Flange Standards
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.

• The pressure class for DIN flanges would be start from PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160 and so on . 
• The pressure class for JIS flanges would be start from 2K, 5K, 10K, 16K, 20K, 30K, 40K, 63K and so on . 
• The pressure class for ASME flanges would be start from 150LB, 300LB, 600LB, 900LB, 1500LB, 2500LB and so on . 

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".

ASME standards (U.S.)

ASME type flange on a gas pipeline
Pipe flanges that are made to standards called out by ASME B16.5 or ASME B16.47, and MSS SP-44. They are typically made from forged materials and have machined surfaces. ASME B16.5 refers to nominal pipe sizes (NPS) from ½" to 24". B16.47 covers NPSs from 26" to 60". Each specification further delineates flanges into pressure classes: 150, 300, 400, 600, 900, 1500 and 2500 for B16.5, and B16.47 delineates its flanges into pressure classes 75, 150, 300, 400, 600, 900. However these classes do not correspond to maximum pressures in psi. Instead, the maximum pressure depends on the material of the flange and the temperature. For example, the maximum pressure for a Class 150 flange is 285 psi, and for a Class 300 Flange it is 740 psi (both are for ASTM A105 Carbon Steel and temperatures below 100F).

The gasket type and bolt type are generally specified by the standard(s); however, sometimes the standards refer to the ASME Boiler and Pressure Vessel Code (BPVC) for details (see ASME Code Section VIII Division 1 – Appendix 2). These flanges are recognized by ASME Pipe Codes such as ASME B31.1 Power Piping, and ASME B31.3 Process Piping.

Materials for flanges are usually under ASME designation: SA-105 (Specification for Carbon Steel Forgings for Piping Applications), SA-266 (Specification for Carbon Steel Forgings for Pressure Vessel Components), or SA-182 (Specification for Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service). In addition, there are many "industry standard" flanges that in some circumstance may be used on ASME work.

The product range includes SORF, SOFF, BLRF, BLFF, WNRF (XS, XXS, STD Schedule 20, 40, 80), WNFF (XS, XXS, STD Schedule 20, 40, 80), SWRF (XS STD), SWFF (XS STD), Threaded RF, Threaded FF LJ, with sizes from 1/2" to 16". The bolting material used for flange connection is stud bolts mated with two nut (washer when required). In Petrochemical industries, ASTM A193 B7 STUD ASTM A193 B16 Stud Bolts are used as these have high tensile strength.

European Dimensions (EN / DIN)
Most countries in Europe mainly install flanges according to standard DIN EN 1092-1 (forged Stainless or Steel Flanges). Similar to the ASME flange standard, the EN 1092-1 standard has the basic flange forms, such as weld neck flange, blind flange, lapped flange, threaded Flange (Thread ISO7-1 instead of NPT), weld on collar, pressed collars, and adapter flange such as flange coupling GD press fittings. The different forms of flanges within the EN 1092-1 (European Norm Euronorm) is indicated within the flange name through the type.