Steel Selection for Pressure Vessels Subject to Autorefrigeration
Autorefrigeration should be considered when selecting steels, as described below. In some liquid services such as LPG, a leak could reduce the pressure and cause a drop in temperature of a pressure vessel and its contents as the liquid boils off.
The autorefrigeration of temperature is defined here as the temperature that the contents of the vessel would reach if the vessel is depressured to 40% of its maximum allowable working pressure. If the temperature of autorefrigeration is less than 20°F, then the vessel should be treated as subject to autorefrigeration, and this used as a design basis to avoid brittle fracture.
(Due to an increase in ASME code allowable stresses in 1999, vessels built in 1999 and later will have autorefrigeration temperatures equal to what the contents would reach if the vessel is depressured to 35% of its MAWP. Section 243 of the Corrosion Prevention and Metallurgy Manual discusses this further. Consult a specialist to determine if this change applies to your situation.)
Vessels that are subject to autorefrigeration require additional consideration as follows:
1. Steels from Curve D of Code, Division 1, Figure UCS-66, should be used. Typically, carbon steel plate steel should be normalized SA 516. Forgings may be SA 350, Grade LF 2, and pipe may be SA 333, Grades 1 or 6. These steels have good inherent toughness.
2. Impact testing is not required for autorefrigeration, unless already required at the normal design temperature. SA 350 and SA 333 materials are, however, impact tested in accordance with their respective specifications.
Autorefrigeration is not considered equivalent to a cold design or operating temperature due to the lowered pressure. Therefore, the recommended safeguards against brittle fracture are not as stringent as for a cold operating temperature. The use of SA 516 steel, and equivalent forging and piping grades, should by itself provide ample resistance to brittle fracture during autorefrigeration. Impact testing is not required for autorefrigeration, unless it is required for a cold design temperature without considering autorefrigeration.
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