Can a Relief Valve Set Point be set higher than the MAWP (Dual Stamped)?

[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 to -20F and the other MAWP is 85 psi @ -20 to -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.

 

[GHartmann]

I can't answer your question about ASME UG-125. I can try to explain the dual rating. The vessel is good for 250 psig from -20 to 300 F. At the colder temperatures from -20 to - 50 deg F there are decreased stress ratings for your SA516-70 material; thus the lower MAWP.

Your ammonia temperature at 9" Hg is approximately -40 deg F.

Thus your relief valve should be set at 85 psig.

 

[Tugboat1984]

Thank you GHartmann

 

[chicopee]

The reason for this double rating is I believe that this tank is also suitable for propane service.

 

[Tugboat1984]

chicopee
The vessel was built for and used only in ammonia refrigeration process.

 

[chicopee]

That's fine, however, it is not uncommon to have propane tanks turned into anhydrous ammonia tanks as long as certain external modifications are made; that makes a lot of sense with the tank that you describe above having the double rating which for propane the 250 psig would apply.