PSV Set Pressure vs Operating Pressure 3

[Sharik]

Is there a limit between the PSV set pressure and the operating pressure? For example, if a system was designed with a MAWP of 1000 psig and the safety relief valve has a set pressure of 1000 psig, would this be acceptable if the system operating pressure was reduced to 500 psig? The system is not derated. Is there a limit as to how much lower the normal operating pressure can be below the PSV set pressure?

 

[LittleInch]

The main difficulty is usually that operating pressure is almost impossible to be the same as MAWP due to the under tolerance of your relief valve.

A basic spring relief valve is often quoted at about 8%. Pilot operated can get you to within 3%, but then you also need to make 'space" for instrumented safety systems and alarms/ shutdown

AFAIK, there is no limit below MAWP you can set normal operating pressure as long as the MAWP isn't changed.

It's a bit strange and a bit of a waste of money, but if something else is causing the operating pressure to be set lower then so be it. Also in some cases it gives room for expansion / higher pressure or simply a facility starts at a higher operating pressure then it falls over time, especially for things like oil and gas fields.

Doesn't impact the MAWP and set pressure of a particular Pressure Vessel.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

In general:
[ul]
[li]PSV with metal seats can operate up to 90% of MAWP.[/li]
[li]PSV with soft seats can operate up to 95% of MAWP.[/li]
[/ul]

Good Luck,
Latexman

 

[RVAmeche]

I'm not aware of an issue of having the PSV set pressure significantly lower than the MAWP.

If it's a situation where a vessel with a much higher MAWP is used in a different process because it's available, that may make sense. But if the whole system design is for the higher MAWP pressure and the PSV is set lower, it seems like you could've saved money by just lowering the design pressure of the system.

 

[MortenA]

@Sharik,

You are asking about a max difference between normal (max) operating pressure and SP of the PSV - and the short answer is NO. Code may require that you have a PSV (e.g. for the fire case) and BP recommends a min margin between SP and normal (max) operating pressure (differs with type of PSV) but no rules with regards to a max differennce. Biggest advantage is low risk of lifting the PSV (that may damage the seat and increase leaking).

Best regards, Morten

--- Best regards, Morten Andersen

 

[The Obturator]

You can find the recommended minimum pressure differential (between operating and set pressure) in ASME VIII (2021) Appendix M-10.
For example Spring Operated Pressure-relief Valves
Set Pressures up to 70 PSIG the differential should be at least 5 PSIG
Set Pressures 71 to 1000 PSIG differential 10%
Set Pressure above 1000 PSIG 7%


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sabeau]

Something worth considering which I think has been overlooked here is the seat tightness of the pressure relief valve (SOPRV).
If we consider a standard like API-527 then maintaining a normal operating pressure of 90% (or even 95% of set pressure) is asking for issues