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    CHEMCADPIPING SYSTEMS

    Tutorial

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    CHEMCAD PIPING SYSTEMS

    TABLE OF CONTENTS

    Chapter 1 -Introduction to Piping Networks in CHEMCAD ..................................................................................1Piping Networks ......................................................................................................................................................... 1

    About Piping Network Systems ....................................................................................................................... 1Flowrate as a Function of Pressure for Fluid Flow.......................................................................................2About Modeling Piping Network Systems...................................................................................................... 3

    Chapter 2-UnitOps for Piping Networks .................................................................................................................4Pressure Node............................................................................................................................................................ 4

    Flowrate Options at Node.........................................................................................................................6Fixed Flowrates at Node...........................................................................................................................6Variable Flowrates at Node......................................................................................................................6Mass Balance Limitations for Flowrate Calculation..............................................................................7

    Node as Divider.......................................................................................................................................................... 8Pressure Node Dialog Screen.................................................................................................................................9

    Mode ............................................................................................................................................................ 9Pressure at Node .......................................................................................................................................9Minimum Pressure...................................................................................................................................10Maximum Pressure..................................................................................................................................10Elevation....................................................................................................................................................10Flowrate Options (Inlet)...........................................................................................................................10Stream Number........................................................................................................................................10Flowrate Option........................................................................................................................................10

    Fixed Mole Rate/Fixed Mass Rate/Fixed Volume Rate.............................................................10Flow Set by Pipe/Valve/Pump........................................................................................................10Free Inlet Stream .............................................................................................................................10

    Use Current Stream Rate ...............................................................................................................10Value..........................................................................................................................................................10Flowrate Options (Outlet)........................................................................................................................10Stream Number........................................................................................................................................10Flowrate Option........................................................................................................................................10

    Fixed Mole Rate/Fixed Mass Rate/Fixed Volume Rate.............................................................11Flow set by Pipe/Valve/Pump........................................................................................................11Free Outlet Stream ..........................................................................................................................11

    Value..........................................................................................................................................................11Pipe Simulator..........................................................................................................................................................11

    Description................................................................................................................................................11Piping Network Modes of Pipe Simulator.............................................................................................11

    Pump..........................................................................................................................................................................12Description................................................................................................................................................12Piping Network Modes of Pump UnitOp...............................................................................................12

    Valves ........................................................................................................................................................................12Description................................................................................................................................................12

    Piping Network Modes of Valve UnitOp ...............................................................................................12

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    Control Valve.............................................................................................................................................................12Description.................................................................................................................................................12Piping Network Modes of Control Valve ...............................................................................................12

    Compressor...............................................................................................................................................................12Piping Network Modes of Compressor.................................................................................................12Node as Mixer ...........................................................................................................................................................13Steady State UnitOps ..............................................................................................................................................13Chapter 3-Control Valve Sizing..............................................................................................................................13

    Topics Covered.........................................................................................................................................13Problem Statement..................................................................................................................................13The Simulation..........................................................................................................................................14Control Valve Sizing.................................................................................................................................14

    Rating Case...............................................................................................................................................16Flowrate as a Function of Pressure.......................................................................................................20

    Chapter 4-Simple Flow Example...........................................................................................................................25Topics Covered.........................................................................................................................................25Problem Statement ..................................................................................................................................25

    Creating the Simulation...................................................................................................................................26Using Controllers to Simplify the Problem....................................................................................................27Calculating NPSHA...........................................................................................................................................28

    Chapter 5-Branched Flow Example......................................................................................................................29Topics Covered.........................................................................................................................................29Problem Statement..................................................................................................................................29

    Creating the Simulation...................................................................................................................................30Running the Simulation...................................................................................................................................33Selecting a Pump.............................................................................................................................................33

    Chapter 6-Flow Relief Piping System ...................................................................................................................36Topics Covered.........................................................................................................................................36

    Flare Header Design........................................................................................................................................36

    Problem Statement..................................................................................................................................37One Branch.......................................................................................................................................................38Two Branch.......................................................................................................................................................38Four Branch.......................................................................................................................................................39Specifying the Outlet Pressure.......................................................................................................................41Addition of Pipe to Discharge.........................................................................................................................41Effect of Closed Valve .....................................................................................................................................41Different Approaches to Relief Problems .....................................................................................................41

    Chapter 7-Piping Calculation Methods .................................................................................................................42The General Energy Equation........................................................................................................................42Conservation of Momentum............................................................................................................................44Friction Factor Determination.........................................................................................................................45Acceleration Component.................................................................................................................................45

    The Isothermal Flow Equation................................................................................................................................45The Darcy-Weisbach Equation..............................................................................................................................46The Hazen-Williams Equation................................................................................................................................46The Fritzche Equation..............................................................................................................................................47

    Two Phase Flow.......................................................................................................................................................47

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    Definitions for Two Phase Flow .............................................................................................................................47Liquid Holdup............................................................................................................................................47Gas Holdup or Gas Void Fraction.........................................................................................................47

    No-Slip Liquid Holdup..............................................................................................................................47Two-Phase Density..................................................................................................................................47Superficial Velocity...................................................................................................................................48

    Slip Velocity.......................................................................................................................................................48Modification of the Pressure Gradient Equation for Two Phase Flow.............................................................48The Baker Method for Two Phase Flow Calculation..........................................................................................49

    Establishing the Two-Phase Flow Pattern...........................................................................................49Pressure Losses for Two-Phase Flow..................................................................................................49Dispersed Flow.........................................................................................................................................50

    Bubble Flow..............................................................................................................................................50Slug Flow...................................................................................................................................................51Stratified Flow...........................................................................................................................................51Wave Flow.................................................................................................................................................51Plug Flow...................................................................................................................................................52Annular Flow.............................................................................................................................................52

    The Baker Method Procedure................................................................................................................................53Assumptions .............................................................................................................................................53Procedure..................................................................................................................................................54Two Phase Flow Regime........................................................................................................................54

    Calculate Pv,100........................................................................................................................................54Calculate Lockhart-Martinelli Parameter, X

    2........................................................................................55

    Calculate2..............................................................................................................................................55

    Calculate P2,100......................................................................................................................................56The Beggs and Brill Method for Two Phase Flow Calculations........................................................................56

    Horizontal Flow.................................................................................................................................................56Flow Regime Determination...........................................................................................................................60

    Two-Phase Density..........................................................................................................................................61Friction Factor...................................................................................................................................................62Acceleration Term ............................................................................................................................................63Elevation Term .................................................................................................................................................63

    Vertical Flow .............................................................................................................................................................63Vertical Flow Regimes.....................................................................................................................................64

    Set Up Calculations .................................................................................................................................................66The 3-K (Darby) Method of Specifying Flow Resistances.................................................................................70

    Heat Loss in Piping Systems .................................................................................................................................71Net Positive Suction Head......................................................................................................................................71Net Positive Suction Head..............................................................................................................................71Pumping Saturated Liquids ............................................................................................................................72

