Validity of Procedure for Plastic Collapse Assessment of a Local Thin Area Near Vessel and Nozzle Intersections Subjected to Internal Pressure and External Loadings

This paper develops a procedure for plastic collapse assessment of vessel (run pipe) - nozzle (branch pipe) intersections with an arbitrarily positioned local thin area (LTA) under different loading conditions, namely internal pressure, external moment on a nozzle applied along various directions with respect to the vessel main axis, and pure bending moment on a vessel. Although simplified procedures for plastic collapse assessment based on the p-M (internal pressure ratio and external bending moment ratio) diagram method have been previously proposed for straight cylindrical vessels and pipe bends with an LTA, very few studies have dealt with the determination of plastic collapse load for an LTA in the critical region of intersecting vessels subjected to internal pressure and external moment loading. This is likely due to the complexity of the stresses caused by the applied loads in the critical region, which arises from geometric discontinuities. In this paper, simple and empirical formulae for predicting conservative plastic collapse loads for an LTA in the critical region of the intersecting vessels are proposed based on the analytical results of stresses at defect-free vessel-nozzle intersections by using linear finite element analysis (FEA). Localized elastic stress retardation factors are taken into account in the evaluation by the results of a non-linear FEA. Consequently, a p-M diagram method is developed for application to vessel-nozzle intersections with an LTA.Copyright © 2015 by ASME