Can a Relief Valve Set Point be set higher than the MAWP 1

[Tugboat1984]

I have a pressure vessel that has a U-1 form (as well as the nameplate) that has two different MAWPs and MDMTs.
One MAWP is 250 psi @ 300F and the other MAWP is 85 psi @ -50F. We operate this vessel between -30F and -40F. The vessel contains anhydrous ammonia and is operated at around 9"hg.

Questions:
Which MAWP do I set my RV's to? 250 psi or 85 psi or something else?
Can I operate the vessel at the conditions of -40F (with an MAWP assigned of 85 psi) with 250 psi RV?
If yes, where in ASME UG-125 does it permit this design?

I understand that manufacturers can provide additional MAWP and MDMTs, but how does an owner/operator comply? I have upload the U-1 form.

 

[gr2vessels]

Tuboat1984,
Try read the U-1 again;- it says MDMT is -20 F at 250 psi and -50 F at 85 psi.
That is, you can operate the vessel at 250 psi down to -20 F, but not at -30 - 40 F.

I believe you could interpolate to work out the MAWP at - 40 F and set your PRV at that pressure, not any higher than that.
Cheers,
gr2vessels

 

[Tugboat1984]

Thanks for your response gr2vessels. Am I correct in saying this...

As long as the vessel is operated within the temperatures of -20F to +300F, the MAWP is 250 psi and my RV can be set to 250 psi.
When the vessel is operated between -50F to -20F, the MAWP is 85 psi and it needs an RV with a set point no higher than 85 psi.

Am I am talking about true operating conditions, not upset scenarios where the vessel is seeing deviated conditions from the normal operating parameters.

Where am I wrong in this basic understanding?

 

[gr2vessels]

You are correct.
Cheers,
gr2vessels

 

[GHartmann]

Please don't double post questions. See my explanation in the other thread.