The structure type of the Center Line butterfly valve is the valve seat shaft, with the butterfly plate in the middle, which is simple in structure and convenient to manufacture. The disadvantage is that the butterfly plate and the valve seat are always in the state of extrusion and scraping, and the resistance distance is large and the valve seat is damaged quickly. In order to avoid extrusion and scraping and ensure the sealing performance, the valve seat is basically made of elastic materials such as rubber or PTFE, but it is also limited by the use temperature, which is why the butterfly valve is not resistant to high temperature.

Single Eccentric Butterfly Valve
In order to solve the extrusion problem between the butterfly plate and the valve seat of concentric butterfly valve, the butterfly valve manufacturer produced a single eccentric butterfly valve. Its structural feature is that the valve seat axis deviates from the middle of the butterfly plate, so that the upper and lower ends of the butterfly plate no longer become rotating shafts, which relieves the excessive extrusion between the upper and lower ends of the butterfly plate and the valve seat. However, because the single eccentric structure did not disappear during the whole opening and closing process of the gate valve, it is similar to the concentric butterfly valve in application scope, so it is rarely used.

Double eccentric butterfly valve

On the basis of single eccentric butterfly valve, the widely used double eccentric butterfly valve has been further improved and formed. Its structural type is offset in the middle and middle of the valve seat axis. After the gate valve is opened, the butterfly plate can be quickly separated from the valve seat, which greatly eliminates unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reduces the opening resistance distance, reduces the damage and prolongs the service life of the valve seat.
Scratches are greatly reduced, and the metal valve seat can also be used for double eccentric butterfly valves, which improves the application of butterfly valves in the field of high-temperature applications. However, because its sealing principle belongs to the position sealing structure, that is, the butterfly plate contacts with the protruding surface of the valve seat, and the elastic deformation caused by the butterfly plate squeezing the valve seat produces a sealing effect, so the requirements for the closed position are high, and the bearing capacity is low, which is why the butterfly valve is traditionally considered to be not resistant to high pressure and has a large leakage.