Although the dimensions of ASME and API flanges sometimes are compatible for bolting - they do not share the same pressure rating system.
The difference between ASME/ANSI and API is the fabrication material and a higher rated API operating pressure.
ASME/ANSI flanges are commonly used in industrial process systems handling water, steam, air and gas. API flanges are manufactured for high strength operating refinery systems with products such as oil and explosive gases.
The flange standards API 6A and ASME/ANSI B16.5 are similar dimensionally - but the API 6A flanges are rated for higher pressures as indicated below:
|API vs. ASME/ANSI Flanges|
|Flange||Pressure Class Rating (psi)||Nominal Size Range
|ASME/ANSI B16.5||API 6A||ASME/ANSI 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.