer would be forced to think through why the information isneeded.

However, when the engineer interacts with AFT Impulse, he/she doesnot need to think at the same depth as in the hand calculation. So whenAFT Impulse asks for the additional information, it is not as apparentwhy it is required. This disconnect in thinking can cause frustration forthe user. Fully understanding the concepts in this topic will greatlyreduce the frustration for some users, and will improve the quality ofmodels for all users.

ExamplesThe simplest example is the system shown in Figure 12.9. In AFTImpulse terms, the system has two assigned flow junctions.

Inlet Q = 1 ft3/sEl = 0 ft

Outlet Q = 1 ft3/sEl = 0 ft

L = 100 ftD = 2 in.f = 0.02

Figure 12.9 Model with two assigned flows. This model does nothave a unique solution.

Obviously, the flow in the pipe is known, but, what is the pressure at theinlet? At the outlet? It cannot be determined because there is noreference pressure. The reference pressure is that pressure from which


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other pressures in the system are derived. There can be one or morereference pressures, but there always has to be at least one.

The model in Figure 12.9 can be built with AFT Impulse. If you try torun this model, AFT Impulse will inform you that it cannot run itbecause of the lack of a reference pressure. In AFT Impulse there arenine junctions that can act as a reference pressure: the reservoir,assigned pressure, spray discharge, pump with submerged option, andvalve and relief valve after cracking (with the exit valve option) behaveas reference pressures because that is their nature. Three other junctiontypes, surge tank, gas accumulator, and assigned flow (when the steady-state flow is zero) have a special feature that allows them to act asreference pressures. This feature is found on each junction's Optionaltab, and is discussed at length in Chapter 6.

There are a couple other things worth noting about Figure 12.9. First isthat the model in Figure 12.9 has redundant boundary conditions. Whenone flow is specified in a single pipe, the other end must have the sameflow. Thus the second known flow does not offer any new information.Because the conditions are redundant, there is no unique solution to themodel in Figure 12.9. In any AFT Impulse model, there is always at leastone unknown for each junction in the model – either pressure or flowrate. Sometimes there are two unknowns such as with a pump. It ispossible the user may not know either the flow or the pressure at thepump. In such cases, the user is required to specify the relationshipbetween flow and pressure. This relationship is called a pump curve, andby specifying the relationship between the two, effectively one of thetwo unknowns can be eliminated. Thus we always end up with oneunknown for each junction.


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Inlet P = 10 psigEl = 5 ft

Outlet Q = 1 lbm/sEl = 0 ft

L = 100 ftD = 2 in.f = 0.02


Outlet P = 0 psigEl = 5 ft

L = 100 ftD = 2 in.f = 0.02

L = 100 ftD = 2 in.f = 0.02



a

b

c

Figure 12.0 a-c Top two models with one pressure and one assignedflow, bottom model with two pressures. All of thesemodels have a unique solution.

Second, if the user was allowed to specify two flows as in Figure 12.9,he/she could specify them with different flow rates. Clearly, they musthave the same flow or an inconsistency occurs. The basic reason theinconsistency would be possible is because Figure 12.9 does not have aunique solution.

Figure 12.10a-c shows three other model configuration possibilities. It isnot possible to specify inconsistent conditions for any of the Figure12.10 models, and they always have a unique solution no matter whatinput is specified by the user.

The four models in Figures 12.9 and 12.10a-c contain all logicalpossibilities. In all four cases there are four things we want to know: thepressure and flow at the inlet, and the pressure and flow in the outlet. Ineach case we know two of these four. The problem is that in the Figure12.9 model the lack of a reference pressure makes it impossible todetermine the inlet and outlet pressures, even though we know the flow.The three models in Figure 12.10a-c all have at least one pressure, thusall four of the desired parameters can be determined. Table 12.1summarizes this. Also note in Table 12.1 that in the Figure 12.10c casethe flow can be determined, but it is by iteration.


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Table 12.1 Four logical possibilities of boundary conditions forsingle pipe systems

CASE Inlet Pressure

Inlet Flow

Outlet Pressure

Outlet Flow OK ? Comments

Fig. 12.9 X X No No reference pressure, cannot calculate pressures

Fig. 12.10a X X Yes 1 known flow, can calculate everything directly

Fig. 12.10b X X Yes 1 known flow, can calculate everything directly

Fig. 12.10c X X Yes 2 pressures, can iterate for flow

As we consider multi-pipe systems, there are a host of other possibilitiesthat present themselves. All other configuration possibilities which lacka reference pressure ultimately boil down to the same problem thatexists in Figure 12.9.

The pipe lengths, diameters and friction factors are not included in theremaining models because they do not influence the main point of thistopic. It is assumed that these parameters can be obtained for each pipeand the resulting pressure drop calculated with standard relationships.

Consider the model in Figure 12.11. With three boundaries having aknown flow, this clearly has the same problem as the model in Figure12.9. No unique pressures at any location can be calculated.


Outlet Q = 0.5 ft3/sEl = 0 ft

Outlet Q = 0.5 ft3/sEl = 0 ft

Figure 12.11 Three known flows at boundaries that lack areference pressure, similar to the model in Figure 12.9.This model does not have a unique solution.


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A pressure at any one of the three boundaries in Figure 12.11 would besufficient to allow a unique solution of the system. It is also possible thatthere could be two pressures and one flow, or three pressures. As long asthere is at least one pressure, a unique solution exists.

Now consider the models in Figure 12.12. The Figure 12.12a model doeshave one reference pressure, but in this case there is still a problem. TheFlow Control Valve (FCV) is controlling the flow to 1 ft3/s, and thedownstream flow is demanding 1 ft3/s. The section of the systempreceding the FCV can be solved (because there is a pressure upstream),but the section after the FCV does not have a unique solution. Why? Inorder to obtain a unique solution, the pressure drop across the FCV mustbe known. But any pressure drop could exist and satisfy the conditionsof this model. It is thus not possible to determine the pressure at theoutlet flow demand because it depends on the FCV pressure drop whichis not known. The model in 12.12b does have a pressure downstream ofthe FCV, and thus there is a unique pressure drop across the FCV and aunique solution exists for the model in Figure 12.12b.

If you input the remaining data for the Figure 12.12a model and run it inAFT Impulse, you will get the message shown in Figure 12.13. AFTImpulse identifies the part(s) of the model where a known pressure isneeded.





FCV Q = 1 ft3/s

FCV Q = 1 ft3/s

J1 J3J2

J1 J3J2

a

b

Figure 12.12a-b The model at the top does not have a referencepressure after the Flow Control Valve so thepressure drop across the FCV cannot be determined.The top model does not have a unique solution. Thebottom model has a pressure upstream anddownstream, and a unique solution exists.


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Figure 12.13 AFT Impulse message when you try to run the modelshown in Figure 12.12a.

Sizing pumps with flow control valvesAn analogous situation to Figure 12.12a is when the user is trying to sizea pump using the Pump with an assigned flow. The Pump with anassigned flow is really just a reverse FCV. It controls the flow by addingpressure, rather than by reducing it. Figure 12.14 shows this case and isanother example of a model without a unique solution, requiring morethan one pressure junction. If the user instead entered a pump curve forthis model, it could be solved.


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Pump with Known (assigned) Flow Q = 1 ft3/s

Figure 12.14 A pump modeled as an assigned flow for sizingbehaves identically to the FCV model in Figure 12.12a,for which no unique solution exists.

Increasing in model complexity, Figure 12.15 shows some additionalexamples of models without a unique solution. In particular, Figure12.15b has a pump with an assigned flow that can add as much pressureas it wants, and the downstream FCV can then take out as much as itwants. There are an infinite number of possible solutions.

While it would be highly unusual for a system to have two FCV’s inseries, it is worth considering the model in Figure 12.15b further. It iscommon to have an FCV in series with a pump. How does one size thepump in such a case? There are several ways to do this, one of which isto change the pump from an assigned flow to an actual pump curve.Trying multiple pump curves will guide you to the best pump.

But there is a better way. Consider for a moment the pressure dropacross the FCV in Figure 12.15b. In the installed system, is any pressuredrop acceptable? Usually not. It is typical to have pressure drop limitsbased on the system design and the FCV itself. A common requirementis a minimum pressure drop across the FCV. If no minimum pressuredrop exists, let’s say we choose a pump that results in the pressure dropacross the FCV being 0.01 psid. If the system is built and the pumpslightly underperforms its published curve, the FCV will not be able tocontrol to its flow control point. Even if the pump does operate exactlyon the published curve, eventually fouling in the pipes will result inincreased pipe resistance (and pressure drop) and again the FCV will notbe able to control to it’s flow control point.


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FCV Q = 1 ft3/s

FCV Q = 1 ft3/s



FCV Q = 1 ft3/s

Pump with Assigned Flow Q = 1 ft3/s

a

b

Figure 12.15 a-b Neither model above has a unique solution. Thetop model has two flow control valves in series.The bottom model has a pump modeled as anassigned flow in series with an FCV. In both caseseither a third pressure junction is needed betweenthe two middle junctions, or one of the junctionsmust be changed from a flow controlling device.

