Depressuring Valves to avoid BLEVE during a Fire

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.

From the originalgatevalve
.