ASME Code, Section VIII, Division 1, Design Temperatures
Many pressure vessels are designed to operate at high temperatures, but are usually required to be under pressure at ambient temperature during startup and/or shutdown conditions. A few vessels are required to operate at temperatures below ambient temperatures, and others can be subjected to “autorefrigeration” resulting from operational upsets or gas leaks.
Both Divisions 1 and 2 of ASME Code, Section VIII, require the nameplate of a vessel to display both a maximum temperature and a minimum temperature on the vessel’s nameplate. This has long been a practice for the Company and a requirement of Division 2, but it has only recently become a requirement of Division 1, beginning with the 1986 edition.
The maximum temperature controls the vessel design by establishing the maximum allowable design stresses for the selected materials of construction. Selecting materials based upon maximum temperature assures that the stresses developed in a pressure vessel at the MAWP will not cause failure by ductile bursting or gross yielding during continuous operation at the maximum temperature.
The minimum temperature does not directly affect the design of a pressure vessel. However, the minimum temperature can affect the selection of the materials. The materials must have sufficient toughness at the minimum design temperature to prevent failure by brittle fracture during startup and shutdown.
Division 1 defines both maximum and minimum design temperatures, which in effect, establish a temperature range over which the vessel can be operated at the MAWP.
The maximum design temperature as defined in Division 1 “shall not be less than the mean metal temperature (through the thickness) expected under operating conditions.” It is displayed on a vessel’s nameplate as the temperature for which the MAWP is determined. It is the maximum metal temperature permitted at the MAWP.
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