    Pipe Sizing Using the Kent Method ......................................................................................................................73Variable Definitions ..................................................................................................................................................74

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    Chapter 8-Descriptions of Valves and Fittings ....................................................................................................75Valves.................................................................................................................................................................75Flanged Fittings ................................................................................................................................................75

    Welded Fittings .................................................................................................................................................76Miscellaneous ...................................................................................................................................................76Valve Diagrams ........................................................................................................................................................77

    Gate Valve .........................................................................................................................................................77Globe Seat Flat, Bevel, Plug ..........................................................................................................................78Globe Wing/Pin Guided Disk..........................................................................................................................79Angle, No Obstruction......................................................................................................................................79Angle, Wing/Pin Guided Disk.........................................................................................................................80Y-Pattern Globe 60 Degrees..........................................................................................................................80

    Y-Pattern Globe 45 Degrees ..........................................................................................................................80Ball Valve ...........................................................................................................................................................81Butterfly, 2-8 Inches.........................................................................................................................................82Butterfly, 10-14 Inches.....................................................................................................................................82Butterfly, 14-24 Inches.....................................................................................................................................82Plug, Straight Way............................................................................................................................................83Plug, 3-Way (Straight Run).............................................................................................................................83Plug, 3-Way Through Branch.........................................................................................................................83Foot Valve, Poppet Disk..................................................................................................................................84Foot Valve, Hinged Disk..................................................................................................................................84Swing Check, Clearway-90.............................................................................................................................84Swing Check, Tilting Seat...............................................................................................................................84Tilt Disk, 5 Degrees 2-8 Inches ......................................................................................................................85Tilt Disk, 5 Degrees 10-14 Inches..................................................................................................................85Tilt Disk, 5 Degrees 16-48 Inches..................................................................................................................85Tilt Disk, 15 Degrees 2-8 Inches....................................................................................................................85Tilt Disk, 15 Degrees 10-14 Inches ...............................................................................................................85

    Tilt Disk, 15 Degrees 16-48 Inches ...............................................................................................................85Lift or Stop Check Valve, Globe.....................................................................................................................85Lift or Stop Check Valves, Angle ...................................................................................................................86

    Flanged Fittings ........................................................................................................................................................87Standard Elbow, 90 Degrees .........................................................................................................................87Standard Elbow, 45 Degrees .........................................................................................................................87Standard Elbow, 90 Long R............................................................................................................................87Return Bend, 180 Degrees, Close (Flanged) ..............................................................................................88Standard T, Flow-through Run (Flanged).....................................................................................................88Standard T, Flow-through Branch (Flanged) ...............................................................................................8845 Degrees T, Flow through Run (Flanged) ................................................................................................8945 Degrees T, Flow through Branch (Flanged)...........................................................................................89Elbow 90 Degrees, R/D = 1.0 (For Welded, Flanged, or Screwed).........................................................90Elbow 90 Degrees, R/D = 1.5 (For Welded, Flanged, or Screwed).........................................................90Elbow 90 Degrees, R/D = 2.0 (For Welded, Flanged, or Screwed).........................................................90

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    Welded Fittings.........................................................................................................................................................90Elbow 45 Degrees, R/D = 1.0 (Welded).......................................................................................................90Elbow 45 Degrees, R/D = 1.5 (Welded).......................................................................................................90

    Elbow 45 Degrees, R/D = 2.0 (Welded).......................................................................................................90Return Bend 180, R/D = 1.0 (Welded).........................................................................................................91Return Bend 180, R/D = 1.5 (Welded) .........................................................................................................91Return Bend 180, R/D = 2.0 (Welded) .........................................................................................................91Tee 100% Flow through Run (Welded)........................................................................................................91Tee 100% Flow out Branch (Welded)...........................................................................................................91Tee 100% Flow in Branch (Welded).............................................................................................................92Reducer (Welded)............................................................................................................................................92

    Miscellaneous ...........................................................................................................................................................93

    Entrance, Inward Projecting ...................................................................................................................................93Entrance, Sharp Edged...........................................................................................................................................93Entrance, Slightly Rounded ....................................................................................................................................93Entrance, Well Rounded.........................................................................................................................................93Exit from Pipe...........................................................................................................................................................94

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    CHEMCAD Piping Systems

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    CHAPTER 1 - INTRODUCTION TO PIPING NETWORKS IN CHEMCAD

    This document provides an overview of the piping network design features in CHEMCAD. Thisdocuments covers:

    What is a Piping Network

    UnitOps that calculate flowrate as function of Pressure

    Use of the Node (NODE) UnitOp

    Use of flowrate scaling UnitOps

    Use of normal UnitOps on piping network flowsheets How to model a piping network system on a flowsheet

    Several tutorial examples

    PIPING NETWORKS

    ABOUT PIPING NETWORK SYSTEMSA piping network represents the flow of fluids through several pieces of equipment. If sufficient variables(flowrate and pressures) are specified on the network, the unknown variables may be calculated.

    For fluid flow through equipment, flowrate may be calculated as a function of the inlet and outletpressure. If the user can specify two of the three variables the third is dependent. Specification ofpressure at various points on a piping network diagram allows the system to be described as a system ofdependent equations.

    The piping network models in CHEMCAD allow for the simultaneous solution of such a system. Ifsufficient constraints are specified, the model will simultaneously solve the flowsheet to converge on theunknown pressures/flowrates throughout the model.

    A simple relief flow system is shown in Figure 1-1.

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    Figure 1-1: Simple Relief Flow

    To size the valve, the pressure out of the valve must be calculated. Known variables are geometry ofthe pipe, pressure out of pipe, and flowrate through pipe. A single equation can be used to solve for thepressure into the pipe as a function of the known variables.

    FLOWRATE AS A FUNCTION OF PRESSURE FOR FLUID FLOW

    Fluid mechanics allows the calculation of fluid flowrate through a pipe or nozzle as a function of inlet and

    outlet pressures. The use of performance curves allows the calculation of fluid flow through acompressor or pump as a function of inlet and outlet pressures. Figure 1-2 shows UnitOps that maycalculate flowrate as a function of pressures. These UnitOps are referred to as flow scaling UnitOps inthis document because they scale the flowrate of the process stream.

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    CHEMCAD Piping Systems

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    Figure 1-2

    ABOUT MODELING PIPING NETWORK SYSTEMS

    Piping network systems are used to solve for flowrates and/or pressures around a network of connectedequipment. Typically the user has a flowsheet of equipment connections and various constraints (exitflowrates, pressure limitations on equipment, etc.) but does not have all the flowrate(s) and/orpressure(s) for the system.

    You may solve a piping network system on a CHEMCAD flowsheet. New models in CHEMCAD allowyou to specify the known variables and solve for the unknown variables on a flowsheet.

