Classification of Thermal Piping Loads Using Limit Load Analysis

Abstract

Equipment nozzle loads essentially originate from sustained (gravity) sources and restraint of the free thermal displacement of the attached piping. A common practice has been to assume that these thermal piping loads develop only secondary stresses. That is, a 1.5Sm [2] check on membrane stress intensities arising from thermal piping loads is typically not performed. The key assumption used in support of this approach has been that these loads decay appreciably with local shell deformation such that the associated stresses are truly self-limiting in nature. This paper illustrates that this assumption may not be appropriate in all instances. A typical pressure vessel and piping configuration is examined. In this example, the associated stresses and deformations developed due to thermal piping loads resulted in significant deformation of the shell arrangement. In static evaluations of local stresses in shells, the ASME Code only offers two classifications that may be applied to stresses resulting from thermal piping loads: primary or secondary. Given these results it may be more reasonable to treat thermal piping load membrane stresses as being primary.