Selecting carbon steel pressure vessel flanges requires determining 4 parameters:

Nominal Diameter - Determined by the specified diameter. (DN)

2. Nominal Pressure - Determined by design pressure (PN)

3. Sealing Surface Type - Determined by factors such as pressure and installation location (Flat, Concave-Convex, Keel Groove)

4. Flange Material - Determined based on pressure class, pipe material, heat treatment requirements, and the corrosion allowance of the medium. Also, note that certain materials have thickness limitations.