MDMT versus Pressure

[sheuer]

I have an ASME Div. 1 Sec. VIII SA240-316L SST pressure vessel that has internal coils at -238F. The vessel itself is normally operated at atmospheric pressure or under vacuum at -103F, but is also periodically steam sterilized at 30 psig and 275F. I usually rate these at 30 psig/FV at 275F with an MDMT of -103F at 30/FV. Not actually true conditions since the vessel could never see pressure and the MDMT at the same time.

My customer wants to change the MDMT rating on these to -238F, but this brings in the requirements for impact testing of production welds and qualifying the filler metal. Our weld procedures and welders are qualified down to -320F so this isn't a problem other than the extra testing.

My question is, since the low temperature MDMT is not concurrent with a pressure condition, can the vessel be rated in such a way as to avoid the UHA-51 requirements (Basically consider the vessel as not be code rated during it's cold cycle)?

 

[sanshu1111]

Please refer to UHA 51 (d)(1)(a) which exempt impact test requirements for austenitic steel having carbon content less than 0.1% at MDMT -196 Deg C (-320 Deg F) and warmer.

 

[sheuer]

I'm well aware of the exemption for the base material, BUT UHA-51(f) requires impact testing of the welding consumables and UHA-51(h) might potentially require production impact tests.

Ignore the "when and where" impact testing is required and for what materials, my question is more are these requirements not applicable since the vessel is never pressurized when it sees the low temperature condition? I'm posing this question to my A.I. tomorrow, but wanted other opinions on the subject.

 

[TGS4]

The intent is that the MDMT has a coincident pressure, just like the design temperature has a coincident design pressure. I'm not sure that you could place the MDMT "out-of-scope" because it only happens when the pressure is below 1 bar.

You're doing the right thing by asking your A.I. Please let us know how that goes.

 

[sheuer]

Yea, UG-20(b) with the statements that "the MDMT marked on the nameplate shall correspond to a coincident pressure equal to the MAWP" and "when there are multiple MAWP's, the largest value shall be used to establish the MDMT marked on the nameplate" makes me think that I'm barking up the wrong tree...but I'll defer to my A.I.'s interpretation. I'll be sure and let you know what feedback I get.

 

[sanshu1111]

UG-20(b) requires that for the marking on the nameplate, MDMT to be established for largest value of MAWP. This clause also allow us to report additional MDMT corresponding to other MAWP's.

In case your process ensures that the MDMT of -238 Deg F can not occur at a higher MAWP, you can also report another design condition of MDMT as -238 Deg F @ its corresponding pressure. You may however be required to check the case of internal coil failure as well which may pressurize the vessel beyond the coincident pressure considered for -238 Deg F.

You may like to refer to interpretation no VII-1-01-23. This interpretation is not directly related to your question but will help you to take the decision.