Below about 2 inches, welding a butt joint on a small line is fiddly and often unnecessary. Threaded and socket weld connections do the job, and both are available in reducing form.
Reducing threaded flanges
A reducing threaded flange has the ring, thickness and bolt pattern of the larger size, with a tapered pipe thread tapped for the smaller size. Threads are normally NPT to ASME B1.20.1 unless something else is specified.
It bolts to a larger flange and screws onto a smaller pipe. No welding at all on the pipe side, which is the reason it gets chosen.
Where it belongs
- Retrofit where welding is not permitted. Live plant, hazardous area, no hot work permit. A threaded connection needs no arc.
- Utility and instrument air, water, low pressure service.
- Galvanized systems, where welding would destroy the coating locally.
- Temporary and test connections that get made and broken.
The limits
- Thread engagement. The tapping needs enough wall around it and enough depth to develop full engagement. That is what limits the reduction, and it is why two sizes is the practical drop here as well.
- Not for cyclic service. A threaded joint is a notch and a leak path. Vibration and thermal cycling work it loose.
- Class limits. Many specifications restrict threaded connections above certain classes or exclude them from hydrocarbon service entirely. Check the project specification before assuming availability.
- Sealant dependency. The seal depends on thread compound or tape applied correctly, which is a workmanship variable rather than an inspectable weld.
Reducing socket weld flanges
A socket weld flange has a bored socket that the pipe drops into, with a fillet weld at the hub face. The reducing version bores that socket for the smaller pipe while the ring stays the larger size.
It is the answer where a threaded joint is unacceptable but a butt weld is impractical: small bore, higher integrity service.
Where it belongs
- Small bore process lines that must not be threaded
- Instrument and sample connections on hydrocarbon or toxic service
- Hydraulic and lube oil systems
- Steam tracing and small utility lines at pressure
The details that matter
- Socket depth and the expansion gap. Standard practice leaves a small gap between the pipe end and the bottom of the socket, so that thermal expansion of the pipe does not load the weld in a way it was not designed for. Do not bottom the pipe hard.
- The crevice. A socket weld leaves an unwelded annular gap between pipe OD and socket bore. In corrosive or crevice-sensitive service that is a real limitation, and it is a reason some specifications exclude socket welds.
- Fillet size. Governed by the code and the pipe wall. It belongs on the drawing.
- Not radiographable in any useful way. A fillet weld gets surface examination, dye penetrant or magnetic particle, not volumetric inspection.
Which to choose
| Requirement | Choose |
|---|---|
| No hot work permitted | Reducing threaded |
| Galvanized system | Reducing threaded |
| Small bore hydrocarbon or toxic service | Reducing socket weld |
| Vibration or thermal cycling present | Neither. Reducing weld neck, or a reducer |
| Crevice-sensitive corrosive fluid | Not socket weld. Butt welded connection |
| Volumetric inspection required | Neither. Butt welded connection |
| Frequent make and break | Reducing threaded, or a union |
Ordering
Same rules as the larger patterns, with two additions.
- Both sizes, labelled, flange size and connection size
- Pressure class, from the larger size
- Thread standard on threaded, NPT to ASME B1.20.1 unless stated otherwise
- Schedule on socket weld, because the socket bore follows the pipe OD and the fillet follows the wall
- Facing, material grade and specification
- Design conditions
See reducing slip-on flanges for the next size band up, and reducing weld neck flanges for high integrity work.