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 ANSI/ASME (USA), PN/DIN (European), BS10 (British), JIS (Japanese) and so on.
Many of the flanges in each standard are divided into "pressure classes", allowing flanges to be capable of taking different pressure ratings. These pressure classes also have differing pressure and temperature ratings for different materials.
You could download below flange standards for further more information.
