Multiple MDMT Cases - Nameplate Stamping

[marty007]

Hello all,

Code: ASME VIII-1 (2013)

I am having a disagreement with our AI about the stamping of a nameplate for a pressure vessel where two MDMT cases have been specified. I would appreciate any opinions on the matter.

Customer specified design conditions:
MAWP: 150psi @ 200°F
MAEWP: FV @ 200°F
MDMT-Case 1: 20°F @ 150psi
MDMT-Case 2: -50°F @ 0psi

Per Footnote 10 and UG-20(b), multiple MDMT cases are allowed. So, along with the normal MAWP/MAEWP calculations, we have performed two low-temperature calculations at the conditions indicated above.

For the nameplate stamping, using our standard nameplate (only has one available line for MDMT), we would apply the following stamping:
MAWP: 150psi AT 200°F
MAEWP: FV AT 200°F
MDMT: 20°F/-50°F AT 150psi/0psi

Now, the concern that our AI has, is what happens at the intermediate temperatures (ex. 0°F). He has asked that we add an additional line of text to our nameplate along the lines of:

Pressure not to exceed 0psi from -50°F To 20°F​

Is this interpretation not implied and standard understanding for a nameplate with the stamping I indicated above?

Extending this out to cases where you might have 3,4,5,... MDMTs, I shudder to think what additional text we would be asked to add on the nameplate.

Thanks for any opinions,
Marty

 

[TomBarsh]

I think your example is muddied because the lower MDMT's coincident pressure is "0". This doesn't make sense to me. But let's say that instead of "0" it was 50 psi. Then I think it's reasonable to follow the AI's interpretation that the specified coincident pressure/MDMT are the "lowest bound"...that the indicated MDMT is the coldest temperature at which you can operate the vessel at that pressure.

 

[marty007]

I agree that it is muddled by the 0psi, but unfortunately this was specified by our customer. I think the purpose of this rating is to indicate to the plant that they can start warming up the equipment at atmospheric pressures, but can't start pressurizing until they reach 20°F.

I whole-heartedly agree that it should be the lowest bound. I'm more confused by the requirement to add an additional set of markings: Pressure not to exceed 0psi from -50°F To 20°F

Is the lower bound interpretation not typical? Would some plants assume a linear line of acceptable MDMT/MAWP ratings between two points?

 

[TomBarsh]

My understanding from attending ASME continuing education classes where this situation was discussed is that the multiple markings on the nameplate indicate the lower bound, not a series of data points that can be interpolated directly. So a graph of pressure-temperature would look like stair steps, not an inclined line. This is not to say that the manufacturer couldn't run the appropriate calculations at the intermediate points and report those pressure-MDMT combination as well.

I agree that the phrase "Pressure not to exceed..." is redundant and unnecessary. On the other hand, it does make things crystal clear. If you are reluctant to include this, can you run it up the food chain at the AIA?