advance De Pressuring

8/12/2019 advance De Pressuring

1/12

Depressuring 1

1

Depressuring

1999 AEA Technology plc - All Rights Reserved.

ADV-2 3.pdf


2/12

2 Depressuring

2

WorkshopHYSYS contains a Depressuring utility that lets the user examine

pressure and temperature profiles in process vessels during simulated

gas blowdowns and pressure letdowns. This utility can also be used to

safely size Pressure Safety Valves (PSVs) under normal shutdowns and

emergency conditions.

In this module, two types of depressuring will be examined.Adiabatic

depressurization which is used to simulate the normal gas blowdown of

pressure vessels and/or piping, and the Wetted Fire model which is

used to simulate emergency conditions in a plant.

Learning ObjectivesAfter completing this module, you will be able to:

Use the Depressuring utility of HYSYS

Predict pressures and temperatures inside process vesselsduring pressure letdowns and emergency conditions

Size Pressure Safety Valves to safely meet the conditions inyour plant

PrerequisitesBefore beginning this module, you should be able to:

Navigate the PFD and the Workbook screens

Add Streams and Unit Operations in HYSYS

You must also have access to the flowsheet that was used in the two

previous modules.


3/12

Depressuring 3

3

Information on DepressuringThe Depressuring utility in HYSYS has been created to perform

rigorous time dependant pressure and material reduction calculations.

The model is comprised of three distinct areas: physical description of

the vessel, thermodynamic constraints of the process, and the flow

regime through the valve.

Although depressurization is a dynamic process, the small steps that

HYSYS takes in achieving a solution allow this process to be modelled

in a steady state environment.

Thermodynamic Models

There are four models that HYSYS can use to simulate the

depressurization process, and each has its own particular application.

Using the right model is very important if accurate results are to be

obtained:

Fire Mode- used to simulate conditions that could occur duringa plant fire. The application of an external heat source isconsidered during the calculations for pressure, temperature,and flow profiles. The heat flux into the fluid is user defined.

Wetted Fire Mode- used as above, except that the heat flux

into the fluid is calculated according to API equations for heattransfer from a fire to a liquid containing vessel. A wetted areamust be supplied.

Adiabatic Mode- used to model the normal gas blowdown ofpressure vessels and/or piping. No external heat is supplied.Heat flux between the fluid and the vessel is modelled as thefluid temperature drops. Typically used to model thedepressurization of compressor loops on emergencyshutdown. The adiabatic model requires an isentropicefficiency term. An efficiency of zero means that the system willbehave isenthalpically, i.e. there will be no loss of enthalpyduring depressurization. An efficiency of 100%, on the otherhand, means that the system will behave isentropically, i.e.there will be no loss of entropy during depressurization.

Isothermal Mode- the temperature of the fluid remainsconstant during the depressurization process. The necessaryheat flux is determined by HYSYS.

Experience has shown that avalue of 100% works well formost pure vapour systems,while a value between 40%and 70% works well for mosttwo phase systems.


4/12

4 Depressuring

4

Valve EquationsThere are also four types of valves that can be used to model the PSV.

Having these options allows you to customize the simulation. The

choice of type depends of the information that the user has available

and the physical conditions of the simulation.

Subsonic Valve- can be used only if the flow across the valveis expected to be completely subsonic. This is usually the caseif the upstream pressure is less than twice the backpressure ofthe valve. This valve is modelled by the following equation;therefore, two constants must be given, C1 and C2.

Supersonic Valve- used in cases where little or noinformation is known about the valve. This valve is modelledaccording to the following equation; therefore, again twovariables are needed, C1 and C2. Generally, C2 will never varyfrom 0.5.

Masonellan Valve - used for general depressuring valves toflare stacks. The equation for this model follows. C1 and C2are automatically set by HYSYS; these values should not bechanged without good reason. The C1 term can be changed to

maintain dimensional consistency within the equation. The CVof the value is usually available from the valve manufacturer.

