Lap joint flanges, also known as Van Stone or loose flanges, are unique in design compared to other types of flanges. They consist of two separate components: a stub-end and a backing flange. Here’s a description of each component and how they work together:

  1. Stub-End: The stub-end is essentially a short piece of pipe with a flared or beveled end. This end is designed to slip into the bore of the backing flange. Stub-ends are available in various lengths to accommodate different requirements.
  2. Backing Flange: The backing flange is similar to a standard flange but lacks the raised face or sealing surface. It has a flat face and bolt holes for connection to the mating flange or equipment. The bore of the backing flange is slightly larger than the outer diameter of the stub-end, allowing the stub-end to slip into it.

When using lap joint flanges, the stub-end is first welded to the pipe or fitting, typically with a fillet weld on the outside. Then, the backing flange is aligned with the pipe or fitting and bolted to the mating flange or equipment. The gap between the stub-end and the backing flange allows for movement and alignment adjustments during installation.

Key features and advantages of lap joint flanges include:

  1. Rotational Freedom: The stub-end can rotate within the backing flange, allowing for easy alignment during installation.
  2. Cost-Effectiveness: Lap joint flanges can be more cost-effective compared to other types of flanges, especially in larger sizes or when frequent disassembly is required.
  3. Flexibility: Lap joint flanges allow for easy replacement or modification of the stub-end without replacing the entire flange assembly.
  4. Insulation: Lap joint flanges are sometimes used in applications where thermal expansion or contraction of the piping system is expected, as the gap between the stub-end and the backing flange can accommodate movement.
  5. Low Pressure Applications: Lap joint flanges are commonly used in low-pressure piping systems or applications where a tight seal is not required.

Freedom to swivel around the pipe facilitates the lining up of opposing flange bolt holes.Lack of contact with the fluid in the pipe often permits the use of inexpensive carbon steel flanges with corrosion resistant pipe. In systems which erode or corrode quickly, the flanges may be salvaged for re-use.

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