Socket Weld vs Threaded Fittings
Socket weld (SW) and threaded (THD) fittings are both forged steel fittings per ASME B16.11, designed for small-bore piping (NPS 1/8 to NPS 4). The fundamental difference is the connection method: socket weld fittings require a fillet weld, while threaded fittings use tapered threads (NPT or BSP) and need no welding.
| Feature | Socket Weld (SW) | Threaded (THD) |
|---|---|---|
| Connection | Fillet weld around pipe OD | NPT/BSP tapered threads |
| Welding required | Yes (fillet weld) | No |
| Leak resistance | High (welded seal) | Moderate (thread sealant dependent) |
| Vibration resistance | Excellent | Poor (loosening risk) |
| Disassembly | Requires weld cutting | Easy (unscrew) |
| Crevice corrosion | Possible (gap at socket bottom) | Possible (thread root) |
| NDE after installation | Visual + MT/PT on fillet weld | Visual only |
| Max recommended size | NPS 2 (per most specs) | NPS 1.5 (per most specs) |
| Pressure classes | 3000, 6000, 9000 | 2000, 3000, 6000 |
| Thermal cycling | Good | Poor (thread loosening) |
| Installation time | Longer (welding + NDE) | Shorter |
| Cost (installed) | Higher (welding labor) | Lower |
When to Use Socket Weld
Socket weld fittings are the default for small-bore process piping in oil and gas plants. Specify SW when:
- The service involves flammable, toxic, or high-pressure fluids
- Operating temperatures cycle significantly (thermal expansion loosens threads)
- Piping specifications require a welded, leak-proof connection
- Vibration is present (rotating equipment connections, compressor suction/discharge)
- The piping class calls for Class 6000 or 9000 fittings
The 1/16-inch (1.6 mm) expansion gap between the pipe end and the socket bottom is critical during installation. Without it, thermal expansion can crack the fillet weld. ASME B31.3 and most fabrication procedures mandate this gap.
When to Use Threaded
Threaded fittings suit applications where:
- Frequent disassembly is needed (instrument connections, sampling points)
- Welding is not permitted (e.g., live plant modifications, explosive atmospheres)
- Low-pressure utility services (air, water, nitrogen) where leak risk is tolerable
- Temporary installations or field-assembled piping
- Fire protection systems (per NFPA standards, threaded connections are common)
Materials
Both SW and threaded fittings share the same material specifications:
| Material | ASTM Spec | Application |
|---|---|---|
| Carbon steel | A105 | High-temperature CS service |
| Low-temp carbon steel | A350 LF2 | Service down to -46°C |
| Stainless steel 304/316 | A182 F304/F316 | Corrosion resistance |
| Alloy steel | A182 F11/F22 | High-temp chrome-moly service |
| Duplex SS | A182 F51 | High strength + corrosion resistance |
For material matching with pipe grades, see the BW fittings materials guide.
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