Plugging criteria and stress determination for high-pressure feedwater heater tubes with local wall-thinning

Abstract

Tubes with local wall-thinning defects are often found in the heat exchangers of high-pressure feedwater heaters in thermal power plants. When serious damage is found in a tube, the tube is taken out of service by plugging it to prevent rupturing at the location of the local wall-thinning defect. This paper introduces a theoretical and numerical analysis of the stresses found in feedwater heater tubes with local wall-thinning defects under pressure and thermal loads. The effect of the dimensions of this defect on these stresses is also studied. Using finite element analysis and regression, analytic equations for the hoop and axial stress on the inner surface of a tube with a local wall-thinning defect are proposed. The plugging criteria for a tube with this form of defect are also determined using von Mises stress theory. It is found that maximum stress occurs on the inner surface of the tube. The radial stress on the inner surface does not depend on the c/t and c/b ratio; rather, depth (c) is the main factor influencing overall stress (i.e., hoop, axial, and von Mises stress), with hoop and axial stress increasing as the depth increases. The longitudinal length (2b) also has an effect on hoop and axial stress. When the depth increases (c/t = 0.3-0.5), the effect of b is more prominent. The procedure used in this study is independent of the material properties and operating conditions, so it can be applied to other tube materials under various operating conditions in high-pressure feedwater heaters. © 2020 Elsevier B.V.