What Is the Difference Between Single, Double and Triple Eccentric Butterfly Valves?

1. Center butterfly valve

The structural characteristics of this type of butterfly valve are that the valve stem axis, the center of the butterfly plate, and the center of the body are in the same position. The structure is simple and easy to manufacture. Common rubber-lined butterfly valves belong to this category. The disadvantage is that the butterfly plate and the valve seat are always in a state of extrusion and scraping, the resistance distance is large, and the wear is fast. In order to overcome the extrusion and scraping and ensure the sealing performance, the valve seat is basically made of elastic materials such as rubber or polytetrafluoroethylene, but it is also limited by temperature in use. This is why people traditionally believe that butterfly valves are not resistant to high temperatures.

2. Single eccentric butterfly valve

In order to solve the extrusion problem of the butterfly plate and the valve seat of the concentric butterfly valve, a single eccentric butterfly valve was produced. Its structural characteristics are that the valve stem axis 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 upper and lower ends of the butterfly plate and the valve seat are dispersed and reduced. Excessive extrusion. However, due to the single eccentric structure, the scraping phenomenon between the butterfly plate and the valve seat does not disappear during the entire opening and closing process of the valve, and the application range is similar to that of the concentric butterfly valve, so it is not widely used.

3. Double eccentric butterfly valve

The double eccentric butterfly valve, which is the most widely used one at present, is further improved on the basis of the single eccentric butterfly valve. Its structural feature is that the valve stem axis deviates from both the center of the butterfly plate and the center of the body. The double eccentric effect enables the butterfly plate to quickly detach from the valve seat 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 wear, and increasing the life of the valve seat. The significant reduction in scraping also allows the double eccentric butterfly valve to use metal valve seats, which improves the application of butterfly valves in high temperature fields. However, because its 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 elastic deformation caused by the butterfly plate squeezing the valve seat produces a sealing effect, the closing position requirements are very high (especially the metal valve seat), and the pressure bearing capacity is low. This is why people traditionally believe that butterfly valves are not resistant to high pressure and have large leakage.

4. Triple eccentric butterfly valve

To be resistant to high temperature, hard seals must be used, but the leakage is large; to have zero leakage, soft seals must be used, but they are not resistant to high temperature. In order to overcome the contradiction of the double eccentric butterfly valve, the butterfly valve was eccentric for the third time. Its structural feature is that while the axis position of the double eccentric valve stem is eccentric, the conical axis of the butterfly plate sealing surface is deflected to the cylindrical axis of the body. 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 its 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 biggest feature of this third eccentricity is that it fundamentally changes the sealing structure. It is no longer a position seal, but a torsion seal, that is, it does not rely on the elastic deformation of the valve seat, but completely relies 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 in one fell swoop, and because the contact surface pressure is proportional to the medium pressure, the high pressure and high temperature resistance is also solved.