Flange dimensions are determined by the pipe size and the pressure class required for the application. Flanges are standardized according publications from organizations like ASME, MSS, API and others.
Diameters and bolt circles for standard ASME B16.5 flanges - 1/4 to 24 inches - Class 150 to 2500.
ANSI B16.5
ANSI B16.5 -1996 is the official standard for ANSI Class Pipe Flanges and Flanged Fittings, applied specifically to those made from cast or forged materials, as well as blind flanges and some reducing flanges crafted in cast, forged, or plate. It covers evaluations and standards for the following characteristics, noted in both metric and U.S. units of measure:
Pressure-temperature ratings
Materials
Tolerances
Dimensions
Marking
Testing
Methods of designating openings
ANSI standard flange dimensions are designated as 150, 300, 400, 600, 900, 1500, and 2500, in sizes NPS 1/2 through NPS 24.
ASME/ANSI B16.5 provides
weld neck
slip-on
socket weld
threaded
lap joint
blind
In general you need to specify the schedule (thickness) of the connected pipe when it is being butt welded to the flange. Socket welded and threaded connections is unaffected by wall thickness and should only require the OD.
