1. Single Eccentric Butterfly Valve
In order to solve the problem of concentric butterfly valve butterfly plate and valve seat extrusion, produced a single eccentric butterfly valve, its structural characteristics: stem axis deviated from the centre of the butterfly plate, so that the upper and lower ends of the butterfly plate is no longer a rotary axis, dispersed, reduce the upper and lower ends of the butterfly plate and the valve seat of the excessive extrusion. But the single eccentric structure in the valve throughout the switching process of the butterfly plate and the seat of the scraping phenomenon has not disappeared, so in the scope of application and concentric butterfly valves are similar, not much.
2. Double Eccentric Butterfly Valve
Double eccentric butterfly valve is further improved on the basis of single eccentric butterfly valve, is currently more widely used butterfly valve. Its structural characteristics are: valve stem axis is not only away from the centre of the butterfly plate, but also away from the centre of the body. The effect of double eccentricity makes the valve to be opened after the butterfly plate can be quickly detached from the valve seat, greatly eliminating the butterfly plate and the seat of the unnecessary excessive extrusion, scraping phenomenon, reduce the opening distance, reduce wear and tear, and improve the life of the valve seat. Scratch greatly reduced, double eccentric butterfly valve can also use metal valve seat, improve the butterfly valve in the field of high temperature applications. But because of its sealing principle of position sealing structure, that is, the butterfly plate and the valve seat sealing surface for the line contact, through the butterfly plate extrusion seat caused by the elastic deformation of the sealing effect, so the position of the closure of the requirements are very high (especially metal valve seat), low pressure capacity, which is why traditionally people think that butterfly valves do not withstand high pressure, large leakage.
3. Three eccentric butterfly valve
To withstand high temperature, hard seal must be used, but the leakage is large; to zero leakage, soft seal must be used, but not high temperature. In order to overcome this contradiction of double eccentric butterfly valve, a third eccentric butterfly valve is carried out. The structural characteristics of the three eccentric butterfly valve are: in the double eccentric stem, axis position eccentric at the same time, so that the conical axis of the sealing surface of the butterfly plate is skewed in the body of the cylindrical axis, after the third eccentric, the butterfly sealing section is no longer a true circle, but an ellipse, and the shape of the sealing surface is therefore asymmetric, one side is tilted at the body centre line, and the other side is parallel to the centre line of the body.
Three eccentric butterfly valves fundamentally change the sealing structure, no longer position sealing, but torsion sealing, that is, not relying on the elastic deformation of the valve seat, but relying entirely on the contact surface pressure of the valve seat to achieve the sealing effect, thus solving the problem of zero leakage of the metal valve seat, the contact surface pressure is directly proportional to the pressure of the medium, so high pressure and high temperature resistance is also solved.