Socket weld fittings are forged fittings with a counterbored socket into which plain-end pipe is inserted and joined with a single fillet weld. Made to ASME B16.11 in Class 3000, 6000 and 9000, they are the standard way to build small-bore process, instrument, drain and vent lines from ⅛" to 4", where they are quicker to fit up than buttweld fittings and stronger and more leak-tight than threaded ones.
Raaj Fittings forges and machines socket weld elbows, tees, crosses, couplings, caps and unions in-house at our two plants in India, in stainless steel, duplex, nickel alloys, carbon and alloy steel, and exports them to refineries, petrochemical plants and fabricators across the Middle East, Europe and the Americas.
ASME B16.11 rates socket weld fittings by class. Each class is designed to match the strength of a particular pipe wall, so the fitting is chosen from the pipe schedule:
| Class | Matching pipe | Size range in ASME B16.11 |
|---|---|---|
| 3000 | SCH 80 / XS | ⅛" to 4" |
| 6000 | SCH 160 | ⅛" to 4" |
| 9000 | XXS | ½" to 2" |
The socket bore is sized to the pipe outside diameter, and the bore of the fitting matches the pipe's inside diameter for the corresponding schedule. ASME B16.11 also fixes the socket depth, the wall thickness around the socket and the centre-to-bottom dimensions, so fittings from different makers are interchangeable.
The pipe is pushed into the socket until it bottoms, then withdrawn by about 1.6 mm (1/16") before tack welding. The gap lets the pipe expand into the socket during welding and in service; without it, the fillet weld can crack. The fillet weld is then completed around the outside of the socket, with a size set by the piping code (ASME B31.1 or B31.3). Because the joint is a fillet weld, it is examined by visual, magnetic particle or dye-penetrant inspection rather than by radiography.
| Feature | Socket weld | Threaded | Buttweld |
|---|---|---|---|
| Usual size | Up to 2" (to 4") | Up to 2" (to 4") | 2" and above |
| Joint | Fillet weld, leak-tight | Threads and sealant; no welding | Full-penetration weld |
| Strength | Matches the pipe schedule | Threads reduce the wall | As strong as the pipe |
| Inspection | Surface methods (MT/PT) | Visual, pressure test | Radiography or UT possible |
| Watch out for | The crevice at the socket can trap fluid | Vibration and thermal cycling can loosen joints | More fit-up time in small sizes |
Piping specifications often limit socket welds in services where crevice corrosion or erosion is likely, such as some sour, caustic or high-purity lines, and in severe cyclic service. In those cases, buttweld fittings are normally specified even in small sizes.
Every fitting is forged from PMI-verified bar or billet, heat treated as the material specification requires, and CNC machined. Socket bores, socket depths, wall thicknesses and centre-to-bottom dimensions are inspected against ASME B16.11. Fittings are marked with the manufacturer's mark, material grade, class and size, and the heat number, and supplied with EN 10204 3.1 certificates; EN 10204 3.2 certification, NACE MR0175 / ISO 15156 material and third-party inspection by the buyer's nominated agency are available on request.
Use Class 3000 socket weld fittings with SCH 80 / XS pipe. Class 6000 matches SCH 160 and Class 9000 matches XXS pipe, so the fitting is at least as strong as the pipe it joins.
Withdrawing the pipe about 1.6 mm (1/16") from the bottom of the socket before welding lets it expand during welding and in service. Without the gap, thermal expansion can crack the fillet weld.
ASME B16.11 covers Class 3000 and 6000 socket weld fittings from ⅛" to 4" and Class 9000 from ½" to 2". In practice, socket weld fittings are most used up to 2"; larger lines normally use buttweld fittings.
Socket welds are fillet welds, so radiography is not a practical way to examine them. They are normally inspected visually and by magnetic particle or dye-penetrant testing. Where volumetric examination is required, buttweld fittings are used.
Yes. Raaj Fittings supplies socket weld reducer inserts to MSS SP-79 and socket weld unions to MSS SP-83, in the same materials as our ASME B16.11 fittings.