Lap Joint Flange

Lap Joint flanges are sometimes referred to by other names, including:
Stub end flange
Loose flange
Backing flange
Vanstone and Backing Ring

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.

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.

This arrangement, therefore, has these two advantages:

reduces the overall cost of the pipeline’s flanged joints, as the use of higher-grade materials is minimized;

bolting operations are simplified, as the lap joint flange can be rotated around the pipe to help with bolts alignment.

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.