To avoid these kinds of problems in installed systems, a minimumpressure drop across the FCV is typically required. This minimumpressure drop provides the key to sizing the pump in Figure 12.15b.Rather than model the FCV as a flow control valve, instead model it as apressure drop control valve (PDCV) set to the minimum pressure droprequirement. Then a unique solution exists, the model will run and thepump can be sized. Once the pump is sized and an actual pump curveexists, the pump curve can be entered for the pump and the valve can beswitched back to an FCV. Remember that when we use the PDCV, weare still getting the flow we want for the true FCV, because the pump isnow providing the control.

Think for a moment about what we just did. We went through a thoughtprocess that many engineers have gone through during their handcalculations. Specifically, we size the pump such that the FCV pressuredrop is minimized. If the FCV has a larger pressure drop than theminimum, the pump must pump harder to overcome this excess pressuredrop. In short, it must use more energy and is thus less efficient. Figure12.16 depicts this process.


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FCV Q = 1 ft3/s



PDCV dP = 5 psid

Pump with Known Flow Q = 1 ft3/s

Pump with Known Flow Q = 1 ft3/s



FCV Q = 1 ft3/s

Pump with Curve dP = f(Q)

a

b

c

Figure 12.16a-c. The model at the top is the same as Figure 12.15band does not have a unique solution. To size thepump, change the model to the second one shownabove, which uses a pressure drop control valve(PDCV) rather than an FCV. Once the second modelis run, the pump is sized, a pump curve exists, andthe model at the bottom can be run using an FCV.

Let’s make a slight addition in complexity to the model in Figure 12.16.In this case we have two flow control valves in parallel (see Figure12.17). To size this pump, we apply the process described in Figure12.16. We choose one of the FCV’s, make it a PDCV, size the pump,choose a pump with an actual pump curve and enter it into the model,then return the PDCV to an FCV.


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FCVQ = 100 gpm

FCVQ = 100 gpm

Pump – known flowQ = 200 gpm

P1> P2>

P4^

P6>

P7^

P5^ ^P8

P3>J1 J2 J4

J8

J6

J5

J7J3

Figure 12.17 A pump with an assigned flow in series with twoparallel FCV’s. The model in its current form does nothave a unique solution. To size the pump, first changethe FCV at J8 to a PDCV.

This brings up the question: which FCV should you turn into a PDCV?When FCV’s are put in parallel, frequently the pipe design has one ofthe FCV’s further away from the pump than the others. Because of theadditional piping leading to this FCV, it will be the weakest link in thechain of parallel FCV’s, by virtue of having the lowest pressure dropacross it. The most remote FCV should be chosen as the PDCV. If themost remote FCV is chosen, when the minimum pressure drop requiredis applied to the PDCV all other FCV’s in the parallel system will havegreater than the minimum pressure drop — thus satisfying the pressuredrop requirement for those FCV’s as well.

What if you do not know which FCV is the most remote? In this casemake your best guess, change it to a PDCV at the minimum requiredpressure drop, run the model, then verify whether all other FCV’s have apressure drop that meets or exceeds the requirement. If not, then theFCV with the smallest pressure drop as determined by the first run is inreality the weakest FCV. Choose this FCV, change it to a PDCV, then


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change the original PDCV back to an FCV since it is not the weak link.If the pipe system leading to each FCV is truly identical, and all FCV’shave identical pressure drops, any of the FCV’s will serve as the PDCV.

The preceding process can be extended to cases where there are three ormore FCV’s in parallel, and also cases with more than one pump inparallel supplying the FCV’s.

In Figure 12.17 the most remote valve is J8, the one on the right. If theminimum pressure drop is 5 psid, change J8 to a PDCV with a 5 psidrop. This results in the output shown in Figure 12.18a. Notice how thepressure drop across the other FCV at J5 (6.837 psid) is greater than 5psid because it is closer to the pump. Also notice how the flow throughJ8 is still 100 gpm even though it is not controlling flow. Once again, thereason for this is that the pump is controlling the flow to 200 gpm, andwith the J5 FCV controlling to 100 gpm, the excess must go through J8.The pump is thus sized and its pump head rise is shown in Figure 12.18b(113.5 feet).

a

b

Figure 12.18a-b. AFT Impulse results for the model in Figure 12.17after J8 has been changed to a PDCV. Top resultsshow valve summary and bottom results show pumpsize.

Pressure control valvesThe problem of non-unique solutions can also occur when a PressureReducing Valve (PRV) or Pressure Sustaining Valve (PSV) is used. A


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PRV is used to control pressure downstream of the valve, while a PSVcontrols pressure at the valve inlet. One thing that PRV’s and PSV’shave in common with FCV’s is that the pressure drop cannot be knownahead of time. The pressure drop depends on the balance of the pipesystem.

Consider the simple system shown in Figure 12.19. With the pressurecontrolled downstream of the PRV, the pressure upstream is not known.Without knowledge of the PRV’s pressure drop, the pressure upstreamof the PRV cannot be determined and thus no unique solution exists.



PRV Pdown = 10 psig

Figure 12.19 If a PRV is downstream of a known flow the PRVpressure drop cannot be calculated. This model doesnot have a unique solution.

Figure 12.20 shows the possible cases with PRV’s and PSV’s, whileTable 12.2 comments on the six cases. In summary, at least one knownpressure is always needed on the side of a pressure control valveopposite of the controlled side.


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PRV Pdown = 10 psig


PRV Pdown = 10 psig


PRV Pdown = 10 psig


PSV Pup = 20 psig


PSV Pup = 20 psig


PSV Pup = 20 psig






a

b

c

d

e

f

Figure 12.20a-f. Six possibilities for PRV and PSV configurations.Cases a and e do not have unique solutions. SeeTable 12.2 for comments.


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Table 12.2 Summary of six possibilities for PRV and PSVconfigurations.

CASE Inlet Pressure

Inlet Flow

Outlet Pressure

Outlet Flow OK ? Comments

Fig. 12.20a - PRV X X No No reference pressure upstream of PRV, cannot calculate

Fig. 12.20b - PRV X X Yes 1 known flow, can calculate everything directly

Fig. 12.20c - PRV X X Yes 2 pressures, can iterate for flowFig. 12.20d - PSV X X Yes 1 known flow, can calculate

everything directlyFig. 12.20e - PSV X X No No reference pressure downstream

of PSV, cannot calculate

Fig. 12.20f - PSV X X Yes 2 pressures, can iterate for flow

If a pump is being sized in series with a PRV or PSV, modeling thepump as an assigned flow will not permit a unique solution if the pumpis upstream of a PRV or downstream of a PSV. The reasons for this arethe same as those in the previous section on FCV’s.

In combination with the previous examples, it should be apparent howthe non-unique solution problem can occur in more complicated systems.In all such cases, the reasoning will boil down to the same problemsalready discussed.

Closing parts of a systemA model may be built which has sufficient information to obtain aunique solution. Then a user closes a junction and suddenly the problemhas no unique solution. This is shown in Figure 12.21a-b. The top modelhas a unique solution and will run fine. In the bottom model the valve isturned off and the section of the system downstream of the valve hasonly known flows. No reference pressure exists in this section because itis isolated from the known pressures in the section on the left.


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Open

P1v

P2> P3>

P4^

P5>P6>J7 J6J3 J4

J5J1

J2

a

Closed

P1v

P2> P3>

P4^

P5>P6>

J7 J6J3 XJ4

J5J1

J2

b

Figure 12.21 a-b Top model has reference pressures at J1 and J2.Bottom model has closed valve at J4 whichisolates the J5 and J6 assigned flows. There is noreference pressure for J5 and J6 and no uniquesolution exists for the bottom model.

Effect of pipe sectioning on transient run times

The transient run time for a simulation is related directly to the totalnumber of computations required. The number of computations dependson the total number of time steps (shown on the Transient Controlwindow) and the total number of sections (which can be determinedfrom the display table in the Section Pipes window).

Run times also depend to a lesser degree on how frequently data iswritten to disk, because of the time required for disk access. This isdetermined by the Save Intermediate Output to File… in the TransientControl window.


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To satisfy the Method of Characteristics (MOC) requirement, the timestep and number of pipe sections are related by Equation 5.1. If moresections are desired, then shorter time steps are required. For example, touse ten sections in a pipe rather than five requires twice as manycomputations because there are twice as many sections. However, thetime step must be reduced by half (Equation 5.3); thus to model the pipefor the same time duration (say 1 second) requires twice as many timesteps. Therefore, using double the number of sections results in fourtimes the number of computations.

A related issue is that of the controlling pipe (see Chapter 5 for moreinformation on controlling pipes). If the controlling pipe is very short,this requires more sections in all other pipes in the model. Hence it maybe prudent to either neglect the shortest pipe or identify a way tocombine it into a longer pipe, which will lead to a different controllingpipe. The Section Pipes window will automatically combine pipes ofcommon diameter for you (see Figure 12.22).

For example, consider a case where the controlling pipe is 1 foot longand the next shortest pipe is 10 feet long. If the controlling pipe can beneglected or combined, then the 10 foot pipe becomes the controllingpipe. This will result in a factor of 10 fewer pipe sections and factor of10 fewer time steps, or a run time reduction by a factor of 100!


