Stress and fatigue analysis of pressurised vessel-nozzle intersections with and without local wall thinning.

摘要

Pressure vessel and piping systems are essential components of all power plants. After long-term service, metallic components in plants will experience wear, leading to wall thinning. Severe thinning will affect the structural integrity, and thus a capability is required to assess the thinned components. The greatest weakening is normally near the intersection of two components. In this work, the finite element method is used to determine the remaining structural capacity of components which have been subjected to local wall thinning. The work is divided into two main parts; a stress analysis at the intersection of pressurized vessels and piping having wall thinning, and a fatigue analysis of damaged intersections subject to cyclical loading.;In the second part of the study, a preliminary fatigue analysis is carried out to compare the performance under cyclic loading of sphere-nozzle and cylinder-nozzle intersections, without and with wall thinning damage. The study consists of an elastic-plastic large displacement FEA analysis for simulated seismic action. Results are presented for the major parameters that affect fatigue life. The study ends with some conclusions about the suitability of the FEA for assessing damaged intersections.;In the first part of the study, a finite element analysis (FEA) is carried out for the common sphere-nozzle and cylinder-nozzle intersections, to determine the stress concentration factor (SCF). Axi-symmetric (2D) and 3D elements, respectively, are used for the two geometries. In convergence-validation studies, the SCF values obtained for vessels with uniform wall thickness are compared with previously published experimental and analytical results. Comparisons are also made with results from standard formulae given in the literature, and good agreement is observed. An evaluation is made of the effect on the SCF of the growth of the thinning away from the intersection. Finally, a parametric study is conducted in which the SCF is computed for a number of intersections, initially considered undamaged, and then with wall thinning damage. Charts based on the results are provided for the convenience of engineers.