Different Set pressure PSV in same line 1

[Nasim12345]

Dears
We have 5 PSVs in one line and A & E set with same set pressure & other 3 psv's(B,C,D) in different set pressure. What is the reason behind this different set pressure selection for same size & type of PSVs for same line?

Thanks

 

[The Obturator]

Nasim12345
You most probably have a series of PSV's in line due to a large capacity that must be passed, that cannot be done by one high pressure PSV. Hence smaller valves capable of the pressure are used in parallel. The reason for the difference in set pressures, known as staggering, is to prevent all PSV's trying to open at the same time which would cause adverse operation such as chattering, due to a large relief area at once. The ideology here is to set the first valve at the "set pressure" (normally MAWP) and the other(s) at 5 % above. This is why you get overpressures of 16% specified in such scenarios (105 + 10 % = 116 %). This is further specified in ASME VIII and API-520.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Nasim12345]

Thank you avalveman. Your reply is useful for me.