Stress Classification for Elevated Temperature Service

Abstract

The paper presents two procedures which can be used to classify the stresses originating at the structural discontinuities of pressure vessel and piping components designed for elevated temperature service. The stress classification is based upon the definitions of primary and secondary stress intensities which are provided in the ASME Boiler and Pressure Vessel Code, Sections III and VIII. The intent is not to reclassify primary stresses, but to provide a designer with practical guidance in evaluating the primary characteristics of thermal and structural discontinuity pressure stresses. Two objectives for an appropriate classification of stresses are: (a) to reduce undue conservatism in elevated temperature Code Case N-47 design rules when primary stress is only a small fraction of secondary stress, and (b) to provide a sufficient margin of safety for extreme (Level D) loadings when the primary local and general membrane stress levels are close to the yield point of the material at structural discontinuities.