Article
For small-bore piping, usually 2 inches and below, forged fittings are the normal choice. They are machined from solid forgings, which makes them strong enough for high pressure in a compact size. ASME B16.11 covers their dimensions, pressure classes and tolerances.
Socket weld fittings
The pipe is pushed into a socket in the fitting and fillet welded around the outside. Socket weld fittings are easy to align and do not need the pipe ends to be bevelled, which makes them quicker to install than buttweld fittings in small sizes.
Before welding, the pipe should be pulled back about 1.6 mm (1/16 inch) from the bottom of the socket. This expansion gap stops the weld cracking as it cools. The gap left inside is a crevice, so socket weld fittings are avoided in service where crevice corrosion or solids build-up is a concern.
Classes: 3000, 6000 and 9000.
Threaded fittings
Screwed on with tapered pipe threads (NPT to ASME B1.20.1 is the most common). No welding is needed, which is useful where hot work is not allowed. Threads reduce the effective wall and can leak, so threaded fittings are kept to utility and low-severity service, and are often seal welded where a leak cannot be accepted.
Classes: 2000, 3000 and 6000.
What the class means
As with flanges, the class is a designation, not a pressure. Each class corresponds to a pipe schedule whose pressure rating the fitting is designed to match:
| Pipe schedule | Socket weld class | Threaded class |
|---|---|---|
| SCH 80 / XS | 3000 | 2000 |
| SCH 160 | 6000 | 3000 |
| XXS | 9000 | 6000 |
A threaded fitting is paired with a lower class because threading the pipe removes wall thickness.
Types available
Elbows (90° and 45°), tees, crosses, couplings and half couplings, caps, unions, plugs, bushings and swage nipples. Branch outlets such as sockolets and threadolets are covered by MSS SP-97 rather than B16.11.
Materials
- Carbon steel: ASTM A105.
- Low-temperature carbon steel: ASTM A350 LF2.
- Alloy steel: ASTM A182 F11, F22, F91.
- Stainless steel: ASTM A182 F304L, F316L.
- Duplex: ASTM A182 F51 (UNS S31803) and F53 (UNS S32750).
What to put on the enquiry
The fitting type, size, end connection (socket weld or threaded, and the thread standard), the class, and the material grade.
See the pipe fittings range.
Frequently asked questions
What classes are socket weld fittings made in?
ASME B16.11 socket weld fittings come in Classes 3000, 6000 and 9000.
Why leave a gap in a socket weld joint?
A gap of about 1.6 mm at the bottom of the socket allows for weld shrinkage and thermal expansion, preventing cracking.
What sizes do B16.11 fittings cover?
Typically NPS 1/8" to 4", with socket weld and threaded fittings most common at 2" and below.
Key takeaways
- B16.11 covers forged socket weld and threaded fittings.
- Socket weld: Class 3000, 6000, 9000; threaded: 2000, 3000, 6000.
- Leave a 1.6 mm gap at the socket bottom before welding.
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