I split the unioned arc segment pipes and the straight pipes with a surface in the XZ-plane.
Exploded everything and split the curved surface segments with isocurves, then joinedd everything and capped it.
Another approach to model this T-pipe is by extruding a rectangle with concave corners. The fillets need to be done one by one. Done in one go, Rhino seems to dislike some coincidences.
BooleanUnionTest.3dm (1.2 MB)