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MDMT

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darrenyee88

Mechanical
Jul 17, 2014
56
On occasion, the MDMT is specified by the end user as an arbitrary
value.The vessel fabricator is then responsible to
verify that the actual MDMT of every component used in
that pressure vessel is lower than the arbitrary value
requested for the nameplate stamping. Why lower?
 
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First, the MDMT may not be an arbitrary value it is usually based on lowest anticipated operating temperature or other.

Second, at or below MDMT.
 
100% agreed with metengr. The MDMT is not an arbitrry number.

When you perform calculations for MDMT, rated MDMT can be equal or colder than required MDMT.

Regards.
 
Just as there is the "design pressure" and the maximum pressure rating...MAWP, you can think of the design MDMT as what is necessary for the design requirements, and then a "rated MDMT" which is the coldest permissible operating temperature per Code rules.

ASME even utilizes this concept in VIII-1 Figure UCS-66.2.
 
One item to watch however when it comes to using commercial software and a calculated rated MDMT is the impact on welding.

For example, if you have a low-pressure CS vessel designed to ASME VIII-1 using Compress, it will often tell you the rated MDMT can be as low as -55°F. This however only takes into consideration the base metal, and ignores the impact on welding. With CS, you hit a breakpoint at -20°F when impact tested weld procedures and production impact tests become required. This can add additional cost to a vessel, and is why you often see CS vessels rated to -20°F.
 
You can certainly fabricate a vessel to -55Deg.F without impact tested weld procedures and production impact testing being required.
 
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