Lap Joint Flange (LJF) Explained

Lap Joint Flange (LJF)

A lap joint flange (LJF) is an assembly of two elements involving a stub end and a lap joint ring flange (also called a ‘lap joint flange’). To be technically correct, the stub end is not part of a lap joint flange. However, a lap joint flange is always used in conjunction with a stub end, so both parts are often collectively referred to as a ‘lap joint flange’. 

Due to their design, lap joint flanges always have a flat face with a smooth surface. But, when combined with the stub end, the resulting sealing face is raised. This occurs because the sealing face of the stub end is above the flange’s bolting plane.

The lap joint flange has no sealing face, only the stub end has the sealing face.

The sealing face of a stub end may be plain, serrated, or grooved to allow for a ring type joint.

Lap Joint Flange Components

Lap Joint Flange Components

Lap joint flanges are available in many sizes and classes; they are also suitable for elevated temperatures and pressures. Generally, a lap joint flange can be used for the same applications as a welding neck flange.

Lap Joint Flange Cross Section

Lap Joint Flange Cross Section

To assemble a lap joint ring flange and stub end, the stub end must slide into the bore of the flange ring and then be butt welded to the pipe. One side of the stub end forms the sealing face, whilst the opposite/back side of the stub end presses against the lap joint flange ring (when the flange is assembled). The lap joint flange ring is free to rotate after the stub end is welded to the pipe, this is because it is not physically joined to the stub end. Once the flange joint has been assembled, the lap joint ring is no longer free to rotate.

Lap Joint Flange         Lap Joint FlangeLap Joint Flange

Lap Joint Flange

The advantages with the lap joint flange design are:

  • Inexpensive.
  • Well suited for corrosive applications.
  • Requires only a single full penetration butt weld.
  • Only the stub end has contact with the process medium
  • It can be easily adjusted to align with the mating flange (the flange can be rotated until the bolt holes align).
  • It is available in almost all pipe diameters.
  • Ideal for systems where frequent disassembly of the flange is required e.g. when cleaning or inspection is required.

It is possible to use a cheaper grade material for the lap joint flange e.g. carbon steel, because only the stub end must match the associated pipe material/grade; this can result in considerable cost savings.

There are some limitations on the lap joint flange’s usage, mostly due to its bulkier multiple component design. Lap joint flanges cannot be used for small size piping.


Non-Destructive Testing (NDT) Techniques

Magnetic particle inspection (MPI), dye penetrant inspection (DPI), radiographic testing (RT) and/or ultrasonic testing (UT), can be used to prove the butt weld (same techniques as used for a welding neck flange). 


ASME Standards

Dimensions and tolerances for lap joint flange stub ends are outlined in the ASME B16.9 standard. Note, this differs from the usual ASME B16.5 standard because a stub end is classified as a pipe fitting, not a flange.

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Flange Types, Faces, and Surfaces - Explained!

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