Types of pipe flanges and where each is used
Pipe flanges join pipe, valves and equipment with a bolted, gasketed joint that can be taken apart. ASME B16.5 (NPS ½–24) and B16.47 (NPS 26–60) define the common types. They differ in how they attach to the pipe, which decides strength, cost and where they belong.
Weld neck flange (WN / WNRF)
A long tapered hub is butt-welded to the pipe. The taper carries stress from the flange into the pipe wall, so weld necks are the first choice for high pressure, high or cyclic temperature and critical service. The bore must match the pipe schedule. Every weld can be radiographed.
Slip-on flange (SO / SORF)
The pipe slides into the flange and is fillet-welded inside and outside. Slip-ons are cheaper and easier to align than weld necks, but their calculated strength under internal pressure is roughly two-thirds of a weld neck and fatigue life is much lower. Common in low-pressure utility, water and cooling lines.
Blind flange (BL / BLRF)
A solid plate with bolt holes that closes the end of a pipe, valve or nozzle. Blinds see the highest bending stress of any flange type, which is why they are thick. Used for future tie-ins, test points and pressure-test closures.
Socket weld flange (SW)
The pipe sits in a socket and is fillet-welded on the outside, with a 1.5 mm (1/16 in) expansion gap at the bottom. Limited to small bores (usually NPS 2 and below). The internal crevice makes it a poor choice for corrosive service.
Threaded flange (TH / screwed)
Screwed onto a threaded pipe (ASME B1.20.1 NPT) with no welding. Useful where welding is not allowed — explosive areas, galvanised pipe — but not suited to high temperature, cyclic loads or bending.
Lap joint flange (LJ) with stub end
A loose backing flange slides over a stub end that is butt-welded to the pipe. Only the stub end touches the fluid, so an expensive stainless or nickel-alloy stub end can be paired with a cheap carbon-steel backing flange. The loose ring also makes bolt-hole alignment easy. Common in stainless and frequently dismantled lines.
Plate, orifice and spectacle blinds
Plate flanges are flat discs welded to the pipe for low pressure. Orifice flanges are weld necks with tapped pressure ports for flow measurement. Spectacle blinds (ASME B16.48) combine a blind and a spacer to positively isolate a line.
How to choose
| Service | Usual choice |
|---|---|
| High pressure / temperature, cyclic | Weld neck |
| Low pressure utilities, cost-sensitive | Slip-on |
| Small bore (≤ NPS 2) process lines | Socket weld |
| No welding allowed | Threaded |
| Expensive alloy, frequent dismantling | Lap joint + stub end |
| Closing a line end | Blind |
Frequently asked questions
Which flange type is the strongest?
The weld neck flange. Its tapered hub spreads stress into the pipe and its butt weld can be fully radiographed, so it suits high pressure, high temperature and cyclic service.
Do flanges of different types in the same class bolt together?
Yes. Within an ASME B16.5 pressure class, OD, bolt circle and bolt holes are the same for every type, so a weld neck mates with a slip-on or blind of the same size and class.
What does RF mean in WNRF or SORF?
RF means raised face, the most common flange facing. WNRF is a weld neck raised face flange; SORF is a slip-on raised face flange.
Last updated 2026-11-02