Weld neck flanges are the most popular flange type with a neck extension with a weld bevel at the end. This type of flange is designed to butt weld directly to pipe to provide a superior and relatively natural form connection. In larger sizes and higher pressure classes, this is almost exclusively the type of flange connection utilized. If only one bored flange style existed in modern applications, the weld neck would be your flange of choice.
The weld bevel joins to a pipe end with a similar bevel in a V-type connection which allows for a uniform circular weld around the perimeter to form a unified transition. This allows for the gas or liquid within the pipe assembly to flow with minimal restriction through the flange connection. This weld bevel connection is inspected after the weld procedure to ensure that the seal is uniform and lacking anomalies.
The other noticeable feature of the weld neck flange is the tapered hub. This type of connection provides a more gradual distribution of pressure forces along the transition from the pipe to the base of the flange, helping to withstand some of the shock from use in a higher pressure and higher temperature operating environment. The mechanical stresses are limited given the extra steel material along the hub transition.
As higher pressure classes require this type of flange connection almost exclusively, weld neck flanges are often made with a ring type joint facing (otherwise known as RTJ face). This sealing surface allows for a metallic gasket to be crushed between the grooves of both connecting flanges to form a superior seal and complement the high strength weld bevel connection to the pressurized pipe assembly. An RTJ weld neck with a metal gasket connect is the primary choice for critical applications