slip-on (SO) flange, also known as flat welding flange. The connection of the SO flange to the pipe is to insert the pipe into the flange bore till the proper position and then to weld together. The advantage is that the welding and assembly are easier, it is more suitable to use them for the pipeline system with lower pressure, fluctuation and vibration. SO flange can be divided into slip on neck flange and plate slip on flange.