Choosing Pipe Joints

Quick pick (TL;DR)

≥ DN65 (2½″) → butt-weld.
≤ DN50 (2″) → socket-weld.
Flanged only at equipment, valves and maintenance breaks.
Threaded only for low-pressure utility/instrument air — never in flammable, cyclic or high-pressure service.
Grooved/fusion for water/HDPE.

Method comparison

Method	Best for	Avoid for
Butt-weld	≥ DN65, critical, high pressure.	Frequent dismantling.
Socket-weld	≤ DN50 small bore.	Crevice corrosion / large bore.
Threaded	Low-pressure utility.	Flammable / cyclic / HP.
Flanged	Equipment / maintenance breaks.	Long continuous runs.
Grooved	Firewater / utility water.	Hydrocarbons / high temp.
Fusion / adhesive	HDPE & GRP.	Metallic / hot.

How to choose

Size: BW ≥ DN65; SW ≤ DN50.
Hazard → weld; minimize flanges/threads.
Flanges only at dismantling points.
Set NDE: RT/UT critical; PT/MT small bore.
PWHT / hardness where required.

Frequently asked questions

What should I know about Choosing Pipe Joints?

≥ DN65 (2½″) → butt-weld. ≤ DN50 (2″) → socket-weld. Flanged only at equipment, valves and maintenance breaks. Threaded only for low-pressure utility/instrument air — never in flammable, cyclic or high-pressure service. Grooved/fusion for water/HDPE.

Which standards or codes apply?

full-size or near-size → welded/forged tee; small branch off a large header → weldolet (butt) or sockolet (socket); avoid unreinforced stub-ins on high-pressure or cyclic lines.

Branch connections — Full-size → tee; small branch → weldolet/sockolet; avoid unreinforced stub-ins on HP/cyclic.

Common mistakes

Threaded in flammable/vibrating service.
Too many flanges on a run.
Socket-weld in slurry/corrosive.

Requisition & datasheet

Method per size, end prep, branch type.
NDE, PWHT, hardness, WPS, standard.

Standards — ASME B31.3; B16.25; B16.11; B16.9; ASME IX.