There are many different Flange standards to be found worldwide. To allow easy functionality and inter-changeability, these are designed to have standardised dimensions. Common world standards include ASA/ANSI (USA), PN/DIN (European), BS10 (British/Australian), and JIS/KIS (Japanese/Korean).
ANSI designations such as ANSI 150, ANSI 300 and so on are often followed by a # (pound sign). The ANSI number does not directly relate to a pressure rating, but to a class of Flange. For example, the hash (#) or 'pound' reference; e.g. 300 pound, can be misleading in that an ANSI 300 flange is actually rated for a test pressure of 740 psi (~5100 kPa), and only within a certain working temperature range.
In most cases these are not interchangeable (eg an ANSI flange will not mate against a JIS 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 (eg an ANSI 150 will not mate with an ANSI 300). These "pressure classes" also have differing pressure and temperature ratings for different materials. "Pressure Classes" of Piping are usually developed for a process plant or power generating station; these "pressure classes" may be unique to the specific corporation, Engineering Procurement and Construction (EPC) contractor, or the process plant owner.
The Flange faces are made to standardized dimensions and are typically "flat face", "raised face", "tonque and qroove", or "ring joint" styles, although other obscure styles are possible.Flange designs are available as "Welding Neck Flange", "Slip-on Flange", "Boss Flange", "Lap Joint Flange", "Socket Weld Flange", "Threaded Flange", and also "Blind Flange".