ASME Code Qualification of Pipe Bends With Localized Wall Thinning

This paper addresses the application of the design by analysis rules of the ASME B&PV Code SEC III Division 1 NB-3200 criteria on degraded Class 1 piping. A tight radius pipe bend with a local thin area (LTA) located on the inner surface and super imposed on general thinning region is considered using a detailed FEA model implementing idealized smooth axial and circumferential thickness profiles. A location dependent thinning rate function is developed (based on the smoothed profiles and the assumed original thickness distribution) to predict the wall thickness distribution at the end of an arbitrary evaluation period. Internal pressure and dead weight loads are statically applied. Linear elastic analysis is performed and the results are checked against the ASME Code criteria for the primary stress intensity. It is demonstrated that a “local” wall thickness considerably below the pressure based thickness for the corresponding straight pipe segment meets the requirements of the ASME Code SEC III for the primary stresses. The effect of the extent of the local thin area in the axial direction is explored. This paper also compares the allowable pressure obtained from elastic analysis of NB-3221 and limit analysis of NB-3228.1.