    The NODE UnitOp allows you to specify the pressure on either side of a UnitOp and calculate the

    flowrate as a function of pressure. As an option, you may specify one pressure and the flowrate.Iterative calculations will solve for the unknown pressure based on the specified pressure and flowrate.

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    A series of UnitOps may be connected using several nodes. The flowrate through the chain may bespecified at a single point, or calculated based on specified pressures around a UnitOp. It is not

    necessary to know the pressures around all UnitOps in the series.

    Figure 1-3 shows a simple flare network. There are seven variables of pressure and flowrate. Three ofthe variables must be specified.

    Figure 1-3

    CHAPTER 2 UNITOPS FOR PIPING NETWORKS

    PRESSURE NODE

    The piping network calculations solve for pressure at nodes and then iteratively calculate the flowratesthrough the network as functions of pressure.

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    A NODE UnitOp represents a point in the piping network where a change in pressure occurs due toelevation change, flow through a pipe, or flow through equipment that changes pressure (pump, valve,

    etc). A CHEMCAD flowsheet for a piping network uses the pipe UnitOp for piping effects and UnitOpssuch as the pump, compressor, and control valve.

    For design of a piping network it is necessary to determine pressure between all UnitOps that calculatepressure as a function of flowrate. The NODE UnitOp sets the pressure on one side of a UnitOp thatcalculates pressure as a function of flowrate.

    The pressure at a node may be specified by the user or calculated by CHEMCAD. The flowrate(s) inand out of a node may be specified or calculated. The flowrates may be specified at the NODE

    UnitOp, or calculated as dependent on adjacent UnitOps.The NODE UnitOp sets a fixed value on the flowsheet. For piping network calculations there are pointson the flowsheet where either the pressure or flowrate is known. The NODE UnitOp allowsspecification of the known variable and calculation of the unknown variable.

    To learn the concepts for specifying a node, look at a system of two nodes surrounding a UnitOp. Thisis shown in Figure 2-1.

    Figure 2-1

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    For the system in Figure 2-1 the inlet pressure (P1), outlet pressure (P2), and flowrate (F) through thepipe are the three variables. A single equation constrains the system. Specification of any two of the

    variables allows CHEMCAD to solve for the third variable.

    If pressure is specified at the first node and either node specifies flowrate, the pressure of the secondnode is variable. CHEMCAD will vary the pressure of the second node until flowrate as a function ofpressure around the pipe equals the specified flowrate. The pressure may vary at either node.

    The pressure of a feed or product stream of known flowrate may be adjusted by adjacent nodes. InFigure 2-1, specifying P1 as fixed pressure specifies the pressure of stream 1 as P=P1.

    If pressure at both nodes is specified, the flowrate through the UnitOp is a dependent variable. The

    variable flowrate may be either the feed stream or product stream. In the NODE UnitOp, specify thelocation where flowrate is a variable. Use mode free outlet or free inlet to specify whether the inlet oroutlet flow is calculated. The model will cascade this flowrate upstream and downstream of the UnitOp.

    The pressure of streams attached to a NODE UnitOp will be set to the pressure of the node. Theflowrates through a network will all be set to the calculated flowrate through a node. You may specifyN 1 flowrates on a flowsheet, where N is the total of feed and product streams on the flowsheet. Thecalculated flowrate will be passed through nodes that use the dependent flowrate. You will receive anerror message if you attempt to specify or calculate two conflicting flowrates through a system with two

    separate nodes.

    Flowrate Options at NodeThe flowrate for an inlet or outlet stream may be manipulated by a node. The node acts bymanipulating the flowrate of the adjacent UnitOp. The pressure settings for the nodes on either side ofthe adjacent UnitOp contribute to the flowrate manipulation.

    Fixed Flowrates at NodeUsing a fixed inlet flowrate for a node specifies the flowrate through the upstream UnitOp. The

    pressure on one side (node) of the UnitOp must be variable. An exception is when a node is acting asa mixer or divider for N streams and the one stream is variable. In this situation the pressure can befixed or variable for both nodes.

    The Fixed outlet flowrate for a node specifies the flowrate through the downstream UnitOp. Thissetting is similar to fixed inlet.

    The Current flowrate setting for an inlet stream is similar to fixed inlet. The current flowrate uses theflowrate currently stored for the inlet stream rather than a specified value in the node.

    Variable Flowrates at NodeUsing free inlet for a node specifies that the feed stream flowrate is a calculated variable. The node willmanipulate the upstream feed flowrate to solve the system. The free inlet specification works best on anode connected to a Feed stream but it may be placed elsewhere on the flowsheet.

    If the outlet flow is specified, the free inlet specification allows the feed to be calculated to maintainmass balance. Only one free inlet specification is allowed per feed stream.

    The free outlet stream for a node is similar to the free inlet setting. Using free outlet specifies that theproduct stream flowrate is a calculated variable. The node will manipulate the product flowrate to solvethe system. The free outlet specification works best on a node connected to a product stream but itmay be placed elsewhere on the flowsheet.

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    If the inlet flow to a system is specified, the free outlet specification allows the product to be calculatedto maintain mass balance. Only one free outlet specification is allowed per product stream.

    If you attempt to specify too many free outlet or free inlet streams, CHEMCAD will issue a warningmessage and reset the extra specifications to flow set by UnitOp.

    The flow set by UnitOp setting indicates that the flowrate is controlled by the adjacent UnitOp. TheUnitOp may be calculating flowrate as a function of pressure. The UnitOp may be using the flowratecalculated by another UnitOp.

    Mass Balance Limitations for Flowrate CalculationOnly one UnitOp on a branch of the network may calculate flowrate. If the nodes adjacent to a UnitOp

    both use flow set by UnitOp and fixed pressure, the calculated flowrate may be used as the flowrate ata free inletor free outletnode. If the nodes adjacent to a UnitOp use flow set by UnitOp but do notboth fix pressure, the flowrate through the UnitOp is calculated elsewhere on the flowsheet.

    The behavior of Flow set by UnitOp depends on the flowrate specifications of other nodes on thebranch. To illustrate, we consider a system from Figure 2-1.

    The inlet to SECOND NODE is flow set by UnitOp .

    The node will use the flowrate from the pipe.

    If the feed stream is fixed inlet, this is the flowrate for the pipe.

    If the feed stream is free inlet and the product streams are fixed flowrate, the free inlet feed flowrate isset by mass balance. The free inlet is the flow through the pipe. If the feed stream is free inlet, oneproduct stream is free outlet, and both nodes are fixed pressure, the free inlet and free outlet are set bythe pipe flowrate. The pipe flowrate is set to the critical flowrate for the given pipe with the specifiedinlet and outlet pressures.

    The example job demonstrates various behaviors of flow settings for nodes. Figure 2-2 shows theflowsheet for this job.

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    Figure 2-2

    NODE AS DIVIDER

    A node may be used as a divider. Outlet streams from the node will be at the pressure of the node.Outlet streams will all have the same temperature and composition but flowrates may differ.

