Dyn 006H Depressuring

7/22/2019 Dyn 006H Depressuring

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Dyn-006H Revised: Nov 28, 2012

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Depressuring with Aspen HYSYS V8.0

1. Lesson Objective: To construct a simple case using the Depressuring Analysis in Aspen HYSYS.

2. Prerequisites Aspen HYSYS V8.0

3. BackgroundAny process dealing with gasses has the potential for unsafe pressure buildup. This can happen because of

instrument failure, loss of power, or an unforeseen heat source such as a fire. If pressure does build up, there

must be a depressuring system in place to depressurize in a safe manner. The pressure is bled through a valve

until it reaches a safe level. The excess gas can also be sent to a pressure vessel, but these vessels are also

equipped with valves to prevent overpressure. Gas blowdown valves are common in oil wells. When the wells

are not in use, the pressure can build up. A blowdown valve vents the excess gas to a flare, where the

hydrocarbons can be burned before being released into the atmosphere.

The examples presented are solely intended to illustrate specific concepts and principles. They may not

reflect an industrial application or real situation.

4. Problem Statement and SolutionProblem Statement

A nitrogen stream that has built up an excess pressure

Pressure: 2161 psia Temperature: 62.6 F Molar flow rate: 2.205 lbmole/hr

Use the Aspen HYSYS Depressuring Utility to determine the behavior of the gas in a depressuring process that

takes 100 seconds. Assume an adiabatic case with no external heat source.


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Aspen HYSYS Solution

4.01. Start Aspen HYSYS V8.0. Open a Newcase.4.02. The Component Listswindow will be displayed. Click Addto create Component List-1and add Nitrogen

to the list.

4.03. Click the Fluid Packagesfolder in the Navigation Pane, then click Addto create Basis-1. Select thePeng-Robinsonproperty package.


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4.04. Move to the simulation environment by clicking on the Simulationbutton in the bottom right corner ofthe screen.

4.05. Insert a Material Streamfrom the Model Palette.

4.06. Double click on the stream. In the Worksheettab, enter a Temperatureof 62.6 F, a Pressureof 2161psia, and a Molar Flowof 2.205 lbmole/hr. If necessary, change the Unit Setto Fieldin order to match

the units being used. In theWorksheet | Compositionframe, enter a Mole Fracof 1for Nitrogen.

4.07. In the Hometab of the Ribbon, click on Analysis | Depressuring.


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4.08. Click Addto create Depressuring-Dynamics-1, then click Editto enter in values for the Depressuringutility.

4.09. Select stream 1for Inlets. HYSYS will automatically size the vessel. However, we will choose differentvalues. Make sure the Verticalradio button is selected under Vessel Parameters, then deletethe entry

for Flat End Vessel Volume. Enter a Heightof 5 ftand a Diameterof 0.8957 ft, and the rest of the

values should be calculated. The page should resemble the image below.


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4.10. Click Heat Fluxin the left side of the window, and change the Ambient Temperatureto 62.33 FunderHeat Loss Parameters.


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4.11. Notice that Unknown Vessel Metal Thicknessis displayed in the status bar at the bottom of the window.To address this, select the Conductionradio button. In the table under Metal, enter a Thicknessof

0.9843 inches. Also, under insulation enter a Thicknessof 0 inches.


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4.12. Click on Valve Parameterson the left side of the window. Change the Vapour Flow Equationto General,then ender a Cdof 0.7and an Areaof 4.907e-2 in

2. A Cdthat is less than 1 signifies that the effective

orifice flow area is less than the physical area, which is a common occurrence.

4.13. Click on Operating Conditionsin the left side of the window. Under Operating Parameters, change theTime Step Sizeto 0.05 secondsand the Depressuring Timeto 100 seconds.


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4.14. The Depressuringutility should be ready to calculate. Move to the Performancetab and select StripChartson the left side of the window. There is a premade chart named Depressuring-Dynamics-1-DL.

However, this chart has over 20 variables, and wil l be difficult to read. We will create a new plot that

only contains the most relevant information. Click on the Create Plotbutton to create DataLogger1,

then click Add Variableto select the variables for the plot.

4.15. We are only concerned with the vapour flow, as we down not anticipate any liquid. Depressuring-Dynamics-1should be selected underFlowsheet. Under the Objectcolumn, select Vapour@TPL1and

Mass Flowunder Variable. Also addVapour@TPL1, Pressureand Vapour@TPL1, Temperature.


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4.16. In the Performancetab, change the Sampling Intervalof DataLogger1to 0.05 seconds, then click ViewStrip Chart.


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4.17. Click Displayin the DataLogger1window to display the plot. Right clickon the plot and select GraphControl. In the Axestab, check the boxes marked Automatic Auto Scalein the Auto Scalesection and

Show Allin the Axis Displaysection.

4.18. Move to the Time Axistab, and click on Set-up Logger. Change the Logger Sizeto 2003, and make surethe Sample Intervalis 0.05 seconds.

4.19. We are now ready to run the simulation. Close the Strip Chart Configurationwindow and return to theDepressuring-Dynamics-1window. The sub-flowsheet will run in dynamics until the depressuring time

is complete, and then the system will return to steady state. Click the Runbutton at the bottom of the

Depressuring-Dynamics-1window, and wait for the simulation to complete.


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4.20. View DataLogger1to see how the Vapour Mass Flow, Pressure, and Temperaturebehaved. The TimeAxiscan be adjusted by using the red triangle at the bottom of the chart.

4.21. Additional information can be found in the Summarypage of the Performancetab.4.22. In the Main Flowsheet, double click on Depressuring-Dynamics-1 (Flowsheet),and click the Sub-

Flowsheet Environmentbutton. This will enter the depressuring sub-flowsheet.


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4.23. The sub-flowsheet contains information for the vessel, as well as spreadsheets that can export data.


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5. ConclusionIn this lesson we examined a basic case of an adiabatic depressuring. The adiabatic case describes a system suchas an oi l well where excess pressure has built up but there is no external heat. Pressure buildup can also be the

result of an accident such as a fire. In this case, the Operating Modecan be changed to Fire Mode, which allows

you to specify a heat flux. The Use Spreadsheetoption is also available, which allows a user to edit the duty

spreadsheet without the values being overwritten when the utility runs. Depressuring is important in assuring

the safety of a process.

6. CopyrightCopyright 2012 by Aspen Technology, Inc. (AspenTech). All rights reserved. This work may not bereproduced or distributed in any form or by any means without the prior written consent of

AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH

RESPECT TO THIS WORK and assumes no liability for any errors or omissions. In no event will AspenTech be

liable to you for damages, including any loss of profits, lost savings, or other incidental or consequential

damages arising out of the use of the information contained in, or the digital files supplied with or for use with,

this work. This work and its contents are provided for educational purposes only.


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Dyn 006H Depressuring

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