Effect of superimposed backpressure on PSV reseat pressure
Effect of superimposed backpressure on PSV reseat pressure
(OP)
Hi all,
If a conventional valve is set pressure of 10 barg and has a 10% blowdown, it operates with a superimposed backpressure of 0barg, it should reseat at 9barg.
What if the exact same valve now has a superimposed backpressure of 0.5 barg, would it still reseat at 9barg or would it reseat at 9.5barg because the backpressure helps it reseat?
I'd like to know your thoughts and experiences.
Thanks,
Curious Wolf
If a conventional valve is set pressure of 10 barg and has a 10% blowdown, it operates with a superimposed backpressure of 0barg, it should reseat at 9barg.
What if the exact same valve now has a superimposed backpressure of 0.5 barg, would it still reseat at 9barg or would it reseat at 9.5barg because the backpressure helps it reseat?
I'd like to know your thoughts and experiences.
Thanks,
Curious Wolf





RE: Effect of superimposed backpressure on PSV reseat pressure
This is why it's best to use a bellows PSV when you have superimposed backpressure. As long as the bellows pressure limit isn't exceeded, the valve will reliably open at the correct set pressure regardless of the superimposed backpressure. That's not true when using a conventional valve.
RE: Effect of superimposed backpressure on PSV reseat pressure
Just as superimposed pressure affects the lifting pressure, the reseating pressure is also affected.
Cheers,
acewolf187
RE: Effect of superimposed backpressure on PSV reseat pressure
If the superimposed backpressure is of a constant value, you may use a conventional SRV. The spring setting (CDTP - Cold Differential Test Pressure) at the factory would then be 9.5 barg (+ temperature correction if appl). Conventional design is used since the back pressure is known not to change, it is constant.
If the back pressure is a superimposed variable back pressure, then yes, you would use a balanced design regardless.
Worth noting also, that if there is any built up back pressure (occurring during SRV opening), then this should also be considered in addition to any other back pressure. BU back pressure limit on its own is limited to 10 % of set pressure when using a conventional design.
Per ISO, only the term Safety Valve is used regardless of application or design.
RE: Effect of superimposed backpressure on PSV reseat pressure
I however plan to use a conventional valve with a built-up backpressure of 21% but the overpressure at the valve inlet will be 26%, which the protected system can easily tolerate. I believe this is acceptable according to API520 but do you know any downsides to doing this?
Regards,
acewolf
RE: Effect of superimposed backpressure on PSV reseat pressure
Per ISO, only the term Safety Valve is used regardless of application or design.
RE: Effect of superimposed backpressure on PSV reseat pressure
Avalveman, I agree that a conventional PSV can be adjusted to compensate for any amount of constant superimposed backpressure. The reality though is that there's practically no system that is consistently at a constant pressure. Unless the valve is route to atm, then I'd start with the assumption that a bellows PSV is needed. Then look at the variation of backpressure values (during S/U, S/D, and normal operation) and consider whether an equal variation in set pressure is tolerable. If so, then a conventional PSV is OK. Pay particular attention to S/U and S/D, during which we don't normally have good control of the pressure, and consider the potential for the PSV to lift at a lower pressure due to the absence of that "constant" backpressure.
RE: Effect of superimposed backpressure on PSV reseat pressure
Best Regards,
Acewolf