A Socket Weld Flange is similar to a Slip On Flange, however Socket Weld Flanges have an internal recess on the inside diameter to allow for a smoother flow of the process fluid.

This creates a smooth bore with the proper welding and grinding. However a Socket Weld Flange can be more expensive than a Slip On Flange due to the more intricate machining. Socket Weld Flanges were initially developed for use on smaller sizes of pipe at higher pressure ratings.