Enter the inner diameter of the flange. For integral type flanges, this value will also be the inner pipe diameter. This value is referred to as "B" in the ASME code. The corrosion allowance will be used to adjust this value - two times the corrosion allowance will be added to the uncorroded ID given by the user. See the figure in Flange Diameters.
For blind flanges the Flange ID is 0.0.
For Reverse flanges this is the B` dimension as shown in appendix 2 of the ASME Code.