The Slip-On Flange has a low hub because the pipe slips into the flange prior to welding. It is welded both inside and out to provide sufficient strength and prevent leakage. Slip-on flanges are all bored slightly larger that the O.D. of the matching pipe. They are preferred over Weld Neck flanges by many users due to their lower initial cost, but final installation cost is probably not much less than that of the Weld Neck Flange because of the additional welding involved.