I have a manifold in which all its branch connections are of extruded type. I have requested my manifold vendor to perform the extruded tee calculation together with SIF calculation.
My vendor gave me SIF values based on 2 cases (non-corroded and corroded).
I am not sure which set of SIF I should use for the pipe stress analysis.
Experts, please advise.

Mojo,

Code B 31.3, B31.1 does not address about using the corrosion allowance in calculating the SIF value. But there is a note under appendix-D regarding the special consideration needed for closely loaded branch connection. If we consider that your manifold is closely loaded with the branches, then it is better to check both the options.

Regards,
R.k.

Appendix D in both B31.1 & B31.3 specifies the nominal thickness in setting the stress intensification factor.