How are the ASME B31.3 longitudinal pressure stress calculated in AutoPIPE?
Pressure stresses are different from pressure axial force divided by the cross sectional area. Moreover the sign is reversed for stress and force, why?
The longitudinal pressure stress calculations take in to account the axial force generated due to pressure extension and the longitudinal pressure force or end cap force. Pressure extension is calculated using thick wall formula, which would then give you the axial force which you see in the forces and moments report against pressure case say P1. The stress due to end cap force is added or subtracted from the stress due to pressure extension during the post processing. The following screen capture is from calculations performed for a similar fully restrained pipe and how the stress values are calculated:
Note: AutoPIPE convention for axial forces is that compression forces are positive and tensile forces are negative. Using the above formula will also take in to account the direction of the stress when reporting the net stress due to the pressure case.
ASME B31.3 Piping Code Calculation Issues