1, concentric butterfly valve
The structural features of the butterfly valve are the center of the stem, the center of the butterfly plate and the center of the body. It is simple in structure and convenient in manufacture. The common butterfly valves belong to this kind. The disadvantage is that the butterfly plate and valve seat are always in extrusion, scratch state, large resistance distance, wear fast. In order to overcome extrusion, scraping, sealing performance, the seat is basically elastic materials such as rubber or polytetrafluoroethylene, but it is also limited by temperature in use, which is why traditionally people think that the butterfly valve is not resistant to high temperature.
2. Single eccentric butterfly valve

In order to solve the extrusion of the butterfly valve and the seat of the butterfly valve, the single eccentric butterfly valve, whose structure is characterized by the structure of the valve shaft, deviates from the center of the butterfly plate, so that the upper end of the butterfly plate is no longer a rotary axis, disperses and reduces the overextrusion of the upper end of the butterfly plate and the seat. However, because of the single eccentric structure in the whole switch process of the valve, the scraping of the butterfly plate and the seat has not disappeared, and it is not much used in the application range with the concentric butterfly valve.

3, double eccentric butterfly valve
On the basis of single eccentric butterfly valve, further improvement is the most widely used double eccentric butterfly valve. Its structural characteristics are that the axis of the stem deviates from the center of the butterfly plate and also deviates from the center of the body. The double eccentric effect makes the valve unlocked quickly out of the seat, removes the unnecessary overextrusion and scraping of the butterfly plate and the seat, reduces the opening resistance, reduces the wear and improves the life of the seat. The scraping is greatly reduced, and the double eccentric butterfly valve can also be used with metal seats, which improves the application of butterfly valve in high temperature field. But because its sealing principle belongs to the position sealing structure, that is, the sealing surface of the butterfly plate and the seat of the valve seat is wire contact, the elastic deformation caused by the extrusion of the valve seat through the butterfly plate produces the sealing effect, so the closed position is very high (especially the metal seat) and the pressure bearing capacity is low. This is why it is traditionally considered that the butterfly valve is not resistant to high pressure. The cause of the large amount of leakage.
4,triple eccentric butterfly valve
To withstand high temperature, must use hard seal, but large leakage; zero leakage, must use soft seal, but not high temperature. In order to overcome the contradiction between double eccentric butterfly valve and butterfly valve, it has made third eccentricities. The structure is characterized by the eccentric position of the double eccentric valve stem axis, while the conical axis of the seal surface of the butterfly plate inclines to the axis of the cylinder, that is to say, after third eccentricity, the seal section of the butterfly plate is no longer true, but an ellipse, and its dense cover is asymmetrical in the body and the side is tilted in the body. The heart line and the other side are parallel to the center line of the body. The biggest characteristic of the third eccentricity is that it fundamentally changes the sealing structure, no longer the position seal, but the torque seal, that is, it does not depend on the elastic deformation of the seat, but depends entirely on the contact surface pressure of the valve seat to achieve the sealing effect. Therefore, it solves the problem of the zero leakage of the metal seat, and the contact surface pressure. It is directly proportional to the pressure of the medium, and it can be readily solved by high pressure and high temperature.
Key words of this article:triple eccentric butterfly valve