Effect of Defect Geometry of Localized External Erosion on Failure of Boiler Tubes

Abstract

Boiler tubes experience reductions in wall thickness due to erosion and corrosion mechanisms while in use. Due to this localized thinning, the tube becomes susceptible to gross plastic deformation, which eventually causes the tube to rupture. This study presents a non-linear finite element analysis of different geometric configurations of localized boiler tube defects. A defected boiler tube with three variants of the localized thinned area, having different geometrical shapes (flat, n-shape and u-shape) was modeled and subjected to a simulated internal pressure. The effect of the defect geometrical shapes and their dimensions (shape aspect ratios, defect length, width and depth) on the failure of the tube while in use were investigated. From the numerical results, the stress concentration factors (SCF) associated with each defect were obtained, and it was observed that these play a more significant role than the amount of material removal in influencing the failure of the tubes. This relationship between the SCF and the defect geometry characteristics helps to predict which tube with a localized thinning geometry is safe for continued operation or will fail, and hence to categorize the severity of defects to prioritize maintenance spending. The result of this work will serve as a guide to categorize the severity of external boiler tube defects. This is relevant whenever a constrained economic environment does not allow for all boiler tubes with defects to be replaced.