Threaded or Socket Weld? Small Bore Connection Choice

Small bore lines offer two options: a threaded connection or a socket weld. The choice depends on service and vibration, not only on convenience.

How each seals

A threaded connection seals by the tapered threads wedging together, usually helped by PTFE tape or a liquid sealant. In a socket weld the pipe enters a recess in the fitting and is joined with an external fillet weld, so the seal is in the metal itself.

When threaded

• Where welding permits cannot be obtained or a flammable atmosphere exists
• Temporary lines and test connections
• Instrument and gauge connections
• Small connections opened frequently

Threading consumes wall thickness, so SCH 80 pipe is normally required. The thread system (NPT, BSPT) must be stated.

When socket weld

• On vibrating lines around pumps and compressors
• High pressure process lines
• Flammable and toxic fluids
• Permanent installations

A gap of about 1.5 mm is left between the pipe end and the bottom of the socket. That gap allows for thermal expansion; if the pipe bottoms out, the weld root risks cracking.

Mixed use

Many projects use socket weld at 2" and below while keeping instrument connections threaded. The piping class list defines this, and where a list exists it governs.

See our forged steel fittings page and NPT versus BSPT threads.

FAQ

Why leave a gap in a socket weld?
To allow for thermal expansion; if the pipe bottoms out in the socket, cracks can form at the weld root.

Which pipe is used with threaded connections?
Normally SCH 80 or heavier, because threading reduces the wall section.

Are threaded connections acceptable on vibrating lines?
Not recommended; vibration loosens them over time and leakage follows.