What is a lap joint flange?

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 hole 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 what ever it is joining to.
The “Stub-end” can normally be purchased in two lengths. There is a short version, about 3″ long and a long version of about 6″ long. It is prudent for the piping designer to know which version is in the piping specification.
Because of it’s two piece configuration, the Lap Joint Flange offers a way to cut cost or simplify work. The cost saving comes when the piping system requires a high cost alloy for all “wetted” parts to reduce corrosion. The sleeve or Stub-end can be the required higher cost alloy but the flange can be the lower cost forged carbon steel.
The lap joint flange can be rotated which can be useful when fixing issues with bolt hole alignment. When designing a piping system, lap joint flanges should not be considered solely to alleviate poor alignment during construction. Good design practice should not need to include poor construction quality. However, if the piping needs to be frequently dismantled for inspection or cleaning, consideration should be made for lap joint flanges. They give the ability to swivel flanges and to align bolt holes which simplifies the assembly of large diameter or unusually stiff piping.
Lap joint flanges are usually used in low pressure applications and are not suitable when there are high loads on the flange pair. Some types of piping require the use of lap joint flanges. For example, metallic pipe that has been plastic lining may have lap joint flanges.
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 wetted 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.

Key Features:

• Recommended in applications that require frequent dismantling of the flanges and pipe.
• Used with a matching stub-end insert.
• Can rotate to allow for an easy alignment of bolt holes.
• Not recommended in extreme or high-pressure temperature applications.

Advantages of Lap Joint Flanges:
Lap Joint flanges have certain special advantages over other flanges. These are as follows:

• Lap Joint flanges have the freedom to swivel around the pipe. This facilitates the lining up of opposing flange bolt holes.
• Lack of contact with the fluid inside the pipe leads to the greater durability of these flanges.
• In piping systems which corrode or erode quickly, the lap joint flanges may be salvaged for re-use.
• The pressure-holding capacity of lap joint flanges is little. But it is better than that of slip-on flanges.
• The Lap Joint works as a backing ring on the stub.
• The main advantage of lap joint flange is that the bolt holes can be aligned with the matching flange after the welds have been completed.
• They allow rotational alignment capability.

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 face. 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 is an assembly system.

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