My question is does CEASER-II uses calculated Principal stresses, based on the Tresca theory, in determining the B31.3 code limit for longitudinal stresses due to sustained load? Since Principal stresses is a combination of the longitudinal stress, hoop stress and torsional shear stress and this (S1-S3) must not exceed the material yield based on the Tresca theory. Is it that B31.3 is not interested in the principal stress. Is it that the moment calculate the Longitudinal stresses due to sustained load and make sure it is less than the Basic allowable stress due to yield at the max metal temperature, then the piping will not fail for single point load. So what is the significance of principal stresses in this instance?