Hello All,

This is indeed a very nice discussion.

I would like to share my idea on this one.

ASME B31.3 does not put distinction between secondary stress (PL+PB+Q in ASME VIII Div. 2 Part 5) and peak stress (PL+PB+Q+F in ASME VIII Div. 2 Part 5). I think both of this requirement is addressed in one expansion stress requirement which is based on 2Sy limit with certain safety factors and which includes also a stress reduction factor (f) to take into account the number of cycles. Basically, Piping code have a more simplified approach than Pressure Vessel Code. Generally, we know that secondary stress addresses the prevention of ratcheting (incremental plastic collapse) which is approximately limited to 3Sm (or 2Sy) which can be considered a Low Cycle Fatigue criteria (where a material can have plastic strains). While the peak stress (PL+PB+Q+F) addresses the High Cycle Fatigue (where a material can fail even the stress amplitude is below the yield strength) and the allowable criteria is based on S-N curves produced from Fatigue Testing which includes different unaccounted factors during test.

Going back to the original question, I agree to use the peak stress SIF to be used for Piping Calculation especially if the system will experience High Cycle Fatigue.

Please correct me if I have made some wrong statements fellow Stressers.

Cheers!!!