    The flowrates may be specified as set by pipe/valve or fixed flowrates. Only one outlet stream flowratemay be free outlet.

    A Node specified as a divider is shown in Figure 2-3. The second node acts as a divider (two productstreams). For N inlet and outlet streams it is necessary to specify N-1 values.

    For the second node in Figure 2-3, specify the flowrate of two of the three connected streams. Allowthe third stream to be freefor mass balance requirements.

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    Figure 2-3

    If both outlet flowrates are specified, the inlet stream must be calculated as free inlet at node 1 tomaintain mass balance. If one outlet is calculated as free outletby the node, the inlet stream may beflow set by pipeif both nodes are fixed pressure.

    PRESSURE NODE DIALOG SCREEN

    ModeSelect Fixed pressure to set the pressure at the node and allow flowrate to be variable. Select VariablePressureto leave pressure variable at the node.

    Pressure at NodeSpecify the pressure for Fixed Pressuremode. For Variable Pressuremode the calculated pressure isdisplayed. Optionally you may specify an estimate for Variable Pressuremode. The estimate will bereplaced with calculated result.

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    Minimum PressureSpecify lower bound for pressure at the node for Variable Pressuremode. Specifying minimum and

    maximum pressure will speed calculations.

    Maximum PressureSpecify an upper bound for pressure at the node for Variable Pressuremode. Specifying minimum andmaximum pressure will speed calculations.

    ElevationSpecify the elevation at the node. The elevation will add a pressure contribution based on height.Specifications are absolute. The default elevation is zero. Positive and negative specifications areallowed.

    Flowrate options (Inlet)

    Stream NumberThe CHEMCAD stream number for the connected inlet stream is displayed.

    Flowrate OptionSelect the specification for the stream.

    Fixed Mole Rate/Fixed Mass Rate/Fixed Volume Rate

    The stream flowrate is a known variable. The stream flowrate is set to the specified value duringcalculations.

    Flow set by Pipe/Valve/PumpThe stream flowrate is a dependent variable. Stream flowrate will be calculated by the adjacent(upstream) UnitOp to satisfy pressure requirements.

    Free Inlet StreamSpecifies the inlet stream to the node is a dependent variable. The inlet stream flowrate will be

    calculated for mass balance of the node / flowsheet. Only one inlet to a node may be Free inlet.Specification of Free inlet is not allowed for a stream that is downstream of another node. UseFree Inlet Stream to specify a variable feed stream.

    Use Current Stream RateThe stream flowrate is a known variable. The stream flowrate is set to the current flowrate of thestream.

    ValueSpecify a fixed flowrate for Fixed mole rate , fixed mass rate , or fixed volume rate . Fixed Volume Ratespecifies the total Actual Volume Rate of the stream. Engineering Units for flow are displayed; theflowsheet units of mole rate, mass rate, and liquid flowrate are used.

    Flowrate Options (Outlet)

    Stream NumberThe CHEMCAD ID number for the connected outlet stream is displayed. A value of N/A indicates nostream is connected at this outlet.

    Flowrate OptionSelect the specification for the stream.

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    Fixed Mole Rate/Fixed Mass Rate/Fixed Volume RateThe stream flowrate is a known variable. The stream flowrate is set to the specified value during

    calculations.

    Flow set by Pipe/Valve /PumpThe stream flowrate is a dependent variable. Stream flowrate will be calculated by the adjacent(downstream) UnitOp to satisfy pressure requirements.

    Free Outlet StreamThe stream flowrate from the node is a dependent variable. The Free Outlet stream flowrate willbe calculated for mass balance of the node / flowsheet. Only one outlet from a node may be Freeoutlet. Specification of Free outlet is not allowed for a stream that is upstream of another node.Use Free Outlet Stream to specify a variable product stream.

    ValueSpecify a fixed flowrate for Fixed mole rate, fixed mass rate, or fixed volume rate . Fixed Volume Ratespecifies the total Actual Volume Rate of the stream. Engineering Units for flow are displayed; theflowsheet units of mole rate, mass rate, and liquid flowrate are used.

    PIPE SIMULATOR

    DescriptionThe pipe simulator UnitOp in CHEMCAD is used to model pressure drop of a fluid through a pipe.

    Piping Network Modes of Pipe SimulatorSizing Option 5 (Given Size Pin and Pout calculate flowrate )of the pipe UnitOp allows calculation offlowrate through the pipe as function of geometry, inlet and outlet pressure. The outlet pressure of aknown pipe is a function of inlet pressure and flowrate. Any two of these three variables are

    independent variables.

    A NODE UnitOp can use (or calculate) the flowrate from an adjacent pipe as the flowrate for a streamconnected to the node. Use stream option Flow set by pipe/valvefor the node.

    The use of Sizing Option 5 for a pipe UnitOp connected to a node creates one variable on the PipingNetwork. The variable may be the flowrate through the pipe or the pressure at either end of the pipe.

    If the node is fixed pressure, the pressure of the node will be used for the pressure of the attachedpipe. If the nodes on either side of a pipe are fixed pressurewith Flow set by UnitOpfor the pipe

    UnitOp streams, the flowrate through the pipe is calculated based on the inlet and outlet pressures. Ifone of the nodes on either side of the pipe is variable pressure, the variable pressure is calculatedbased on the fixed pressure (from the other node) and flowrate. The flowrate may be a fixed value setby either node, or it may be specified elsewhere on the flowsheet.

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    PUMP

    DescriptionThe Pump UnitOp has a characteristic equation mode that calculates outlet pressure as a function ofinlet pressure and flowrate.

    Piping Network Modes of Pump UnitOpA NODE UnitOp can use (or calculate) the flowrate from an adjacent pump as the flowrate for a streamconnected to the node. Use stream option Flow set by pipe/valve for the node.

    The use of the characteristic equation mode specifies one unknown on the Piping Network. The

    unknown may be the flowrate through the pump or the pressure at either end of the pump. The nodeconnected to the pump acts to constrain the unknown.

    VALVES

    DescriptionThere are two UnitOps that may be used to represent valves in a Piping Network simulation.

    The Valve (VALV) UnitOp allows an arbitrary adiabatic pressure change of pressure between nodes.

    Piping Network Modes of Valve UnitOpUse a valve UnitOp when a valve changes to or from a variable pressure node. The valve UnitOp isused to change the pressure of the stream to match the pressure calculated by the pressure node.The valve does not adjust flowrate unless turned off. Do not specify an outlet pressure for the valve.

    CONTROL VALVE

    DescriptionThe PID control valve may be used in manual control model on a piping network. The valve flowcoefficient (Cv) must be specified. The Control Valve (CVAL) UnitOp has three modes for manualcontrol.

    Piping Network Modes of Control Valve

    Mode Fix valve position and adjust flow rate is used to calculate flowrate as a function of Cv, valveposition, inlet pressure, and outlet pressure. The downstream node is fixed Pand free inletstream, orvariable P and free outletstream with inlet flow set by UnitOp .

