Minimum Design Metal Temperature

The minimum design metal temperature (MDMT) is defined in Division 1 as “the lowest (temperature) expected in service.” This temperature is essentially the same as the minimum pressurizing temperature (MPT) in Company terminology. For most vessels, the minimum design metal temperature is the lowest metal temperature permitted at the MAWP. The Company’s current practice is to limit the operating pressure to 40% of MAWP at temperatures below MDMT.

This practice (allowing pressurizing to 40% of MAWP when below the minimum design metal temperature) has changed for vessels built in 1999 and later. This is due to the fact that ASME increased the allowable stresses for Division 1 vessels in 1999. For the newer vessels, limit operating pressure to 35% of MAWP at temperatures below MDMT. Refer to Section 342 of the Corrosion Prevention and Metallurgy Manual for more information, and consult a specialist to confirm if this practice applies to your situation.

The minimum design temperature should never be above 50°F, unless the circumstances are discussed with an experienced pressure vessel engineer.

The minimum design metal temperature can be established for a vessel in several different ways depending upon how the vessel will be operated, as discussed below. It is necessary to assure resistance to brittle fracture at the minimum design temperature. This is accomplished by either selecting materials that are known to have adequate toughness at the minimum design temperature, or requiring CV-impact testing in order to establish that adequate toughness exists. Preventing brittle fracture is discussed in Section 500, and the Specifications section. The model specifications contain recommendations for procuring equipment with adequate resistance to brittle fracture.

It is preferable to use materials with assured adequate toughness as designated by the Code curves described in Section 500. Materials that require CV-impact testing can be substituted if the vessel fabricator advises that they are more economic or if procurement will be expedited. But the substitution should be discussed with an experienced pressure vessel engineer before it is permitted.

Whenever CV-impact testing is required, the tests must be conducted at the minimum design temperature (for the base metal, weld metal, and heat affected zone) for all weld procedure qualifications. In addition, production test plates may need to be prepared during fabrication of the vessel to verify that the materials and welding consumables actually used will provide the required CV-impact toughness.

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