The autorefrigeration temperature is defined as the temperature that the contents of a vessel would reach if the vessel is depressured to 40% of the MAWP (35% of MAWP for vessels built in 1999 and later). However, the autorefrigeration temperature need not always be used as the minimum design temperature. Autorefrigeration is not considered a critical service condition equivalent to continuous operation at a low temperature, because the loss of pressure that causes the contents of a vessel to autorefrigerate also reduces the operating stresses during autorefrigeration. Furthermore, cooling is likely to be highly localized at the source of a leak, or lag appreciably behind the temperature of the vessel’s contents during a general system loss of pressure.
Pressure vessels subject to autorefrigeration should be fabricated from SA 516 normalized or SA 537 plate (with equivalent pipe or forging grades). See Section 500 for typical selections. These are materials that have inherently good toughness at low temperatures, and should provide adequate resistance to brittle fracture during a transient condition that causes autorefrigeration. CV-impact testing is not mandatory for the autorefrigeration temperature.
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