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MDMT coincident pressure

MDMT coincident pressure

MDMT coincident pressure

(OP)
I'm working on a carbon steel ASME VIII-1 vessel where the design pressure and temperature are up around 200psi/300°F. This design pressure has been chosen as an upset case, and the tank will actually be operating at around -5psig/-45°F, with potential upsets taking us down to around -70°F at deeper vacuums. In this application we're working with a saturated fluid, so the temperature/pressure curve is very well defined.

As I understand for MDMT the fabricator will be required to stamp the MDMT at the vessel MAWP, in which case the rated MDMT will be well above the required -70°F.

That said, they are also free to stamp additional MDMT values at lower coincident pressures to bring down the MDMT using the rules of UCS-66.

My question is this: are we allowed to specify a coincident pressure for a MDMT case that is lower than the vessel design pressure (200psi)?

As an example, could I set a MDMT case at: -70°F @ 50psi for a vessel designed for 300°F/200psi if the process shows that I could never exceed 50psi at -70°F?

If I could, then the coincident ratio of 0.35 from UCS-66 would be met, and the vessel would be exempt from impact testing.

Thank you!

RE: MDMT coincident pressure

Yes you can. But the nameplate will need to be stamped for these 'step' conditions.

RE: MDMT coincident pressure

marty007 (Mechanical)

Agree with TomBarsh (Structural); Also check ASME B31T 2010  

RE: MDMT coincident pressure

I would check if it has to be stamped for vac as well.

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