Rings of forged or plate stainless steel that bolt pipe, valves and equipment together so a joint can be opened again. Flanges are made to dimensional standards (ASME B16.5, EN 1092-1) and material standards (ASTM A182 for forgings); in the Gulf they are a staple of desalination, oil and gas, chemical and food plants.
A flange joint is two flanges, a gasket between them and a ring of bolts. It is used wherever a pipe must be connected to a valve, pump, vessel nozzle or instrument, or wherever a line has to be opened for maintenance. Stainless flanges are chosen when the fluid, the outside atmosphere or a hygiene requirement rules out carbon steel — and the flange is normally matched in grade to the pipe it is welded to.
Pressure flanges are forged: a billet or bar is hot forged into a ring or a hubbed blank, solution annealed, then machined to the standard's dimensions and facing. ASTM A182 is the usual material specification, with grades marked F304L, F316L and so on. Low-pressure flanges and large plate flanges are also cut from stainless plate (ASTM A240, EN 10028-7); EN 1092-1 recognises plate flanges as type 01, but most piping codes require forgings for weld neck and high-pressure flanges.
The flange rating is a pressure class (ASME Class 150, 300, 600, 900, 1500, 2500) or a PN number (EN), and the allowable pressure falls as temperature rises. The tables in ASME B16.5 and EN 1092-1 are grade specific: a 316L flange has a lower rating than a 316 one, because the low-carbon grade is weaker at temperature. Faces are raised face (RF), flat face (FF) or ring type joint (RTJ) for high-pressure service.
In the GCC stainless flanges are used in great quantity in seawater desalination — where seawater and brine call for duplex, super duplex (F53, F55) or 6Mo (F44) rather than 316L — in oil and gas plant to operators' material specifications and ISO 15156 (NACE MR0175) for sour service, and in dairy, beverage and pharmaceutical plants. Most are imported forged and machined from India, China, Italy and Korea, with regional stockists and fabricators holding standard sizes.
A flange is specified by type, nominal size, pressure class, facing, the pipe schedule it is bored to (for weld neck and socket weld) and material grade — for example 'WNRF 6 in Class 300 Sch 40S, ASTM A182 F316L, ASME B16.5'.
| Region | Standard | What it covers |
|---|---|---|
| USA | ASME B16.5 ASME |
Pipe flanges and flanged fittings NPS ½ through NPS 24 — dimensions, Class 150–2500 ratings by material group |
| USA | ASME B16.47 ASME |
Large diameter steel flanges, NPS 26 through NPS 60 — Series A and Series B |
| USA | ASME B16.48 ASME |
Line blanks — spectacle blinds, spacers and paddle blanks |
| USA | ASTM A182/A182M ASTM |
Forged or rolled alloy and stainless steel pipe flanges, forged fittings, valves and parts — grades F304L, F316L, F51, F53 and others |
| Europe | EN 1092-1 CEN-CENELEC |
Flanges and their joints — circular flanges for pipes, valves, fittings and accessories, PN designated — steel flanges |
| Europe | EN 10222-5 CEN-CENELEC |
Steel forgings for pressure purposes — martensitic, austenitic and austenitic-ferritic stainless steels |
| International | ISO 15156 (NACE MR0175) ISO |
Materials for use in H2S-containing environments in oil and gas production — limits on hardness and grade for sour service |
| Japan | JIS B2220 JSA |
Steel pipe flanges — 5K to 30K ratings, used for Japanese-designed plant |
| China | GB/T 9124 SAC |
Steel pipe flanges — PN and Class designated series |
| Russia / CIS | GOST 33259 | Flanges for valves, fittings and pipelines — types and dimensions for PN up to 300 |
Grade equivalents and the standards bodies: Steel grades & standards
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