Slip on flange dimensions are covered in ASME B16.5 – which covers Pipe Flanges and Flanged Fittings for size NPS ½” to 24” for above NPS 26” to 60” it should be as per ASME B16.47.
During dimensional inspection of flange, you should check for
Outer & Inner Diameter of body
Bolt Circle & Bolt Hole Diameter
Length of the Hub
Straightness and alignment of the bolt hole
