In order to solve the extrusion problem between the butterfly plate and the valve seat of the concentric butterfly valve, the Dalian marine valve manufacturer has produced the single eccentric butterfly valve. Its structural feature is that the shaft center of the valve stem deviates from the center of the butterfly plate, so that the upper and lower ends of the butterfly plate no longer become the rotation axis, and the excessive extrusion between the upper and lower ends of the butterfly plate and the valve seat is reduced. However, since the single eccentric structure does not eliminate the scraping phenomenon between the butterfly plate and the valve seat during the whole opening and closing process of the valve, and is similar to the concentric butterfly valve in the application range, it is rarely used.
On the basis of the single eccentric butterfly valve, the double eccentric butterfly valve which is widely used at present is further improved. Its structural feature is that the shaft center of the valve stem deviates from both the center of the butterfly plate and the center of the body. The double eccentric effect enables the butterfly plate to be separated from the valve seat immediately after the valve is opened, greatly eliminating the unnecessary excessive extrusion and scraping between the butterfly plate and the valve seat, reducing the opening resistance, reducing the wear and improving the service life of the valve seat.
At the same time, the double eccentric butterfly valve can also adopt metal valve seat, which improves the application of butterfly valve in high temperature field. However, because the sealing principle belongs to the position sealing structure, that is, the sealing surface of the butterfly plate and the valve seat is in line contact, and the sealing effect is produced by the elastic deformation caused by the compression of the valve seat by the butterfly plate. Therefore, the requirements for the closing position are very high (especially the metal valve seat), and the pressure bearing capacity is low. This is why people traditionally think that the butterfly valve is not resistant to high pressure and has large leakage.
The three eccentric butterfly valve must be high-temperature resistant and must be hard sealed, but the leakage is large; To achieve zero leakage, soft seal must be used, but it is not resistant to high temperature. In order to overcome the contradiction of double eccentric butterfly valve, the third eccentric of butterfly valve was carried out. Its structural feature is that the conical axis of the sealing surface of the butterfly plate is inclined to the cylindrical axis of the body while the double eccentric valve stem is eccentric. That is to say, after the third eccentricity, the sealing section of the butterfly plate is no longer a true circle, but an ellipse, and the shape of the sealing surface is asymmetric. One side is inclined to the center line of the body and the other side is parallel to the center line of the body.
The characteristic of this third eccentricity is that it fundamentally changes the sealing structure, no longer the position seal, but the torsion seal, that is, it does not rely on the elastic deformation of the valve seat, but completely depends on the contact surface pressure of the valve seat to achieve the sealing effect. Therefore, the problem of zero leakage of the metal valve seat is solved at one stroke, and because the contact surface pressure is proportional to the medium pressure, the high-pressure and high-temperature resistance is also solved.
The main features of three eccentric butterfly valve are as follows:
1. Small opening moment, flexible and convenient, energy saving.
2. The three eccentric structure makes the butterfly plate more and more tightly closed, with good sealing performance, so as to achieve no leakage.
3. High pressure resistance, corrosion resistance, wear resistance, long service life, etc. Welding two-way pressure three eccentric butterfly valve welding two-way pressure three eccentric butterfly valve is made of the same material as the valve body and is electrically or pneumatically driven. The applicable temperature depends on the material selection of the valve body. It is generally applicable to the ventilation and regulation of the medium flow in industrial, metallurgical and environmental protection pipelines, so that the disc plate can rotate freely within 90 degrees to achieve the purpose of opening and closing or regulating the medium flow.