Trace the history of non eccentric butterfly valve and single eccentric butterfly valve

Non eccentric butterfly valve:

The structural feature of non eccentric butterfly valve (or concentric butterfly valve) is that the stem axis, butterfly plate center and body center are in the same position. The structure is simple and the manufacture is convenient. The common rubber lined butterfly valve belongs to this kind. The disadvantage is that the butterfly plate and the valve seat are always in the state of extrusion, scraping, large resistance distance and fast wear. In order to overcome extrusion, scraping and ensure 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 traditionally people think that butterfly valve is not resistant to high temperature.

With the appearance of natural rubber and the appearance of artificial rubber lining in the 1950s, rubber lining began to be used as sealing material. The early butterfly valve design was non eccentric butterfly valve, and the common rubber lined butterfly valve was one of them.

Structural features: the stem axis, butterfly plate center and valve body center are in the same position (as shown in the figure).

Advantages: simple structure, because both sides of the center line of the butterfly plate are equal, the torque and the pressure load through the butterfly plate are balanced, so that the butterfly valve can achieve two-way tight sealing when closed, there is no unbalanced area, and the unilateral leakage is reduced.

Disadvantages: because it is a symmetrical structure centered on the axis of rotation. Its sealing surface is located on both sides of the shaft. In this structure, only half of the valve plate is consistent with the pressure of the sealing surface and the medium, and the other half is in the opposite direction. Therefore, the sealing performance of butterfly valve is not high, that is to say, its sealing performance is completely dependent on the initial interference between the sealing surfaces.

Single eccentric butterfly valve:

In order to solve the extrusion problem between the butterfly plate and the valve seat of concentric butterfly valve, the single eccentric butterfly valve is produced. Its structural feature is that the shaft axis 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, which disperses and reduces 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 does not disappear, and its application scope is similar to that of concentric butterfly valve, so it is rarely used.

In the early sixties, this phenomenon was corrected by staggering the center line of the valve stem and the center line of the seal, so as not to pass through the sealing surface of the rubber liner, thus producing the single eccentric butterfly valve.

Structural features: the axis of the valve stem deviates from the center of the butterfly valve, as shown in the figure.

Advantages: the center line of valve stem and the sealing center line are staggered to form asymmetric structure so that the sealing surface of butterfly valve is made into a complete and continuous circular arc surface without passing the rubber lining sealing surface. This is very convenient for the processing and manufacturing of sealing surface, and the valve plate and valve seat can also be ground, which has a sufficient guarantee for the sealing performance of butterfly valve. The two-point sealing problem of valve stem through lining is solved. The upper and lower end of butterfly plate is no longer a rotating axis, which disperses and reduces the excessive extrusion of the upper and lower end of the butterfly plate, can leave the sealing surface quickly and reduce the friction between mating surfaces.

Disadvantages: because the scraping phenomenon between butterfly plate and valve seat is not disappeared in the whole process of valve switch of single eccentric structure, it is not used much.