Plate flat weld flanges are suitable for connecting steel pipes with a nominal pressure not exceeding 2.5MPa. The sealing surface of a flat weld flange can be made into three types: full plane, concave and convex, and tongue and groove. The neck height of a necked flat weld flange is relatively low, which improves the rigidity and bearing capacity of the flange. Compared with butt weld flanges, the welding workload is large, the consumption of welding rods is high, and it cannot withstand high temperature and high pressure, repeated bending and temperature fluctuations, but it is more convenient to install on site, and the process of beating and rubbing the weld can be omitted, so it is more popular.