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How to select the triple eccentric butterfly valve and double eccentric butterfly valve?

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The sealing pair of the sealed butterfly valve has a metal-to-metal hard seal and a metal-to-rubber or plastic soft seal. The sealing ring can be placed on the butterfly plate or on the valve body. According to the position of the butterfly plate in the valve, butterfly valves are divided into centerline butterfly valves, single eccentric butterfly valves, double eccentric butterfly valves, and triple eccentric butterfly valves. This article will briefly introduce their structural characteristics and applications.

Table of Contents

Centerline butterfly valve

Structural features: The shaft center of the valve stem, the center of the butterfly plate, and the center of the body are in the same position.
The centerline butterfly valve (pictured below) is in the same plane as the center plane of the butterfly plate and perpendicularly intersects the centerline of the valve body pipe, and the area on both sides of the butterfly plate is symmetrical to the axis of the valve stem. The centerline butterfly valve is generally made of rubber-lined form. Because of its simple structure, the center symmetrical (I type) two-way sealing effect is the same, and the flow resistance is small, the switching torque is also small, so it is widely used in medium and small butterfly valves. However, because the shaft head is often in a friction state, it wears faster than other parts and is easy to leak here. Therefore, the rubber-lined butterfly valve is sometimes lined with a PTFE film to reduce friction or increase springs to compensate for wear. See Figure 2. Obviously, if the centerline type is made of metal-to-metal, it is difficult to seal. The inclined plate and the offset plate butterfly valve shaft head have no friction, but their flow resistance and sealing torque are larger than the center symmetric butterfly plate.


in conclusion:
Advantages of the centerline butterfly valve: simple structure and convenient manufacturing. Common plastic-lined butterfly valves belong to this category;
Disadvantages: Because the butterfly plate and the valve seat are always in the state of squeezing and scraping, the resistance torque is large and the friction is fast. Temperature limitation of working conditions (silicone rubber: -60~200℃, PTFE: -30~180℃, PEEK: -46~280℃).
The centerline butterfly valve produced by our factory has a caliber range of DN50-DN1200 and a nominal pressure ≤PN16. It is suitable for control of normal temperature, high temperature, and low-pressure air or other low-pressure gases.  

Single eccentric butterfly valve

Structural features: the axis of the valve stem deviates from the center of the butterfly plate
The single eccentric butterfly valve (left in the figure below) has an eccentric h when the valve shaft deviates from the center plane of the butterfly plate, so that the upper and lower ends of the butterfly plate against the valve shaft no longer become the rotation axis, eliminating the friction and wear of the upper and lower shaft ends and causing leakage.
The single eccentric butterfly valve has a better sealing effect, but the sealing effect in the two directions is inconsistent. Generally, it is easy to seal in the forward direction (flow from the valve shaft to the sealing surface), and the reverse direction is easy to leak because there is no supporting effect of the sealing surface. However, the rubber soft seal is adopted, and the two-way seal can be well realized by using its elasticity. Because its sealing contact mechanism belongs to “slope sealing”; therefore, there are always two points of the butterfly plate that cannot be out of contact at each opening. When used for adjustment, these two points will wear more than other parts, which will be unfavorable for sealing. influences. If it is made of a metal hard seal, and the two positive cones are matched, the sealing surface will interfere when the butterfly plate is rotated, and it will be difficult to switch.

in conclusion:
Advantages of a single eccentric butterfly valve: Dispersion reduces the excessive extrusion between the upper end of the butterfly plate and the valve seat;
Single eccentric butterfly valve Disadvantages: Due to the single eccentric structure during the entire opening and closing process of the valve, the scratching phenomenon between the butterfly plate and the valve seat has not disappeared. It is not much different from the center butterfly valve in the application range, so it is not used much.
The single eccentric butterfly valve produced by our factory has a caliber range of DN650-DN2700.

Double eccentric butterfly valve

Structural features: the shaft center of the valve stem deviates from the center of the butterfly plate and the center of the body
Double eccentric butterfly valve (above right), in order to improve the situation of single eccentricity, the valve shaft is offset by a distance e, so that the connecting line between the shaft center and the sealing point and the sealing surface is at an obtuse angle (greater than 90) so that the sealing is not Interference occurs, and the more closed the tighter, the greater the sealing surface pressure. The feature of double eccentricity is that when the valve is opened, the sealing surface of the butterfly plate can be quickly separated from the valve seat, and the contact and scraping effect is greatly reduced. At the same time, it can be made into a metal sealing valve.
The double eccentric butterfly valve can be designed in the form of rubber, PTFE, and metal hard seals. For example, C-shaped, O-shaped, or U-shaped are in the form of a sealing ring.
If you want to make a cone-to-cone metal hard seal without interference, sometimes the secondary eccentricity needs to be large, resulting in a large eccentric torque, so that the valve opening torque is too large. In order to reduce the eccentricity, it is introduced Triple eccentric butterfly valve.

in conclusion:
Advantages of double eccentric butterfly valve: The effect of double eccentricity enables the butterfly plate to be quickly separated from the valve seat after the valve is opened, which greatly eliminates unnecessary excessive squeezing and scratching of the butterfly plate and the valve seat, reducing wear and improving Valve life; the scraping is greatly reduced, and the double eccentric butterfly valve can also use the metal seat, which improves the application of the butterfly valve in the high-temperature field.
Disadvantages of a double eccentric butterfly valve: The double eccentric valve produces a sealing effect through the elastic deformation of the valve seat caused by the butterfly plate squeezing. It is a line seal, so it is used in occasions where the closing position is not high and the pressure bearing capacity is low.
The double eccentric butterfly valve produced by our factory has a caliber range of DN50-DN4600 and a pressure rating of ≤PN40.

Triple eccentric butterfly valve

Structural features: The shaft center of the valve stem deviates from the center of the butterfly plate and the center of the body; the conical axis of the sealing surface of the butterfly plate deviates from the cylinder axis of the body, which is a face seal.
The triple eccentric butterfly valve (above) is to rotate the positive cone angle by one angle to an oblique cone angle so that the eccentricity e can be reduced and the opening torque is also reduced. Of course, this is only an intuitive understanding, where should the actual axis be set, or should be used for three-dimensional motion analysis to determine whether the sealing pair will interfere. It is worth pointing out that the sealing ring of the triple eccentric butterfly valve can not only be designed as a multi-layered type but also can be made as a U-shaped or O-shaped ring-like Neles. In some cases, non-metallic materials such as rubber and PTFE can be used, but non-metallic materials can be used. It is questionable whether it is necessary to make the elastic sealing material into triple eccentricity (double eccentricity is sufficient).

in conclusion:
Advantages of triple eccentric butterfly valve: The sealing structure is fundamentally changed. It is no longer a positional seal, but a torsion seal, that is, it does not rely on the elastic deformation of the valve seat, but completely depends on the contact surface of the valve seat to achieve the sealing effect, so it is solved in one fell swoop The problem of zero leakage of the metal valve seat is solved, and because the contact surface pressure is proportional to the medium pressure, the high temperature and high-pressure resistance are also easily solved.
Disadvantages of triple eccentric butterfly valve: None.
The triple eccentric butterfly valve produced by our factory has a caliber range of DN100-DN5000 and a pressure rating of ≤PN63.

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