Design pressure of PSV discharge piping
Design pressure of PSV discharge piping
(OP)
Our client is insisting that PSV discharge piping must have the same design pressure as the PSV set pressure of 1200 kPag (175 psig). Normally PSV discharge piping would have lower design pressure since it's open pipe to flare, ie pressure is limited to backpressure from hydraulic losses in the piping and flare system only. We are designing new pressure vessels with design pressure of 1200 kPag, with PSVs set at 1200 kPag, and flare discharge piping connects to an existing flare system. The existing flare system piping design pressure is 1200 kPag (ie unnecessarily high).
The client says the actual max backpressure is lower, at 500 kPag. We are checking the selected balanced bellows type PSVs as to whether they will operate with this backpressure as it's right on the 40% limit (but that's another issue). These PSVs will definitely not work if they ever needed to relieve against an actual backpressure of 1200 kPag.
My question is, should we accept a design with a PSV discharge piping design pressure of 1200 kPag? Given that, if this pressure actually existed, the PSVs would not work. We are under pressure to sign off and issue this design, and are currently being seen as the "bad guy" since I'm refusing to issue our package with piping design pressures of 1200 kPag to match the flare system we are connecting into.
The client says the actual max backpressure is lower, at 500 kPag. We are checking the selected balanced bellows type PSVs as to whether they will operate with this backpressure as it's right on the 40% limit (but that's another issue). These PSVs will definitely not work if they ever needed to relieve against an actual backpressure of 1200 kPag.
My question is, should we accept a design with a PSV discharge piping design pressure of 1200 kPag? Given that, if this pressure actually existed, the PSVs would not work. We are under pressure to sign off and issue this design, and are currently being seen as the "bad guy" since I'm refusing to issue our package with piping design pressures of 1200 kPag to match the flare system we are connecting into.





RE: Design pressure of PSV discharge piping
Your only concern should be the second part of your question, i.e. whether any of the PSV's will see an actual backpressure in the ranges that could affect their performance. If yes, you need to ensure that the design will address those issues. If there is an existing flare network simulation model, you can see the calculated backpressure for the maximum combined simultaneous relief and ecide on which PSV type to use. If there is no such information, you can either charge the client for doing these calculations, or go for the design which ensures reliable PSV performance regardless of the actual backpressure.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Design pressure of PSV discharge piping
However, in this case you're not talking about a high pressure. 1200 kPa = 174 psi, and that is easily within the limits of a 150# outlet connection (unless the temperature is very high). So, except for cases where the temperature is very high (check the tables in API526 to see the limits) you'll get valves with 150# inlet and 150# outlet connections. That's means you and your client can both be satisfied, despite it being wrong to say the outlet must always be able to tolerate the same pressure as the inlet. Perhaps this is just a miscommunication - possibly the client is saying they want 150# outlets and this has been morphed into a statement that the outlet must always be equal to the inlet.
RE: Design pressure of PSV discharge piping
As you've realized, you also need to check what the bellows limits are on the PSVs you are purchasing. If the bellows' backpressure is limited to less than 1200 kPag in a static or non-relieving state, I would want to make sure the client is aware of this 'weak link".
I know of at least one client that I've worked with that stipulates a piping minimum design pressure of 150 psig and that is what goes on the line list. Full stop be it process piping or the 3/4" section of piping downstream of the drain valve that you run to the sewer hub.
RE: Design pressure of PSV discharge piping
Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.
RE: Design pressure of PSV discharge piping
One reason for 12barg design pressure may be to withstand detonation pressure (due to flashback at the flare tip) in the event of loss of purge. Industry position on this is that connected RVs' are not expected to be in operation if there is a detonation within the flare network.
RE: Design pressure of PSV discharge piping