What I tried to say is that- for piping point of view, and only as my opinion- using elastic-plastic analysis of pressure vessel together with B31 approach means a mix of concepts. Better to remain on the safe part, considering stress classification method, as an engineering approximation, simple and mature, even in some cases brings certain difficulties in correct classification. But the last is not the case with Table 5.6.

Maybe the future will be to consider elast-plastic approach in all system- piping and pressure-vessels, reflecting structural damage limit state, which will release also Sam's concerns....