Butterfly valves, also called as flap valves, are a kind of valves having simple structure. Butterfly valves can also be used for low pressure pipelines to regulate media. The disc of butterfly valves is a plate, swinging with valve shaft to turn on or off valves. Butterfly valves are used for cutting off and throttling in pipelines. Industrial specialized butterfly valves have high temperature resistance and wide applicable pressure range. They have large nominal diameter. The body is made of carbon steel. Sealing ring in disc is made of metal ring, instead of plastic ring. Large high temperature butterfly valves are welded by using steel plate, which are mainly used for high temperature air flue and gas pipe. The structure of concentric butterfly valves is that axis of stem, centre of disc and centre of body are in the same position. Butterfly valves have simple structure and convenient manufacturing.  

1. Turning on-off conveniently and rapidly with less force. Having less fluid resistance and can be operated frequently.
2. Simple structure, small size. Having short face to face dimension, less volume and light weight, suitable for large diameter valves.
3. Can be used for transmitting mud, less liquids are stored at apertures of pipe.
4. Under low pressure, butterfly valves can realize good sealing.
5. Butterfly valves have excellent regulation performance.
6. When valves turning on completely, passageway of seat has large effective flow area and less fluid resistance.
7. Less torque. The pressure on disc at the two sides of spindle is almost equal, causing contrary torque. Thereby, the valves can be opened with less force.
8. Sealing face is generally med of rubber or plastic. For this butterfly valves have good sealing under low pressure. 

The materials used for butterfly valves are classified into polymer and metal. Seat made of polymer is mainly used under low temperature. Although metal sealing butterfly valves are satisfied with conditions having high temperature and pressure, elasticity on metal seat is less than that on polymer seat. So, metal sealing butterfly valves have poor sealing.