How to detremine the Adjusted MDMT for auto-refrigeration condition

[jtseng123]

Dear all,

We have many CS equipment will undergo auto-refrigeration with pressure down to 0 psig and the temp can drop to -150F.

The “Basic MDMT” from Table UCS-66 will be calculated based on the nominal noncorroded thickness. Then the reduction from Fig UCS-66.1 will be subtracted from the Basic MDMT to become the Adjusted MDMT.

My question is: If material is impact tested at -49F, will the “basic MDMT” becomes -49F instead of value from Table UCS-66 ?

One case we have, 2.25” thick SA-516-70N impact tested at MDMT = -49F. The reduction from Fig UCS-66.1 is 140F during auto-refrigeration:

(1)Using basic MDMT per Table UCS-66, it will be 0F, the adjusted MDMT = 0-140 = -140F.
(2)However, it is impact tested at -49F. If basic MDMT can be treated as -49F, then adjusted MDMT = -49-140= -189F but not to be lower than -155F per UCS-160. So the final adjusted MDMT will be -155F.

Which of the above calculation is the correct interpretation of the code ?

 

[pdiculous963]

I would limit the adjusted MDMT to -155.

"the MDMT shall not be colder than the impact test temperature less the allowable temperature reduction as determined from Figure UCS-66.2"