What is Triple Eccentric Butterfly Valve?

Butterfly valve is simple in structure, small in size and light in weight. It consists of only a few components and is easy to operate. Its structural principle is suitable for making large-diameter valves and has good flow control characteristics. Therefore, it is widely used in petrochemical, metallurgical steelmaking, water treatment, Electric power and other industries have been widely used. The currently widely used triple eccentric butterfly valve, the sealing section of its disc is no longer circular, but ellipse, and its sealing surface shape is asymmetric. Due to the innovative structural characteristics of the triple eccentric butterfly valve, on various process pipelines, whether it is an on-off valve or a regulating valve, as long as it is properly selected, it can be safely used and the cost is low.

1.Development of triple eccentric butterfly valve

The early butterfly valve could not withstand high temperature and high pressure and had large leakage, so its application range was greatly limited. In order to meet the requirements of various working conditions, the butterfly valve has experienced the evolution from concentric to single eccentric, double eccentric and triple eccentric.

The structural feature of the concentric butterfly valve is that the shaft center of the valve stem, the center of the disc and the center of the body are in the same position, which is simple in structure and convenient in manufacture. The disadvantage is that the disc and the valve seat are always in a state of extrusion and scraping, the resistance distance is large, and the wear is fast. In order to solve the extrusion problem between the disc and the valve seat of the concentric butterfly valve, the single eccentric butterfly valve is produced. Disperse and reduce the excessive extrusion of the upper and lower ends of the butterfly plate and the valve seat. However, since the single eccentric structure only reduces the scraping phenomenon between the disc and the valve seat to a certain extent during the entire opening and closing process of the valve, it is similar to the concentric butterfly valve in terms of application scope, so it is not used much.

On the basis of the single eccentric butterfly valve, the double eccentric butterfly valve, which is widely used at present, is further improved and formed. Its structural feature is that the shaft center of the valve stem is not only deviated from the center of the disc, but also deviated from the center of the valve body.

The effect of double eccentricity enables the disc to break away from the valve seat immediately after the valve is opened, which greatly eliminates unnecessary excessive extrusion and scratching between the disc and the valve seat, reduces the opening resistance, reduces wear, and prolongs the life of the valve. Seat life. The scraping is greatly reduced, and at the same time, the double eccentric butterfly valve can also use a metal valve seat, which improves the application of the butterfly valve in the high temperature field. However, because the sealing principle is a position sealing structure, that is, the sealing surface of the disc and the valve seat is in line contact, and the elastic deformation caused by the disc extrusion of the valve seat produces a sealing effect, so the closing position is very demanding (especially metal valve seat), the pressure bearing capacity is low, which is why traditionally it is believed that butterfly valves are not resistant to high pressure and have large leakage.

To withstand high temperature, a hard seal must be used, but with a large amount of leakage; to zero leakage, a soft seal must be used, but it is not resistant to high temperature. In order to overcome the contradiction of double eccentric butterfly valve, the butterfly valve was eccentric for the third time.As shown in Figure 1 .

Figure 1 Structure diagram of triple eccentric butterfly valve

As shown in Figure 1, its structural feature is that the shape of the valve plate is an elliptical cone. While the position of the double eccentric valve stem is eccentric, the conical axis of the sealing surface of the butterfly plate is deviated from the cylinder axis of the body. That is to say, the sealing section of the triple eccentric butterfly plate is no longer a circle, but an ellipse. The biggest feature of triple eccentricity is that it fundamentally changes the sealing structure. It is no longer a position seal, but a torsion seal, that is, it does not rely on the elastic deformation of the valve seat, but completely relies on the contact surface pressure of the valve seat to achieve the sealing effect. as shown in picture 2 .

Figure 2 Schematic diagram of sealing shape of triple eccentric butterfly valve

2.Features of triple eccentric butterfly valve

（1）Available in on off type and regulating type

The design of the new triple eccentric butterfly valve adopts a double safety structure. In order to prevent the deformation of the disc, the dislocation of the valve stem, and the occlusion of the sealing surface caused by the influence of fluid pressure and temperature, two independent stoppers are installed on the upper and lower sides of the disc. The push ring ensures the normal operation of the valve under any working conditions; in order to prevent the valve stem from being damaged and flying out and causing unexpected accidents, independent valve stem flying-out prevention mechanisms are designed at the lower end of the valve inside and outside.

（2）No dead zone design, small opening torque.

