The early butterfly valves could not withstand high temperature and high pressure and had a large amount of leakage, which greatly limited their application range. In order to meet the requirements of various working conditions, the butterfly valve has undergone the evolution from concentric to single eccentricity, double eccentricity and triple eccentricity.

The structural characteristics of the concentric butterfly valve are that the shaft of the valve stem, the center of the butterfly plate and the center of the body are in the same position, which is simple in structure and convenient to manufacture. The disadvantage is that the butterfly plate and the valve seat are always in a state of extrusion and scraping, with a large blocking distance and fast wear. In order to solve the problem of the extrusion of the butterfly plate and the valve seat of the concentric butterfly valve, the single eccentric butterfly valve is produced, and its structural characteristics are that the shaft of the valve stem deviates from the center of the butterfly plate, so that the upper and lower ends of the butterfly plate no longer become the rotary axis, and the excessive extrusion of the upper and lower ends of the butterfly plate and the valve seat is dispersed and reduced. However, due to the single eccentric structure in the whole opening and closing process of the valve, it only reduces to a certain extent and does not eliminate the scratching phenomenon of the butterfly plate and the valve seat, and is similar to the concentric butterfly valve in the application range, so it is not used much.

On the basis of the single eccentric butterfly valve, the double eccentric butterfly valve, which is further improved and formed, is currently widely used. Its structural characteristics are that the stem axis deviates from both the center of the butterfly plate and the center of the valve body.

The effect of double eccentricity makes the butterfly plate quickly detach from the valve seat after the valve is opened, which greatly eliminates the unnecessary excessive squeezing and scratching between the butterfly plate and the valve seat, reduces the opening resistance, reduces wear and extends the life of the valve seat. The significant reduction of scratching also makes the double eccentric butterfly valve also available with metal seats, which improves the application of butterfly valves in high-temperature fields. However, because its sealing principle is a positional sealing structure, that is, the sealing surface of the butterfly plate and the valve seat is in linear contact, and the sealing effect is produced by the elastic deformation caused by the butterfly plate extruding the valve seat, so the closing position is very high (especially the metal seat), and the pressure bearing capacity is low, which is why people traditionally think that the butterfly valve is not resistant to high pressure and has a large leakage.

To withstand high temperatures, a hard seal must be used, but the leakage is large; To have zero leakage, it is necessary to use a soft seal, but it is not resistant to high temperatures. In order to overcome the contradiction of double eccentric butterfly valve, the third eccentricity of the butterfly valve was carried out.

As shown in Figure 1, the structural characteristics of the valve plate are elliptical cones, and the conical axis of the sealing surface of the butterfly plate is deviated from the cylindrical axis of the body while the axial position of the valve stem of the double eccentricity is eccentric. That is to say, the sealing section of the triple eccentric butterfly plate is no longer circular, but elliptical, and the shape of the sealing surface is asymmetrical, one side is inclined to the center line of the body, and the other side is parallel to the center line of the body. The biggest feature of the three eccentricities is that the sealing structure is fundamentally changed, no longer position sealing, but torsion sealing, 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 is shown in Figure 2

Features of eccentric butterfly valves

The triple eccentric butterfly valve is a rotary valve, and its three eccentricities are placed on the valve plate. In the process of valve fabrication, the outer edge of the valve plate adopts a partial outer spherical structure (see Fig. 2), the outer eccentric spherical surface is polished to achieve roughness R a 04, and the spherical surface is chrome-plated or surfacing Stellite alloy, and the hardness reaches HRC 45~60. The triple eccentric disc valve has a maximum pressure resistance of 26MPa, a temperature resistance as low as -196 °C and a temperature resistance of up to 700 °C, and a seal with zero leakage. It can be seen that it is precisely because of the innovative structural characteristics of the triple eccentric butterfly valve that it has the following main characteristics.

1. It integrates adjustment and cutting, and has strong controllability.

In order to prevent the deformation of the butterfly plate, the dislocation of the valve stem and the occlusion of the sealing surface caused by the influence of fluid pressure and temperature, two independent thrust rings are installed on the upper and lower sides of the butterfly plate, so as to ensure 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 sudden accidents, independent valve stem fly-out prevention mechanisms are designed at the inner and outer parts of the lower end of the valve.

