Differences among Single Offset Butterfly Valves, Double-offset Butterfly Valves and Triple Offset Butterfly Valves

​1.Centerline butterfly valve
A centerline butterfly valve features in the structure that the axle center of valve stem, valve disc and body are in the same position. It has simple structure and is easy to manufacture. The common-used rubber lined butterfly valve is also a sort of centerline butterfly valves. Since the valve disc and seat keep squeezing and scraping each other, the butterfly valve have the shortcomings of big frictional resisting moment and quick wearing. In order to overcome squeezing and scraping problems and ensure the sealing performance, we basically adopt elastic materials like rubber and PTFE for the valve seat. However, the valve is restricted to the temperature. This is the reason why butterfly valve is concerned, in the traditional view, as a  valve without high temperature resistance property.

2. Single offset butterfly valve
Single offset butterfly valves are designed to solve the squeezing problem between disc and seat of butterfly valve. The axle center of valve stem is deviated from center of the disc, making the top and bottom of the disc no longer become the center of revolving axle, and the overpressure caused by squeezing between the top&bottom disc and the seat dispersed and alleviated.

However, the scraping problem still exits between the disc and seat in the whole process of opening and closing of single offset valve. Its application range is similar to concentric butterfly valve’s, so it is not often used.
 
3. Double-offset Butterfly Valve
A Double-offset butterfly valve, which has the widest range of application these days, is an improved valve based on the structure of the single offset butterfly valve. The double-offset structure features in the stem center deviated from both the centers of the disc and valve. With this structure, the valve disc can leave the valve seat quickly, reducing the unnecessary over squeezing and scraping between them greatly. This structure also helps decrease the wearing process and prolong the operation life of the valve. Furthermore, the reduction of scraping also makes the seats of double-offset butterfly valves can be made from metal, expanding the application range of butterfly valves to the industries of high temperature working condition. However, the sealing of double-offset valves is position sealing, which means the sealing surface of the disc and seat is contact by a line. And the sealing effect is achieved by the elastic deformation caused by the squeezing of the disc, making a high requirement to the valve closing part(metallic valve seat especially) with poor ability of pressure resistance. Thus, double-offset valves have high requirements on the closing position, especially when the valves are with metallic valve seats and bad resistance to pressure. This is also the reason that people always believe in a traditional way that butterfly valves operate with large leakage and not suit for high pressure condition. 
 
4.Triple Offset Butterfly Valves 
For having high-temperature resistance property, it is necessary to use hard sealed double-offset butterfly valves, which may often leak. For preventing leakage occurring, we must choose soft sealed double-offset butterfly valves which cannot resist high temperature.  In order to overcome this shortcoming of double-offset butterfly valves, triple offset butterfly valves are designed based on the structure of the double-offset butterfly valves.

After improvement, when the axle of the double-offset valve stem is deviating, the conic axis of the disc surface deviates from the cylinder axis of the valve at the same time, which means the sealing section of the disc forms a oval shape instead of a circle shape. Due to this, the shape of the sealing surface becomes asymmetric, part of it inclined to the axis of the valve and another part of it parallel to the axis of the valve.
 
The most outstanding characteristic of the triple offset butterfly valves is the sealing structure has been changed fundamentally, which is not the position sealing any more and becomes torque force sealing. Torque force sealing means the sealing effect is realized only by the pressure of contact area of valve seat, instead of elastic deformation. This sealing structure assures zero leakage of metallic valve and solves the problem under high pressure and temperature because the pressure of contact area is just proportional to medium p
May 08,2017
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