Practical Process Engineering

Make sure select "Sizing Case". What does it mean? It means this scenario is the main one. Sometimes, various scenarios are probabale. But, one of them is governing as it needs largest orifice area. Here, we have only one scenario.

The same flowrate and the same orifice size, but no error shows up. Please note that discharge coeeficient is 0.975 per API-520 recommendation for priliminary sizing. You can change it to vendor specified ammount.

One more PRD is added. The pilot operated safety valve is a good option here as the relief load is too high.

Move back to "PRD Data" Tab and do two changes: 1- Press "Add PRD" button 2- Change the valve type to Pilot-Operted safety valves. Do it for both valves. Now, return to sizing page. No error shows up and you see green bar at the bottom.

Although Orifice T is selected, but we still see an error mesaage. HYSSY guides us how to resolve this option. We can slpit the whole flow between two PSV's. one is set at design pressure and the second is set at 1.05 * design pressure. Let's do it.

Press select and HYSYS automatically selects proper standard orifice size per API- 526

The error message says total bp exceeded 10%. It is because we enterd 2 bar for constant superimposed bp. . Move back to "PRD Data" tab and change constant superimposed bp from 2 bara to 1 bara. You will see the safey valve is sized now and a green message shows up at the bottom.

Only one step is remained. You have to enter or specify relief load per the message you see at the yellow ribbon at the bottom of the page. Like other sections, if "Reference" is selected, HYSYS obtain relief load directly from the flowsheet. You can enter a nubmer manually as well. Make sure to choose proper units. Select "Reference" here. Sizing should be done at this point. But, you see an error message in the yellow ribbon.

We already sntered 2 bar as the constant superimposed bp. You can enter the variable superimposed (if any), or change the maximum allowable overpressure (please note that max allowable overpressure for a conventional PSV limits the max allowable overpressure per API-510). Kb for conventional and pilot-operated PSV's is 1. If "Calculated" is selected, HYSYS automatically use proper value. However, for some installations you get this coefficient from the vendor. HYSYS allows you to enter it manually as well. Change nothing and close the window.

It has two tabs: Scenario Setup Fluid Properties in "Scenario Setup" tab, you enter/Change the Scenario Name or you can select/change Scenario Type. In "Scenario Reference Stream" section: Reference Stream: As a default, HYSYS shows the name of the stream that is used to obtain or calculated require properties for PSV sizing. As a default, you see the name of the stream on which you already installed the PSV. But, if you select "Override", you will be able to select another stream. It means, HYSYS obtains/calculates required properties from another stream. Enter the relieving temperature. If "Reference" is selected, normal operating temperature shows up. I choose "Manual" and enter 50 C. Relieving pressure is summation of set pressure and overpressure. Set pressure= 45 barg Overpressure for conventional PSV is 10% Relieving pressure= 45 + 0.1 * 45= 49.5 barg Press "Edit" to change default value of overpressure you already entered in preferences. Leave it as is. Total Backpressure: Superimposed bp+ built-up bp. It shows 10% of the set pressure as default. Press "Edit". Another window shows up

Move back to "Scenarios" tab and double click on the "Scenario 100". A new window shows up. You are only a few steps away from finishing a preliminary sizing.

PRD type: Relief Valve or Rupture Disk (Select Relief Valve for this example. We will talk on rupture disk in future) Discharge to: You have several optins here. I choose "Flare" If "Design P" is selected, set pressure= design pressure. We already specified deisgn pressure in "Equipment" tab. You can unselect it and enter a diffent number (for example for multiple PSV's or supplemental PSV, you have to enter something higher than design pressure). Leave as is. Constant superimposed BP: Change it to 2 bar. Relief Valve Type: Select "Conventional". Rememeber the maximum allowable overpressure for this kind of PSV is 10%. You can change it in "Preferences" as I explained yesterday. RD Combination (Kc): This is one of the main coefficient we need to size a safety valve. If "PSV+Rupture Disk" is selected, you see "0.9" per API recommendations. However, you can enter a different number. Leave it as is.

It would not be hard to enter or change data here if you are familiar with safety valve sizing terminologies and process. If not so, please review API-520 part 1 or review recent posts here.

Go to next tab "PRD Data". to add more data.

Choose Scenario type here. It is optional. You can select it later. I select Blocked Outlet

You are able to add several scenarios to calculate orifice area for them. Here I add only one. Scenario type shows "User Defined". Click on it to see table of scenarios. It is optional to select a scnario. It means you can leave it as is and move forward to next stage.

HYSYS helps you to find out what is the next step. There is a message in the yellow ribbon at the bottom of the page :"Create an emergency scenario". Move to next tab; "Scenarios" and hit "Create Scenario" button.

Here you see the data entries for this tab

Here you have to select or enter: Operating Temperature Design Temerature Operating Pressure Design Pressure If you select "Reference", the operating temperature and pressure from the main flowsheet show up and if you select "Manual", you have to enter the values manually. Please note that set pressure= design pressure as default in HYSYS ( You change it later in PRD Data tab) and design temperature can potentially affect safety selection as it affects pressure rating of the flanges. HYSYS uses operating temperature for relieving temperature and it can affect calculated orifice area directly. So, if relieving temperature differs from the normal operation or you would like to see how temperature affects PSV sizing, enter it manually and do not select "Reference".

Double click on the PSV. A new page shows up. It has 5 tabs: Equipment, Scenarios, PRD Data, Rating and Line Sizing (not active till PSV get sized)