2. No dead zone design, small opening torque.

The triple eccentric butterfly valve disc is shaped with an oval cone and its surface is welded with cemented carbide for excellent wear resistance, and the floating U-shaped stainless steel seat has the function of automatic centering. When the valve is opened, the oval cone seal face disc first detaches from the U-shaped elastic seat and then rotates; When closed, the disc rotates, and under the action of the eccentric shaft, the disc automatically adjusts the center to the elastic seat, and presses the seat to deform the seat until the seat and the oval cone sealing surface of the disc are tightly matched. When the valve is opened and closed, the butterfly plate does not scratch the valve seat, and the torque of the valve stem is directly transmitted to the sealing surface through the butterfly plate, and the opening torque is small, so as to eliminate the common jumping phenomenon when opening the valve, that is, to eradicate the dead zone (no induction band). This non-dead zone design allows the triple eccentric butterfly valve to enter the adjustable zone from almost 0 opening to 90 opening, and its control ratio can be up to more than 1001, which is twice that of a general butterfly valve.

3. Zero leakage, high temperature and high pressure resistance.

The biggest feature of the three eccentricities is that the sealing structure is fundamentally changed, no longer position sealing, but torsion sealing, 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 allows the triple eccentric butterfly valve to achieve a hard seal V I class and achieve true zero leakage. At the same time, due to the selection of metal seats and the structural characteristics of face seals, the problem of high temperature and high pressure resistance is also solved. In addition, the triple eccentric butterfly valve makes the valve structure more compact due to the body seat structure, and reduces the opportunity of the valve seat to directly contact the medium, reduces the degree of erosion of the valve seat, and can effectively prevent the influence of small solids in the medium and the sealing surface occlusion that may be caused by thermal expansion, thereby prolonging the service life of the valve seat. The new triple eccentric butterfly valve not only has the body valve seat replaced, but also the butterfly plate sealing surface is independent from the butterfly plate, and the butterfly plate sealing surface can also be replaced, so that the maintenance cost is greatly reduced.

Application of eccentric valve in petrochemical plant

In large-diameter pipelines, the cost of using shut-off valves is very high and zero leakage cannot be achieved, and in the case of emergency shut-off, it is necessary to install a shut-off valve next to the shut-off valve. Although V-Ball Valves can also be shut-off (zero leakage) and regulated, the cost is much higher than that of Triple Offset Butterfly Valves. At present, butterfly valves, which are generally 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 000mm as an example, the butterfly valve is about 2t and the gate valve is about 35t, and the butterfly valve is easy to combine with various driving devices, which has good durability and reliability. Because the triple eccentric butterfly valve has significant advantages compared with globe valves, ball valves, and gate valves, the triple eccentric butterfly valve has been successfully applied in many occasions in the petrochemical industry.

1. The eccentric butterfly valve combines regulation and shut-off (to achieve zero leakage), and has a successful typical application in polyethylene plants. The reactor cooling water temperature regulation circuit of the polyethylene plant is the most important component of the plant, and whether the temperature regulation loop can run smoothly directly determines whether the whole polyethylene plant can run safely and smoothly. The temperature control water pipeline is a large diameter (DN450 mm or more), under normal working conditions, the triple eccentric butterfly valve is used as a regulating valve because of its high control ratio, which is more stable and reliable than the ordinary regulating valve, and when the interlock occurs, the triple eccentric butterfly valve can also play a shut-off role, and completely cut off (zero leakage). From the perspective of practical application, it not only saves investment costs, but also controls it very stably.

2. The coke cleaning valve at the outlet of the cracking furnace of the ethylene plant is a kind of valve used in the petrochemical industry with harsh conditions, and the carbon particles in the process medium may accumulate on the sealing surface of the valve, causing the sealing surface to occlude and make the valve unable to open and close normally, or even shorten the service life of the valve. Compared with other types of valves, the triple eccentric butterfly valve is the most ideal choice due to its unique structural characteristics.

3. The cut-off and adjustment of the inlet pipeline of the flue gas turbine of the catalytic cracking unit of each refinery, due to the large diameter of the process pipeline, the process medium is high-temperature flue gas (up to 750 °C), the use of the valve mainly adopts the triple eccentric butterfly valve, the maximum valve size of the domestic device has reached DN1 800mm, and the closing time of the hydraulic actuator is less than 05 s; Others, such as the main fan inlet and outlet pipes and venting valves, air compressor inlet and outlet pipelines, de-flare venting valves, oil slurry pump inlets and outlets, and the adjustment and shut-off of various high, medium and low pressure steam pipelines are also widely used in triple eccentric butterfly valves. Liquefied petroleum gas is very prone to leakage, and it is more reliable to use a triple eccentric butterfly valve on the main line of its storage tank inlet and outlet to meet the large flow capacity and avoid leakage. In addition, in sulfur recovery units, the liquid sulfur that needs to be transported in a molten state is currently a triple offset butterfly valve with a steam jacket.

summary

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 technology of the valve, has been valued by the majority of users and designers and has been widely used.

(The article comes from the Internet)