A flange is an external or internal ridge, or rim (lip), for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel. Thus flanged wheels are wheels with a flange on one side to keep the wheels from running off the rails. The term "flange" is also used for a kind of tool used to form flanges. Pipes with flanges can be assembled and disassembled easily.
There are many different flange standards to be found worldwide. To allow easy functionality and interchangeability, these are designed to have standardised dimensions. Common world standards include ASA/ASME (USA), PN/DIN (European), BS10 (British/Australian), and JIS/KS (Japanese/Korean). In the USA, ANSI stopped publishing B16.5 in 1996, and the standard is ASME B16.5.
In most cases these are interchangeable as most local standards have been aligned to ISO standards, however, some local standards still differ (e.g. an ASME flange will not mate against an ISO flange). Further, many of the flanges in each standard are divided into "pressure classes", allowing flanges to be capable of taking different pressure ratings. Again these are not generally interchangeable (e.g. an ASME 150 will not mate with an ASME 300).
These pressure classes also have differing pressure and temperature ratings for different materials. Unique pressure classes for piping can also be developed for a process plant or power generating station; these may be specific to the corporation, engineering procurement and construction (EPC) contractor, or the process plant owner. The ASME pressure classes for Flat-Face flanges are Class 125 and Class 250. The classes for Ring-Joint, Tongue & Groove, and Raised-Face flanges are Class 150, Class 300, (Class 400 - unusual), Class 600, Class 900, Class 1500, and Class 2500.
The flange faces are also made to standardized dimensions and are typically "flat face", "raised face", "tongue and groove", or "ring joint" styles, although other obscure styles are possible.
Flange designs are available as "weld neck", "slip-on", "lap joint", "socket weld", "threaded", and also "blind".