Practical Process Engineering

The results for 3 pilot operated PSV's. After you add the PRD and change its type to pilot-operated in "PRD Data" tab, move to "Rating" tab and press "Equalize Flow" and "Select Orifice". The result shows 100% of the first valve plus about 85% of the other valves are being used. Please note that orifices are P not Q or even T. Smaller PSV is cheaper.

Results for valve B with no error. Outlet pipe size decreased from 46" to 30". I would like to add 3rd valve and see how it affects the sizing.

Results for valve A with no error.

Move to "Line Sizing" Tab. We have to valves" 100 PSV 001 A 100 PSV 001 B Choose Valve A from drop down list in "Valve Name". Change the inlet and outlet line size (Length, Diameter and Schedule) so that no error shows up. Do this for the 2nd valve as well. Here are the results.

Press "Select Orifice" button. System will change orifices from T to Q. Now, % orifice area of 2nd valve increase to 55.55%

Orifice T for both valves, but % orifice area of the 2nd one is zero.

Move back to "PRD Data" tab, press " Add PRD". Change the "Relief Valve Type" form conventional to Pilot-operated. Move to "Rtaing" tab and press "Equalize Flow" button. Software divides the required flow evenly between two PSV's. Orifice size for both is T. No error shows up, but % Orifice Area for 2nd valve is zero.

Outlet pipe size : 46" STD with no error. However, this is very large and we have to optimize the sizing. I add one more pilot operated PSV but with smaller size than T.

Let's work more on last night example. Now we have only one pilot-operated PSV 8T10. Two line sizing criteria met, but one still needs more attention. You have to increase line size gradually untill no error shows up. The first size with no error is 46" that is very large.

I selected some rough numbers to control velocity at outlet not to exceed 90% of the critical velocity.

I changed equivalnet length from 50 m to 5 m and pressure drop went down 3%.

Three error shows up. The first one is for pressure drop in inlet line that exceeded 3%. I can reduce equivalent length or increase pipe diameter.

As the first estimation, I select an 8" inlet line and 10" outlet line with schedule 60. Let's see the error messages.

Click on "Constraint Setting" to see major constraints. If you did not adjust them in preferences, here you can change them and see the effects.

Move to last tab: "Line Sizing". This tab activates only after the sizing is done.

Something is wrong? Yes. There are two tables in the above picture. The bottom table shows %percentage orifice area for both valves. It shows 66.11% for first valve and 0% for the 2nd one. It means we do not need the 2nd one. Total required area is 110.9 cm2 and selected orifice area is 167.7 cm2. Go back to "PRD Data" tab and remove the 2nd PSV. Following picture for "Rating" tab" after I deleted the 2nd valve. No specific error shows up.

Move to "Rating" tab. Here, you see a summary of the calculation settings. As default, flow is devided evenly. But, you have this option to change this 50/50 flow division here.