The Structure and Performance of Four Kinds of Butterfly Valves

According to different structures, butterfly valve can be classified into concentric butterfly valve, single offset butterfly valve, double offset butterfly valve and triple offset butterfly valve.

Structure and performance of concentric butterfly valve  
The concentric butterfly valve is featured by the same center position of the axis of stem, disc and ontology; simple structure and convenient manufacturing. Common lined butterfly valve belongs to this category. Defects are big resistance and fast wear due to that the butterfly plate and valve seat remain the state of extrusion and scratches. In order to overcome the extrusion, and scratches, in general, elastic material such as rubber or PTFE is used as seat material. However, these materials are limited by temperature in use, which is why people traditionally deem butterfly valve is not resist to high temperature. 

Structure and performance of single offset butterfly valve 
To solve the problem of extrusion between butterfly plate and valve seat that exist in concentric butterfly valve, single offset butterfly valve was created. Its structural characteristic is that the valve stem axis deviates from the disc center, so that the top and bottom of butterfly plate is no longer a rotary axis, scattering and reducing the excessive extrusion between butterfly plate and valve seat. But its application scope almost the same as concentric butterfly valve, because the extrusion between butterfly plate and valve seat did not disappear in the whole switching process. 

Structure and performance of double offset butterfly valve 
Currently the most widely used double eccentric butterfly valve is improved on the basis of a single offset butterfly valve. Its structure characteristics is that the axis of stem deviates from both butterfly plate and the center of ontology. The effect of double eccentric is that butterfly plate can immediately deviates from the seat as soon as the valve opens, greatly eliminating the unnecessary excessive extrusion and scratches between butterfly plate and valve seat, narrowing resistance distance, reducing abrasion and improving the seat life. At the same time, slash scraping also makes the double eccentric butterfly valve can use a metal seat, which improves the application in the field of high temperature. However, its sealing principle is that the sealing surface of butterfly plate and valve seat are line contact, getting sealing effect through elastic deformation caused by the butterfly plate extruding seat. Therefore, there is a high requirement for closed position and low bearing capacity. This is why people traditionally deem butterfly valve cannot work under high pressure and leak a lot. 

Structure and performance of triple offset butterfly valve 
To get high temperature resistant, hard seal must be used, while the large amount of leakage; To get zero leakage, soft seal must be adopt, but it is not resistant to high temperature. So, in order to overcome this contradiction, triple offset butterfly valve was created. On the basis of double offset butterfly valve, the cone axis of sealing face of butterfly plate deviates from the cylindrical axis of ontology, that is to say, after the third offset, the sealing section of butterfly plate is no longer a true circle, but the ellipse, so the sealing face shapes are asymmetric, one side tilts to the center line of ontology, the other side is parallel to the center line of ontology. 

The biggest breakthrough is fundamentally changing the valve sealing surface and internal structure, no longer sealing at a single position, but using J shaped elastic sealing ring and triple eccentric, multi-level metal hard seal structure. This is not dependent on the elastic deformation of the seat, but completely rely on the contact pressure of the seat to achieve sealing effect and to achieve the goal of zero leakage. Also, because the pressure of contact surface is proportional to that of medium, triple offset butterfly valve is resistant to high temperature and pressure. 

In a word, triple offset butterfly valve is the upgrade version of double offset and single offset butterfly valve.

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