Lap Joint Flanges, Standard : ASME B16.5

Similar to Slip-on flange, a socket-weld flange however has a recession machined into the hub side of the flange with width equal to the thickness of the pipe. Usually developed for small sizes, these socket-weld flanges are beneficial for pressure applications. Socket Weld Flange is welded to a pipe with one fillet weld over the socket with no primary alignment practice required.
The passive Strength of the socket-weld flange is 2/3rd that of weld neck and fatigue strength is equal to slip-on flange.
While welding the pipe to the socket-weld flange one must mind a clearance gap between the pipe and the seat of the recession in order to reduce the residual stress at the root of the weld. The space between the pipe and the seat however allows settlement of the fluid and induces erosion and corrosion at the gap which is a major disadvantage of using a socket-weld flange.

Lap Joint Flanges(Loose Flanges) are used with a corresponding stub-end that "inserts" in the inside of the flange. The main benefit of this type of flange, is that once the pipe welds to the insert stub-end, the flange then can rotate for easier alignment of the bolting holes. However, lap joint flanges and their stub-ends' ability to hold pressure is about the same as slip-on flanges. They are not suitable for situations with a high external or changing loads of force. They do provide an advantage in a piping system that needs frequent dismantling for inspection or to remove build-up

Manufacturing
Lap Joint Plate Flange can be manufactured as forged or directly machined from plates. Whereas Stub has to be manufactured as wrought from parent Pipe, seamless or welded by cold press or hot press method.

Applications
lap Joint Flanges are preferred for low cost pipeline projects specially civil water pipeline projects, Chemical Processing, Paint & Food Processing units & some sector of Oil & Gas pipelines.