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Figure 12.22 The Section Pipes window alerts you to pipes thatcould be combined and possibly reduce modelruntime.

Artificial transients

An artificial transient occurs when the steady-state results do not matchthe transient calculation initial step. During the first time step AFTImpulse tries to bring the mismatched conditions to what it considerssteady conditions which generates a transient pressure. This transient isnot a true physical transient, but a false “artificial” one based onmismatched data. If undetected, the artificial transient will interact withthe physical transient being modeled and corrupt the results.

Before AFT Impulse initiates the transient simulation, it performs a“zero time step computation” and compares the predicted results withthe results from the steady-state solution. If the difference exceeds theartificial transient criteria (0.5% by default), then AFT Impulse willdisplay a warning in the output. If Stop Run if Artificial TransientDetected is selected, the simulation will be immediately terminated. This


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feature is especially helpful for simulations that take a long time to run.For example, it is wasteful to run a large model for an hour only to findat the end that an artificial transient input error was present at the verybeginning, thus nullifying the results.

The artificial transient detection feature is an automatic way for the userto identify problems of this nature. A manual process can also be used,and it is instructive to consider the manual process to better understandartificial transients.

In some cases where pipes are initially not flowing, the first transientcalculation is some very small but non-zero flowrate. This can appear tobe an artificial transient as compared to a flow of zero. To avoid thesetypes of spurious warnings, one can specify a flow rate threshold belowwhich artificial transients will be ignored.

Manually detecting artificial transientsFor those interested in better understanding artificial transients, it isinstructive to consider how one can manually detect an artificialtransient.

Once the AFT Impulse model is created, run the model but do not useany of the transient input (e.g., by setting the Transient SpecialConditions to None). This will, for example, make all valves stay at theiroriginal positions, all pumps stay at their original speed, and all pressureand flow boundary conditions steady. Then run the model for 10 to 100time steps by specifying the appropriate Stop Time in the TransientControl window. If no artificial transients exist, all pressure and flowsolutions will remain steady with time because nothing transient isoccurring. (It is easiest to see if the pressures or flows change bygraphing some of the results.) If pressures and/or flows change andtransients are evident, then an artificial transient exists. It is artificialsince no physical transients are being modeled, thus all transients thatoccur are non-physical artificial ones.


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A P P E N D I X A

Keyboard Modifiers and Shortcuts

The keyboard modifiers and shortcuts for the Workspace aresummarized here for convenient reference

Selection Drawing Tool

Double-clicking toolIf you double-click the Selection Drawing Tool, the Selection DrawingTool remains active after you have completed the selection. It remainsactive until you click it again a single time

Control keyIf you hold down the CTRL key when completing the selection drawing(just before releasing the mouse button), the Selection Drawing Toolremains active. The CTRL key thus performs a similar function todouble-clicking the tool.

Shift keyIf you first double-click the Selection Drawing Tool or hold down theCTRL key, you can also hold down the SHIFT key while drawing theselection box and the enclosed pipes and junctions will be deselected.


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Alt keyIf you first double-click the Selection Drawing Tool or hold down theCTRL key, you can also hold down the ALT key while drawing theselection box and the enclosed pipes and junctions will toggle. That is,any pipes or junctions that were selected will become deselected, andvice versa.

Drag left to rightIf you select the objects by dragging left to right, you will select allobjects completely inside the box.

Drag right to leftIf you select the objects by dragging right to left, you will select allobjects completely or partially inside the box (if this feature is enabledin the Workspace Preferences window).

Pipe Drawing Tool

Double-clicking toolIf you Double-click the Pipe Drawing Tool it remains active until youclick it again a single time. This allows you to draw a series of pipeswithout returning to the Toolbox each time.

Control keyIf you hold down the CTRL key when completing the pipe drawing, thePipe Drawing Tool remains active, and you can draw a series of pipeswithout returning to the Toolbox each time. The CTRL key thusperforms a similar function to double-clicking the tool.

Shift keyIf you hold down the SHIFT key down while drawing pipes on theWorkspace, you can draw pipes that are perfectly horizontal or vertical.This may improve the aesthetics of your model.


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Zoom Select Tool

Shift keyIf you hold down the SHIFT key down while selecting the zoom area,the zoom state will always be 100%.

Panning the Workspace

If the Workspace has been enlarged beyond the default one screen size(Workspace scrollbars will be visible), then the Workspace can bepanned by holding down the CTRL key, pressing the mouse down on theWorkspace, and then dragging in the direction you want to move theWorkspace.

Dragging junctions from Toolbox

Control keyA junction can be “morphed” from one type to another. To morph ajunction, hold down the CTRL key while selecting a junction from theToolbox. Drop the junction onto an existing Workspace junction. Thejunction type will change to the new junction type, and data that can bekept will be copied into the new junction.

Shift keyIf you hold down the SHIFT key while selecting a junction from theToolbox and then drop the junction onto an existing pipe, the pipe willsplit into two pipes. The physical length of the original pipe will beautomatically halved, and the new pipe will be assigned the balance.Thus the sum of the two pipe lengths will equal the length of the originalpipe. Any additional losses in the original pipe will be left in the originalpipe, and the new pipe will have no additional losses.


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Inspecting pipes and junctions

Shift keyIf you hold down the SHIFT key when you perform an inspection, onlythe items not yet defined are displayed.

Control keyIf you hold down the CTRL key when you perform an inspection, theinspection window will show the output data for the pipe or junction.

Moving junctions with connected pipes

Control keyBy default, when you move an existing junction icon, any connectedpipes will move along with the junction and retain the connection. If youhold down the CTRL key as you release the junction, only the junctionwill move. All pipe connections will be broken. This behavior can bereversed such that connections will not be retained unless you hold downthe CTRL key. This reversal can be specified on the WorkspacePreferences window.

Pipe and junction specifications window

F2 function keyWhen in a pipe or junction specifications window you can toggle thehighlight feature by pressing the F2 function key. You can also togglethis by from the Options menu.

F5 function keyWhen in a pipe specifications window, if you press the F5 function key,you will jump to the next highest numbered pipe. If you press the CTRLkey while pressing the F5 function key, you will jump to the next lowestnumbered pipe.


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The F5 function key also behaves this way for junction specificationswindows.


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A P P E N D I X B

Limitations

The size of model you can create with AFT Impulse is limited by theamount of RAM on your PC. When solving a system, AFT Impulsecreates a square matrix that is double-precision. The size of the matrix isdetermined by the total number of tee/wye and branch junctions in yourmodel.

For example, if you have 100 tee/wye and branch junctions, a 100x100array is created. With double precision, the amount of RAM needed is16x100x100, or 160KB. A 1000x1000 array would require 16MB.

In addition to this memory requirements, a relatively small amount ofmemory is required for other Steady-State Solver and modelingactivities. AFT Impulse itself requires approximately 15MB of RAM torun.

Window 95, 98 and ME are limited to displaying a maximum ofapproximately 350-400 junction icons as a result of the way in whichthese operating systems handle graphics resources. Windows NT, 2000and XP, on the other hand, has no such limit. One can determine theamount of free graphics resources by selecting Help from within AFTImpulse, then clicking on About AFT Impulse and then System Info.Among the information displayed will be GDI Memory Available(Graphical Device Interface). As this value approaches zero, Windows95, 98 and Me will become unstable and, ultimately, crash.

It is important to note that this Windows 95, 98 and ME limitation doesnot limit the number of junctions or, as a result, the model size, sincejunctions may also be displayed as a solid box, an outline box or not atall (the junctions are still there even if not displayed, and may be edited


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by double clicking on them within the Model Data window). Theseoptions are selected from the Display Pipes and Junctions item under theView menu, which allows you to selectively specify how you wantjunctions displayed. For example, by displaying all branches in a largemodel as an outline, a large number of icons can be made available forpump, valve, accumulator and other junction types where display of theicon is of greater advantage.


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A P P E N D I X C

Installation Issues

Customization files

As you customize AFT Impulse, two new files are created in yourWINDOWS directory:

IMPULSE3.INI

IMP_USER3.DAT

The IMPULSE3.INI file contains the information to customize thefeatures in AFT Impulse. The IMP_USER3.DAT contains all customdatabase information. If you invest significant time in customizing AFTImpulse, especially in the area of custom databases, you are stronglyadvised to keep backup copies of these files.

Special file control features

What is the Control.aft File?Control.aft is read as AFT Impulse begins to load and instructs AFTImpulse where certain key files and copy protection tokens are located.Typically there is little need to change the settings, however this file canbe used to override the internal defaults and customize your workingenvironment.


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Control.aft is typically located in the application folder. However, onnetworks, it may be necessary to locate it in the Windows\System folderof each client. If AFT Impulse finds the file in the Windows\Systemfolder it will use it and ignore any file in the application folder.

Control.aft can be edited using any text-based editor. Notepad, suppliedas part of the Windows installation, is a good choice. Be careful to avoidchanging any of the headings or key words. If these are changed AFTImpulse will not find the important data.

Specifying where Chempak is locatedBefore AFT Impulse can use the Chempak Fluid Database it needs toknow where both the data and the copy protection token are located.These paths are specified in the control.aft file. Typically the token is inthe same location as the data files, however, this is not a requirement(see Special Network Installations).

