What is the difference between three-eccentric butterfly valves and two-eccentric butterfly valves?

In a double-eccentric butterfly valve, the rotation center of the butterfly plate is offset by a certain distance from the sealing surface of the butterfly plate, and it is also offset by a certain distance from the axis of the valve body’s passage. Compared to centerline butterfly valves, this type of valve can disengage from the valve seat sealing surface more quickly during opening and closing; as a result, the relative mechanical wear between the butterfly disc and the valve seat sealing surface is reduced, and the angle of compression is smaller. This leads to less mechanical wear and compression, and thus its sealing performance and service life are improved compared to the previous two types of valves. Double-eccentric butterfly valves can use synthetic rubber and PTFE, or metals as sealing materials, which allows them to operate at higher temperatures. However, they require a precise closing position, which results in a relatively lower pressure resistance; as a result, they are less likely to leak under high-pressure conditions. Three-eccentric butterfly valve: The three-eccentric butterfly valve has an additional angular offset between the axis of rotation of the valve seat and the axis of the valve body channel, as compared to the double-eccentric butterfly valve. Its feature is that it comes into contact with and presses against the valve seat sealing surface only at the moment when the butterfly plate is opened or closed. It eliminates the reduction or disappearance of the sealing pressure in conventional eccentric butterfly valves, which is caused by factors such as aging of the elastic material in the valve seat, cold flow, and elastic failure. Moreover, it allows for arbitrary adjustment of the sealing pressure by applying an external torque, thereby improving its sealing performance and **extending its service life. Such valves often use metal as the sealing material.

I’ve learned it; it would be more intuitive if there were pictures.

I. Centered butterfly valve: The structural feature of this type of butterfly valve is that the axis of the valve stem, the center of the butterfly disc, 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 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. II. Single-eccentric butterfly valve: To address the problem of compression between the butterfly disc and the valve seat in concentric butterfly valves, the single-eccentric butterfly valve was developed. 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; this reduces the excessive compression between those ends and the valve seat. However, due to the fact that the scraping between the butterfly disc and the valve seat does not cease 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, which is further improved upon the basis of the single-eccentric butterfly valve, is the most widely used type in use today. 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 enables 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 double-eccentric butterfly valves to use metal seat inserts, thereby expanding the application 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 handle high pressures and suffer from high leakage rates. IV. Three-eccentric butterfly valves: They need to be able to withstand high temperatures, require hard sealing, but 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 cylindrical 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 by relying on the elastic deformation of the valve seat, but entirely 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 can also be handled with ease.

What was described above is the application in high-temperature conditions; how is it in low-temperature/cryogenic conditions?

If you want to know about the structures of various butterfly valves and the differences in their applications, you can send me a WeChat message or an email at manager@tomoe-valve.com; I’m happy to help