Hi Martin,

The SIF is irrelevant because beam theory computer programs cannot calculate stresses for the "block Y" fittings. The fitting at issue is not round on its exterior surface and since it is irregular in cross section software designed to calculate stresses in pipe (round in cross section with uniform wall thickness) cannot be applied. The "block Y" fitting is illustrated in Chapter 2 of the B31.3 book by Glynn Woods and Roy Baguley (CASTI Publishing). Since the "Block Y" fitting is designed for very high internal pressure it is very robust when compared to pipe.

The "block Y" fitting is very much more rigid than the pipes that are attached to it and we typically model the fitting as three rigid elements meeting at a node at their point of intersection. We might include an SIF for each of the three butt welds where the pipes meet the fitting but if they are rather "normal" butt welds the SIF would just be 1.0.

Regards, John