Relief Valve Sizing in Aspen HYSYS ® V8.3
Demo File Walkthrough Guide
Wilfried Mofor, Product Management, Aspen Technology, Inc.
Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc.
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Relief Valve Sizing in Aspen HYSYS ® V8.3
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pressure Safety Valve Workflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Sizing a PSV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Line Sizing for PSVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Documenting PSVs Created in Aspen HYSYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix: Theory for Critical Flow Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 1. Descriptions of Variables in Equation 2 – Equation 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 2. Conditions for PSV Sizing Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 3. Derived Information to Calculate PSV Orifice Size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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Relief Valve Sizing in Aspen HYSYS ® V8.3
Introduction
Using the downloaded demo file and this walkthrough guide, the user will learn how to properly use relief valve sizing in
Aspen HYSYS to design, size, and document pressure safety valves for a simulation. In order to access these features,
Aspen HYSYS V8.3 must be installed.
If any questions arise during the use of this demo file, contact Wilfried Mofor, the AspenTech Product Manager, for relief
valve sizing in Aspen HYSYS at Wilfried.Mofor@aspentech.com.
Process Description
The demo file simulation includes a mixed phase hydrocarbon feed that is approximately 34% vapor at 16.56°C
and 2 barg. The feed is then flash separated into its liquid and vapor components. The vapor stream passes through
a heat exchanger where it is heated, then fed to a compressor. The liquid stream is fed to a pump and then passed
through a heat exchanger.
The unit operations present in the simulation provide ample opportunities for pressure safety valve placement on the
flowsheet. Also, having both liquid and vapor streams allow for different calculation methods of relief valve sizing
to be performed.
Pressure Safety Valve Workflow
Sizing a PSV
1.)
Download the demo package and unzip the file to a folder. This walkthrough file and an Aspen HYSYS simulation
named ‘Pressure Relief Starter’ are extracted.
2.)
Open Aspen HYSYS. Once Aspen HYSYS has loaded, open the case “Pressure Relief Starter.HSC” from the location
that the file was extracted. The simulation will open to the main flowsheet in the Simulation environment, as shown
in Figure 1.
Figure 1. Main Simulation Flowsheet for Pressure Relief Starter HYSYS Case File
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Relief Valve Sizing in Aspen HYSYS ® V8.3
3.)
Continue to the Safety Analysis environment by clicking either the “Safety Analysis” option situated below the
navigation pane, or the “Pressure Relief” icon located in the home ribbon.
Figure 2. Safety Analysis Option and Pressure Relief Icon to Open Safety Analysis Environment
Figure 3. Flowsheet in Safety Analysis Environment
4.)
In the Safety Analysis environment, pressure safety valves can be added to streams or equipment and overpressure
protection contingencies can be analyzed. Click the “Add PSV” button under the Home tab of the ribbon, shown in
Figure 4.
Figure 4. Add PSV Option in Home Tab of Ribbon
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Relief Valve Sizing in Aspen HYSYS ® V8.3
5.)
Drag the cursor to the flowsheet. Hover the cursor over the stream “5.6.3.2 – CritVapflow Example” until a small red
box appears (Figure 5). This box indicates that a PSV can be added at this location. Left click on the red box to add
the safety valve to that location.
Figure 5. Where to Add a PSV to a Stream and an Attached PSV
6.)
Double click the PSV block added to the flowsheet. The Valve Design form opens.
Figure 6. Valve Design Form
7.)
In the Valve Design tab, the valve’s material of construction, type, design temperature, set pressure, superimposed
backpressure, and discharge location should be specified. Enter 75 psig (5.171 barG) for the set pressure, and leave
all other parameters as they presently are.
8.)
Create an emergency scenario by clicking the “Create Scenario” button, shown in Figure 7. Clicking this populates the
table with a new scenario.
Figure 7. Creating an Emergency Scenario in the Valve Design Form
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Relief Valve Sizing in Aspen HYSYS ® V8.3
9.)
Double click the newly created scenario in the table. The Scenario Setup form, shown in Figure 8, opens.
Figure 8. Scenario Setup Form for Emergency Scenario
10.) Leave all scenario parameters defaulted. Specify the relief load scenario type as ‘User Defined’ and enter 53500 lb/h
(24270 kg/h) for the relieving flow.
