The difference between centerline, double and triple eccentric butterfly valves

Centerline Butterfly Valve

Centerline butterfly valve is that the shaft center of the valve stem, the center of the disc and the center of the body are in the same position. The structure is simple and the manufacture is convenient. 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 , so the resistance distance is large and the wear is fast. In order to overcome extrusion and scratching and ensure the sealing performance , the valve seat is basically made of elastic materials such as rubber or PTFE .
But it is also limited by temperature in use , which is why traditionally people think that butterfly valves are not resistant to high temperature.

 

 

Double eccentric butterfly valve

On the basis of the centerline butterfly valve, the most widely used double eccentric butterfly valve is further improved and formed. Its structural feature is that the shaft center of the valve stem deviates from the center of the disc and the center of the body. The effect of double eccentricity enables the disc to break away from the valve seat immediately after the valve is opened , which greatly eliminates unnecessary excessive extrusion and scratching between the disc and the valve seat . The opening resistance distance is reduced, the wear is reduced , and the valve seat life is improved.

The scraping is greatly reduced , and at the same time, the double eccentric butterfly valve can also use a metal valve seat , which improves the application of the butterfly valve in the high temperature field.
However, because its sealing principle is a positional sealing structure , that is, the sealing surface of the disc and the valve seat is in line contact , and the elastic deformation caused by the disc pressing the valve seat produces a sealing effect , so the closed position is required to be high , and the pressure bearing capacity is high. low .
This is why traditionally it is believed that butterfly valves are not resistant to high pressure and have large leakage.

Triple eccentric butterfly valve

To withstand high temperature , a hard seal must be used , but the amount of leakage is large; to zero leakage , a soft seal must be used , but it is 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. The structure is characterized in that the conical axis of the sealing surface of the butterfly plate is deviated from the cylinder axis of the body while the position of the double eccentric valve stem is eccentric . That is to say , after the third eccentricity , the sealing section of the disc is no longer a true circle , but an ellipse .
The shape of the sealing surface is also asymmetrical , one side is inclined to the centerline of the body, and the other side is parallel to the centerline of the body.

 

The biggest feature of this third eccentricity is that the sealing structure has been fundamentally changed , and it is no longer a positional 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 easily solved.