How to Choose the Welding Method of Pipe Flanges

Pipe flange welding includes flat welding, butt welding, and socket welding.

1. Choose the appropriate flange material according to the material of the pipe. For example, stainless steel pipes are definitely not allowed to use carbon steel flanges.
2. Pick the proper flange according to the nominal diameter (DN) of the pipe and the nominal pressure (PN) of each pipe section in the whole piping system. At present, American standards and European standards are basically adopted for the pipe flanges in the world.
3. Choose flange structure forms according to specific working conditions, such as slip-on welding and lap joint flange. The sealing face include the raised face, the full face, the male face, the female face, etc.

Welding methods of the flange

A socket welding flange is attached by inserting a pipe into the recessed area of the flange. The welding neck flange is typically butt welded to a pipe. For socket welding, the radiographic inspection can't be done. In contrast, butt welding can do the test. Therefore, welding neck flanges are recommended for welds with high testing requirements.

Generally, butt welding requirements are higher than socket welding requirements as well as the testing requirements. For butt welding, it is necessary to do radiographic inspection, but for socket welding, magnetic particle testing or penetrant testing is enough. If the fluid in the pipe does not have high welding requirements, socket welding is recommended for its convenient testing.

Socket welding is mainly used for welding small diameter valves and pipes, pipe fittings and pipes. Small diameter pipes are generally thin-walled so that unfitness and ablation easily occur, and the pipes are difficult to be butt welded. Thus, they are more suitable for socket welding. In addition, the socket weld bell end has strengthening action, so this welding method is also widely used under high pressure. However, socket welding also has some shortcomings. One is that the stress is not good after welding, which is prone to incomplete penetration and there is a weld bead in the piping system. Therefore, socket welding should not be used for the piping system with medium susceptible to crevice corrosion and the piping system with high cleaning requirements. Furthermore, the ultra-high pressure pipe, even with a small diameter, is relatively thick-walled , so butt welding should be used as far as possible.

In short, socket welding forms a fillet weld, while butt welding forms a butt weld. Butt welding is better than socket welding from the analysis of weld strength and stress condition, so the butt welding is suitable to be used when the pressure is higher and the service condition is bad.