Socket welding (SW) flange, looks similar to SO flange in appearance. the main difference is the socket welding flange is bored out match a specific pipe inside-diameter. The back side of this center opening is then counter bored to form the appropriate size socket to take the pipe outside diameter. Because of the gap between the flange and the pipe, weld a seam from inside to avoid corrosion. The diameter of the smaller bore has to be the same as the inner diameter of the matching pipe. It is primarily used in the smaller pipes with sizes below DN80.