How to Select Concentric Butterfly Valve, Double Offset Butterfly Valve and Triple Offset Butterfly Valve?

Butterfly valves are partial rotary valves, which can be used to cut off the medium, and can also be used to adjust the pressure, temperature and flow of the medium. Butterfly valves are widely used in all walks of life, especially in the fields of water systems and gas systems, because of their uncomplicated structure, relatively small valve weight, and easy installation. Butterfly valves can be divided into four categories only according to the structure, namely centerline butterfly valve, single offset butterfly valve, double offset butterfly valve and triple offset butterfly valve.

For centerline butterfly valves, the shaft center of the valve stem and the center of the butterfly plate must be in the same plane, and at the same time must be perpendicular to the centerline of the valve flow channel, and the areas on both sides of the valve core are symmetrical to the axis of the valve stem. Centerline butterfly valves are generally made of soft seals. Due to their simple structure, the center-symmetric two-way sealing effect is the same, and the flow resistance is small, and the opening and closing torque is small. It is widely used in medium and small caliber butterfly valves. However, because the shaft head is often in a state of friction, it wears faster than other parts, and it is easy to leak here. Therefore, in fluorine-lined butterfly valves or rubber-lined butterfly valves, sometimes the shaft head is often lined with PTFE to reduce friction or add springs to compensate for wear. Wait. In addition, in fact, the centerline butterfly valve is difficult to seal, so metal-to-metal valve seals are rarely used. Generally, eccentric butterfly valves or offset butterfly valves are used for metal-to-metal sealing. Compared with the centerline butterfly valve, the shaft head friction loss of the eccentric butterfly valve is smaller, but the flow resistance and sealing torque will increase.

For single eccentric butterfly valve, the valve stem of single offset butterfly valve deviates from the center line of the butterfly valve, resulting in an eccentricity h between the two, and the rotating shaft of the butterfly plate no longer becomes the center of the rotating shaft, eliminating the friction and wear at the upper and lower shaft ends leakage. The single eccentric butterfly valve has a better sealing effect than the centerline butterfly valve, but the sealing effect in both directions is asymmetrical. Generally, when the medium flows from the valve shaft to the sealing surface, the sealing effect is better; on the contrary, the medium flows directly to the butterfly valve. If the plate is sealed, the sealing effect of the sealing surface will be weakened, resulting in an increased chance of leakage. However, with the continuous development of valve technology, it is found that the use of rubber soft seals can achieve a good two-way sealing effect by utilizing the elastic properties of rubber. Similarly, single eccentric butterfly valves rarely use metal-to-metal hard seals, because when the butterfly plate is turned, the sealing surface will interfere with the opening and closing of the valve, which is not conducive to opening and closing.

Double eccentric butterfly valve, the birth of double offset butterfly valve, the main purpose is to further improve the performance of single eccentric butterfly valve, offset the valve shaft by a distance e, so that the connection line between the axis line and the sealing point and the sealing surface are greater than 90°, In this way, when the valve is sealed, it can avoid mutual interference, and it will become tighter and tighter as the pressure increases, resulting in greater sealing surface pressing force. The characteristic of double eccentricity is that when the valve is opened, the sealing surface of the butterfly plate can be separated from the valve seat quickly, and the contact scraping effect is greatly reduced. Of course, it can also be made into a metal-to-metal sealing butterfly valve. The double eccentric butterfly valve can be designed in two forms: polymer material soft seal and metal material hard seal.

Triple eccentric butterfly valve, triple offset butterfly valve rotates the positive cone angle by an angle and changes it to an oblique cone angle, so that the eccentricity e can be reduced, and the torque to rotate and open the valve will also be reduced accordingly. Of course, this is just an intuitive understanding. In reality, where should the axis be set or should three-dimensional motion analysis be used to judge 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-layer type, but also other types of sealing types, and sometimes polymer non-metallic soft sealing materials can also be used. The final solution can be determined according to the process requirements. The purpose of designing the sealing of the triple offset butterfly valve is to better reduce the excessive opening torque of the valve, so that the operator can open and close the valve more conveniently, and at the same time, from a certain point of view, it can also prolong the service life of the valve stem.

The selection of the butterfly valve needs to be selected according to the actual process, generally divided into two major directions, the first direction is the centerline butterfly valve, and the second direction is the triple eccentric butterfly valve. If the pressure is not high, you can choose a centerline butterfly valve. If the pressure is high, it is recommended to choose a triple eccentric butterfly valve. In addition, it is recommended to choose a metal seal for high temperature. If the temperature is not higher than 150 ° C, you can choose a soft seal butterfly valve. The above is just a simple initial selection. If you want to make the best selection, you need to provide relevant parameters such as specific process media.

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