ASME B16.5 - ANSI Pipe Flanges and Fittings

These are commonly referred to by their pressure rating class, (e.g ANSI 150 or 150lb).  This standard specifies that for the standard 304(L) and 316(L) grades, the flanges must be forged, (except for blind flanges which can be made from plate).
The most commonly stocked flanges are slip-on, weld neck and blind.  (Note that the slip-on flange has a hub similar to the lap joint flange and they should not be misinterpreted).
The standard covers sizes up to DN 15 (NPS 1/2") to DN 600 (NPS 24"), in Class ratings of 150, 300, 400, 600, 900, 1500 and 2500.