Schedule
The wall thickness designation of pipe and fittings — SCH 10 through SCH 160 and XXS — set against the nominal pipe size, not the outside diameter.
Nominal pipe size fixes the outside diameter; the schedule fixes the wall, so a NPS 6 SCH 40 and a NPS 6 SCH 80 fitting share an outside diameter and differ in bore. Because ASME B16.9 permits a minus tolerance on wall, a fitting is accepted on the minimum wall at the thinnest point rather than on the nominal figure — which is exactly what an incoming inspection should measure.
Related terms
Where this applies
butt-weld-fittings
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