longitudinal weld joint efficiencies in ASME B31.1

Dears

I do have two clarification, why using longitudinal weld joint efficiencies reduce expansion stresses in ASME B31.1?.

whether it is a recommended practice to utilize the longitudinal weld joint efficiencies in such case?

thanks for your support

it means reduce the ratio of calculated expansion stresses to allowable expansion stress?

It seems a little pit strange to me to increase the allowable stresses when the longitudinal weld joint efficiencies are added, it suppose to reduce the allowable!!

Weld joint efficiency (E) does not change calculated stress. E is used to increase the required (pressure) wall thickness where the weld quality is uncertain. But this E is not used in setting the allowable stresses for EXP, SUS & OCC.
B31.1 and B31.3 handle this E differently. The allowable stress tables in B31.3 hold the material basic allowable stress. In B31.1, the allowable stress table holds SE, not just S.
In CAESAR II, if you enter E for a B31.1 model, the program will divide the entered allowable stresses (SC, SH1,...,SH9) by this E - thus increasing the allowable stress (since E<1). This will also reduce the (calculated stress/allowable stress) ratio.

Many thanks Dave for your support.