General Valve- use this model if the effective throat area ofthe valve is known. The model makes limiting assumptionsconcerning the characteristics of the orifice. The equation usedby this model follows. Gcwill be 1 if SI units are used, and32.17 if field units are used. C1 will vary with the geometry ofthe valve, when modelling orifices, C1 is equal to theCoefficient of Discharge of the orifice. C2 will be equal to 0.5,as usual.

More information on these valve equations can be found in the

Reference Section of this folder.

Constant rate depressur-ization can be modelled withthis valve equation by settingthe C1 term to be equal to theflow, and the C2 term to 0.

FLOW C 1Pup Pback+( ) Pup Pback( )up

Pup----------------------------------------------------------------------------

C2

=

FLOW C 1 Pup up( ) C2=

FLOW C 1 CV CF YF Pup up( ) C2=

FLOW C 1 432 00 AV Kterm G( c Pup up k)C2=


5/12

Depressuring 5

5

Generalized DepressurizationProcedure

The following steps are meant as a general outline for using the

Depressurization utility in HYSYS.

1. Access the Depressurization utility by selecting Tools/Utilitiesfrom the Menu Bar, or by pressing the hot key.

2. On the Connectionspage, change the name of the utility ifdesired.

3. In the Stream Connectionsgroup box, attach a stream of knowncomposition. (Up to four streams can be connected).

4. If desired, specify the vessel and liquid volumes. If these values

are not specified, HYSYS will size them to hold one hours worthof flow. This applies to both the liquid volume and the totalvolume of the vessel.

5. A wetted area must be entered if theWetted Firemodel is to beused as the thermodynamic model. Otherwise, it is not necessary.However, a known parameter should never be left out of asimulation.

6. Once all the streams are attached, press the Settle Outbutton.HYSYS now calculates the combined vessel information. Theresults of this calculation can be seen on the Vesselpage.

7. On the Parameters page, specify a thermodynamic model. Thefour options are listed on page 3 of this module.

8. Enter a relief pressure. A pressure build-up can be simulated by

specifying a relief pressure greater than the settle out pressure.

9. Make changes to the depressuring time, the final pressure, andthe number of steps if desired.

10. On the Heat Fluxpage, enter the parameters governing the heattransfer between the fluid and the vessel. This page will changedepending on the thermodynamic model chosen.

11. On theValvepage, select a valve to model. The choices aredescribed on page 4 of this module.

12. Once the valve is chosen, the Equation Parameters and/orEquation Unitscan be modified.

13. Press the Calculatebutton. HYSYS now runs the utility.

14. The results can be seen on either the Plotstab, or the Tabletab.

The information contained on both tabs is identical; however, thepresentation form is different.

The streams attached to theDepressurization utility mustbe fully defined.

Pressing the Equation Helpbutton on the Heat Flux orValve Eqns page will bring updetails on the availableequations.


6/12

6 Depressuring

6

Adding the Utility1. Open the Utilitymenu by pressing the hot key combination

.

2. Highlight Depressuringand press theAdd Utilitybutton.

Defining the Utility

1. In the Stream Connectionsbox, add the streams To LP SepandTo IS Sep. Add the following information to the current view.

The screen should now look like this:

In This Cell... Enter... Enter...

Stream To LP Sep To IS Sep

Vessel Volume, m3(ft3) 25 (880) 20 (700)

Liquid Volume, m3(ft3) 0 (0) 0 (0)

Wetted Area, m2(ft2) 45 (480) 35 (375)

Both streams here are vapouronly; therefore, there is noliquid present. In other words,the Liquid Volume is 0.


7/12

Depressuring 7

7

2. Press the Settle Outbutton and check the calculated values on theVesselpage (on the Resultstab).

3. Move to the Parameterspage on the Designtab, and specify thethermodynamic model asAdiabatic. Supply a relief pressure of70 bar. Set the final pressure to 15 bar. The page should looksomething like this.


