Fire case PSV flow
Fire case PSV flow
(OP)
Hello everybody:
In case of a fire with the inlet flow on to a vessel, the PSV will "pop" open at 21% accumulation.
My question is:
* If the generation of vapour is not large open, will the PSV opening be between 121 and 100 like a modulating (if that is the right word) control valve OR
will it keep popping shut and open like a on-off valve.
Thanks
In case of a fire with the inlet flow on to a vessel, the PSV will "pop" open at 21% accumulation.
My question is:
* If the generation of vapour is not large open, will the PSV opening be between 121 and 100 like a modulating (if that is the right word) control valve OR
will it keep popping shut and open like a on-off valve.
Thanks





RE: Fire case PSV flow
Good luck,
Latexman
RE: Fire case PSV flow
If the vapor generation rate is more than the rated capacity of the PSV at about 93% of set pressure but less than the rated capacity of the PSV at 21% accumulation, the PSV flow steady at some intermediate pressure.
Good luck,
Latexman
RE: Fire case PSV flow
Have you read API 520? There are some good figures in this standard that explains how it works.
It will actually denpend on your tyoe of PSV
A standard spring loaded valve will open sufficiently to allow the fluid to escape (until it is full open off course).
A pilot operated my open fully and then close again - modulating pilot operated valves does however exists.
Please note that the max 3% dP in the inlet is required in order to avoid a situation where the drop in apperant pressure AT THE VALVE will cause the valve to re-seat - thus allowing pressure to rise again causing the valve to open cause pressure drop to increase again and thus eneter a cycle of opening and closing.
Best regards
Morten
RE: Fire case PSV flow
First, the valve will open at 100% of the set pressure (+ or -), and not at 121%. Allowable accumulation during a fire event dictates the upper limit of pressure that may develop in the vessel of 121% of design (or MAWP).
Secondly, there are two types of valves in terms of their relieving characteristics. You can have a "pop" type of valve that jumps between fully open and fully closed, OR you can have a modulating valve which operates over a range of valve positions.
HTH,
Doug
RE: Fire case PSV flow
kamakal