API vs. ASME/ANSI Flanges
Compatibility between pressure classes and size
 API and ASME/ANSI flanges are not necessarily interchangeable, however some of the RTJ styles from ASME B16.5 do match up dimensionally with existing API flanges. API 6A flanges in 5000# class and below are often made from A105 carbon steel with a 45K minimum yield strength, whereas the API 6BX flanges of 10000# class and above require a steel with anA even higher yield strength. The most common of those is AISI 4130 with a 75K yield.
You are more likely to find ASME/ANSI flanges in general pipeline assembly work and vessel service, whereas API flanges are often found in oilfield equipment as originally intended. B16.5 flanges will typically deal with materials such as air and high flow liquids, whereas API will handle primarily unrefined crude oil, refined petrochemicals, and high energy gases.

API and ASME/ANSI flanges - comparing dimensions and pressure ratings
Although the dimensions of ASME and API flanges may sometimes be compatible for bolting - using the same bolt circle and the same number of bolts - they do not share the same pressure rating system.

• ASME rates the pressure of a class based on the material of construction and the design temperature
• API specifies allowable materials and gives it a specific pressure rating

• 1 inch = 25.4 mm

API 6A FLANGE