8/12

8 Depressuring

8

4. On the Heat Fluxpage (on the Equationstab), specify theisentropic efficiency as 70%. The vessels specific heat as 0.42 kJ/kg oC (0.1 BTU/lb oF), and the vessels mass as 8000 kg (17640 lb).Leave the value for the Heat Transfer Coefficient as .

5. On theValve Eqnspage, change the value of C1 to 16.663, and usethe drop-down menu to select the "Upstream Pressure Units" asbar. Press the Calculate button.

6. Move to the Plotspage, and answer this question.

7. Return to theValve Eqnspage, and enter a Cvvalue of 15.2.Recalculate the utility.

What is the pressure inside the system after the 15 minuteshave passed? __________

What is the pressure after 15 minutes now? __________

Recall that the C1 term can beused to maintain dimensionalconsistency in the valveequation. Here, we will usebaras the pressure unit so weneed to use a C1 value that is10 times larger than theoriginal value.

Save your case!


9/12

Depressuring 9

9

ExerciseFor this exercise, it is desired to use the depressuring utility to size a

PSV. Use the following data for this exercise:

Variable Name Value

Connections

Streams To LP Sep

Vessel Volume, m3(ft3) 25 (880)

Liquid Volume, m3(ft3) 0 (0)

Wetted Area, m2

(ft2

) 10 (110)

Parameters Page

Mode Fire Wetted

Relief Pressure 68 bar

Final Pressure 50 bar

Depress. Time 15 minutes

Pressure Steps 100

Heat Flux Page

C1 37

C2 0.82

C3 0

Units kW and m2

Valve Eqns Page

Valve Equation Masonellan

C1 16.663

C2 0.5

CV 2

Units kg/h, kg/m3, and bar

It is not necessary to delete theprevious depressurizationutility; just build a new onefrom the beginning.


10/12

10 Depressuring

10

Leave all unspecified variables at their default values. Calculate the

utility.

This simulation shows a gradual rise in system pressure up to the

specified relief pressure (68 bar) and then a gradual drop in pressure as

the valve opens and material is released.

A properly sized PSV will be able to maintain the system pressure at the

relief pressure without too much of a decline. Therefore, decrease theCv of the valve until you find a properly sized PSV. Note: do not make to

large a change in the Cv, start by making a 0.1decrease and observe the

effect that this has on the simulation.

What is the final pressure in the system at the end of thedepressuring process? __________

What Cv is required to maintain the system pressure at therelief pressure? __________

Does the system pressure stay at the relief pressureperfectly? __________

Would this be possible in "real life"? __________

Save your case!


11/12

Depressuring 11

11

Analysing the ResultsThe results of the Depressuring utility can be seen under either the

Plotstab or the Tabletab. The information on either tab is identical;

however, the method of presentation is different.

On the Plotspage, there are five different plots that can be displayed.

These five plots are defined here:

Temperature- The temperature of the fluid, the vessel, andthe fluid exiting the valve are all plotted against time. Thisallows you to see if the thermodynamic model is acting

properly. Pressure- The pressure of the fluid inside the vessel is plotted

against time. This provides you with a quick reference as to therelief pressure, final pressure, and the depressurization time.

Inventory- The total number of moles of material remaining inthe system is plotted against time.

Valve Rate- The molar flow rate of material through the valveis plotted against time. The user can quickly see the maximumflow rate that occurs during the depressurization.

Composition- The composition of the material remaining inthe vessel during the depressurization is plotted against time.

On the Tablepage, all of the information is presented in tabular format.

The table headings are: Pressure, Fluid Temp., Valve Outlet, Inventory,

Vapour Fraction, Vessel Temp., Valve Rate, Sys. Den. (Vap), Sys. Den.(Liq), andHeat Flux.

From the table headings it can be seen that the information presented

on the Tablepage is identical to the information presented on the Plots

page.

The composition of material remaining in the vessel during the

depressurization process can be seen in tabular format by pressing the

Compositionsbutton.


12/12

12 Depressuring

12

advance De Pressuring

Documents

Transcript of advance De Pressuring