    COMPRESSOR

    Piping Network Modes of Compressor

    The Compressor UnitOp in mode 5 Specify Performance Curves calculates outlet pressure as afunction of volumetric flowrate, efficiency, and head of gas. Volumetric flowrate and head to thecompressor are functions of the mass flowrate and inlet pressure of the compressor.

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    NODE AS MIXER

    A node may be used as a mixer. The inlet streams to the node will all have the same pressure as thenode. Only one inlet stream flowrate may be free inlet.

    STEADY STATE UNITOPS

    Regular steady state UnitOps may be used on a piping network diagram. A constant pressure drop may

    be entered for a (non-scaler) steady state UnitOp. Adjacent nodes will recognize pressure dropspecifications on the UnitOp.

    Between two nodes there must be one flowrate scaler. A heat exchanger and a pipe can be betweentwo nodes, as the heat exchanger does not calculate flowrate as a function of pressure. A constantpressure drop may be specified for the heat exchanger and it will affect the pressure drop between thetwo nodes. A heat exchanger cannot be the only UnitOp between two nodes, as the heat exchangerdoes not have an effect on pressure.

    CHAPTER 3 CONTROL VALVE SIZING

    Topics Covered

    Control Valve Sizing

    Control Valve

    Using Nodes

    Problem StatementThe example is to size control valves for handling a flow of 113,000 lb/hr of Liquid Ammonia in each linecoming from vessel D-1.

    We wish to select appropriate sized valves and then determine the percent open for each valve at therated service.

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    D-1

    -9F

    225 psig

    D-2

    -1F

    15 psig

    D-3

    -28F

    0.2 psig

    Figure 3-1

    This example is located in the CC5DATA \Examples\Piping\Chapter 3 -Contro l Valve Sizing folder.

    To size the valves using CHEMCAD, we have to convert the problem statement into a simulation. LetCHEMCAD calculate the properties for us, and then let CHEMCAD calculate the valve requirements.

    The Simulation

    To do the initial sizing, all we need are streams with the correct properties. It is not necessary to modelthe tanks:

    1

    1 2

    3

    Figure 3-2

    In the flowsheet shown above, all three streams are at the inlet conditions of -9 degrees F, 225 psig.The divider splits the 226,000 lb/hr flow into 2 equal flows of 113,000 lb/hr of ammonia.

    Control Valve SizingTo do the initial sizing, run the simulation (Run menu>Run Al l) to calculate the flow information forstreams 2 and 3. Both streams should be at -9 degrees F, 225 psigand 113,000 lb/hrof ammonia.

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    Next select stream 2 by left clicking on it. The stream is selected when it is shown bracketed by blacksquares. Go to the Sizingmenu, and select Contro l valve. The following screen will appear:

    Figure 3-3

    Enter 15 psigas the Downstream pressureand press the OKbutton. On the screen will appear thefollowing report:

    Figure 3-4

    Pi i S CHEMCAD

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    CHEMCAD reports the properties of the stream, and the calculated parameters for the valve. We repeatthe procedure for stream 3 using 0.2 psig as the Downstream pressure. The following screen will

    appear:

    Figure 3-5

    In the next section we will rate these valves to see what their performance is in this service.

    Rating CaseOur next task is to rate these valves in a simulation. We want to know what the valve position is forthese valves in this service at 113,000 lb/hr. Since this task models the behavior of the control valveswe will need a slightly larger flowsheet:

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    1

    2

    3

    1

    4

    5

    6

    7

    4

    8

    3

    5

    9

    2

    Figure 3-6

    The flash UnitOps at the end are not necessary, they are included so we could see our vapor and liquidflowrates in separate streams if flashing occurs.

    The divider is still set to 113,000 lb/hrand the flash tanks are set to mode 2 (specify T and P)FlashUnitOp #2 is s et to -1 degrees F, 15 psigand Flash UnitOp #3 is set to -28 degrees F,0 .2 psig.

    Open the control valve #4 by double-clicking on it. The control valve screen is shown below:

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    Figure 3-7

    Enter the Valve flow coefficientof 36, the Downstream pressureto 15 psig, and set the Valve modesetting to Fix flow rate, adjust valve position. Press OKand move on to Valve #5. Valve #5 is set in thesame way, with a Valve flow coefficientof 54, Downstream pressure of 0.2 psig, and Valve modesetto fix flow rate, adjust valve position.

    Run the simulation by going to the Ru nmenu and selecting Run All. To view your results, go to theResul tsmenu, and select UnitOps. You should see this dialog asking for what UnitOps to view:

    Figure 3-8

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    If you dont see this, then you already have a unit selected in the flowsheet, and it is showing you areport for that unit. Close the report, deselect the UnitOps by holding down the shift key while clicking on

    the units, and go back to Results, UnitOps. You can also deselect UnitOps by left-clicking on a blanksection of the worksheet.

    Select units # 4 and 5 and press the OK button. You will see the following report:

    Figure 3-9

    This report shows that valve #4 is at 72.5% open, and valve #5 is at 53.3% open.

    By right-clicking on Control valve #5, and selecting View stream pro per t ies, we can see how muchvaporization occurs across the valve:

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    Figure 3-10

    This volume change is why CHEMCAD chose a larger valve for the second stream. With thevaporization occurring in the valve, a smaller 2-inch valve body would be approaching sonic velocitythrough the valve body.

    Flowrate as a function of PressureIn typical CHEMCAD simulations information flows in one direction: downstream. Upstream conditionsdetermine the downstream conditions. In most simulations, you simply set the flowrates and pressuresof feed streams. Pressure drops are either calculated based on flow or specified through UnitOps. Thedownstream pressures, flowrates, etc. are calculated when the simulation is run.

    For piping simulations, flowrate and pressure are dependent on each other. The backpressure onvalves, pipes, and other UnitOps affects the flowrate through the valve. Likewise, the flowrate through avalve, (or pipe, or pump) determines the downstream pressure.

    In flow models like the control valve sizing model, sometimes it is useful to let flowrate vary as a function

    of the pressure.

    For example, assume a process upset caused the pressure in vessel D-2 to rise from 15 psig to 30 psig.Assuming the valve positions dont change, what is the new flowrate from D-1?

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    D-1

    -9 F

    225 psig

    D-2

    -1 F

    30 psig (UPSET condition)

    D-3

    -28F0.2 psig

    Cv=36

    72.5 % open

    Cv=54

    53.3% open

    Figure 3-11

    In order to answer this question, we need to introduce a special UnitOp called a node. A node is a pointin the simulation that has a pressure, flow coming in, and flow going out. The node units create anetwork, solving for flowrate at each point based on the fixed pressures. Nodes are placed on theflowsheet before and after the control valves. For the above system, the flowsheet is shown below:

    6 7

    8

    9

    10

    10 11

    12

    13

    14

    11

    15

    18

    12

    16

    19

    20

    17

    Figure 3-12

    The function of the divider (to split the incoming flow) is now handled by NODE #6. The node willbalance the flowrates such that all streams entering and exiting the node are at the same pressure.

