In HG, butt-welded flanges, flat-welded flanges and socket welded flanges have different standards. Applicable occasions are different, in addition, butt-welding flange is the pipe diameter and wall thickness of the interface end and the same as the pipe to be welded, and the two pipes are welded as well.
Flat welding flange is a concave platform which is slightly larger than the outer diameter of the pipe at the interface, and the pipe is inserted into it.
The welding performance of butt welding is better and the corrosion is smaller.
Flat welding flanges are generally used for low and medium pressure pipelines, butt-welding flanges are used for the connection of medium and high pressure pipelines, butt-welding flanges are generally at least PN2.5Mpa, the use of butt welding is to reduce stress concentration.

Features of various flanges:
Socket welding flange is suitable for small diameter, high pressure, high temperature pipe connection.
Flat welding is the same as butt welding with a boss, the boss is cut into a groove, and then the pipe is inserted into the welding.
Socket welding is no boss, in the flange body directly cut a groove, just like a hole on the blind flange, and then cut a groove.
Flat welding is slightly better than socket welding.