What Is a Socket Weld Flange?

A socket weld (SW) flange has a recessed bore that accepts the pipe end. The pipe is inserted into the socket until it bottoms out, then pulled back approximately 1.6 mm (1/16 in) before a single fillet weld is applied around the outside. This design is used almost exclusively for small-bore piping—NPS 1/2 through NPS 4—where butt-weld fit-up on small diameters would be impractical.

Property	Detail
Standard	ASME B16.5
Size range	NPS 1/2 to NPS 4
Pressure classes	150 through 2500
Face types	RF, RTJ
Common materials	A105 (CS), A182 F304/F316 (SS), A182 F11 (alloy)
Weld type	Single fillet weld (outside of socket)
Socket bore	Counter-bored to match pipe OD
Gap	1.6 mm (1/16 in) expansion gap between pipe end and socket shoulder

How It Works

The socket creates a self-aligning connection. The pipe slides into the counter-bore, which acts as a guide, eliminating the precise bevel preparation and fit-up required for butt welds. A single fillet weld around the pipe-to-flange junction completes the joint.

The 1.6 mm gap is critical. Without it, thermal expansion of the pipe into the socket bottom can crack the fillet weld during high-temperature service.

Common Mistake

Skipping the 1.6 mm expansion gap. The pipe should NOT bottom out against the socket shoulder. Insert the pipe fully, mark the outside, then pull the pipe back 1/16 inch before welding. ASME B31.3 and ASME B16.11 both require this gap.

When to Use Socket Weld Flanges

SW flanges are the practical choice for:

• Small-bore process piping (NPS 1/2 to NPS 2) in high-pressure service
• Instrument connections and gauge taps
• Drain and vent lines off main headers
• Chemical injection points
• Lines where forged fittings (socket weld elbows, tees, couplings) are used throughout

For sizes above NPS 2, most specifications switch to weld neck flanges with butt-weld connections.

SW Flange vs Threaded Flange

Both serve small-bore piping, but with different trade-offs:

Feature	Socket Weld	Threaded
Connection	Fillet weld	NPT threads (no welding)
Leak resistance	High (welded seal)	Lower (thread path = potential leak)
Fatigue strength	~75% of butt weld	~50% of butt weld
Disassembly	Requires cutting	Easy—unscrew
Best for	Permanent high-P/T connections	Low-pressure, non-critical, or temporary

Pro Tip

On projects with extensive small-bore piping (instrument connections, sampling systems), standardize on socket weld fittings and flanges. Mixing SW and threaded in the same system creates procurement complexity and potential for incorrect installation.

Read the full guide to flanges