    Nodes are also placed between the flash vessels and the control valves. At the nodes we can fix thepressures, and let the flowrate vary as a function of valve position and pressure difference.

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    Open NODE #6 by double-clicking on it:

    Figure 3-13

    We are assuming the pressure at this node is fixed at 225 psig. The inlet flow is set to Free inlet streamand the two outlet streams are set to Flow set by UnitOp . Flow into each control valve will bedetermined by the control valve Cv valve opening position, and pressure difference across the valve.

    The other two NODE UnitOps are set in a similar fashion.

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    Figure 3-14

    The pressure in is set to 30 psig for NODE #9, 0.2 psig for NODE #10. Flow into the node is controlledby the control valve (Flow set by UnitOp), flow out is a Free Outlet Stream .

    The control valves need to be changed to fix the valve position, and calculate flowrate.

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    Figure 3-15

    We need to set the Valve mode for each valve to Fix valve position, adjust flowrate in order for theflowrate to change.

    Now we can run the simulation. Go to theRunmenu, select Run All. We can view the streams aroundNODE #6 by right-clickingon the node and selecting View stream compo si t ions from the menu. Thefollowing report will appear:

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    Figure 3-16

    The flowrate from D-1 to D-2 dropped from 113,000 lb/hr to 109138 lb/hr. So we can see the effect ofback pressure on the flowrates through the valves.

    CHAPTER 4 SIMPLE FLOW EXAMPLE

    Topics Covered

    Control Valve Sizing

    Feedback Controllers NPSH

    Orifice Sizing/Rating

    Pipe Sizing/Rating

    Pipe UnitOp

    Problem StatementThe piping system shown must be designed to transport 120 gpm of glacial acetic acid at 70-140F.The pressure at the inlet is known at 20 psia, the outlet must be no less than 20ps ia. The pipingsystem and its individual elements must be sized for design conditions and then rated at operatingconditions. Our goal is to determine the NPSHaand head requirements for future pump selection.

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    Figure 4-1

    This example is located in the CC5DATA \Examples\Piping\Chapter 4-Simple Flow folder.

    CREATING THE SIMULATION

    1. Convert the piping isometric to a CHEMCAD flowsheet. Pipe UnitOps are used to represent entiresections of piping, including fittings:

    2. Using CHEMCADs pipe sizing utility (Sizing Menu>Pipes) size the pipes in the network for thedesign flowrate of 120 gpm, at 70 F. Use standard schedule 40 pipe. Since the fluid in this system issubcooled liquid and all flows are constant, this requires us to do only one calculation for thedischarge side. As a rule of thumb, use 1 size larger pipe on the suction of the pump.

    3. Size the orifice (Sizing menu>Orifice) on the discharge side of the pump. Use 120 gpm, the pipe sizedetermined in step 2 above, d and d/2 pressure taps and 100 inches of water differential pressure.

    Using the calculated bore hole, determine the flow resistance factor of the orifice as shown:

    C=Cd/(v(1-4))

    KrK=(1-2)/(C

    2

    4)

    In the orifice sizing report, Kr is given, so we can just enter this value into the pipe dialog.

    4. Size the control valve. To size the control valve, select stream #1, and then go to Sizingmenu>Contro l Valve. Enter an outlet pressure of 15 psia, and a single seat valve. Since stream #1is at 20 psia, we are simply calculating a control valve to give us a 5 psi pressure drop. Enter theValve flow coefficient, Cv, of the valve from the sizing report, set the Valve positionto 50%open,and set the Valve modeto Fix flow, valve position, calculate Pout.

    5. Assume a 2 psi pressure drop across the heat exchanger. Set the outlet temperature to 140F.

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    6. Tabulate the valves, fittings, pipe lengths, and elevation changes of each section of pipe. All of ourpipes will use the Single phasemethod, and the modeshould be set to rating. We will be usingflanged fittings throughout.

    Pipe #11 entrance, well rounded2 ball valve4 std elbow 90 degree1 tee, flow through branch35 feet piping-8 foot elevation change

    Pipe #2 (before control valve)1 swing check valve, clearaway2 ball valves2 tee, flow through run1 orifice plate (as determined above)14 feet piping14 foot elevation change

    Pipe #3 (after control valve)

    2 ball valve2 tee, flow through run3 Std elbow 90 degree1 exit from pipe24 feet piping2 foot elevation change

    Pipe #4 (after E-1515)1 Ball valve

    4 std elbow 90 degree1 tee, flow through run1 well rounded entrance1 exit from pipe157 feet piping5-foot net elevation change

    7. Specify the pump outlet pressure at some arbitrary value (25 psia) and make a trial run. Check thecalculated outlet pressure. We can iterate to find the required pump head.

    USING CONTROLLERS TO SIMPLIFY THE PROBLEM

    While manually changing the pump outlet pressure will get us where we need to be, it is easier to letthe program do the work. Were going to use a UnitOp called a feedback controller (CONT) to adjustthis pressure for us.

    A Feedback controller in CHEMCAD has nothing to do with process control valves or PID settings. Ina CHEMCAD steady-state model, when we use the term feedback control were talking about amathematical controller. Its a math tool used to adjust a variable on a flowsheet until a target value

    reaches our specified value.

    Change the flowsheet to include a feedback controller just before the product arrow:

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    1 2

    3 4 5 6 71

    2 3

    4

    56

    7 8

    8 9

    Figure 4-2

    Specify the Controller modeas a feedback controller. Adjust pump outlet pressure until the pressureof stream 8 is equal to a constant target of 20 psia. When you are finished, the controller screenshould look like so:

    Figure 4-3

    When you run the simulation, the controller will automatically change the pump outlet pressure until thepressure leaving the last pipe unit is equal to 20. We now know the head requirements for our pump.

    CALCULATING NPSHA

    NPSH is the Net Positive Suction Head, and it is defined as the total pressure available at the pumpsuction minus the pumping fluids vapor pressure. It is almost always reported in feet of pumped fluidor water.

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    Every pump has a specified NPSH requirement (NPSHr) at a given operating speed. To ensure reliableoperation the available NPSH (NPSHa) must be greater than the NPSHr. If not, cavitation andshortened service life may result.

    Calculate NPSHa. The net positive suction head is defined as pressure available at pump suction minusthe fluid vapor pressure, expressed in feet of fluid. To calculate this in CHEMCAD is an easy task.Open the Pump dialog, and put a checkmark where it says Check here to Calculate NPSHa. Rerun thesimulation, and the calculated NPSHa will appear.

    It is important to the NPSHa calculation that the inlet piping to the pump be correctly specified. If thepiping is not correct, then the pressure at the inlet may not be correct, and the NPSHa may not becorrect.

