Dear Friends,

While checking the axial stress due to differential expansion in jacketed piping, is the allowable is to be taken as
Sa = f (1.25 Sc + 0.25 Sh)
or
Basic allowable stress 'S' in table A1 of ASME B31.3.

Thanks

Tricky one in terms of definition... it is a thermal stress which infers limit to f(1.25Sc + 0.25Sh), but it is not a 'self limiting' stress which would fail only by fatigue. It could lead to direct load (probably tensile)failure if not controlled. Consequently it should be limited to the basic allowable in my opinion.

In my opinion this should be considered a secondary stress and the stress range resulting from the combination of this axial stress and secondary bending & torsion should be compared with Sa. This is my interpretation of B31.3 para 319.2.3-c.
For this purpose I would enforce "Add F/A in Stresses" directive in Configuration-->SIF & Stresses. Caesar will, however, not consider the stress intensification for the axial stress component.
Better approach will be to refer Appendix "P" which addresses the combination of axial stress with other components and also the SIFs for axial stresses.

If this axial stress component is considered primary (non limiting) stress then this alone will not be compared with S, rather its combination with other primary stresses (SL of B31.3) will have to be compared with S. In certain cases this combination will be approaching or exceeding the limit and may dictate basic design changes e.g change in wall thickness etc.