Hello all,
Perhaps a more important issue for non-standard fittings would be Code pressure design.
I you use a standard B16.11 (only up to NPS 4) socket welding fitting, the fitting is prequalified for the pressure/temperature class (assume the class is chosen correctly). If you use a B16.9 butt welding fitting of a certain schedule, it is similarly prequalified for the matching pipe schedule. Sometimes we take this for granted.
If you use a non-standard fitting in a B31.3 piping system, it must be qualified for pressure design to the requirement$ of paragraph 304.7.2 and thi$ colul get very expen$ive.
Regards, John.