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/ball valve. These PCVs tend to have air fail closed actions. Are there any comments on this state of affairs?

What about a fusible link design that opens with spring actuator when link melts? This would only open in the event of a fire and are independent of the air system.

I prefer to have any ESD system go to a de-energized state or position during emergencies.  That means the blowdown valve should go to the open position on loss of instrument air and/or signal to the solenoid controlling the valve.

I know that some companies go to an energized state during emergencies but I fail to see the logic and advantages.

In my opion a BLEVE is a catastrophic condition to the point that inadvertant release of organics is something you may have to put up with.

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