What Is ASME B16.11?
ASME B16.11 is the standard for forged steel fittings used in socket weld and threaded piping systems. It covers the dimensions, tolerances, ratings, and marking requirements for fittings from NPS 1/8 to NPS 4. These fittings serve small-bore, high-pressure piping applications across oil and gas, petrochemical, and power generation industries.
When to Use ASME B16.11 Fittings
Forged fittings per B16.11 are the standard choice for pipe sizes NPS 2 and below. Above NPS 2, buttweld fittings per ASME B16.9 take over. The cutoff is not absolute; B16.11 covers sizes up to NPS 4, but most piping specifications limit socket weld fittings to NPS 2 or NPS 1.5 and threaded fittings to NPS 1.5 or lower.
| Feature | Detail |
|---|---|
| Standard | ASME B16.11 (Forged Fittings, Socket-Welding and Threaded) |
| Size range | NPS 1/8 to NPS 4 |
| Connection types | Socket weld (SW), threaded (NPT or BSP) |
| Pressure classes | 2000, 3000, 6000, 9000 |
| Fitting shapes | Elbows (45°/90°), tees, crosses, couplings, half couplings, caps, unions, bushings |
| Materials | ASTM A105 (CS), A182 (SS/alloy), A350 LF2 (low-temp CS) |
| Manufacturing | Forged and machined from solid steel bar or billet |
Pressure Classes
B16.11 defines four pressure classes. Each class corresponds to a specific pipe schedule that the fitting’s socket bore and wall thickness are designed to match.
| Class | Matching Pipe Schedule | Typical Application |
|---|---|---|
| 2000 | Threaded only (no SW) | Low-pressure utility |
| 3000 | SCH 80 (SW), SCH 80 (threaded) | Standard process piping |
| 6000 | SCH 160 (SW), SCH 160 (threaded) | High-pressure systems |
| 9000 | XXS (SW only) | Very high-pressure, compact piping |
Class 3000 is by far the most common in oil and gas projects. Class 6000 is specified for high-pressure instrument connections and chemical injection lines.
Socket Weld vs Threaded
Socket weld fittings require a fillet weld to join the fitting to the pipe. The pipe inserts into the fitting socket, is pulled back 1/16 inch (1.6 mm) to allow for thermal expansion, and then a fillet weld is applied around the circumference.
Threaded fittings use NPT (National Pipe Taper) or BSP (British Standard Pipe) threads. No welding is needed, making installation and disassembly faster. However, threaded connections are prone to leakage under vibration and thermal cycling, so most piping codes restrict their use to non-critical, non-toxic services.
Materials
The most common forged fitting materials:
- ASTM A105: Carbon steel for high-temperature service (matches A106 Gr. B pipe)
- ASTM A182 F304/F316: Austenitic stainless steel
- ASTM A182 F11/F22: Chrome-moly alloy steel
- ASTM A350 LF2: Carbon steel for low-temperature service (down to -46°C)
For a comprehensive material reference, see the buttweld fittings materials guide, which covers matching pipe-to-fitting grades.
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