Development and Application of Three-Eccentric Butterfly Valves

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　　Development and applications of triple-eccentric butterfly valves? In many countries in the Asia-Pacific region, due to limitations in local valve design and manufacturing capabilities as well as the influence of traditional textbooks, there remains a significant prejudice against butterfly valves. Meanwhile, the advanced industrial countries in Europe and the United States are vigorously promoting and actively using butterfly valves. 　　The reason is simple: butterfly valves are not what they used to be. Three-eccentric butterfly valves can operate under pressures as high as 2500 pounds per square inch, in temperatures ranging from -196 °C to 700 °C, with zero leakage at the seal, and offer a control ratio of over 100:1. In other words, on various harsh and critical process control pipelines, whether it’s switch valves or control valves, as long as the right type is selected, butterfly valves can now be used with confidence, and they are also cost-effective. 　　The excellent performance of butterfly valves is closely related to their continuous eccentricity, evolution, and development. To meet the requirements of various operating conditions, butterfly valves have evolved from concentric design to single-eccentric, double-eccentric, and triple-eccentric designs. The evolution of butterfly valves, as well as the development and application of triple-eccentric butterfly valves, are briefly described as follows: 1. Concentric butterfly valve: The structural feature of this type of butterfly valve is that the axis of the valve stem, the center of the butterfly plate, and the center of the valve body are all in the same position. It has a simple structure and is easy to manufacture. Common rubber-lined butterfly valves fall into this category. The disadvantage is that the butterfly plate and the valve seat are constantly in a state of compression and scraping, resulting in a large flow resistance and rapid wear. To overcome compression and scraping and ensure sealing performance, the valve seat is generally made of elastic materials such as rubber or polytetrafluoroethylene; however, this also imposes temperature limitations on its use, which is why butterfly valves are traditionally considered unsuitable for high temperatures. 2. The single-eccentric butterfly valve was developed to address the problem of compression between the butterfly disc and the valve seat in concentric butterfly valves. Its structural feature is that the axis of the valve stem is offset from the center of the butterfly disc, thereby preventing the upper and lower ends of the butterfly disc from serving as the rotation axis and reducing the excessive compression between these ends and the valve seat. However, due to the fact that the scraping between the butterfly disc and the valve seat does not disappear throughout the entire opening and closing process of the valve in a single-eccentric design, and its application range is similar to that of concentric butterfly valves, it is not widely used. 3. The double-eccentric butterfly valve is the widely used type in use today, and it was developed as an further improvement on the single-eccentric butterfly valve. Its structural feature is that the axis of the valve stem is offset from both the center of the butterfly plate and the center of the body. The effect of double eccentricity allows the butterfly disc to quickly disengage from the valve seat once the valve is opened, significantly reducing unnecessary excessive compression and scraping between the butterfly disc and the valve seat, decreasing the opening force, minimizing wear, and extending the lifespan of the valve seat. Scratching is significantly reduced, and this also allows dual-eccentric butterfly valves to use metal seat assemblies, thereby enhancing the applicability of butterfly valves in high-temperature environments. However, because its sealing principle relies on a positional sealing mechanism – that is, the sealing surfaces of the butterfly plate and the valve seat are in line contact, and sealing is achieved through the elastic deformation of the valve seat caused by the pressure exerted by the butterfly plate – high requirements are placed on its closed position (especially for metal valve seats), and its pressure resistance is low. This is why it is traditionally believed that butterfly valves cannot withstand high pressures and suffer from high leakage rates. 4. Tri-eccentric butterfly valves must be able to withstand high temperatures, require hard sealing, but have high leakage rates ； Zero leakage is required, soft seals must be used, but they cannot withstand high temperatures. To overcome the contradiction of the double-eccentric butterfly valve, a third eccentricity was applied to the butterfly valve. Its structural feature is that, in addition to the double eccentricity of the valve stem axis, the conical axis of the butterfly valve’s sealing surface is also tilted relative to the cylindrical axis of the body; in other words, after the third level of eccentricity, the sealing cross-section of the butterfly valve is no longer circular but elliptical, and as a result its sealing surface becomes asymmetric – one side being inclined relative to the body’s centerline while the other side remains parallel to it. 　　The characteristic of the triple-eccentric butterfly valve is that it fundamentally changes the sealing mechanism: instead of relying on positional sealing, it uses torque sealing. In other words, sealing is achieved not through the elastic deformation of the valve seat, but through the pressure at the contact surface between the valve components. This approach solves the problem of zero leakage in metal valve seats. Moreover, since the contact surface pressure is proportional to the medium pressure, high pressure and high temperature conditions are also easily handled.