When Chempak is installed the setup procedure automatically updatesany control.aft file specified with the path to the data and the token.

You can manually edit the file by opening it in Notepad (or any texteditor) and adding the following lines to the [SEARCH PATHS] section(replace C:\AFT Products\Chempak Data with the path informationbased on your system):ChempakTokenPath = C:\AFT Products\Chempak Data

ChempakDataPath = C:\AFT Products\Chempak Data

This will instruct AFT Impulse to look in the C:\AFT Products\ChempakData directory for the data and also for the token.

Special network installations (remote tokens)At times it may be more convenient to have AFT Impulse installedlocally on each client but have the token located on a network somultiple users can access AFT Impulse concurrently. This is especiallytrue for dialup and remote login networks where access to the server isrelatively slow. With AFT Impulse installed locally on the client, it willload quickly and the only network traffic is the checking for tokens.

The token normally resides in a hidden, system folder of the applicationfolder. The hidden folder for AFT Impulse is named 0930.092. The keyfile, cimp.dll, is located in the application folder.


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However, to setup a remote token, where the token is placed in adifferent location than the application, first create a folder on the remotelocation or server named AFT Impulse 3.0 (for example,R:\Engineering\AFT Products\AFT Impulse 3.0). Then copy the key file,cimp.dll, from the Master Floppy Disk into it. Then move the tokenusing CCMOVE and specify the folder you created(R:\Engineering\AFT Products\AFT Impulse 3.0) as the destination. Thehidden folder will automatically be created as the token is moved.

In order for this remote token method to work, AFT Impulse on theclient has to know where to look for the tokens. The path to the tokens(actually the folder were the key file is located) must be specified foreach client. Manually edit the control.aft file to specify the folder wherethe cimp.dll file is located. Control.aft should be located on the client inthe application folder or the Windows\System folder. Continuing withthe above example, if cimp.dll is located on R:\Engineering\AFTProducts\AFT Impulse 3.0 then add/edit the following line to the[SEARCH PATHS] section:ImpulseTokenPath = R:\Engineering\AFT

Products\AFT Impulse 3.0

The application can be installed both on the network and the client. Inthis case the clients that reference the application on the server do notneed to have their control.aft file modified.

As an extension of the remote token method, the data files for theChempak database can also be installed on the local client and theChempak token can be placed on the server. You will need to modify theinformation in control.aft as described in the Specifying WhereChempak is Located section.

Network installations

AFT Impulse 3.0 requires a number of auxiliary files to run properly.Because of how 32-bit Windows operating systems work (Windows 9xand NT), certain files must be registered in the Windows registry. Forsoftware installed directly on the client PC, this situation works well.However, it creates a dilemma and an inconvenience for softwareinstalled on a local or wide area network.


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The dilemma is where to locate the auxiliary files – on the network inthe AFT Impulse folder or on the client PC local hard drive. Theimplications of this dilemma are discussed at length below.

The inconvenience is that the installation becomes a two-step process.The first step is the network administrator’s installation to the network,and the second step is the registration of auxiliary files on each clientPC.

Following is a discussion of the this issue as related to AFT Impulseinstallation and operation.

Locate auxiliary files on the client PC hard drive – The installationprocess is a two-step process. The first step is to install AFT Impulse onthe network using the Network option in the setup program. Thisperforms a typical network installation except that the auxiliary files arecopied into a subfolder below AFT Impulse. The second step is for eachclient to run the AFT Impulse Client Setup program installed into theAFT Impulse network folder. The Client Setup program automaticallydetects that the Network option was chosen in the first step, and installsthe auxiliary files from the network to the client’s SYSTEM folder andregisters them there. This process uses standard installation protocolwhereby the auxiliary files are copied to the SYSTEM folder only if theydo not exist, or are newer versions than those that already exist on theclient PC.

Advantages of this approach are: 1) Since the auxiliary files for eachuser are installed and registered on the client PC, the network folder canbe more easily relocated on the server because the auxiliary files will notneed to be registered again. 2) Problems that occur with loading theGraphics Server will be avoided.

Disadvantages of this approach are: 1) Later re-installs of AFT Impulsemay use more up to date files than the ones in the current installation,and these files will also need to be copied to all client PC’s. 2) All clientPC’s will have these files on the local hard drive, thus using extra diskspace.

Although this approach is not a perfect solution, we believe it is the bestbecause the client PC registry points only at local files (as far as AFTImpulse is concerned).


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Problems loading the Graphics Server

AFT Impulse uses a third party tool for graphing called the GraphicsServer. There are four files that are used to load and run the GraphicsServer:

• GRAPHS32.OCX

• GSW32.EXE

• GSWDLL32.DLL

• GSWAG32.DLL

Because of how the Graphics Server is written, multiple copies ofGSW32.EXE or GSWDLL32.DLL in your Path will cause problemswhen your try to load AFT Impulse 3.0.

In practice, the most common problem is a network installation where allAFT Impulse files have been installed into the network folder along withAFT Impulse. Thus, GSW32.EXE and GSWDLL32.DLL will exist inthe AFT Impulse network folder. A client PC which happens to havecopies of either or both of these files on the local hard drive (usually inthe WINDOWS\SYSTEM folder on Windows 95/98 andWINNT\SYSTEM32 on Windows NT) will fail to load properly. Themost likely reason copies exist on the local hard drive is anotherapplication installed locally uses them.

A complete list of your options is given below, along with anexplanation of the impact of each option. In the following explanations,the term conflicting files means the GSW32.EXE and GSWDLL32.DLLfiles. Your WINDOWS\SYSTEM folder is just called your SYSTEMfolder.

1. Delete the conflicting files on your hard drive – This will solvethe problem for AFT Impulse 3.0, but may cause other software youuse to stop working. This is not a permanent solution in that if youinstall other software in the future that installs copies of theconflicting files into your SYSTEM folder, the current problem withAFT Impulse will occur again.

2. Change the names of the conflicting files on your hard drive –This will solve the problem for AFT Impulse 3.0, but may causeother software you use to stop working. However, when you try torun the other software and it fails to load because of these missing


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files, you can change the names back to the original names. This willthen cause AFT Impulse 3.0 to stop working until you rename themonce again, but will allow you to run your other software that needsthese files. This will also require you to remember which files yourenamed. This is not a permanent solution in that if you install othersoftware in the future that installs copies of the conflicting files intoyour SYSTEM folder, the current problem with AFT Impulse willoccur again.

3. Move the conflicting files on your hard drive into the applicationfolder that uses them – This will solve the problem for AFTImpulse 3.0 and still allow the other software you use to runproperly. However, you will need to know which software uses theconflicting files so you can copy the files into their folder. If youremember what you do, you can move the files out of the SYSTEMfolder (or wherever conflicting files exist) and into a temporaryfolder. When a software application that used to work fails becausethese files are missing, you can copy the files into its folder at thattime. This is not a permanent solution in that if you install othersoftware in the future that installs copies of the conflicting files intoyour SYSTEM folder, the current problem with AFT Impulse willoccur again.

4. Allow AFT Impulse 3.0 to automatically rename the files for you– This is not a perfect solution, but it does have advantages. Thisoption enables a feature whereby AFT Impulse searches for theconflicting files in both your SYSTEM folder and WINDOWSfolder before the full AFT Impulse application is launched. If it findsthem, it automatically renames them and thus only the copies in theAFT Impulse network folder are found. When you exit AFTImpulse, the files will be renamed back to their original names. Theadvantages are, 1) you can continue to run your other software thatuses these files and AFT Impulse with no conflicts (as long as theyare not run at the same time) and, 2) even if you install othersoftware in the future, this option will continue to work for you. Thedisadvantages are, 1) you are permitting AFT Impulse to changefiles on your local hard drive every time it is run, whichunderstandably makes some users uncomfortable and, 2) you cannotrun AFT Impulse 3.0 at the same time as the other software whichneeds these files.


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5. Reinstall AFT Impulse with the Network option to run auxiliaryfiles on the client (recommended) – This option is the best long-term option. To do this, first uninstall AFT Impulse from thenetwork and reinstall it with the Network option. See Information onSoftware Installation for more help.


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A P P E N D I X D

Chempak Technical Information

© Copyright 1998 Madison Technical Software Inc. Used withpermission.

The theoretical basis for Chempak is a lengthy topic and has beenincluded in the AFT Impulse 3.0 Help system. To learn more aboutChempak, open the Help system and search for Chempak.


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A P P E N D I X E

Obtaining Technical Support

In order to receive technical support, the Licensing Agreement returncard must be signed by an authorized representative of your companyand returned to Applied Flow Technology within 30 days of purchase.AFT Impulse is a sophisticated software application intended for trainedengineers, and in order to provide effective support, Applied FlowTechnology needs to know who is using the software.

AFT's three levels of support

1. Free Software Support for 180 DaysAFT provides free support for the first 180 days to those who sign andreturn the Licensing Agreement. This support is for resolving installationproblems and clarifying usage issues.

2. Annual Support, Upgrade and Maintenance (SUM)After the first 180 days, AFT provides support to those sites thatpurchase an annual SUM. This support will also be for resolvingquestions on how to use specific features in the program.

