Consideration of Thermally Induced Bending Load in Failure Assessment of Piping Components

Abstract

The primary goal of this paper is to demonstrate and validate the applicability of the failure assessment diagram (FAD) approach for predicting failure behavior of piping subjected to thermal loading caused by thermal stratification. The analysis is performed for an austenitic surge line of a PWR circuit where thermal gradients over both the pipe length and circumference produce high bending moments. For the most critical location, circumferential through-wall cracks of variable length are assumed and assessed with respect to their initiation and possible unstable propagation. The assessment result is shown to be decisively influenced by the modeling approach, which in effect depends on the categorization of the bending stresses as either primary or secondary loading. To achieve realistic fracture mechanics assessment, the whole surge line is described by a finite-element model, whereas thermal loading is imposed by mapping the temperature distribution measured under service conditions. Subsequently, elastic-plastic stress and fracture mechanics calculations are performed for the entire surge line model containing cracks of different size in the most stressed location. The analysis clearly confirms the LBB behavior. The numerical results are then compared to those based on the failure assessment diagram (FAD) approach, whereas the section bending moment determined from pipe calculations is categorized as either primary or secondary loading. It is concluded that most realistic analytical failure prediction is achieved when considering the bending moment as secondary loading. Even though the analytical approach is found in this case to produce conservative assessment results, its accuracy is concluded to be acceptable within a wide range of crack sizes considered.