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 (tmin) in the straight pipe region (STR1) between Grayloc and Bend 1. The (tmin) is 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) using 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 separately qualified under NB-3600 using a general purpose piping stress analysis software. The piping stress analysis is based on average thickness equal to tmin 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 tmin. In contrast, the finite element analysis models are thinned to 0.75 tmin (in increments of 0.05 tmin) all-around the circumference in the straight section region allowing for flexible inspection requirements. In Darlington NGS there are 22 different types of feeder pipes which have been grouped into 6 types based on similarities in pipe size, straight section length, bend angle and orientation. Thickness versus the allowable axial extent curves were developed for 6 representative feeder types. The stress analysis results show that all Darlington NGS outlet feeder pipes are fit for service with locally thinned regions in STR1 with down to 75% of the pressure based minimum thickness.
This paper demonstrates the methodology and effectiveness of finite element analysis in extending the useful life of degraded piping components.