For D/t of 64 and 80 I am getting higher SIFs and hence higher stress ranges for elbows when I reduce the pressure, everything else being equal. This is because the pressure stiffening option is at default (ie on). B31.3 Appendix D note 1 says SIFs from table D300 are valid for D/t <=100 so why is this option default on ? If I switch off pressure stiffening then the SIFs are much larger as I believe the k and SIF values tare not then reduced by note 7 values.

Should the pressure stiffening be switched off when considering stress ranges for pipes where D/t is <=100?