3. Waterhammer Modeling Consulting SupportAFT offers specialized consulting support to users who have generalwaterhammer analysis questions. For questions that do not pertainstrictly to AFT Impulse, but instead pertain to the general nature of how


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to do waterhammer analysis, AFT offers high quality consulting servicesby experienced engineers at an hourly rate. Call for an hourly rate quote.

Contacting AFT

Telephone supportApplied Flow Technology can be reached at:

(719) 686-1000 (Voice)

(719) 686-1001 (FAX)

Web siteYou can download the latest maintenance releases of Impulse, find outwhat new things are happening at AFT, and get the latest information byvisiting our website at:

http://www.aft.com

E-Mail supportAFT can also be reached by E-mail at:

[email protected] or www.aft.com

Mail supportYou can send mail to AFT at:


P.O. Box 6358

Woodland Park, CO 80866-6358 USA

You can send courier mail to AFT at:


400 W. Hwy. 24, Suite 201

Woodland Park, CO 80863 USA


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References

Brown R.J. , and D.C. Rogers, “Development of Pump Characteristicsfrom Field Tests”, Journal of Mechanical Design, ASME, vol. 102, pp.807-817, Oct., 1980.

Carthew, G.A., Goehring, C.A., van Teylingen, J.E., "Development ofdynamic head loss criteria for raw sludge pumping", Journal WaterPollution Control Federation (WPCF), Volume 55, Number 5, (May1983).

Chaudhry, M. H., Applied Hydraulic Transients, 2nd edition, VanNostrand Reinhold Company, New York, NY, 1987.

Crane Co., Flow of Fluids Through Valves, Fittings, and Pipe, TechnicalPaper No. 410, Crane Co., Joliet, IL, 1988.

Donsky, B., "Complete Pump Characteristics and the Effects of SpecificSpeeds on Hydraulic Transients", ASME Journal of Basic Engineering,pp. 685-699, Dec. 1961.

Fox, R. W., and A.T. McDonald, Introduction to Fluid Mechanics, 3rdedition, Wiley & Sons, New York, NY, 1985.

Idelchik, I. E., Handbook of Hydraulic Resistance, 3rd edition, CRCPress, Boca Raton, FL, 1994.

Ingersoll-Dresser Pumps, Cameron Hydraulic Data, Liberty Corner, NJ,1995.

Jeppson, R. W., Analysis of Flow in Pipe Networks, Ann Arbor SciencePublishers, Ann Arbor, MI, 1976.

Karassik, I.J., J.P. Messina, P. Cooper and C.C. Heald, Pump Handbook,3rd Edition, McGraw Hill, New York, NY, 2001.


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Karney, B.W., and D. McInnis, "Efficient Calculation of Transient Flowin Simple Pipe Networks", ASCE Journal of Hydraulic Engineering,Vol. 118, No. 7, July, 1992.

Kittredge, C.P., “Hydraulic Transients in Centrifugal Pump Systems”,ASME Journal of Basic Engineering, Vol. 78, No. 6, pp. 1307-1322,Aug. 1956.

Lyons, J. L., Lyons' Valve Designers' Handbook, Van NostrandReinhold, New York, NY, 1982.

Miller, D. S., Internal Flow Systems, 2nd edition, Gulf PublishingCompany, Houston, TX, 1990.

Pipe Line Rules of Thumb Handbook, Gulf Publishing, Houston, TX.

Swaffield, J. A., and Boldy, A. P., Pressure Surge in Pipe and DuctSystems, Avebury Technical, Hampshire, England, 1993.

TAPPI, Generalized method for determining the pipe friction loss offlowing pulp suspensions, TAPPI, TIS 0410-14, Atlanta, GA, 1988.

Tilton, J. N., Perry's Chemical Engineer's Handbook, Seventh Edition,Chapter 6, McGraw-Hill, New York, NY, 1997.

Thorley, A.R.D., and A. Chaudry, “Pump Characteristics for TransientFlow”, Pressure Surges and Fluid Transients, BHR Group, 1996.

Walski, T. M., Analysis of Water Distribution Systems, Van NostrandReinhold Company, New York, NY, 1984.

Walters, T.W., "Analytical Approaches to Modeling Transient VaporousCavitation in Multi-Pipe Fluid Systems", ASME FED-Vol. 116, NewYork, NY, 1991.

Watters, G.Z, Modern Analysis and Control of Unsteady Flow inPipelines, Ann Arbor Science, Ann Arbor, MI, 1979.

White, F. M., Fluid Mechanics, McGraw Hill, New York, NY, 1979.

Wylie, E.B., V.L. Streeter & L. Suo, Fluid Transients in Systems,Prentice Hall, Englewood Hills, New Jersey, 1993.


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Glossary

accumulator A chamber in a pipe system that offers elastic relief topressure waves. Can be either liquid filled or gas filled. Gasaccumulators are used as surge suppression devices.

artificial transient A false transient pressure that is generated becausethe initial conditions do not represent a steady-state condition.

BATCH.LOG File created to log events when a batch run isperformed.

Base scenario The top level scenario in a multiple scenario model.Single scenario models always work in the base scenario.

Best Efficiency Point See BEP.

BEP Best efficiency point. Flow rate at which pump operates atmaximum efficiency.

Bingham plastic See non-Newtonian fluid.

Brecht and Heller method A model for pipe pressure drop in pulpand paper stock flow. See non-Newtonian fluid.

cavitation A phenomenon that occurs when flowing liquids passthrough restricted areas resulting in reduced static pressures andlocalized boiling. Can be very destructive to pipe systems. Transientcavitation differs from the traditional cavitation that occurs in steadyflow systems. The concern in transient flow is that cavities form which,when they collapse, produce a large pressure spike. This is sometimesreferred to as liquid column separation.

checklist The window that tracks the overall completeness of themodel.


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communication time The time it takes a disturbance to propagate tothe nearest boundary and return.

compatibility equations The basic finite difference equations whichsolve the mass and momentum balance.

controlling pipe The pipe with the fastest end-to-end communicationtime in a pipe system. Its properties govern the choice of time step.

convergence The state of the solution when the correct answer hasbeen obtained.

custom database A customizable database that allows pipe system datato be saved. Custom databases are read in during startup and used as ifthey were part of AFT Impulse.

Database Manager The window that allows creation and connectionto network databases.

defined objects Objects whose properties and connectivityrequirements have been completed.

design alert A value assigned to a pipe parameter for which AFTImpulse will warn the user when exceeded.

distributed loss A hydraulic loss that is modeled as occurring over afinite distance in the pipe system (typical of pipe losses).

Duffy method A model for pipe pressure drop in pulp and paper stockflow. See non-Newtonian fluid.

EGL Abbreviation for Energy Grade Line.

elevation The vertical coordinate system (in parallel with body force)that is assigned to junctions. Elevation differences between junctionsmake possible calculations of hydrostatic or gravity head pressurechanges.

Energy Grade Line The sum of the pressure head, elevation head andvelocity head. Quantity is relative to the elevational coordinate systemspecified by the engineer.

exit valve In AFT Impulse, a valve that is located at the exit from apipe system. An exit valve passes fluid to an outside fixed pressure.

flow into system The flow rate that enters the pipe system through ajunction.

fluidhammer Same as waterhammer.


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fluid transient Same as waterhammer.

Graph Results window The graphical window that allows preparationof plots.

head In AFT Impulse, synonymous with piezometric head andHydraulic Grade Line.

head loss The irrecoverable loss in head that occurs in a pipe system.Head loss is equal to the frictional pressure loss converted into units oflength.

HGL Abbreviation for Hydraulic Grade Line.

history The information on how the solution scheme progresses.

Hydraulic Grade Line The sum of the pressure head and gravity head.This quantity is relative to the elevational coordinate system specified bythe engineer.

hydraulic transient Same as waterhammer.

ID number The unique number associated with a pipe or a junction.The ID number is always greater than zero but less than 10,000.

incompressible A fluid whose density changes are small enough that aconstant density can be safely assumed.

infinite pipe A junction which does not reflect and pressure or flowwaves. Available in Assigned Flow and Assigned Pressure junctions.

inspection A feature in AFT Impulse that allows you to obtainread-only information about objects.

instantaneous waterhammer A waterhammer event that occurs sorapidly that it is effectively instantaneous, thus yielding the highestpossible pressure surge.

iteration A single step of the Steady-State Solver, during which a newsolution is obtained based on the previous solution state.

Jump A feature that allows lateral changes to be made betweenSpecifications windows.

junction In AFT Impulse, a pressure/head solution point that connectsand balances flow from pipes.

lock To fix an object's position on the Workspace in order to preventaccidental movement.


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loss factor A parameter that relates the amount of irrecoverablepressure loss through a component. Synonymous with K factor.

Model Data window The text-based window where the input for themodel is given in text form and input data can be entered.

Net Positive Suction Head (or Pressure) See NPSH.

Newtonian Fluid A fluid whose viscosity is independent of thedynamics of the flow. Temperature or pressure dependence may exist.

Non-Newtonian Fluid A fluid whose viscosity is dependent on thedynamics of the flow such as velocity.

NPSH Net Positive Suction Head. A property of pumps that relates thesupply head needed for proper performance.

