Buttweld vs forged fittings

Buttweld fittings

Buttweld fittings under ASME B16.9 are made to match pipe outside diameter and schedule, so the bore is continuous and the joint can be fully radiographed. They dominate larger bore process piping.

• Bevelled ends welded directly to pipe
• Smooth bore, low turbulence and pressure drop
• Wall thickness matched to the pipe schedule
• Common items: elbows, tees, reducers, caps, stub ends

Forged fittings

Forged socket weld and threaded fittings under ASME B16.11 are rated by class — typically 3000, 6000 and 9000 — rather than by schedule. They are normally used on small bore lines, up to about NPS 2.

• Socket weld: pipe inserted into a socket and fillet welded
• Threaded: NPT or BSP connections, no welding needed
• Class rating relates to the matching pipe schedule
• Common items: elbows, tees, couplings, unions, plugs, bushings

Choosing between them

Large bore, high-integrity or radiographed lines generally use buttweld. Small bore utility, instrument and drain lines often use forged fittings because they are quicker to fabricate. Some specifications prohibit threaded joints in certain services, so check the piping class first.

Common questions

What does Class 3000 mean on a forged fitting?

It is a pressure class designation from ASME B16.11 that corresponds to a matching pipe schedule. It is not a direct pressure in psi at all temperatures.

Can socket weld fittings be radiographed?

A socket weld fillet joint cannot be radiographed the way a full penetration butt weld can. Where full volumetric examination is required, buttweld construction is normally specified.