Q. Minimum Design Metal Temperature?

The MDMT and coincident pressure shall be selected in consideration of the operating phases. Reliable administrative procedures to control the pressure/coincident temperatures during transient operations (e.g. startup and shutdown), are often appropriate from a materials of construction selection point of view.  For example, when considering the effects of auto-refrigeration on carbon and low alloy steels, such procedures make it appropriate to consider operations below the minimum design metal temperature (MDMT) stamped on the nameplate.

When atmospheric temperatures govern the metal temperatures during startup or normal operations, the lowest one-day mean atmospheric temperature at the installation site should be considered. Fig 2-2 from API Standard 650 or NBC of Canada may be used to establish the lowest one-day mean temperatures, insofar as applicable, or obtained from the Design Questionnaire. The mean metal temperature during shop and future field pressure testing shall also be considered during the vessel design stage.  The temperature during the pressure test of the pressure resisting components and attachments judged to be essential to the structural integrity, shall be at least 30oF (16^oC) warmer than the minimum design metal temperature to be stamped on the nameplate, but shall not exceed 120oF (49^oC).