The API 6A specification is a standard developed for the American Petroleum Institute and shares many characteristics with ASME flanges. They are similar in dimensions, but their minimum design requirements for working pressure begin in the class 2000, which is derived from the ASME class 600.
Certain work requires the use of pressure classes as high as the 6BX class 25000, although this is much less common than the typical 6B range of class 5000 and below.
All API flanges require ring type joint facings with proper gaskets for optimum integrity of their application. These basic higher-pressure requirements are due to their overwhelming popularity in petrochemical and volatile oilfield applications. Upstream assemblies with API flanges include wellheads, drilling equipment and more.
The downstream sectors of refining and processing also need these flanges for developing crude oil into products for everyday consumers.
Compatibility between pressure classes and size
API and ASME flanges are not necessarily interchangeable, but some RTJ styles of ASME B16.5 do match existing API flanges in terms of dimensions. API 6A flanges of class 5000 and below are often made of A105 carbon steel with a minimum yield strength of 45K, while API 6BX flanges of class 10000 and above require steel with an even higher yield strength. The most common is AISI 4130 with a yield strength of 75K.
You are more likely to find ASME flanges in general pipeline and pressure vessel assembly work, while API flanges are often used in oilfield equipment, as originally intended. ASME B16.5 flanges are typically used for materials such as air and high-flow liquids, while API flanges are primarily used for unrefined crude oil, refined petrochemicals and high-energy gases.
The flange standards API 6A and ASME B16.5 have the same dimensions - but the API 6A flanges are rated for higher pressures as indicated below..
|API vs ASME Flanges with the same Dimensions|
|Flange||Pressure Class Rating
|Nominal Size Range
|ASME B16.5||API 6A||ASME B16.5||API 6A1)|
|Weld neck||600||2000||1/2 - 24||113/16 - 11|
|Blind and Threaded||600||2000||113/16 - 211/4|
|900||3000||113/16 - 203/4|
|1500||5000||113/16 - 11|
1) In the old API standard, flanges ranged from 1.1/2 to 10(20) inches