A weld neck flange is a type of flange that is designed to be welded to a piping system. It has a long tapered neck, which is used to transfer stress from the flange to the pipe. The weld neck flange is attached to the pipe by a butt weld, which provides a strong and permanent connection.

Weld neck flanges are commonly used in high-pressure and high-temperature piping systems, as they provide a secure and reliable connection. They are available in a variety of materials, including carbon steel, stainless steel, and alloy steel, and can be designed for both standard and high-pressure applications.

The weld neck flange is typically used in combination with a gasket and a set of bolts. The gasket is placed between the flange faces to prevent leakage, and the bolts are tightened to a specified torque to hold the flanges securely together.

One advantage of weld neck flanges is that they provide a smooth flow of fluid or gas through the piping system, as there is no protrusion inside the flange that can cause turbulence. However, they are generally more expensive than other types of flanges, and require skilled labor to install properly.