(2) zdas04 (Mechanical)
16 Oct 12 7:24
This really is both more complex and simpler than you think.
First the complex bit. You need to look at the system and come up with "credible scenarios". "Scenarios" are events like "shut block valve", "runaway reaction", "failure of controls", etc. and API 521 has a list of over 20 categories of scenarios. Let's say that it is possible for something like a "shut block valve" to happen. Then you have to look at what pressure could be seen in the vessel if a "possible scenario" happened. If the maximum pressure that could be developed is greater than the vessel MAWP, then the scenario moves from "possible" to "credible". For every credible scenario you need to determine the flow rate out the PSV that would prevent an overpressure event. Generally an overpressure event is defined as exceeding 110% of MAWP (but there are special cases like the Fire Case which allows 122%).
Now the simple bit. Size and set your PSV such that in the worst cast credible scenario you will not exceed the limiting overpressure. You can set a PSV at 120% of MAWP if your only credible scenario is Fire Case and AND your are absolutely certain that it will open quick enough AND you are absolutely certain that it will not lift a tiny bit higher than the set point AND you are foolish.
My default setting is MAWP. I have had specific cases where I set the PSV well below MAWP. I've had other cases where I set the PSV at 105% of MAWP (for example when I have two credible scenarios, one with very high flow requirements and one with very low flow requirements, I will use two PSV's, a very small one set at or below MAWP and a large on set 5-10% higher for the high flow requirement scenario--NOTE: never install two PSV in parallel with the "same" setpoint)
David Simpson, PE
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.