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StoneCold (Chemical)
17 Jul 07 17:36
I am trying to size relief valves for a pressure vessel that is rated for 250 psig @ 115 F it was originally designed to store liquid Propane. The vessel is made of carbon steel (SA616 for the shell and SA516 for the heads). The vapor pressure of the contents, in this case Tetrahydrofuran is only about 6 psia at 115 F. This causes the relief valves to be extremely large.  I am wondering if there are any guidelines (API, etc) about relief temperature.  Is it allowed to exceed the vessels rated temperature so that the relief valves could be set at a higher pressure?  (The only relief case for this storage tank is the fire case.)"
Latexman (Chemical)
17 Jul 07 17:58
TTHF @ 317.2 psia (1.21 x MAWP) = 402oF.  Most Allowable Stress Values of CS's are not reduced at all at this temperature.  There is no Code requirement that your relieving temperature be < MAWT.  I see no problem.

Good luck,
Latexman

StoneCold (Chemical)
18 Jul 07 18:17
Thanks Latexman.  
I could not find any reference that mentioned the MAWT and relief temperature and this low MAWT was giving me problems.

Thanks

StoneCold
Latexman (Chemical)
18 Jul 07 19:09
Yeah, I've come across that exact issue many times in the last 28 years and I know for a fact there is no Code requirement.  I've even discussed this, and some other topics, in the past with a member of the ASME Code Board.  He was from my alma mater and we worked in the same building, so it was very easy and convenient to ask a bona fide ASME Code board member these things in between our post game analysis and Coach critique of our team's latest game.

Good luck,
Latexman

kkimic (Petroleum)
28 Aug 07 14:56
Does this mean that the vessel will collapse in a relief scenario?

I have come accross this issue many times and it is true that sometimes the relieving temperature is higher than even the design temperature of the relief valve itself... any explanations? Still do not understand.

Thanks.
Latexman (Chemical)
28 Aug 07 17:36
"Most Allowable Stress Values of CS's are not reduced at all at this temperature" means the vessel will probably not collapse as long as liquid remains in the vessel during the fire.  If it goes dry, all bets are off!

Good luck,
Latexman

MortenA (Petroleum)
29 Aug 07 1:31
Stone this questions has been raised before.

As far as I understand it the API520 fire case is a scenario where deluge fails and the question is now to prevent further escalation. The vessel (and equipmennt) are assumed lost anyway. Off course the temp must not be so high that the vessel bursts but a temperature higher than the design temperature is acceptable.

Best regards

Morten
kkimic (Petroleum)
29 Aug 07 18:19
Yes well I undertand that the aim for having the PSV is to avoid further damage in this fire case scenario, by avoiding a "bomb" and keeping the situation as stable as possible... but yes equipment is assumed lost anyway I agree.

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