    CHAPTER 5 BRANCHED FLOW EXAMPLE

    Topics Covered

    Node UnitOp

    Pipe Networks

    Pump Selection Criteria

    Pump UnitOp Performance Curves

    Problem StatementThe piping system from the previous section has been changed. Due to the branched flow to the twoheat exchangers, the problem is no longer a simple one.

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    Figure 5-1

    This example is located in the CC5DATA \Examples\Piping\Chapter 5-Branc hed Flow folder.

    The branched flow is a difficult problem to solve using our controller approach. The two exchangershave different piping, which gives them different flowrates. What we need is an approach where we splitand recombine flows, and have the simulation calculate the pressure and flowrates in an iterativemanner. The node UnitOp gives us this flexibility.

    A node is a point where pressure is uniform. There may be multiple inlets and outlets. The flowrates foreach stream will be balanced by CHEMCAD to reach a single pressure. Pressure may be specified orallowed to vary.

    CREATING THE SIMULATION1. Convert the piping isometric to a CHEMCAD flowsheet:

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    1

    24

    5

    6

    7

    9 10 13 14

    1 2

    11

    16

    17 18 19

    12

    20

    15 3

    21

    3

    4

    5

    6

    20

    19

    18

    17

    1615

    14

    13

    1211

    10

    9

    8

    7

    8

    22

    Figure 5-2

    2. Pipe UnitOps are used to represent entire sections of piping, including fittings. NODE UnitOps areplaced where pressure or flowrate are unknown.

    3. Assume a 2 psi pressure drop across each heat exchanger.

    Tabulate the valves, fittings, pipe lengths, and elevation changes of each section of pipe. We will beusing flanged fittings throughout.

    Pipe #11 entrance, well rounded

    2 ball valve4 std elbow 90 degree1 tee, flow through branch35 feet piping

    Pipe #2 (before control valve)1 swing check valve, clearaway2 ball valves2 tee, flow through run

    1 orifice plate (as determined above)14 feet piping

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    Pipe #3 (after control valve)1 ball valve1 tee, flow through run2 Std elbow 90 degree10 feet piping

    Pipe #4 (to E-1514)1 tee, flow through branch1 ball valve1 exit from pipe3 std elbow 90 degree26 feet piping

    Pipe #5(from E-1514)1 ball valve1 well rounded entrance3 std elbow 90 degree1 tee, flow through branch30 feet piping

    Pipe #6 (to E-1515)

    1 ball valve1 exit from pipe1 tee flow through run1 std elbow 90 degree14 feet piping

    Pipe #7 (from E-1515)1 std elbow 90 degree1 ball valve

    1 well rounded entrance1 tee flow through run10 feet piping

    Pipe #8 (to V-1522)3 s td elbow 90 degree1 exit from pipe147 feet piping

    4. Pump- At this time we dont know the pump specifications, so we will set the pump to Specify OutletPressure and leave the pressure specification blank. The NODE UnitOps will solve for thepressure increase, and set the pump outlet pressure accordingly.

    5. Setup nodes with appropriate information:

    Node 3 Variable pressure, use current stream rate for inlet, flow set by UnitOp foroutlet. Elevation = 20 feet

    Node 5 Variable pressure, flow set by UnitOp for both inlet and outlet.Elevation = 34 feet

    Node 7 Variable pressure, flow set by UnitOp for both inlet and outlet.Elevation = 34 feet

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    Node 9 Variable pressure, flow set by UnitOp for both inlet and outlet.Elevation = 34 feet

    Node 12 Variable pressure, flow set by UnitOp for both inlet and outlet.

    Elevation = 50 feet.Node 16 Variable pressure, flow set by UnitOp for both inlet and outlet.

    Elevation = 38 feet.Node 18 Variable pressure, flow set by UnitOp for both inlet and outlet.

    Elevation = 42 feet.Node 20(last node)

    Fixed pressure, 20 psia, flow set by UnitOp for inlet, free outlet stream foroutlet. Elevation = 43 feet.

    RUNNING THE SIMULATION

    To run the simulation, press the Run All button or go to the Ru nmenu, and select Run All.

    If the simulation doesnt converge, check all the input settings and run it again. Simulations can besensitive to initial estimates and min/max settings in nodes. Convergence is also an iterative process, soyou might need to increase the max number of iterations for a given flowsheet.

    Once the simulation has run, check the pump UnitOps pressure increase. This tells us the head

    requirements of our pump. Using this information and our flowrate (120 gpm) we can consult a pumphandbook to determine the correct pump size.

    SELECTING A PUMP

    Below is an appropriate pump for our application.

    Pump Curve

    1750 rpm

    1450 rpm

    1150 rpm

    20

    3040

    50

    60

    70

    80

    90

    0 40 80 120 160 200

    Flow (gpm)

    Head

    (ft)

    Figure 5-3

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    1750 rpm

    1450 rpm

    1150 rpm

    0.3

    0.35

    0.4

    0.45

    0.5

    0.55

    0.6

    0 40 80 120 160 200

    Flow (gpm)

    Efficiency

    Figure 5-4

    To enter this curve into our pump, select Specify Performance Curve for the pump Mode. Once you doyou have the opportunity to enter multiple speed lines and an operating speed. For our purposes, wellassume we want the pump to be at 1750 RPM, so we enter a single speed line. Press the OKbutton,and you will see the following entry screen:

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    Figure 5-5

    In this screen we can enter as many points as we wish to define our curve. CHEMCAD will fit a

    quadratic equation to the points, creating a smooth curve fit to our data.

    Now that we have entered the pump performance curve we must add node unitops to the suction side ofthe pump to allow the flowrate to vary. In this way we can calculate the max flowrate of our system.(Note:when computing the max flow for the system, be sure to open the control valve to 100%.)

    Piping Systems CHEMCAD

    CHAPTER 6 FLOW RELIEF PIPINGSYSTEM

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    CHAPTER 6 - FLOW RELIEF PIPING SYSTEM

    Topics Covered Branched Network

    Compressible flow

    Degrees of Freedom

    Flare header systems

    Node UnitOp

    Pipe UnitOp

    Valve UnitOp

    FLARE HEADER DESIGN

    Flare headers are specialized piping networks desiged to convey relief valve flow to a flare whereproducts are consumed before release to the atmosphere. Using CHEMCAD, we can design andevaluate flare header networks for many relieving scenarios.

    Figure 5-1 shows a simplified flowsheet for a relief header network. The flows and pressures from reliefdevices RD0001-RD0008 are previously calculated. The piping network has been designed but notcertified or built. The pipes on this flow diagram represent continuous sections of pipe with various

    fittings (elbows, etc).

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    Figure 6-1

    Problem Statement

    Calculate pressures throughout the network when RD0001, RD0002, RD0007, and RD0008 arein a relief scenario

    Determine if an additional section of 8 inch pipe can be added to the discharge

    Determine effects of a two phase flow scenario through RD0007

    Calculate pressures throughout the network if only three segments of the network have a reliefevent

    These tasks may be solved using the NODE UnitOp and Piping Network. The problem will be solvedone section of piping at a time. It is easier to tackle the problem if we divide it into smaller problems.Using results from smaller cases as estimations we can gradually build a complex network, reducingcalculation time.

