Basic knowledge of eccentric
Eccentric butterfly valve can be divided into various types, generally single eccentric sealing butterfly valve, double eccentric sealing butterfly valve, and three eccentric sealing butterfly valve.
Features of single eccentric sealing butterfly valve:
In order to solve the extrusion problem between the butterfly plate and the valve seat of the concentric butterfly valve, a single eccentric butterfly valve is produced. Its structural feature is that the stem axis deviates from the butterfly plate center, so that the upper and lower ends of the butterfly plate no longer become the rotation axis, disperse, and reduce the excessive extrusion between the upper and lower ends of the butterfly plate and the valve seat. However, due to the single eccentric structure in the whole process of valve opening and closing, the scraping phenomenon between the butterfly plate and the valve seat has not disappeared, which is similar to the concentric butterfly valve in the scope of application, so it is rarely used. The single eccentric sealing butterfly valve has the following characteristics:
- It has a sealing structure with reliable performance and long service life.
- Truss structure with small flow resistance, good rigidity and small pressure loss.
- The sealing ring of valve plate is easy to be replaced and maintained.
- The sealing ring of valve plate adopts “t” structure, and the sealing pair is linear.
- Bimetal thrust gasket is used at the bottom of the valve, with excellent thrust performance.
- The valve can be installed either vertically or horizontally with good applicability.
- This kind of valve is recommended to be used in one way, and it can be used in two ways when the reverse sealing requirements are not strict.
- When the valve is fully closed, the leakage of the seal test is zero.
- Replace the valve plate seal ring, O-ring, valve body, valve plate, shaft and other materials, applicable to a variety of media and different temperatures.
Features of double eccentric sealing butterfly valve:
On the basis of the single eccentric butterfly valve, the double eccentric butterfly valve, which is widely used at present, is further improved. The structure is characterized in that the valve stem axis deviates from both the center of butterfly plate and the center of body. The effect of double eccentricity makes the butterfly plate break away from the valve seat quickly after the valve is opened, greatly eliminating the unnecessary over extrusion and scraping between the butterfly plate and the valve seat, reducing the opening resistance, reducing the wear and improving the valve seat life. At the same time, the double eccentric butterfly valve can also use metal seat, which improves the application of butterfly valve in the field of high temperature. However, because the sealing principle belongs to the position sealing structure, that is, the sealing surface of butterfly plate and valve seat is in line contact, and the sealing effect is produced by the elastic deformation caused by the butterfly plate pressing the valve seat, the requirements for the closing position are very high (especially for the metal valve seat), and the pressure bearing capacity is low, which is why people traditionally think that butterfly valve is not resistant to high pressure and has a large leakage. Its main features are as follows:
- The full flow design is adopted, which is not affected by the jamming of impurities. The joint “O” type sealing line at both ends makes the pipeline installation reliable without gasket, and different materials can be selected according to different use requirements.
- The butterfly plate material adopts streamlined design, which can be used in both directions, with small flow resistance and excellent flow characteristics.
- The shaft pin adopts the pin withdrawing structure, which can tightly combine the butterfly plate and valve rod without weakening the strength of the shaft and has good interchangeability.
- The valve body connection can be applied to ansi125 / 150, din2501 PN10 / 16, jisb2210 10K, JIS g5527 7.5K standard.
Three eccentric sealing butterfly valve features:
- API specification
- Dual security structure
- Dead zone free design
- Body seat structure
- Structure of thin layer valve seat
- Replaceable sealing pair
- Balanced fixed structure
- Intrinsic refractory construction
- High packing construction
The structure is characterized in that the conical axis of the sealing surface of the butterfly plate deviates from the cylinder axis of the body when the shaft axis of the double eccentric valve rod is eccentric, that is to say, after the third eccentric, the sealing section of the butterfly plate is no longer true circle, but ellipse, and the sealing surface shape is also asymmetric because of this, one side is inclined to the center line of the body, the other side is 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 sealing, but the torque sealing, 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, so it solves the problem of zero leakage of the metal valve seat at one stroke, and because the contact surface pressure is proportional to the medium pressure, it also resists high pressure and high temperature readily solved. Since its appearance, triple eccentric butterfly valve has experienced the process of self-improvement and continuous development in order to meet the increasingly severe working conditions. Even if the highest zero leakage is required, three eccentric butterfly valve can be achieved in theory, but in fact, it still depends on careful design and precise manufacturing.