can someone halp me to understan the aplication of weld quality factor on B31.3

The weld quality factor dont need to apply to SL and SA ???? on par 302.3.5 note 3 ?????

I m involved on the flexibility design of a hot gas duct for a cement plant made with carbon steel A36.

I interpret tht the cuality factors apply only for pressure design not for longitudinal and displacement stresses SL and SA is correct ??????

Assuming you have CAESAR II Version 4.50, check the Technical Reference Manual, Chapter 2, the "Allowable Stress" section. This section details which codes use the joint efficiency and how.