Proposal for Inclusion of Torsion in Section XI Flaw Evaluation Procedures for Pipes Containing Surface Crack-Like Flaws

Abstract

Piping installations in power plant may experience combined bending–torsion loads in the presence of crack-like flaws. ASME Boiler and Pressure Vessel code Section XI non-mandatory Appendix C provides flaw evaluation procedures for pipes with flaws under fully plastic fracture. These are currently limited to straight pipes under pressure and bending loads and no provision is made for torsion loading. The Working Group on Pipe Flaw Evaluation is developing guidance for combining the torsion load within the existing solutions provided in the Appendix C for bending loading on a straight pipe. This paper reports on the finite element analyses performed on the straight pipe containing surface crack-like flaw with pipe diameters ranging from 4″ to 24″ nominal OD and R/t ranging from 6 to 14. Results for pipes with internal pressure are also included. The effect of torsion is notable only for large diameter pipes subject to large torsion loads. For the purpose of non-mandatory Appendix C flaw evaluation the torsion loads can be combined with bending loads using the root of the sum of the squares method as per Section III of the ASME Boiler code for small diameter pipes. For large diameter pipes a weighting factor of 1.2 on torsion moment is recommended.