Pressure Vessels Thermal Stresses
Thermal-expansion problems can occur whenever there is: (1) a considerable difference between the vessel operating temperature and the temperature of the environment surrounding the vessel; (2) restricted expansion or contraction; or (3) a temperature gradient within a vessel component that creates a differential expansion. Problems due to external constraint are solved differently than those due to internal constraint.
Thermal stresses are secondary stresses (see Section 115). They will not cause failure in ductile materials on their first application, but they can cause failure after repeated cycling, because of thermal fatigue.
Because the difference in temperature between the inside and outside of a vessel depends mainly on the thickness of the shell and insulation, thick-wall and uninsulated vessels are more susceptible to failure caused by thermal stresses. The stresses are compressive at the inner surface, where the temperature is the highest, and tensile at the outside. Failure from fatigue most likely initiates at the outer surface, where thermal stresses add to the tensile stresses from internal pressure.
Another location where thermal stresses are likely to occur in a hot pressure vessel is the support skirt. At the shell-skirt junction the temperature of the shell and the skirt will be nearly the same. However, the skirt temperature will decrease from the joint down. The temperature difference causes a rotation of the skirt end, which is restrained by the welded joint. In addition to the thermal stresses, radial deformation of the shell under internal pressure will cause discontinuity stresses. In order to minimize thermal stresses at this location, the shell insulation is usually extended below the skirt-to-shell weld. The skirt should also be long enough to minimize the temperature difference between the bolted-down base of the skirt and the concrete foundation, in order to prevent any distortion and local thermal stress at this location.
Under certain conditions, application of a steady mechanical load (like internal pressure) to a vessel subject to cyclic operating temperature may produce cycling of combined thermal and mechanical stresses and a progressive increase in the plastic (permanent) strain in the entire vessel. The action of cyclic, progressive yielding is called thermal ratcheting. It may lead to large distortions and ultimately to failure.
In practice, thermal stresses can be minimized by reducing external constraints, providing local flexibility capable of absorbing the expansion, selecting proper materials (or a combination of materials), and by selective use of thermal insulation.
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