Socket Weld Flanges

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The socket is the difference from a slip-on. On a socket weld flange, the bore is counterbored to a precise socket diameter and depth: the pipe bottoms into the socket, is backed off a fraction for thermal expansion, and is fillet welded at the hub face. That gives a smooth internal bore and a joint strength close to a butt weld, in sizes where butt welding thin-wall pipe is slow and distortion-prone. A typical catalogue execution: NPS 3/4, Class 150, raised face, Schedule 80 socket, in A105.

Socket weld flanges are a small-bore product — in general practice the type concentrates on NPS 1/2 to 3 — and a high-integrity one: steam, condensate, hydraulic, instrument air and chemical dosing lines are their home ground, alongside the socket weld fittings that complete the run.

Socket Weld or Threaded?

Both serve small bore. The threaded flange avoids welding but relies on a thread that can weep under cycling; the socket weld joint is a welded pressure boundary with a smooth bore, at the cost of a welder and a crevice at the socket root that some services (certain corrosive or sanitary duties) exclude. Our butt weld vs socket weld guide covers the fitting side of the same decision.

The Expansion Gap Matters

Socket weld joints are assembled with the pipe pulled back roughly 1.5 mm from the socket bottom before welding, so thermal expansion does not stress the fillet. It is a small instruction that decides whether the joint lasts; it belongs on the installation drawing, and the flanges are machined with the socket depth that assumes it.

Ordering

State NPS, class, facing, grade and the pipe schedule — the socket bore is machined for a specific wall series, so like the weld neck, this flange cannot be specified from NPS and class alone. The catalogue execution is bored for Schedule 80; a socket for another schedule is confirmed per product at enquiry.

Frequently Asked Questions

Why does a socket weld flange need the pipe schedule?

The socket is bored to the pipe's outside diameter and the bore step is set for the wall series. Schedule 80 and Schedule 160 pipes of the same NPS need different socket machining at the bore. The catalogue execution on this page is bored for Schedule 80; state the schedule your line needs on the enquiry and a socket for that schedule will be confirmed for the specific product before order.

Are socket weld flanges suitable for vibration?

Yes — socket weld joints are widely used on small lines subject to vibration, such as steam tracing and compressor auxiliaries, where threaded joints would loosen. The fillet weld and guided socket make a rigid joint.

What sizes do socket weld flanges cover?

In general practice the type covers NPS 1/2 to 3; the catalogue execution on this page is NPS 3/4, and the size for a specific product is confirmed at enquiry. Above the small-bore range, weld neck flanges take over — butt welding becomes practical and economical at larger bore.

Do socket weld flanges come in stainless steel?

No stainless socket weld flange execution is among the catalogue executions referenced on this site; name the grade on the enquiry and it will be assessed for the specific product before order. The socket dimensions follow the same practice as carbon steel, so the two systems assemble identically.

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