Detailed Finite Element Analysis of Darlington NGS Feeder Pipes With Locally Thinned Regions Below Pressure Minimum Thickness

Abstract

Feeder Pipes in CANDU nuclear stations are experiencing wall thinning due to flow accelerated corrosion (FAC) resulting in locally thinned regions in addition to general thinning. In Darlington NGS these locally thinned regions can be below pressure based minimum thickness (t_min), required as per ASME Code Section III NB-3600 Equation (1). A methodology is presented to qualify the locally thinned regions under NB-3200 (NB-3213 & NB-3221) for internal pressure loading only. Detailed finite element models are used for internal pressure analysis using ANSYS v11.0. All other loadings such as deadweight, thermal and seismic loadings are qualified under NB-3600 using a general purpose piping stress analysis software. The piping stress analysis is based on average thickness equal to t_min along with maximum values of ASME Code stress indices (Table NB-3681(a)-1). The requirement for the use of this methodology is that the average thickness of each cross-section with the locally thinned region shall be at least t_min. The finite element analysis models are thinned to 0.75 t_min (in increments of 0.05 t_min) all-around the circumference in the straight section region allowing for flexible inspection requirements. Two different thicknesses of 1.10 t_min and 1.30 t_min are assigned to the bends. Thickness vs the allowable axial extent curves were developed for different types of feeder pipes in service. Feeders differ in pipe size, straight section length, bend angle and orientation. The stress analysis results show that all Darlington NGS outlet feeder pipes are fit for service with locally thinned regions up to 75% of the pressure based minimum thickness. This paper demonstrates the effectiveness of finite element analysis in extending the useful life of degraded piping components.