Effect of superimposed backpressure on PSV reseat pressure

[acewolf187]

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

 

[don1980]

Superimposed pressure changes the set pressure of conventional valves, if the spring is not adjusted for that superimposed pressure. So, if no spring adjustments are made and you have a superimposed pressure of 0.5 bar, then the PSV will now open at 10.5 bar and reclose at a pressure that is 10% below that value.

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.

 

[acewolf187]

well explained, thanks don1980.

Just as superimposed pressure affects the lifting pressure, the reseating pressure is also affected.

Cheers,
acewolf187

 

[The Obturator]

Just to advise and clarify further.
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.

 

[acewolf187]

Thanks avalveman, I agree with all above.

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

 

[The Obturator]

acewolf187 I am a little thrown with your last scenario quoting a 26 % overpressure. Why this figure and are you within the vessel code requirements (I do not know where this installation is or what the application is). You need also to engage the SRV vendor on this since this higher overpressure may affect the SRV selection. I take it also that this is a vapour/gas relief ? How is a 21 % built up back pressure determined ? Too many open items to properly advise best solution.

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

 

[don1980]

The allowable built-up backpressure is the amount of overpressure. So, acewolf is correctly applying API 520. If the overpressure is 26%, then you're allowed 26% built-up backpressure. Engineers often mistakenly think that there's a fixed allowable limit (10%) for built-up backpressure, but that's only true when the overpressure is 10%. A conventional PSV can be designed for any amount of built-up backpressure, as long as it doesn't exceed the overpressure.

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.

 

[acewolf187]

Thanks very much for the advice don1980 and avalveman.

Best Regards,
Acewolf