Differences in the use of various butterfly valves

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Differences in the use of various butterfly valves: Butterfly valves can be classified into center-line butterfly valves, single-eccentric butterfly valves, double-eccentric butterfly valves, triple-eccentric butterfly valves, etc., based on their structural design. The differences among them are as follows: 1. Center-line butterfly valve (concentric butterfly valve): The structural feature of a center-line 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 under compression and scraping, resulting in high 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, the sealing materials are limited by temperature in terms of their usability, which is why butterfly valves are traditionally considered unsuitable for high-temperature environments. II. Single-eccentric butterfly valve: The structural feature of the single-eccentric butterfly valve is that the axis of the valve stem is offset from the center of the butterfly plate, as a result of which the upper and lower ends of the butterfly plate are no longer the rotation axes. This reduces the excessive pressure exerted on these ends by the valve seat, thereby solving the problem of compression between the butterfly plate and the valve seat that occurs in concentric butterfly valves. 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. III. Double-eccentric butterfly valve: The double-eccentric butterfly valve is an improved version of the single-eccentric butterfly valve, and it is the most widely used type. 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 separate 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 seats, thereby enhancing the applicability of butterfly valves in high-temperature environments. However, since 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 resulting from the pressure exerted by the butterfly plate on the valve seat – 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 handle high pressures and suffer from high leakage rates. IV. Three-eccentric butterfly valves: They need to be able to withstand high temperatures, thus require hard-sealing, but they have high leakage rates ； To achieve zero leakage, 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 cylinder axis of the valve 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 center line of the valve body while the other side remains parallel to it. The most significant feature of this third type of eccentric design is that it fundamentally changes the sealing mechanism: it is no longer a position-based seal but rather a torque-based seal. In other words, sealing is achieved not through the elastic deformation of the valve seat, but entirely through the pressure on its contact surface. 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 can also be handled with ease.