The shape of the valve plate of the triple eccentric butterfly valve is an elliptical cone, and its surface is surfacing with hard alloy, which has excellent wear resistance. The floating U-shaped stainless steel valve seat has the function of automatically adjusting the center. When the valve is opened, the valve plate of the elliptical cone sealing surface first separates from the U-shaped elastic valve seat, and then rotates; when the valve is closed, the valve plate rotates, and the valve plate automatically adjusts the center to the elastic valve seat under the action of the eccentric shaft. Seat pressure deforms the seat until the seat is in close contact with the oval conical sealing surface of the valve plate. When the valve is switched on and off, the disc does not scratch the valve seat, the torque of the valve stem is directly transmitted to the sealing surface through the disc, and the opening torque is small, thus eliminating the common jumping phenomenon when opening the valve, that is, eradicating the dead zone (no sense band) . This design without dead zone enables the triple eccentric butterfly valve to enter the controllable area almost from 0 opening to 90 opening, and its control ratio can be as high as 100:1 or more, which is twice that of ordinary butterfly valves.

（3）No leakage, high temperature and high pressure resistance.

  The biggest feature of triple eccentricity is that it fundamentally changes the sealing structure. It is no longer a position seal, but a torsion seal, that is, it does not rely on the elastic deformation of the valve seat, but completely relies on the contact surface pressure of the valve seat to achieve the sealing effect. This sealing structure enables the triple eccentric butterfly valve to achieve a hard seal of Class VI, achieving true zero leakage. At the same time, due to the selection of metal valve seats and the structural characteristics of the face seal, the problem of high temperature and high pressure resistance is also easily solved.

In addition, the triple eccentric butterfly valve adopts the body seat structure, which makes the valve structure more compact, and reduces the chance of the valve seat directly contacting the medium, reduces the degree of erosion of the valve seat, and can effectively prevent small solids in the medium. The impact of the material and thermal expansion may cause the sealing surface to bite, thereby extending the service life of the valve seat.

The new triple eccentric butterfly valve can not only replace the body seat, but also the disc sealing surface and disc are independent, the disc sealing surface can also be replaced, which greatly reduces the maintenance cost.

3.Application of triple eccentric valve in petrochemical plant

On large-diameter pipelines, the cost of using a shut-off valve is very high and it is impossible to achieve no leakage. In the case of emergency shutdown, a shut-off valve must be installed beside the shut-off valve. Although a V-shaped ball valve can also achieve shut-off (no leakage) and regulation, the cost is also much higher than that of a triple-eccentric butterfly valve. At present, butterfly valves above DN300 mm have gradually replaced gate valves. Compared with gate valves, butterfly valves have the characteristics of short opening and closing time, small operating torque, small installation space and light weight. Taking DN1 000 mm as an example, the butterfly valve is about 2 t, and the gate valve is about 3.5 t, and the butterfly valve is easy to combine with various driving devices, and has good durability and reliability. Because triple eccentric butterfly valve has significant advantages compared with globe valve, ball valve and gate valve, triple eccentric butterfly valve has been successfully applied in many occasions in petrochemical industry.

（1）The temperature regulation loop of the reactor cooling water of the polyethylene plant is the most important part of the plant. Whether the temperature regulation loop can run smoothly or not directly determines whether the entire polyethylene plant can run safely and smoothly. The temperature-adjusting water pipeline is of large diameter (above DN450 mm). Under normal working conditions, the triple-eccentric butterfly valve is used as a control valve because of its high control ratio, which is more stable and reliable than the ordinary control valve. When the interlock occurs, the triple-eccentric butterfly valve Butterfly valve can also play the role of cut off, and completely cut off (zero leakage). Judging from the actual application situation, it not only saves the investment cost, but also the control is particularly stable.

（2）The coke cleaning valve at the outlet of the cracking furnace of the ethylene unit is a kind of valve with harsh operating conditions in the petrochemical industry. Carbon particles in the process medium may accumulate on the sealing surface of the valve, causing the sealing surface to bite and the valve cannot be normally opened and closed. , and even shorten the service life of the valve. Compared with other types of valves, triple eccentric butterfly valve is the most ideal choice because of its unique structural characteristics.

（3）Pipeline cutting and adjustment, due to the large diameter of the process pipeline, the process medium is high temperature flue gas (up to 750 ℃), the valve used mainly adopts triple eccentric butterfly valve, the maximum valve size of domestic installations has reached DN1 800 mm, equipped with hydraulic actuator to close The time is less than 0.5 s; other such as the inlet and outlet pipes of the main fan and the vent valve, the inlet and outlet pipes of the air compressor, the vent valve to the torch, the inlet and outlet of the oil slurry pump, and the adjustment and cutting of various high, medium and low pressure steam pipelines are also widely used. Tri-eccentric butterfly valves are also widely used. . Liquefied petroleum gas is very easy to leak. To meet the large flow capacity and avoid leakage, it is more reliable to use a triple eccentric butterfly valve on the main line of the inlet and outlet of the storage tank. In addition, in the sulfur recovery device, the liquid sulfur that needs to be transported in a molten state is currently a triple-eccentric butterfly valve with a steam jacket.

4.Conclusion

Through the evolution process and structural characteristics of the above butterfly valve, it can be seen that the triple eccentric butterfly valve, as the crystallization of the latest valve technology, promotes the advantages of various valves and avoids the shortcomings of various valves, and has been valued and widely used by users and designers.