    This example is located in the CC5DATA \Examples\Piping\Chapter 6-Flow Rel ief Piping Systemfolder.

    Piping Systems CHEMCAD

    ONE BRANCH

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    ONE BRANCH

    We begin by drawing one segment of the network as shown in Figure 5-2, below. The intermediatepressures are unknown / variable and will be solved for. The flowrate and composition from RD0002 isknown. We can specify either the discharge pressure to the system or the entrance pressure at the firstnode. We arbitrarily specify the pressure at the first node, making the outlet pressure variable. Whenthe entire network is drawn we will specify the outlet pressure and recalculate the inlet pressure.

    Figure 6-2

    From known flow, known pressure, and known pipes the calculation of pressures is straightforward for acontinuous section; this flowsheet converges quickly. From the given conditions of fixed inlet pressurethe flare header is more than adequate for this single valve, as the outlet pressure to the flare stack is

    greater than atmospheric pressure.

    TWO BRANCH

    Figure 5-3 shows a second section of pipe added to the network representing the flow from RD0001. AT branch fitting is added to the pipe UnitOp after the node which connects the two sections.

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    Figure 6-3

    The flowrate from RD0001 is known. The pressure at the first node for RD0001 is variable. If weconsider the RD0001 section a separate section of pipe, we do not have a degree of freedom to specify.The pressure at the node where RD0001 flow com bines with the RD0002 flow is a dependent variablefor RD0001 flow; it is calculated by the RD0002 section.

    FOUR BRANCH

    A third and fourth branch are added to the network, shown in Figure 5-4 and Figure 5-5, below. As ourflowsheet becomes more complex, the upper/lower bounds for pressure in nodes become moreimportant. A more complex network may require more careful settings of upper and lower bounds.

    Piping Systems CHEMCAD

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    Figure 6-4

    Figure 6-5

    The flowsheet is run and converged after adding the third and fourth sections . All four branches are on

    the network. The calculated discharge pressure is above atmospheric; the network is adequate for thisrelieving scenario.

    CHEMCAD Piping SystemsSPECIFYING THE OUTLET PRESSURE

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    We can now perform a rating case on this network, to determine the minimum pressure at RD0002 forrelief under this scenario.

    The pressure on the first section node is changed to variable pressure. The outlet pressure for thenetwork is changed to fixed pressure. The boundaries on variable pressures are tightened and theflowsheet is converged to the same results, but with the fixed pressure node changed from inlet to outlet.

    To drop the pressure to 2 psig, one should relax the lower pressure boundaries on variable nodes. Bymaking several minor adjustments from the current pressure to 2 psig, fewer calculations will berequired.

    ADDITION OF PIPE TO DISCHARGEA pipe is added to the discharge of the network. The pipe is set to rating mode in CHEMCAD; there isno need to use nodes around this final pipe. The flowsheet is run, the final pipe calculates outletpressure. The design modes of the pipe can be used to determine a required diameter for the pipe if therequired length of pipe is known.

    Figure 6-6

    EFFECT OF CLOSED VALVEThe simple valve (VALV) UnitOp in CHEMCAD has a closed setting. This can be used to shutoff theflow to a section of pipe. In this example we set the valve on RD001 to closed and run the process.

    DIFFERENT APPROACHES TO RELIEF PROBLEMS

    This has not been an exhaustive study of the use of the piping network calculations in CHEMCAD. Thisexample process does not represent the only way to study a relief header system; the approach usedwas selected for instructional purposes. CHEMCADs methods are flexible, you can use a differentapproach (fixed pressure inlets, fixed flow on one section, etc) to suit your purposes.

    Piping Systems CHEMCAD

    CHAPTER 7 PIPING CALCULATION METHODS

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    THE GENERAL ENERGY EQUATIONAn expression for the balance or conservation of energy between two points in a system can be derivedlike so:

    iU = internal energy

    ii VP = energy of expansion or contraction

    2gc

    Mv2i

    = kinetic energy

    gc

    ZMg i = potential energy

    q = heat additions (losses)

    sW = work done on the fluid

    Z = elevation above datum

    QovsWq

    1 2

    MgZ

    2gcMv

    VP

    U

    1

    2

    1

    11

    1

    MgZ

    2gc

    Mv

    VP

    U

    2

    22

    22

    2

    CHEMCAD Piping SystemsThus

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    gc

    ZgM

    2gc

    Mv

    VPUWqgc

    ZgM

    2gc

    Mv

    VPU2

    22

    222s1

    21

    111

    +++=++

    +++ (Equation 1)

    Divide by M and write in differential form (to place on a per unit of mass basis):

    0WddqZdgc

    g

    gc

    vdvPdUd s =++++

    +

    (Equation 2)

    Equation 2 is difficult to apply because U is hard to evaluate. Therefore, equation 2 is converted into the

    mechanical energy form.

    dPTdShd

    Pd-hdUd

    +=

    =

    and;

    P

    d

    dP

    TdSUd

    += (Equation 3)

    For an irreversible processf

    Pdq-TdSorT

    -dqSd +=

    Where Pf = losses due to irreversible processes like friction.

    Assuming no work is done on the system; Equation 3 becomes,

    ( )

    0dPZdgc

    g

    gc

    vdvdP

    00qdZdgc

    g

    gc

    vdvdPqd

    Pd

    f

    f

    =+++

    =+//++++//+

    (Equation 4)

    Z = dPfsin F = some angle of elevation

    0dPsinLdgc

    g

    gc

    vdvdpf =+++

    (Equation 5)

    Multiplying byLd

    0WddqZdgc

    g

    gc

    vdvPd

    Pd

    dPTdS s =+++

    /

    //+

    /

    //+

    Piping Systems CHEMCAD

    0dL

    dPsin

    g

    dL

    dvv

    dL

    dp f =+++

    (Equation 6)

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    dLgcdLgcdL ( )

    CONSERVATION OF MOMENTUM

    Conservation of Linear Momentum

    ( ) osinA

    d

    Ld

    Pdv

    Ld

    dv

    Ld

    dg

    2

    =

    +

    where

    Ld

    Pd

    = pressure loss

    A

    D

    = shear stress loss (friction)

    osing = gravitational loss

    At steady state

    osinA

    D

    Ld

    Pd

    Ld

    vd

    g

    2

    =

    ( )Ld

    vdv

    Ld

    vdv

    Ld

    vd2

    +=

    ( )osin

    A

    D

    Ld

    Pd

    Ld

    vdv

    Ld

    vdv g

    =+

    ( ) osinA

    D

    dL

    dvv

    Ld

    vdv

    Ld

    Pdg

    +++=

    at steady state( )

    ;0Ld

    vd=

    and

    osin

    A

    D

    Ld

    vdv

    Ld