Hello
I come back to this topic, there was a similar query years ago, but I wanto to go a little bit more in detail to be sure that I fully unserstand. Please, correct me if anything I state is wrong.
Let's assume that I want to specify a new conventional PSV (even though for this example I would have chosen a balanced one).
I attach a diagram with backpressure values for different scenarios (no discharge and multidischarge).
PSV set pressure is 23.3 barg.
I have a variable superimposed pressure (0.3 to 1.37 barg). In GEPF, built up backpressure is 2.73-1.37= 1.36 bar.
In a conventional PSV, as per API 520, if allowable overpressure is 10% of Pset, then, allowable built up backpressure is 10% Pset. For this particular PSV that would be 2.33 bar
If in the process datasheet I specify maximum superimposed backpressure as 1.37 barg, then, I could specify maximum total backpressure as 1.37 + 2.33 = 3.7 barg. Is this correct?
Then, for my case, since maximum total backpressure is 2.73 barg, PSV would work (built up, which is 1.36 bar, is less than 2.33 bar).
Now, in reality, would it be ok to specify a conventional PSV like this, giving the range of variable superimposed backpressure and the value for maximum built up backpressure of 2.33 bar? Would the PSV open as long as total backpressure is less than 3.7 barg? Or is it wrong to specify a conventional PSV when superimposed pressure is that variable?
In reality, what I am doing is checking an existing system with flarenet (because some new discharges have been added). I came across this conventional PSV (and it discharges to flare system, where superimposed pressure is variable). With flarenet I can get the backpressure values for the different scenarios, and I wanted to check if the PSV is still valid for multi-relieving scenario.
Thank you for your help
Edited by Butterfly, 21 January 2025 - 12:47 PM.