Vessels, process tensile strength

When the shell of a vessel is exposed to extreme heat on the outside and the inside is in contact with vapor, metal temperatures may reach levels where tensile strength will be reduced such that rupture may occur even though the pressure does not exceed the set point of the pressure relief valve [for ordinary carbon steel, this temperature is 900°F (482°C)]. Vapor depressuring provides fire protection for process units by reducing the contained pressure at such a rate that ... 

In the design of process vessels and pressure equipment, two basic modes of failure may be assumed elastic failure based on the theory of elasticity and plastic failure based on the theory of plasticity. Except for thick-walled vessels, elastic failure is usually assumed for the design of pressure vessels. It is considered to occur when the elastic limit of the material is reached. Beyond this limit, excessive deformation or rupture is expected. These limits are usually measured in terms of tensile strength, yield strength, and, to some degree, rupture strength. 

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