A lap joint flange, abbreviated as LJF, is a flange that consists of two parts, a stub end and the backing flange.  The flange slides over the pipe and the stub end is the connection where the gasket sits.  The end where the gasket sits is the same outside diameter as the face of a raised face flange.  In general, the thickness of the hub is between ¼” to 3/8”.

The flange can be rotated which can be useful when fixing issues with bolt hole alignment.  When designing a piping system, lap joint flanges should not be considered solely to alleviate poor alignment during construction.  Good design practice should not need to include poor construction quality.  However, if the piping needs to be frequently dismantled for inspection or cleaning, consideration should be made for lap joint flanges.  They give the ability to swivel flanges and to align bolt holes which simplifies the assembly of large diameter or unusually stiff piping.

Lap Joint Flange Standards

ASME Standards

• ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard

Lap Joint Flange Sizes - 1/2" through 24"
	Flat Face ANSI Class 150, B16.5 (in) ANSI Class 300, B16.5 (in) ANSI Class 400, B16.5 (in) ANSI Class 600, B16.5 (in) ANSI Class 900, B16.5 (in) ANSI Class 1500, B16.5 (in) ANSI Class 2500, B16.5 (in)