Lap Joint Flange

Foraged flanges both connect pipe systems to various equipment and components while also providing added flexibility when it comes to assembly, disassembly, and maintenance.

Lap joint flanges are a type of flange popular in both piping and duct system design.

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

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.

The advantages of the lap joint flange design are:

• 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.

There are some limitations on the lap joint flange’s usage, mostly due to its bulkier multiple-component design. Lap joint flanges are not suitable for small-sized piping.

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.

Lap Joint Flange Production Process