NPSHA Net Positive Suction Head Available. See NPSH.

NPSHR Net Positive Suction Head Required. See NPSH

NPSP Net Positive Suction Pressure. A property of pumps that relatesthe supply pressure needed for proper performance.

object A pipe system component (either a pipe or a junction) that iscreated, exists, and can be viewed on the Workspace.

object status The state of the object, whether defined or undefined.

Output window The text-based window that shows the results of theanalysis.

piezometric head Synonymous with Hydraulic Grade Line.

pipe In AFT Impulse, a conduit for incompressible, steady-state fluidflow. All pipes have constant diameter and are completely filled withfluid.

point loss A hydraulic loss that is modeled as occurring at a singlepoint in the pipe system. Typical of junction losses.

positive flow direction See reference positive flow direction.

Power law See non-Newtonian fluid.

Pressure Reducing Valve A type of valve that maintains a constantpressure/head at a location, independent of the flow rate.

primary window One of AFT Impulse's two input or three outputwindows.


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PRV See Pressure Reducing Valve.

Pump configuration A particular pump which contains data forrotational speed and impeller size.

reference positive flow direction The flow direction you assign to apipe, which by convention is positive. Flow in the opposite direction isconsidered negative.

relaxation A parameter that affects how the iterative solution schemeapproaches a pipe flow solution.

relief valve A type of valve that is typically closed but opens to releasefluid when a set pressure occurs. Used as a protective device.

resistance A convenient term relating the head loss to flow rate (R =H/Q2).

Scenario Manager Window that allows one to create and managemultiple pipe flow model scenarios. These include different equipment,sizes, and operating conditions.

Solver Either Steady-State or Transient. The Steady-State Solver is thepart of AFT Impulse that contains the pipe steady flow network solutionmethod. The Transient Solver contains the transient waterhammersolution algorithms.

Specifications window The window in which an object's propertiesare entered.

stagnation pressure Sum of static and dynamic pressures.

station A location in a pipe where a transient computation occurs.Form computational reasons pipes are broken into sections. Becausecomputation is required at the pipe inlet and outlet, there will always beone more station than there are sections.

steady-state The condition of a system in which the rate of change ofall parameters is negligibly small.

surge flow Same as waterhammer.

tolerance The value at which the Steady-State Solver should considerthat convergence has been obtained.

Toolbox The area on the Workspace window that offers tools forbuilding pipe flow models.


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valve state The position of the valve control device which regulates theflow through the valve.

Visual Report window The graphical window that allows integrationof the analysis results with the pipe system schematic.

waterhammer A transient phenomenon in pipe systems where arapidly changing operating condition causes large pressure surges.

wavespeed The speed at which a transient wave propagates in a pipe.

Workspace The area of the Workspace window where models arevisually assembled.

Workspace window The graphical window where model assemblyoccurs and input data can be entered.


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2/24/2003 1:35 PM Page 489 c:\aft manuals\i3\impulse3 manual 1b.doc

IndexAAbsolute roughness 62, 186, 187, 295,

323

Absolute tolerance See SteadySolution Control window

Additional Pipe Losses 63. See Losses

customizing 288

design factors 192

in Solver 340

Model Data window 87

Non-Newtonian flow 329

specifying 62

splitting pipes 463

when splitting pipes 124

window 189

ADDLOSS.DAT 289

Adjust Pipes Automatically 81

Affinity laws 232

for pumps 352, 378, 382

AFT DEFAULT INTERNALDATABASE 298

AFT Fathom

Converting models from 10

AFT Impulse

converting custom databases from 1.09

engineering assumptions 4

Example models 7

Getting started 6

installation 5

New features in version 3.0 11

Overview 7

qualifications for use 4

summary of capabilities 2

using online help 7

AFT IMPULSE LOCAL USERDATABASE 298

AFT Impulse window 16, 71

AFT Mercury

Converting models from 10

AFT Standard fluid 29, 134

Change Fluid Data window 294

Database 4, 141, 353

Database editing 146

Aligning objects 81

Analysis type

steady-state vs. transient 175

Animate Using Output File SeeAnimation

Animate Using Solver See Animation

Animation 53, 107

Annotation Manager 78

Annotation tool 17, 78

Annotations 125

Annotion Manager 125

Area Change


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as Additional Pipe Loss 190

Area Change junction 340

abrupt transition 208, 342

conical transition 208, 342

loss factors 209, 342

Specifications window 208

Artificial transients 46, 153, 459

manual detection 460

Stop run when detected 153

ASME Steam Tables 29

Assigned Flow junction 339

behavior in hydraulic solution 339

contrast with pressure-type junction59, 339

example 443

loss factors 62

source/sink indicator on Workspace280

Special features for steady flow 211


Transient Data 211

waterhammer theory 368

Assigned Pressure junction 338

How pressure junctions work 439

loss factors 62

periodic data 214


the role of pressure junctions 442

Transient Data 214

use in open and closed systems 318

using static and stagnation pressure318


Atmospheric pressure 144

Autosave 286

Auxiliary Graph Formatting window111, 288

BBase area 61, 208, 217, 230, 263, 347

Specifying 207

Batch mode 172

BATCH.LOG 173

Bends

loss factors 343

Bernoulli equation 313

Bingham Plastic See non-Newtonian

Black and white junction icons 172

Branch junction 18, 262, 340

flow source or sink 216

loss factors 62

Specifications window 34, 214

Transient Data 216


Brecht & Heller method See non-Newtonian


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Index 491

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CCalculator 175

Cavitation 140, 293

transient 153

Check Valve

theory 395

Check Valve junction 340

Delta Pressure to Re-Open 218

Forward Velocity to Close Valve218

inherent event logic 418

loss factors 218

Special Condition check valve 218


Transient Data 218

Checklist 26, 132

Chempak 29, 134, 141, 353

accuracy options 353

mixtures 29

specifying Chempak file locations470

chopped sine wave 211, 214

Clipboard 90, 111

Colebrook-White 324

Color Map See Visual Report window

column separation See vapor cavitation

communication time 43, 65, 66, 358,375, 437

compatibility equations 363, 367, 368,369, 370, 373, 375, 379

Component Database 296

computing stations 149, 409

graphing 52

in output file 92

max/min results in output 96

numbering convention 149

results in output window 99

Connectivity 126, 128, 129

graphical connection 127

in Junction Specification window201

in Pipe Specifications window 184

model connection 128

Constant Pressure Drop Valve SeePressure Drop Control Valve

Continuity equation

steady-state 312

transient 359

Control Valve junction 338

action if control is lost 221

Control Transient 223

failure to control issues 436

Failure Transient 222

Open Percentage Table 222

sizing pumps with flow control valves448

special condition options 167, 221


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Special Conditions 221


static and stagnation pressure 318

Transient Data 222

types 220


Control.aft file 469

controlling pipe 43, 44, 147, 366, 458

Conventions See also Base area

flow entering and exiting 60

Convergence 336

Converting models from AFT Impulse1.0 9

Copy 79, 80, 122, 124, 126

Create Mixture window 141

Crude oil 324

CTRL key See Keyboard modifiers.See Keyboard modifiers

Custom databases See Databases

Customize Graph window 110

Customizing AFT Impulse

additional pipe losses 288

databases 290

files created 469

fluids 292

graph styles 288

junction (component) databases 296


Output window 92, 287

Parameter Preferences 271

pipe materials 294

Toolbox Preferences 284

Unit Preferences 273

Visual Report window 288

Workspace window 82, 276

Customizing AFT Impulse SeeDatabases. See Model Data Controlwindow. See Parameters and UnitPreferences window. See ToolboxPreferences window. See WorkspacePreferences window. See OutputControl window. See Visual ReportControl window

Cut 79, 126

cyclic dual events 416. See Eventtransients

DDarcy-Weisbach 187, 312, 324

Database Manager 160, 291, 297

Benefits of shared databases 302

Connecting and disconnectingdatabases 298

creating network databases 305

DATABASE.LIB 298, 304, 307

Editing databases 300

Enterprise network databases 304

IMP_DBUSER.LIB 305

Making databases available 298


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Index 493

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sharing databases usingDATABASE.LIB 307

DATABASE.LIB 298, 304, 307, 309

Databases 32, 290

Benefits of shared databases 302

Component database 201, 232, 296,297

Connecting to external shareddatabases 309

converting from AFT Impulse 9

Creating an enterprise-wide networkdatabase system 304

custom junctions 296

fluid See AFT Standard fluid orChempak

fluid database 141, 146, 292

fluid properties 146, 290

Friction Data Set 295

IMP_USER3.DAT 271, 291, 300,306, 469

pipe materials 185, 294

special control files 302

Specifying common locations 286

Dead End junction 339



Defining objects

Checklist - Define All Pipes andJunctions 30, 134

object status 80

Delete 79, 126

Density 140

Design Alerts See Pipe Specificationswindow

cross-plotting against results 106

Design factors

defaults 272

junctions 206, 342

pipes 192, 329

Discrete Vapor Cavity Model 364, 409

Distributed loss 62, 340

Drag-and-drop 3

Duffy method See non-Newtonian

Duplicate 79, 122, 124, 126

Dynamic Pressure 317

Dynamic viscosity 140

EEditing

on Workspace 79

EGL 106

Elbow

See Bend 343

Elevation 106, 200

default 273

Event messages 103, 420

Event transients 414

Example models 7

Exit valve 373. See Valve junction


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Explicit friction factor 62, 188

