Relief Properties for valve sizing at Set pressure or set pressrue + over pressure? 1

[jamesbanda]

The purpsoe of this is to understand peoples conventions on this.. generally it does'nt impact the calcs but on a revalidation i'm doing it is close to needing a new valve.. so i'd like some input please..

So, when you size for the fire case
Q= FA^0.82 : Fire duty = Q/ Latent heat.
Is the latent heat/ temp at
1. Pressure = PSET pressure
2 Pressure = Set pressure + over pressure margin.. (as you approach the critical pressure latent heat really drops off..)

I think it is at 2, because in a fire the vessel will be operating at the pressure = PSET + P over pressure.

(I'd rather it be no 1 as my RV is smaller but think that is not correct..)

 

[Latexman]

For fire case, sizing pressure = 1.21 x PSET. The sizing temperature is the boiling temperature of the liquid at the sizing pressure.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[don1980]

It's hard to justify using Hvap at a pressure other than the relieving pressure. That said, there's a lot of "wiggle room" in choosing Hvap when you have a chemical mixture. That's a decision that requires judgment - there's no single right answer because the relief composition is changing with time.

As for the shrinking Hvap as you approach critical, that's a real concern that should cause one to question whether the PSV can provide meaningful protection from fire. If the vessel is all vapor or supercritical at relief P, then a PSV isn't going to do any good at protecting from fire, and there's certainly no justification for replace the existing PSV with a bigger one. The question is whether or not other protective measures need to be taken to reduce the risk or consequences of vessel failure during a fire.