Fire case PSV flow

[Kamakal]

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

 

[Latexman]

As the vessel contents heats up, and pressure increases, the PSV will "pop" open at *about* the set pressure. Code allows us to size the PSV in the fire case such that the pressure does not continue to build-up to more than 21% accumulation.

Good luck,
Latexman

 

[Latexman]

If the vapor generation rate is less than the rated capacity of the PSV at about 93% of set pressure (7% blowdown), then yes the PSV will cycle open and closed.

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

 

[MortenA]

With regard to HOW it open between 100% and 121%:

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

 

[djack77494]

I think there are two points worthy of noting here.
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

 

[Kamakal]

Thanks everybody.

kamakal