1. Flat welding: only welding the outer layer, without welding the inner layer; Generally used in medium and low pressure pipelines, the nominal pressure of the pipeline is less than 0.25mpa. There are three kinds of sealing surface of flat welding flange, which are smooth type, concave and convex type and tenon groove type. Among them, smooth type is the most widely used, and the price is affordable and cost-effective.
2. Butt welding: both the inner and outer layers of the flange should be welded, generally used in medium and high pressure pipelines, and the nominal pressure of the pipeline is between 0.25 ~ 2.5mpa. The sealing surface of the welding flange connection is concave and convex, and the installation is more complicated, so the labor cost, installation method and auxiliary material cost are relatively high.

3. Socket welding: generally used for nominal pressure is less than or equal to 10.0MPa, nominal diameter is less than or equal to 40mm in the pipeline.
4. Loose sleeve: Generally used in the pipeline with low pressure but corrosive medium, so this kind of flange has strong corrosion resistance, and the material is mainly stainless steel.
This kind of connection is mainly used for the connection of cast iron pipe, bushing hose, non-iron metal pipe and flange valve, etc., and the connection of process equipment and flange are also connected with flange.