Socket weld and threaded (screwed) fittings are both forged small-bore fittings covered by ASME B16.11. They share the same family of shapes (elbows, tees, crosses, couplings, caps) but differ in how they join the pipe and in the meaning of their pressure classes.
Socket weld vs threaded at a glance
| Socket weld | Threaded / screwed | |
|---|---|---|
| Joint | Pipe inserted into a socket and fillet welded | Male / female taper pipe threads (e.g. NPT to ASME B1.20.1) |
| Pressure classes (ASME B16.11) | 3000, 6000, 9000 | 2000, 3000, 6000 |
| Welding at site | Yes, one fillet weld per end | No (seal welding optional) |
| Leak tightness | High: weld seals the joint | Lower: relies on thread engagement and sealant |
| Dismantling | Cut out | Can be unscrewed |
| Typical use | Process, hydrocarbon, steam and high-temperature small-bore lines | Utility, instrument, air and water small-bore lines |
Pressure classes and matching pipe schedules
ASME B16.11 assigns each class to a pipe wall thickness. The same number means different pipe for the two end types: a Class 3000 socket weld fitting matches Schedule 80 pipe, while a Class 3000 threaded fitting matches Schedule 160.
| End type | Class 2000 | Class 3000 | Class 6000 | Class 9000 |
|---|---|---|---|---|
| Socket weld | — | Schedule 80 / XS | Schedule 160 | XXS |
| Threaded | Schedule 80 / XS | Schedule 160 | XXS | — |
The class number is not a pressure rating in psi. B16.11 rates a fitting at the pressure calculated for straight seamless pipe of equivalent material and the matching schedule, under the piping code that applies (for example ASME B31.1 or B31.3).
When to use each
- Socket weld where leak tightness matters: flammable, toxic or high-temperature service. Leave about 1/16" between the pipe end and the socket bottom before welding.
- Threaded where welding is impractical or not allowed, for low-risk utility and instrument lines, and where joints must be taken apart. Many piping specifications restrict threaded joints in cyclic, vibrating or hazardous service.
- For larger sizes or full-penetration welds, use butt weld fittings to ASME B16.9 instead.
This guide is a general reference summarised from the named standards. Always design, specify and inspect to the current edition of the applicable standard and your project specification. Published 2026-10-19.