11.) The Valve Results table will display a calculated orifice size of 37.18 cm2. Select an actual orifice for the valve, which is
usually the next largest value on the dropdown list in the table. For this demo, 41.161 cm2 is the appropriate size to
choose. Once an actual orifice is chosen, the Valve Results table completes with sizing specifics for the valve (Figure 9).
12.) Check the “Sizing Case” box in the Scenario Setup form.
Figure 9. Entering a Relief Load, Choosing an Orifice, and Specifying Sizing Case
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Relief Valve Sizing in Aspen HYSYS ® V8.3
13.) Click the Fluid Properties tab, depicted in Figure 10, to view the fluid properties of the load at relief.
Figure 10. Fluid Properties for Relief Load
Line Sizing for PSVs
14.) To begin line sizing for a valve, return to the Valve Design form and then click the “Line Sizing” tab, shown in Figure 11.
Figure 11. Navigating to Line Sizing Functionality
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Relief Valve Sizing in Aspen HYSYS ® V8.3
15.) Select a nominal diameter of 6 and schedule 40 piping for each the inlet and outlet line.
Figure 12. First Iteration of Line Sizing
16.) As can be seen in Figure 12, a nominal diameter of 6 and piping schedule of 40 results in pressure drop errors for the
inlet and outlet line and an average fluid velocity error for the outline line. Increase the inlet and outlet diameters to 8
inches to obtain an approved line sizing. The Line Sizing Results table is completed, showing exact pressure drop and
fluid velocity results.
Figure 13. Approved Line Sizing Within Default Limits
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Relief Valve Sizing in Aspen HYSYS ® V8.3
17.) View the Line Sizing Default Values window by navigating to the “Customize” tab in the ribbon, and selecting “Line
Design Preferences”. This shows the constraints on line sizing, as set forth by API 520, 521.
Figure 14. Editing Line Sizing Constraint Limits
Documenting PSVs Created in Aspen HYSYS
18.) Access the documentation builder by going to the “Reports” tab on the ribbon and clicking “Documentation Builder”,
when in an editing window for a valve. If viewing the flowsheet, navigate to the “Home” tab on the ribbon, and click
“Documentation Builder”.
Figure 15. Navigating to the Documentation Builder Option
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Relief Valve Sizing in Aspen HYSYS ® V8.3
19.) Microsoft Access will open, and display the interface depicted in Figure 16. Click “Reports and Project
Customization” to edit pertinent report formatting. Upon clicking “Reports and Project Customization”, the form in
Figure 17 is opened.
Figure 16. Documentation Builder Interface in Microsoft Access
Figure 17. Common Settings for Valve Documentation
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Relief Valve Sizing in Aspen HYSYS ® V8.3
20.) Specify the fields shown under the “Common Settings” tab similar to the one shown in Figure 17. Then, specify the
fields for each data sheet as shown in Figure 18 using the information present under the “Mechanical Data Sheets” tab.
Figure 18. Mechanical Data Sheet Tab
21.) Return to the main menu of the documentation builder by clicking the “Start” option in the top left corner of the
customization form. Then, from the main menu of the documentation builder, select the “Edit” option for a process
data sheet.
22.) View the edit page for a process data sheet as shown in Figure 19. The values present are automatically populated
directly from the process simulation and valve design and sizing in Aspen HYSYS.
Figure 19. Sample Process Data Sheet Edit Page
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Relief Valve Sizing in Aspen HYSYS ® V8.3
23.) Open the edit form for a mechanical data sheet, obtainable by clicking “Edit” under the mechanical data sheet
heading. Figure 20 displays the values imported automatically from Aspen HYSYS.
Figure 20. Sample Mechanical Data Sheet Edit Page
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Relief Valve Sizing in Aspen HYSYS ® V8.3
24.) Return to the main menu of the documentation builder after completing editing. Select the “View” option under the
mechanical data sheets header to see a completed mechanical data sheet.
25.) When viewing the front cover page for a document, use the arrows on the bottom left of the documentation builder
to scroll through the different pages of the report, as shown in Figure 21. Note that information input in report
customization in step 20 is present on the cover.
