A fitting only fits if its ends match the pipe it is welded or screwed to. Yet many enquiries give the pipe size and leave the wall thickness to be "as per pipe" — which means the supplier has to guess. This page explains which matching dimensions to state on a fitting RFQ, and which ones should never be inferred.
Why "As Per Pipe" Is Not Enough
The same nominal pipe size (NPS) can be supplied in many wall thicknesses. A 4" elbow in Schedule 40 and one in Schedule 80 have the same outside diameter but a different bore and a different weld preparation. If the fitting wall does not match the pipe, the weld joint will have a step on the inside, and the fitting may not meet the design pressure. The RFQ should therefore carry the wall thickness of every end.
Buttweld Fittings (ASME B16.9): What to State
- NPS of each end — for reducers, reducing tees and reducing elbows, both the large and small end
- Wall thickness of each end, as a schedule (Sch 10S, 40, 80, 160, XS, XXS) or as a dimension in mm or inches
- Different walls on each end where they apply, e.g. a reducer from 6" Sch 40 to 4" Sch 80
- Elbow radius: long radius (1.5D), short radius (1D) or 3D / 5D bends
- End preparation: standard ASME B16.25 bevel, or a special bevel for heavy walls
- Material grade and seamless or welded construction
Stainless steel "S" schedules (10S, 40S, 80S) are not always the same as carbon steel schedules of the same number in every size. State the schedule exactly as it appears on the pipe specification.
Forged Fittings (ASME B16.11): Class, Not Schedule
Socket weld and threaded forged fittings are ordered by pressure class — 2000, 3000, 6000 or 9000 — rather than by fitting schedule. Each class is intended for use with a particular pipe wall:
- Socket weld Class 3000 is normally matched with Schedule 80 / XS pipe, Class 6000 with Schedule 160, and Class 9000 with XXS
- Threaded Class 2000 is normally matched with Schedule 80, Class 3000 with Schedule 160, and Class 6000 with XXS
- For socket weld fittings, the socket bore depends on the pipe size; state the pipe schedule too, so the bore of the fitting suits the pipe
- For threaded fittings, state the thread form: NPT (ASME B1.20.1) or BSP
These pairings are the usual practice in ASME B16.11; your piping specification governs, so always quote the class and the matching pipe schedule from it.
Branch Outlets and Special Items
For weldolets, sockolets and threadolets (MSS SP-97), give the run pipe size, the branch size and the branch wall or class. For heavy-wall or non-standard items, attach a drawing with the bore, wall and bevel details.
Fitting RFQ Checklist
- Every line has the fitting type, NPS and wall / class of every end
- Reducing items state both ends
- Elbow radius is stated
- End preparation is stated if it is not the standard bevel
- Material grade, seamless / welded and standard are stated
- Documents, tests and marking requirements are listed
For the difference between connection types, see butt weld vs socket weld vs threaded fittings. Browse our pipe fittings, or send your fitting list with the details above.
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