Dual MDMT Rating for P V at Different Temp.

[bonkers]

I am making alterations to an old pressure vessel and re-rating it. The original U-1A form has a duel MDMT rating which I do not understand. It is rated MAWP 250 PSIG at 125 F with an MDMT of 20 F at 250 PSIG and -40 F at 125 PSIG. I do not see where the USC-66 chart has diferent temps for different pressures. Can someone explain it to me?

Thanks for any help!
Bonkers

 

[SnTMan]

bonkers, have you reviewed UCS-66(b)?

Regards,

Mike

 

[jtseng123]

Looks like it has two process conditions, at 125F and 40F respectively, which is not unusual. We have a lot of vessels have dual stamping design conditions. The actual MDMT in my interpretation is only 20F and set at the highest design pressure 250 psig, which is also very common practice to do so. From re-rating purpose, MDMT 20 at 250psig is the worst case you shall consider and you can ignore 40F at 125psig even it says it is another MDMT that I think it is just another process condition.