Figure 21. Sample Mechanical Data Sheet Cover Page
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Relief Valve Sizing in Aspen HYSYS ® V8.3
26.) Scroll through the pages until page 3 is reached. All the information input on the edit page is visible on the final copy
of the data sheet, which is shown in Figure 22. Note that information input in report customization in step 20 is
present on the document.
Figure 22. Final Version of Mechanical Data Sheet
27.) The example HYSYS file provided in this example can be used for further exploration of relief valve sizing capabilities
inside Aspen HYSYS, including sizing for more overpressure scenarios and design of PSVs for different streams.
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Relief Valve Sizing in Aspen HYSYS ® V8.3
Appendix: Theory for Critical Flow Example
As seen in the “Fluid Properties” form, upon relief, the fluid was undergoing critical flow. Critical flow is defined as the flow
at which the sound velocity in the medium is reached under a given differential pressure—even though the differential
pressure is higher for a given nozzle or restriction, the flow will not increase.
The critical flow pressure ratio is defined as:
(Eq. 1)
where Pc f is the critical flow nozzle pressure in psia, P1 is the upstream relieving pressure in psia, and k is the ratio of
specific heats (Cp/Cv).
As per API 520, 8th edition standard, calculation of PSV orifice size under critical flow conditions is completed using the
following equations, with variable descriptions in Table 1. These calculation methods from API RP 520, 8th edition are all
built into Aspen HYSYS.
(EN units)
(Eq. 2)
(SI units)
(Eq. 3)
(Eq. 4)
Table 1. Descriptions of Variables in Equation 2 – Equation 4
W = required load through the PSV (lb/hr) [kg/hr]
P1 = upstream relieving pressure (psia) [kPaA]
Kd = effective coefficient of discharge. For preliminary sizing use:
= 0.975 when PSV is installed with or without Rupture Disk in combination
= 0.62 when PSV is not installed and sizing is for a Rupture Disk (where applicable)
Kb = capacity correction factor due to backpressure
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Relief Valve Sizing in Aspen HYSYS ® V8.3
For Balanced Bellows: Value should be obtained from manufacturer; preliminary sizing curve is provided in API 520
For Conventional or Pilot Operated valves: Kb = 1
Kc = combination correction factor for installations with a Rupture Disk upstream of PSV
= 1.0 when Rupture Disk is not installed
= 0.9 when a Rupture Disk is installed and the combination factor does not have a published value
T = relieving temperature of the inlet gas or vapor (°R = °F+460) [°K = °C+273.15]
Z = compressibility ratio @ PSV inlet conditions, to consider deviation from perfect gas
M = molecular weight of the gas
A = calculated orifice (in2) [mm2]
k = Cp /Cv - ratio of Specific Heats
The example in this guide is based on an application example taken from API 520 with the following conditions:
Table 2. Conditions for PSV Sizing Example
Emergency scenario = local power failure (not included in example for simplicity)
W = 53500 lb/h [24,260 kg/h]
Composition = 1:1 mixture of n-butane and pentane with M = 65
Relieving temp = 167 °F (627 °R) [348 °K]
PSV set pressure = 75 psig [517 kPag], corresponding to design pressure
Total back pressure = 14.7 psia [101.3 kPaa (0 kPag)] and builtup BP = 0 barG (not included in example for simplicity)
The following can be derived from API 520:
Table 3. Derived Information to Calculate PSV Orifice Size
Permitted accumulation = 10%
Relieving pressure, P1 = (5.171-1.013)*1.1 + 1.013= 5.688 psia [670 kPaa]
Calculated compressibility, Z = 0.84
Specific heat ratio, k = 1.09
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Relief Valve Sizing in Aspen HYSYS ® V8.3
The critical flow pressure, Pc f , can then be calculated from Equation 1 as:
(Eq. 5)
Using the Equation 3, the valve coefficient can be calculated as:
(Eq. 6)
Kb would be equal to 1, since the selected valve is a conventional one, making the back pressure lower than 10% of the set
pressure (7.5 psig). Kc would be equal to 1 as well since no upstream Rupture Disk is installed.
Using the calculated valve coefficient and other defined variables, the valve orifice can then be calculated from
Equation 2 as:
(Eq. 7)
Once the required Orifice has been calculated, the next commercially available orifice can be selected with the aid of API
526. In this case, the orifice “P” (6.38 in2) will apply.
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