Slip On (Slip-On) Flanges:
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 welding neck flanges by many users due to their lower initial cost, but final installation cost is probably not much less than that of the welding neck flange because of the additional welding involved.
Lap joint (Lap-Joint) Flanges:
The lap joint flange is practically identical to a slip-on flange except it has a radius at the intersection of the bore and flange race. This radius is necessary to have the flange accommodate a lap joint stub end. Normally, a lap joint flange and a lap joint stub end are mated together in an assembly system.