A flange is a ring of steel welded or screwed to the end of a pipe, valve or vessel nozzle so that it can be bolted to its neighbour with a gasket between them. Flanged joints can be taken apart for maintenance, which is why every refinery, pipeline, water plant and process unit uses them by the thousand.
A flanged joint is two flanges, a gasket and a set of bolts. Tightening the bolts compresses the gasket between the two flange faces and seals the joint; undoing them lets a valve, a pump or a length of pipe be removed. Because the joint has to stay tight at the design pressure and temperature, flanges are standardised by pressure class: the class fixes the outside diameter, the thickness and the bolting, so a Class 300 flange from one maker bolts to a Class 300 flange from any other.
Most flanges are forged. A billet or bar is hot forged to a ring or a hubbed blank, then normalised where the standard requires it and machined: bore, face, bolt holes and, for a weld neck, the bevel that is butt-welded to the pipe. Large and low-pressure flanges are also cut from plate or rolled from bar and welded. Forging gives the grain flow and soundness that a pressure-retaining part needs, which is why plate flanges are not accepted in many piping specifications.
The weld neck flange, with its long tapered hub butt-welded to the pipe, is used for high pressures, high temperatures and cyclic service. The slip-on flange slides over the pipe and is fillet-welded inside and out; it is cheaper to fit but weaker. Socket weld and threaded flanges are for small bore piping, lap joint flanges turn loose on a stub end, and the blind flange closes the end of a line. The face may be flat, raised (the usual choice) or machined with a groove for a ring-type joint (RTJ) at high pressure.
The GCC is one of the largest flange markets in the world because of its oil, gas, petrochemical, power and desalination plants. Projects are specified to ASME B16.5 and B16.47 with API and ASME piping codes, and the national oil and gas companies add their own material and inspection requirements and approved-manufacturer lists on top of them. Flanges are machined and stocked in the region and imported in large volumes from India, China, Italy and other forging centres. Matching gaskets, fasteners and valves & fittings are bought to the same class.
A flange is ordered by type, size, pressure class, facing, bore (to match the pipe schedule for weld neck and socket weld flanges) and material grade — for example '6 in Class 300 weld neck flange, RF, bore to Sch 40, ASTM A105N'.
Full dimensions: ASME B16.5 Class 150 flange dimensions (NPS ½–24)
| Region | Standard | What it covers |
|---|---|---|
| USA | ASME B16.5 ASME |
Pipe flanges and flanged fittings, NPS ½ through NPS 24, Classes 150 to 2500 — dimensions, pressure–temperature ratings, materials and tolerances |
| USA | ASME B16.47 ASME |
Large-diameter steel flanges, NPS 26 through NPS 60; Series A (the former MSS SP-44 pattern) and Series B (the former API 605 pattern), which do not bolt to each other |
| USA | MSS SP-44 | Steel pipeline flanges, including high-yield flanges for pipeline service |
| USA | AWWA C207 AWWA |
Steel pipe flanges for waterworks service, 4 in to 144 in, for low-pressure water lines |
| USA | ASTM A105/A105M ASTM |
Carbon steel forgings for piping applications — the standard material for carbon steel flanges |
| USA | ASTM A350/A350M ASTM |
Carbon and low-alloy steel forgings requiring notch toughness testing (grade LF2 for low-temperature flanges) |
| USA | ASTM A694/A694M ASTM |
Carbon and alloy steel forgings for pipe flanges, fittings and valves for high-pressure transmission service (F52 to F70) |
| Europe | EN 1092-1 CEN-CENELEC |
Flanges and their joints — circular flanges for pipes, valves, fittings and accessories, PN designated — steel flanges |
| Germany | DIN 2633 replaced by EN 1092-1 |
Weld neck flanges PN 16, one of the DIN 2500 series of flange standards |
| UK | BS 4504 replaced by EN 1092-1 |
Circular flanges for pipes, valves and fittings, PN designated |
| Japan | JIS B2220 JSA |
Steel pipe flanges, nominal pressures 5K to 63K |
| China | GB/T 9124 SAC |
Steel pipe flanges (PN- and Class-designated series) |
| Russia / CIS | GOST 33259 | Flanges for valves, fittings and pipelines for nominal pressures up to PN 250 |
Grade equivalents and the standards bodies: Steel grades & standards
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