Engineer's Cheat Sheet: Comparing 4 Common Industrial Pipe Flanges (WN, SO, Threaded, SW)

Flange connections are the critical detachable joints of any piping system, and four types cover most engineering duty: weld neck, slip-on, threaded and socket weld. Choosing the right one — together with the correct flange gaskets, fasteners and bolting discipline — decides whether the joint holds for decades or leaks under the first thermal cycle.

Weld Neck and Slip-On Flanges

The weld neck flange carries a long tapered hub that flows smoothly into the flange disc. The hub end is beveled and butt-welded to the pipe with a full-penetration weld that can be radiographed, so flange and pipe become one continuous structure. The hub disperses stress concentration at the weld, which makes this the preferred type for high-pressure, high-temperature and severe cycling duty on flammable, toxic or otherwise hazardous media — petrochemical, thermal power and nuclear lines included. Typical rating is above PN 2.5 MPa across a temperature span from cryogenic -196°C up to 800°C.

The slip-on flange is a flat disc of uniform thickness without a hub; its bore slips over the pipe. It is joined by two fillet welds — inside the disc and outside on the pipe wall — with small weld volume and no special beveling. Its strength and rigidity are lower than the weld neck type, so it stays in low to medium pressure service, typically below PN 2.5 MPa and at temperatures up to 300°C: water, air, inert gases and mildly corrosive media in building services, HVAC and general circulating water systems.

Threaded and Socket Weld Flanges

Threaded and socket weld flanges solve specific construction problems.

• Threaded flange — a flat disc with internal pipe threads that screw onto an externally threaded pipe; no welding required, with sealant or PTFE tape added where needed. It is the answer where hot work is prohibited: operating plants, explosive areas and field tie-ins, plus high-pressure hydraulic gas and fire-water lines. Thread accuracy limits it to DN150 and below, up to PN 25 MPa, and temperatures below 250°C to avoid thread relaxation.
• Socket weld flange — a stepped bore forms a socket: the pipe is inserted to the bottom, then backed off about 1.5-2 mm for weld thermal expansion, and fillet-welded at the outer end face. The bore stays smooth and free of weld residue, which suits high-purity service. It covers small bore up to DN50 at ratings to PN 42 MPa in chemical, pharmaceutical, food-grade and steam systems; the socket gap can invite crevice corrosion in aggressive media.

Selection by Service Condition

Table 1. Quick comparison of the four common industrial flange types.

Selection follows three axes. Diameter and pressure: large bore and high pressure favor weld neck; small to medium bore at moderate pressure favors slip-on; small bore at high pressure favors socket weld; small bore where hot work is banned favors threaded. Medium: flammable, toxic or radioactive service mandates weld neck; clean media favor socket weld; corrosive media force the material choice alongside the type. Construction conditions: on-site welding bans and field work favor threaded flanges, while shop prefabrication makes weld neck and socket weld quality easier to control.

Installation Discipline

Whatever the type, the joint is a system: the pipe flanges, the fasteners and the tightening procedure must be selected with the flange itself. Field practice often puts the split at 30% selection and 70% installation — even the best type fails under uneven bolting, so use torque-controlled tightening for critical service and keep a lifecycle record for every flanged joint.

Frequently Asked Questions

Which flange is best for high pressure and high temperature?

Weld neck. Its tapered hub handles stress concentration and butt-welded joints stay serviceable from cryogenic -196°C up to 800°C.

How do you connect pipe in an explosive area without welding?

Use threaded flanges. The screwed connection eliminates hot work entirely, which makes it suitable for operating plants and field tie-ins.

Which flange suits small-bore high-pressure lines?

Socket weld. The socket supports the pipe, a single fillet weld seals it, and the smooth bore suits high-purity media up to PN 42 MPa.

What limits slip-on flanges?

Lower structural strength and rigidity. Keep them below PN 2.5 MPa and 300°C, away from severe thermal or pressure cycling.

Why back the pipe off 1.5-2 mm before socket welding?

The gap absorbs weld thermal expansion. Without it, shrinkage stress can crack the fillet weld as the joint cools.