A flange joins two lengths of pipe, or a pipe to a valve, pump or vessel, with a bolted connection that can be taken apart for inspection and maintenance. Choosing the right type affects cost, strength, ease of installation and how the joint behaves under pressure and temperature swings. Here are the types you will meet on almost every project.
Weld neck flanges
A weld neck (WN) flange has a long tapered hub that is butt-welded to the pipe. The taper spreads stress from the flange into the pipe wall, which makes it the first choice for high pressure, high or sub-zero temperature and cyclic service. Because the joint is a full-penetration butt weld, it can be radiographed. The bore is matched to the pipe schedule, so flow is smooth.
Slip-on flanges
A slip-on (SO) flange slides over the pipe and is fillet-welded inside and outside. It is cheaper to buy and easier to align than a weld neck, which is why it is popular for lower-pressure utility, water and cooling lines. Its strength under internal pressure and its fatigue life are lower than a weld neck, so it is not the right choice for severe service.
Blind flanges
A blind flange has no bore. It closes the end of a pipe, valve or nozzle and makes future extension or pressure testing easy. Because it carries the full pressure load across its face, a blind flange is usually the most heavily stressed flange in a system.
Socket weld flanges
The pipe is inserted into a socket and fillet-welded on the outside. Socket weld flanges are used on small-bore, high-pressure lines. Leave an expansion gap of about 1.6 mm (1/16 in) between the pipe end and the bottom of the socket before welding to avoid cracking. The small crevice that remains makes them less suitable for highly corrosive service.
Threaded flanges
Threaded (screwed) flanges need no welding, which makes them useful where hot work is restricted, such as in hazardous areas, and on galvanised pipe. They are best kept to small sizes and services without significant temperature cycling or vibration, which can loosen the threads.
Lap joint flanges
A lap joint flange is used with a stub end that is welded to the pipe; the flange itself is loose and can rotate. This makes bolt-hole alignment easy and allows frequent dismantling. It is also economical for expensive alloys: only the stub end has to match the pipe material, while the backing flange can be carbon steel.
Special-purpose flanges
- Long weld neck — an extended hub used as a nozzle on vessels and columns.
- Orifice flanges — supplied in pairs with pressure tappings for flow measurement with an orifice plate.
- Spectacle blinds, spades and spacers — used to positively isolate a line (see ASME B16.48).
- Reducing flanges — connect pipes of different sizes without a separate reducer.
- Plate flanges — flat plates for low-pressure applications.
How to choose
| Service | Usual choice |
|---|---|
| High pressure, high temperature, cyclic | Weld neck |
| Low-pressure utility lines | Slip-on |
| Small-bore, high pressure | Socket weld |
| No welding allowed | Threaded |
| Frequent dismantling or costly alloy pipe | Lap joint with stub end |
| Closing a line | Blind |
Whichever type you choose, match the flange standard, pressure class, facing and material to the rest of the system. If you are unsure, send us the line details and our team will recommend the right flange.
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