Allowabel Tail pipe back pressure - fire case 1

[Bill3752]

In the event of a fire case the allowable pressure is 21% of set (psig). I am doing tail gas calculations and am not certain if the built up back pressure needs to be less than 21% or 10%. thanks

 

[Latexman]

For fire case the inlet to the PSV can be as high as 21% over the set pressure. It is called the overpressure. This has nothing to do with the allowable backpressure from the tail pipe.

The allowable backpressure depends on the type of PSV (conventional, bellows, remote pilot operated). No matter what type is selected, the backpressure cannot be so high as to reduce the capacity of the PSV below the requirement.

Good luck,
Latexman

 

[lizking]

If tail pipe is being sized on the rated capacity of the RV, then 10% is the limit (conventional RV).

 

[JoeWong88]

There are at least three aspects a 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"

JoeWong
Chemical & Process Technology

 

[CJKruger]

JoeWong88 is correct, with the clarification that (i) applies to built-up back pres only. See API520 Sec 3.3.3.1.3.

 

[Bill3752]

Thanks so much for your responses. I had read 3.3.3.1.3, but I wanted to be sure to be catious about this.