Extended Model Check 175

FFanning friction factor 323

FCV See Flow Control Valve

Find utility 163

Flip workspace See Workspacewindow

Flow Control Valve 220, 339, 436,447, 450

Flow rate relaxation See SteadySolution Control window

Fluid database See AFT Standard fluidor Chempak

Fluid Database window 292, 294

fluid transients

See waterhammer

fluidhammer

See waterhammer

Fouling 187

Friction Data Set 186, 295

Friction factor 145, 187

changes during transient flow 153

Friction models 62, 322

Absolute roughness 323

Frictionless pipe 326

Hazen-Williams 324

Hydraulically smooth 324

Miller Turbulent method 325

MIT Equation 324

non-Newtonian 326

Relative roughness 323

Frictional Losses See Losses

Frictionless pipe 62, 188, 326

GGas Accumulator junction 340

connector pipe 227

flow restrictor 227

Graphing transient data 229

in output file 155

Initial Gas Volume 225

lumped inertia 407

Maximum and minimum volumes226

orifice 227

Polytropic Constant 225



surge reduction 427


Gaussian Elimination 336

With pivoting 336

General Component junction 340

loss factors 229


General Preferences 286

Global Junction Edit window 268


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Index 495

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Global Pipe Edit window 267

Glossary 483

Graph Results window 8, 51, 72, 104,288

Animation 53, 107

Import Graph Data 105

Save Graph Data 105

Select Data 52

Select Graph Data 104

Graphics Server

problems loading 473

Gravitational acceleration 5, 145

Grid

On Workspace 24, 115, 279

Grouping objects 80, 125

HHazen-Williams 188, 324

H-Equation method 311

HGL 106, 312

Hide Empty Columns


Highlight feature 32, 129, 180

Homologous pumps See Affinity Laws

Hydraulic Grade Line See HGL

hydraulic transients

See waterhammer

Hydraulically smooth 62, 188

Hydrocarbons - roughness 325

IIcon size 278

ID numbers 22, 31, 122, 124, 146, 183,281

ID Reduction - Scaling 186

IMP_DBUSER.LIB 305, 308, 309

IMP_USER3.DAT 271, 286, 291, 300,306, 469

impedance 363

Impulse Examples.hlp 7

IMPULSE3.INI 271, 286, 469

Infinite Pipe

example 437


theory 375

Initial guesses

flow guess 191

generation 205

pressure guess 204

using for troubleshooting models435

Inspection feature 33, 130, 197, 199

Showing only undefined items 464

Installation 469

Network installations 471

special network installations 470

Intermediate Pipe Elevations 193

Irrecoverable losses See Losses

Iteration history 336, 435


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JJacobian matrix 314

Jump 128, 179, 202

Junction icons

printing black and white 172

Junction label position 281

Junction Results table 90

Junction Specifications window 194

Area Change 208

Assigned Flow 209

Assigned Pressure 212

Branch 214

Check Valve 217

connected pipes 201

Control Valve 219

Database list 201

Dead End 223

design factor 206

displaying name 206

Elevation 200

format 197

Gas Accumulator 224

General Component 229

Initial pressure guess 204

Jumping to next higher or lowernumber junction 129

Liquid Accumulator 231

Name 199

Notes 207

Number 199

Pump 232

Relief Valve 248

Reservoir 250

Same As Junction 201


Spray Discharge 253

Status 208

Surge Tank 255

Tee/Wye junction 261

Vacuum Breaker Valve 264

Valve 263

Volume Balance 266

Junctions 59

and the Steady-State Solver 338

Base area 207

comparison of types 195, 338

connectivity 126

creating 124

customizing icons 285

defining 128

displaying data in Output window96

displaying in Model Data 88

displaying in Visual Report 115

displaying on Workspace 83

displaying transient data in GraphResults 106


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Index 497

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displaying transient results in Outputwindow 103

editing 79, 126

finding 163

flow specifying 339

Global Editing 268

icons on Toolbox 79

ID numbers 124

keyboard shortcuts 463

loss factor 62

negative flow rates 197

pipe joining 340

placing on Workspace 124

pressure losses 340

pressure modifying 340

pressure specifying 338

specifying data 128

KKeyboard modifiers 461

CTRL 73, 74, 121, 126

SHIFT 73, 74, 77, 80, 121, 125

LLaminar flow 145, 323

Legend 114, 118

Liquid Accumulator junction 340

Elasticity 231

Initial Volume 231



Lock 79, 125

Loss factor 60, 62, 189, 197, 198, 208,214, 217, 221, 229, 251, 261, 289,341

loss models 36, 38, 341

Losses 60, 62, 313, 323, 340. SeeAdditional Pipe Losses

LU Decomposition 336

Lumped inertia pipe with orifice

theory 407

MMatrix Method 336

Merging models 170

with multiple scenarios 170

Method of Characteristics 43, 358,359, 458

reliability with cavitation 410

Miller Turbulent Method 188, 325

minor losses See Losses

MIT Equation 188, 324

Mixtures 4, 29, 141, 142, 353

Model Data Control window 88

database connections 89

showing junction data 88

showing pipe data 88

showing selected pipes and junctions89


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using saved format files in DatabaseManager 302

Model Data window 8, 41, 71, 86

Additional Losses 87

customizing 88

opening pipe and junctionspecifications windows from 128,179

Pipe Detail Summary 87

Pipe Loss Data table 87

Pipe table 87

print format 174

transient junction data 422


Model Status Light 132

modulus of elasticity 187

Momentum equation

steady-state 312

transient 359

Moody friction factor 323

morph 124, 463

NNetwork databases 201. See Database

Manager

Newton-Raphson method 3, 312, 314,321

transient solution for gasaccumulators 400

non-Newtonian 143, 144, 322, 326

Additional Pipe Losses 329

Bingham Plastic 143, 328

Brecht & Heller method 327

Duffy method 326

junction losses 329

Power Law 143, 328

pulp and paper 326, 327

NPSH 140, 234, 237

constant for variable speed 234

NPSHR 237

NPSP 140

NPSPR 237

NPSP See NPSH

OObject status 80, 130, 146, 194, 208

One-dimensional flow 59

Open Pipe/Jct Window 86, 128, 179

Orifice

loss factors 345

round edged 345

sharp edged 345

Output Control window 27, 50, 92,134, 136, 287, 321


Format and Action 95

General Output Control 94

Pipe and Junction parameters 92, 94

scenarios 160


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Index 499

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Show Selected Pipes/Junctions 96


