A Lap Joint Flange (Loose flange) is a device which consist of two pieces , it looks like a weld neck flange together, and it is butt welded with the pipes, it also like aloose slip-on flange. … Backing flange dimensions are same with other commonflanges.

First piece of the lap joint flange is a sleeve called a ‘Stub-end” and is shaped like a short piece of pipe with a narrow shoulder on one end and a butt weld end on the other end which called the hub. The narrow shoulder dimensions is like the rase face of the flange, the butt weld end of the stub-end is like the neck of the weld neck flange, the diameter and thickness is according to the pipe for connection.  The back face of the hub has a rounded transition that joins the hub to the sleeve.