What should be the air fail action for depressuring valves installed, or nominated, to avoid a BLEVE when fire surrounds a pressure vessel?

If air fail closed, and if fire also causes damage to air system then depressuring will not happen and BLEVE possible.

If air fail open, then vessel depressurizes when there is no fire and at a far higher frequency than fire, leading to undesired/ unacceptable release of organics.

As a rider to the general problem above, we have, in the past, nominated certain PCVs to also serve as depressuring valves. These PCVs tend to have air fail closed actions. Are there any comments on this state of affairs?

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This is a very common problme/ question. I used to work on the design of a NGL plant and there we had the same discussions again and both arguments are true. But I guess that one would certainly want to have the Blow down valves be opened during a fire event. So I tend to make them Air FAil open. But I will then make sure that I have a relibale source of Air that doesn't fail every now and then.

Other thing is to look for a similar design that your company has with other systems.

I don't think that there is any one answer that is correct for all situations.  As the others have indicated I think the typical action would be depressuring valves fail open and shutdown valves fail closed but that does not have to be so in every case.

I believe what you are doing is the correct way to approach the problem and you need to evaluate the advantages and disadvantages of your options.  Pick the fail position that makes the most sense for your system but expand your evaluation to more than just the fail action of the depressuring valve.

I can't offer any thorough reference for depressuring systems but API RP-521 does have a little information that you may want to review.  Basically, the key points they make include...
"The valves should remain operable for the duration of the emergency or should fail in a full open position."
I would put the emphasis on the "or" and don't necessarily interpret that to mean that all depressuring systems should fail open.

API 521 goes on to say...
"Fireproofing of the power supply and valve actuator may be required in a fire zone."
So there are other factors that should be part of your design that should affect your decision.

For example, I have seen emergency air supplies (small pressurized containers) sufficient to actuate an air operated valve located close connected with valve(API Cast Steel Valves).  Maybe you could locate your depressuring valve outside the fire zone or maybe install redundant valves.  A fusible link sounds interesting, some relief systems on compressed gas cylinders include a combination rupture disc and fusible plug to ensure release only during a fire event.  What about depressuring your system to some type of disposal system (flare or scrubber).

I happened to find this article on the Instrument Society's website that talks about using a Safety Integrity Level (SIL) selection method relating to depressuring of a hydrocracker reactor.  You might want to consider using an approach like this to help decide which fail action is best for your system.

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