You should be seeing allowable stress values. For B31.8 VIII the allowable is just a constant times Sy.

The issue with the minimum wall thickness is that B31.8 VIII doesn't need the longitudinal joint factor ("eff") for the allowable stress, so it is grayed out in the input. Unfortunately this value is needed for the minimum wall thickness calculation.

We'll have to change the status of this "edit box" in the Input Processor so that you can define joint factor for B31.8 VIII.