3 replies to this topic

[Bill B]

I am a little confused by API 520. When doing tail pipe calcs, for a fire scenario is the allowed back pressure 21% or set or 10% of set. Thanks

[fallah]

I am a little confused by API 520. When doing tail pipe calcs, for a fire scenario is the allowed back pressure 21% or set or 10% of set. Thanks

I think,if your question is about conventional PSV and its build-up back pressure,as per paragraph 3.3.3.1.3 of API 520 (SEVENTH EDITION),build-up back pressure for fire case could be up to 21% of set pressure.

[Bill B]

Thanks. I reread the standard and think you are correct. Appreciate your help.

[JoeWong]

At least three aspects how backpressure affecting a Pressure Relief Valve (PRV).

i) Maximum allowable accumulated pressure of protected vessel.
If allowable overpressure is 21% of set pressure, then allowable back pressure can be as high as 21% of set pressure

ii) PRV Capacity
Shall ensure PRV capacity is not affected by back pressure for any overpressure i.e. 10% or 21%

iii) PRV Stability
Shall ensure stability of PRV for back pressure exceeded 10%.

For conventional spring loaded PRV, you may read more in "Several Impact of Backpressure on Conventional PRV"