A lap joint flange is a type of flange that is used to connect pipes or fittings of different materials, sizes, or shapes. It consists of two parts: the stub end and the backing flange. The stub end is a short piece of pipe or tube that has a welded or threaded connection to the pipe, while the backing flange is a circular disc with bolt holes that is used to connect the lap joint flange to another flange or pipe.

The stub end of a lap joint flange is made of the same material as the pipe, while the backing flange can be made of different materials, such as carbon steel, stainless steel, or alloy steel, depending on the specific application and the fluid or gas being transported. The backing flange can also be of a different pressure rating than the lap joint flange, which makes it a popular choice for systems that require frequent dismantling or maintenance.

Lap joint flanges are commonly used in low-pressure systems, such as water supply or drainage pipelines, as well as in piping systems that require flexibility or frequent maintenance. They allow for easy assembly and disassembly of the pipeline or system, without having to remove the entire flange or pipe. Proper installation and torque of the bolts are crucial to ensure a tight and leak-free seal.