In a previous paper, it was demonstrated that a tight radius bend with uniform wall thickness equal to the pressure based thickness of the corresponding straight pipe does not meet the linear elastic criteria of NB-3221 Design Loadings. It was also demonstrated that the allowable wall thickness circumferential distribution of the ASME B&PV Code SEC XI Code Case N-597-2 achieves a uniform linear elastic stress intensity over the entire bend that meets the linear elastic criteria of NB-3221. As stated by the ASME B&PV Code under NB-3221, the provisions of NB-3228 may provide relief from certain of the linear elastic stress limits if plastic analysis techniques are applied.
In this paper, the plastic collapse analysis approach of the COG FFSG is adopted to explore the relative safety margin when analyzing various Class 1 tight radius pipe bends’ configurations made of Carbon Steel SA-106 Grade B. In addition to the power law material model, three elastic plastic material models are constructed based on ASME B&PV Code Section II Part D material properties. The pressure-based thickness for the same size straight pipe is uniformly used in modelling the tight radius bends (no increased thickness on the intrados). It is demonstrated that the results from the ASME B&PV based flow stress model are marginal and that the ASME B&PV based bi-linear material model and the limit analysis results are not meeting the collapse pressure criterion. It is also demonstrated that the uniform pressure-based thickness along with the power law material model meets the requirements for the adopted plastic collapse pressure providing a considerable life extension.
