Stress Evaluation for Class 2 and 3 Pipes With Reinforcement and Wall-Thinning Using Equivalent Stiffness Concept

Abstract

Abstract Due to various reasons, wall-thinning can occur in nuclear piping under the operating environment. When wall-thinning occurs in large-diameter Class 2 and 3 pipes, reinforcement is typically applied before replacement. To check structural integrity of reinforced pipes, stress analysis should be performed, which is not straightforward due to three-dimensional nature of reinforced pipes. Full 3-D FE analysis would not be so useful because calculated stresses would not be compatible to the equations given in ASME BPVC Sec. III NC/ND-3650 for Class 2 and 3 piping system. In this paper, stress evaluation methods for reinforced wall-thinned pipes are presented using the equivalent straight pipe concept. The equivalent straight pipe is defined as the pipe having the same elastic stiffness as reinforced wall-thinned pipe. Analytical expressions for the stiffness of wall-thinned pipe with reinforcement are given for axial tension and bending moment, using which the dimensions of the equivalent pipe can be obtained. The equations in ASME BPVC Sec. III NC/ND-3650 for Class 2 and 3 piping system are then used using the dimensions of the equivalent pipe. Finally, a method to quantify the stress concentration factor of reinforced wall-thinned pipe for fatigue evaluation is presented using the structural stress method.