Output file 47, 154

transient data 91

Output window 8, 48, 71, 90

customizing 287

Event messages 420

exporting output 90

number formatting 95

print format 174

setting defaults 97

updates 104

Overview of AFT Impulse 7

PParallel Pipes

in Pipe Specifications window 192

Parameter and Unit Preferences window271


default roughness model 188

Design Factors 272

elevation 273

setting defaults 276

setting preferred units 275

unit preferences 273

Use Most Recent Pipe Size Data 272


Paste 79, 80, 122, 124, 126

PDCV See Pressure Drop ControlValve

Piezometric head 311

Pipe Detail Summary


Pipe Drawing tool 17, 22, 74, 121, 462

Pipe flow direction 210

Pipe Loss Data table


Pipe Material Database window 185,294

Pipe Results table 90

pipe sectioning 43, 365, 366. SeeSection Pipes

effect on run time 457

Pipe Specifications window 38, 39,180

Additional Losses 189

connected junctions 184

Design Alerts 191

displaying name 192

Fluid Properties 194

friction model 187

ID Reduction - Scaling 186

initial flow 191

jumping to next higher or lowernumber pipe 129


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material 185

Notes 194

optional input 191

Parallel Pipes 192

pipe diameter 186

Pipe Length 185

Pipe Line Size and Color 192

pipe modulus of elasticity 187

Pipe Name 183

Pipe Number 183

pipe Poisson ratio 187

pipe support 189

pipe wall thickness 186

roughness model 187

Same as previous pipe 184

Same Specs as Pipe 184

Status 194

wavespeed 189

pipe stations See computing stations

Pipe Support 189

Pipe table


Pipes 22, 59

connectivity 126

creating 121

defining 128

Design factors 192

displaying data in Output window96

displaying in Model Data 88

displaying in Visual Report 115

displaying on Workspace 83

displaying transient data in Outputwindow 99

editing 125

finding 163

flow direction 60, 122, 124, 164,184, 197

Global Editing 267

handles 75, 125

ID numbers 122

length on Workspace 122, 185

loss factor 62

losses 60, 61, 62, 323, 324, 340

number of connections 122

Reverse direction 164

segmenting 75

specifying data 128

Splitting 124, 463

stretching 125

Point loss 62

Poisson Ratio 187

Popup menu 80

positive flow direction 23

Power Law See non-Newtonian

Pre-mixtures 142


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Index 501

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Pressure

defining system pressure 155, 442

Pressure Drop Control Valve 220, 450

Pressure Reducing Valve 220, 436,453

static and stagnation pressurediscussion 318

Pressure relaxation See SolutionControl window

Pressure Sustaining Valve 220, 436,453, 455

static and stagnation pressurediscussion 318

Primary windows 7, 71

Print Format 88, 90, 174

Print Preview 170

Print Special 170

PRV See Pressure Reducing Valve

PSV See Pressure Sustaining Valve

pulp and paper stock See non-Newtonian

Pump junction 339, 340, 448

as assigned flow 246

check valve 234

controlled discharge pressure 234,352

controlled flow 234, 352

Controller transient 244

efficiency and power usage 238

Estimating the pump inertia 243

Graphing pump data 248

in output file 155

Known speed transients 238, 239

NPSHR 237

problems from poor curve fits 431

pump configurations 234

Pump Startup With Inertia 243

Pump trip with inertia – four quadrant241

Pump trip with inertia and nobackflow 240, 241

sizing with flow control valves 448

Special Conditions 246, 248


Submerged pump 233

Transient Data 238

variable speed 232, 234, 352

viscosity corrections 247


pump speed 379

Pumps 140, 352

efficiency 234, 238

power usage 234, 238

Pump configurations 234

sizing method 440

sizing with flow control valves 448

submerged 381

viscosity corrections 247


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RREADME.TXT 5

Reference Information 94, 174

Reference positive flow direction

See positive flow direction

Reference Pressure 443

References 481

Relative location 18

Relative roughness 62, 186, 188, 323

Relative tolerance See Steady SolutionControl window

Relaxation See Steady SolutionControl window


Relief Valve junction 21, 338, 340

entering transient data 249

Exit 394


Internal 394

Internal vs. exit relief valves 249

loss factors 62, 217


surge reduction 426

Transient Data 250

Transient valve cracking 249


Renumber Wizard 85

Renumbering See Workspace window.See Workspace window

Repeat transient 420

Reservoir junction 17, 60, 338

Difference between a Reservoir andSurge Tank 260

How pressure junctions work 439

loss factors 62

Specifications window 31, 36, 250

the role of pressure junctions 442

Transient Data 253

use in open and closed systems 318


resistance 363, 367

variable during transient 63

Resistance Curve 230

Reverse Direction 164

Reynolds number

effects during transient flow 153

transition 145, 324

SScale/flip workspace See Workspace

window

Scaling See ID Reduction - Scaling

Scenario Manager 156

re-establishing broken links 161

Scenarios

merging models with multiplescenarios 170


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Index 503

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same as parent junction 201

same as parent pipe 184

Section Pipes window 43, 135, 147,152, 176, 366, 457, 458

Select Flow Path 80, 125

Select Graph Data window 105

Select Special 80, 97, 164

Selection Drawing tool 16, 73, 80, 125,461

selecting objects completely orpartially in box 278

sequential dual events 417. See EventTransients

Setup program 6

SHIFT key See Keyboard modifiers.See Keyboard modifiers. SeeKeyboard modifiers

Show list box

in Output window 90

Show Object Status 146

single event transients 414

Solution Balance Summary table 321

Solution Progress window 46, 90, 173,336

iteration history 336

View Results button 90

Solver

discussion of Steady-State andTransient 46

output file for transient data 47, 91

Recommendations on Steady Solver176

Steady-State 134, 175, 311, 329

Transient 176, 358

Transient Solver control 151

verification of steady-state 321

working with the steady and transientsolvers 175

Sort output feature 90


Sparger 254

Special Conditions 165, 436

Ignore 169

junctions 206

No reflections - infinite pipe 169

Pipe/Junction Symbol on Workspace281

Transient 169

Workspace appearance 278

Specifications windows 86, 179

accessing 179

inspecting in 131

Jump feature 180

Splitting pipes 124, 463

Spray Discharge junction 338

Graphing Spray Discharge data 255

in output file 155



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Transient Data 255


Spreadsheet

exporting data to 90

Stagnation pressure

at pressure control valves 318

explanation 315

Static pressure

at pressure control valves 318

explanation 315

Status Bar 132

Steady Solution Control window 134,135

absolute tolerance 331

convergence 330

false convergence 335

flow rate relaxation 334

Matrix Method 336

maximum iterations 336

parameters 329

pressure relaxation 335

relative tolerance 331

relaxation 333

relaxation example 334

tolerance 330

tolerance example 332

Submerged pumps 381

Support 479

Annual Support 479

e-mail 480

surge flow

See waterhammer

surge suppression 425

Surge Tank junction 20, 340

as a branch during steady-state 259

as a finite size reservoir duringsteady-state 260

as a pressurizer during steady-state259

connector pipe 257

Difference between a Reservoir andSurge Tank 260

flow restrictor 257

Graphing Surge Tank data 261

in output file 155

lumped inertia 407

Model as dipping tube vessel 260

Only Allow Outflow 258

orifice 257



surge reduction 427

Tank area 256

Tank geometry 256

Tank height 256

Transient Data 258



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Index 505

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surge transients

See waterhammer

System pressure 155, 442

System Properties window 28, 134,138

AFT Standard fluid database 141

atmospheric pressure 144

Chempak database 141

creating mixtures 141

fluid list 293

gravitational acceleration 145

scenarios 160

Transition Reynolds number 145

TTechnical Support See Support

Tee/Wye junction 198, 340

convergence problems andrecommendations 434

loss factors 62, 262, 346

Simple, no loss 262



Terminology 1

time step 365, 458, 459

Time-based transients 413

Tip of the Day 286

Title 94, 114

Tolerance

balance verification 321


Toolbars 119

Toolbox 72, 79, 121, 124

customizing See ToolboxPreferences window

Toolbox Preferences window 284

Current Toolbox/Unused Tools 285




ToolTips 79

Transfer Results to Initial Guess 90,172

Transient Control window 45, 135,151, 176, 229, 244, 248, 255, 261,266, 413, 457

artificial transients 459

Estimated file size and run time 155

Include in transient output file 154

Model transient cavitation 153

Save Output to File Every Time Step152

Saving junction output 155

Saving pipe output 154

scenarios 160

specifying content for output file 91

Start and stop time 151


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Stop run if artificial transient detected153

variable resistance 64, 152

Transient Data

Absolute vs. relative 419

Repeat transient 420

Transient output file See Output file

Transient special conditions SeeSpecial Conditions

Transient vapor cavitation See vaporcavitation

Transition flow 145, 324

Transition Reynolds number 145

Transport properties 353

Troubleshooting steady-state models430

Turbulent flow 145, 324

UUndo 79, 126

Unit Preferences See Parameter andUnit Preference window

Unspecified fluid 28

VVacuum Breaker Valve junction 340

Graphing Vacuum Breaker Valvedata 266

in output file 155



surge reduction 427

Valve coefficient 350

Valve junction 21, 60, 338, 340

Exit valve 263, 338

loss factors 62, 343, 345



Transient Data 264


vapor cavitation 46, 150

transient 364, 409

vapor cavitation waterhammer theory

assigned flow 369

assigned pressure 368

branch 371

check valve 395

dead end 370

gas accumulator 401

infinite pipe 376

liquid accumulator 393

pump 381

Relief Valve 395

spray discharge 378

surge tank 398

valve 374

Vapor pressure 140

Variable fluid properties 139, 194


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Index 507

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variable pipe resistance 152

Verification

Verification against published results7

Verifying steady-state networksolutions 321

View Results button See SolutionProgress window

Viscosity corrections 247

Visual Report Control window 112,288

scenarios 160


Visual Report window 8, 55, 71, 111,288

Color Map 115

Display Parameters 113

General Display 113

input-only mode 115

legend 114, 118

Save Options 118

Show Selected Pipes and Junctions115

Visual Report Control 112

Volume Balance junction 340


WWalk Through Example 15

wall thickness 186

Water Data from ASME Steam Tables29

waterhammer 1, 64

description 64

instantaneous 65, 355

Waterhammer Assistant 162

wavespeed 39, 43, 64, 66, 147, 148,189, 356, 358, 365, 366, 409

assumption that it remains constant5

Calculated 185

User Specified 186

Web site 480

Workspace

grid 279

Reverse Direction 23

Workspace Preferences window 125,146, 166, 169, 206, 276

Action When Dragging Junctions279

Allowable Workspace LabelMovements 281

colors and fonts 282


display options 279

displaying name and/or ID numbers281

icon size 278

pipe line options 277

Sample Workspace 283


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source/sink indicator 280

Special Condition Pipe/JunctionSymbol 281

special conditions 278

Transient indicators on theWorkspace 421


Workspace window 8, 16, 71, 72

changing junction icons 207

colors and fonts 282

customizing 82, 276

Displaying name and/or ID numbers281

editing 79

finding pipes and junctions 163

flipping 84

pipe lengths on 185

relative locations 18

renumbering 84

scaling 84

setting the junction label positionSee Workspace Preferenceswindow

size 83

Special Condition Pipe/JunctionSymbol 281

zoom feature 83

ZZoom feature 83, 111

Zoom Select tool 17, 77, 463


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Impulse 3.0 Users Guide

Documents

Transcript of Impulse 3.0 Users Guide