A Lap joint flange is a two piece device that is much like a weld-neck flange but also like a loose slip-on flange. One piece is a sleeve called a ‘Stub-end” and is shaped like a short piece of pipe with a weld bevel on one end and a narrow shoulder on the other end called the hub. The hub is the same outside diameter as the raised face (gasket contact surface) of a weld neck flange. The thickness of the hub is normally about ¼” to 3/8″. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.
The other piece of a lap joint flange is the backing flange. This flange has all the same common dimensions (O.D., bolt circle, bolt hole size, etc.) as any other flange however it does not have a raised face. One side, the backside, has a slight shoulder that is square cut at the center or pipe hole. The front side has flat face and at the center holes an outside fillet to match the fillet of the “Stub-end” piece. The flange part of the lap joint flange assembly is slipped on to the stub-end prior to the sleeve being welded to the adjoining pipe or fitting. The flange itself is not welded or fixed in any way. It is free to spin for proper alignment with whatever it is joining to.
Using lap joint flanges might be an option for saving costs when the piping is made of exotic materials. By using a lap joint flange, the whetted materials would consist of the exotic materials and the flange would be carbon steel. Since the flange doesn’t ever come in contact with the process fluid, it would not be affected by the fluids.