For B31 Codes, we determine sustained, occasional and expansion stresses relative to allowable stresses.

My approach has been to: include operating case with the relief valve force for calculation of occasional stress; include operating case without the relief valve force for calculating the expansion stress.

Realizing that a relief event will normally result in a displacement (rotation) of the relief valve, also displacements of the relief stack or discharge piping may occur, and understanding that displacements result in secondary stresses (fatigue), I am concerned that my approach does not adequately address fatigue as a result of the occasional load events.

I think I need to include an expansion stress case based on operating cases with the relief valve force (OPE+F)-(SUS). Is this the approach other analysts use to address fatigue for relief valve events?