Aspen Plus / Aspen HYSYS V8.
6 Pressure
Relief Device Workshops
Workshop 1 – Adding and sizing a PSV ........................................................................................................... 2
Workshop 2 – Fire Relief Calculations .............................................................................................................. 5
Basic Wetted Area Fire Calculation .......................................................................................................... 5
Semi-Dynamic Flash Fire Calculation ....................................................................................................... 5
Workshop 3 – Control Valve Failure Calculations ............................................................................................ 6
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Workshop 1 – Adding and sizing a PSV
1. Start by opening the workshop starter file in either Aspen HYSYS or Aspen Plus. If you prefer
working with HYSYS open “PRESSURE RELIEF STARTER.hsc”. If you prefer working with
Aspen Plus open “Safety Analysis Without PRD.bkp”. If using Aspen Plus, click the Next button
(N→) to run the simulation
2. Switch to the Safety Analysis environment and then select the Home ribbon. Make sure
you have selected SI units. Click on the Add PSV button.
3. This switches you into PSV “attach mode”.
In HYSYS move the cursor over the vapour connect on the top of Separator Vessel 100 and click on
the blue square. In Aspen Plus move the cursor over the end of stream VAPOUT and click on the
blue square.
4. You should now see the PSV added to the flowsheet. Double-click on the PSV to open the input
forms.
5. Enter the Design Pressure and temperature shown above. Then switch to the Scenarios tab.
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6. Click the Create Scenario button to create a PSV sizing scenario – it will be called Scenario100 in
HYSYS or SCEN-1 in Aspen Plus. Select the new scenario by clicking on the small arrow at the left
end and then click Open Scenario. (Note: an alternative ways to open the scenario are to double click
on the scenario in the input form, or by using the Navigation Pane on the left).
7. Enter the following data for this scenario
The calculated orifice size in 37.91 cm2 so select the next largest standard size 41.161 (P).
8. Explore some of the options e.g. changing Relieving method or Clicking on Edit next to the
Relieving Pressure and Total backpressure fields. But return to the data shown above before
continuing with the workshop.
9. In the Navigation Pane to the left, select the PSV (not the scenario) to go back to the PSV input form.
10. Normally you would consider other relief scenarios e.g. fire, control valve
failure etc. but for now make this scenario the sizing case by clicking the
Sizing Case check box.
11. Switch to the Line Sizing tab to do some preliminary line size checks.
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12. The inlet piping leading to the PSV is estimated to be 5m long and the tailpipe downstream of the
PSV is estimated to be 50m. As the nominal flanges sizes for this relief valve are 4/6 inches try
4inch schedule 40 for the inlet line and 6inch schedule 40 for the outlet tailpipe.
There are problems!
13. What lines sizes should be used?
14. Check the “Calculate Equivalent Length” box and experiment with entering fittings into the line.
15. Use File > Save As to save your simulation file with a new name.
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Workshop 2 – Fire Relief Calculations
Basic Wetted Area Fire Calculation
1. Continue with the simulation from Workshop 1. Add a second Scenario to the PSV and set the
Scenario Type to Fire. Open the input form for this new Scenario
2. Because it is a fire calculation, the reference stream should be the feed to the vessel. Check the
“Override” box and select the feed stream (HYSYS:Inlet Stream; Aspen Plus: S2)
3. How has the Relieving Pressure been calculated? Click on the Edit field to see this.
4. For this scenario the Calculation Method should be Wetted (API) and the Relieving Phase – Method
should be set to Vapor.
5. Use the following data:
- Drainage & Firefighting equipment are present
- Estimate Latent Heat
- Horizontal Vessel; Diameter = 3m, Tan-Tan length = 6m, Liquid Level = 1.5m
- Environment Factor F = 1
6. What is the corrected selected Orifice Size for this Scenario?
7. Save your Simulation.
Semi-Dynamic Flash Fire Calculation
1. We want to make a copy of the above Scenario. In HYSYS you can use the Duplicate & Rename
button with the List of Scenarios. With Aspen Plus you will need to make a new scenario and add
the above data.
2. Here are the changes for this second Fire Scenario:
- Calculation method is Semi-Dynamic Flash
- The Max Wall Temperature can be left at the default: 593.3 C
- Click on the Edit Flash Table field and set the Max Iteration Temperature to 65 C
3. How does this change the selected Orifice Size?
4. Save your Simulation.
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Workshop 3 – Control Valve Failure Calculations
1. Open a new starter file for this workshop: Control Valve Failure STARTER.hsc (or Control Valve
Failure STARTER.apwz for Aspen Plus).
2. In this workshop we will calculate the relief load if Valve VLV-100 fails open. If using Aspen Plus
run the simulation before proceeding.
3. Go to the Safety Analysis Environment and add a PSV on to the VapOut line. The design pressure
is 5 barg and the Design Temperature is 175 C.
4. Create a Scenario and set the Scenario type to Control Valve Failure. Open the input form for this Deleted:
Scenario.
5. The Reference stream should be the Feed stream upstream of the Control Valve so override the
default stream and select Feedstream / FEED. The Relieving Phase – Method should be set to
Vapor.
6. The input table for this calculation is given at the bottom of the form:
7. The first three fields Control Valve Inlet Pressure, Temperature and Normal Flow are the values for
the Feed stream into the Valve. Open the input form for this stream and copy/paste the values.
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8. The Control Valve CV at 100% can be found in the input data for Valve VLV-100 – copy/paste the
value into this field.
9. You need to enter the Mol Wt for the feed stream into the Valve and its Compressibility. In HYSYS
you can find these in the Properties page for the feed stream. In Aspen Plus, the starter file already
has a Property-Set containing these two properties (ZAMW) and this has been selected in the Stream
Report options to be included in the Results for all streams. So you can find these two properties in
the Results for the FEED stream. Enter the appropriate values.
10. What is the calculated Relief rate?
11. Finally the Relieving Temperature for the PSV sizing calculation will not be temperature of the
reference stream. The temperature will have dropped slightly as the pressure falls from the CV inlet
to the Relieving Pressure. So go back to the simulation environment and copy/paste the Valve plus
its Feed and product streams.
Change the valve from a Rating calculation using the Cv to a fixed pressure drop with an outlet
pressure of 5.5 barg i.e. the Relieving pressure. What is the calculated temperature for the stream
leaving the valve?
12. Change the setting for the Relieving Temperature in the form above from Reference to Manual.
Enter the temperature calculated above.
13. What is the correct Relief Valve size (select the next standard size above the calculated area)?
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