Application of Code Case N-597 for Local Thinning Assessment for Class 1 Piping

Process piping components such as elbows, bends and tees experience local wall thinning due to Flow Accelerated Corrosion (FAC). Fitness for service studies of the piping components that are affected by FAC for continued operation have gained importance in light of the plant life extension resulting in economic benefits for the operating utilities. ASME Code Case N-597 provides the guidelines for acceptance for continued service of Classes 2 and 3 piping components experiencing wall thinning during operation. However, for Class 1 systems, the Code Case recommends that the owner develop the methodology and criteria for the assessment of wall thinning. This paper establishes a criteria and methodology for the assessment of thinned Class 1 piping system components. The rules of NB-3221 and NB-3650 of ASME Boiler & Pressure Vessel Code (Ref.1) have been considered in establishing the criteria. The requirements of NB-3221 have been utilized for evaluating the adequacy of pressure design. After meeting this requirement, further evaluation of the piping with thinned area is carried out as per NB-3650 requirements for all loadings corresponding to design and service conditions. To illustrate the criteria and the methodology, an assessment of a sample problem of a locally thinned region in a pipe bend is provided in this paper. Finite element analysis and piping analysis are used for the assessment of the thinned region of the pipe bend. Loading considered includes pressure, deadweight, thermal transients and seismic effects.