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

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

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

(OP)
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.

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

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.

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

(OP)
Thank you GHartmann

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

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

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

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

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

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.

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