A weld neck flange (WN flange) means a flange has a neck for weld with the pipe, it is easily to recognized by their long tapered neck on the flange (The neck is also called hub in the weld neck flange).
Weld-Neck flanges or welding-neck flanges can be differentiated from all other types of flange by their long tapered hub.
The Welding neck flanges is normally referred to as the high hub flange. It is designed to transfer stresses to the pipe, thereby reducing high stress concentrations at the base of the flange.
A weld neck flange (also known as a high-hub flange and tapered hub flange) is a type of flange. There are two designs. The regular type is used with pipes. The long type is unsuitable for pipes and is used in process plant. A weld neck flange consists of a circular fitting with a protruding rim around the circumference. Generally machined from a forging, these flanges are typically butt welded to a pipe. The rim has a series of drilled holes that permit the flange to be affixed to another flange with bolts.