• Structure of Triple Eccentric Metal Seated Butterfly Valves

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Structure of Triple Eccentric Metal Seated Butterfly Valves

  • Jan 27, 2023
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With the development of the national economy, the demand for large-diameter valves has increased, and the requirements for pressure, temperature, sealing, corrosion resistance, and reliability have also become higher and higher. In recent years, triple eccentric metal seated butterfly valves have shown irreplaceable advantages in meeting customer requirements. They have light weight, compact structure, quick opening and closing, good sealing, high-temperature resistant sealing materials, and long service life. Therefore, they are widely used in petroleum, chemical industry, metallurgy, hydropower and other fields.
 
Structural features of triple eccentric metal seated butterfly valves
1. The so-called triple eccentricity means that an axial eccentricity (the dimension is a) is set in the rotation axis of the disc plate and the sealing surface of the plate, and a radial eccentricity (the dimension is b) is set in the valve body channel axis; an angular eccentricity is formed at the axis of rotation of valve seats and the axis of the passage of the valve body.
 
2. An angle B is formed between the rotation axis of the valve seat and the channel axis of the valve body based on the application of double eccentricity. If the butterfly valve with an eccentric seal is fully opened, it will be separated from the sealing surface of the valve seat. A gap c is formed between the sealing surface of the plate and the valve seat. For the offset B, a track tangent line will be formed in the large half circle and small semicircle of the rotating plate with a long radius and short radius, and form different angles with the sealing surface of the valve seat; when the butterfly valve is in different opening or closing state, compared with the sealing surface of the valve seat, the sealing surface of the plate is gradually separated or compressed to avoid mechanical friction or damage caused by frequent opening or closing of the butterfly valve.
 
The triple eccentric butterfly valve is the same as the single eccentric and double eccentric butterfly valves. The opening and closing of the triple eccentric butterfly valve are realized between 0 and 90 degrees. Because of the angle in the triple eccentricity, the sealing surface of the plate is immediately separated from the sealing surface of the valve seat at the moment of opening, and only at the moment of opening and closing, the sealing surface of the plate will contact and press the sealing surface of the valve seat. At the same time, the sealing specific pressure between the two sealing surfaces of the sealing pair is mainly generated by the external driving torque of the valve stem, eliminating the sealing specific pressure drop between the two sealing surfaces of the sealing pair caused by factors such as elastic valve seat, aging of elastic material aging, cold flow, and elastic failure in common eccentric butterfly valves. The sealing specific pressure can be adjusted arbitrarily by applying an external driving force, thereby improving the sealing of the butterfly valve and greatly increasing the service life.
 
The designer chooses this type of sealing structure for the sealing of the triple eccentric metal seated butterfly valve through the research of the working conditions in the piping system. When the triple eccentric butterfly valve is in a fully closed state in the pipeline system, the butterfly plate is close to the valve seat under the pressure of the flow direction of the pipeline medium (the most favorable direction for valve design) due to the angle B and offset b in the triple eccentricity. At the same time, the laminated metal sheet and graphite on the plate will be closely attached to the valve seat, and the laminated steel sheet can withstand high pressure to ensure the working pressure of the pipeline system. High-temperature resistant materials such as interlayer graphite are in close contact with the valve seat under the specific pressure of the pipeline, and their micro-deformation compensates for machining accuracy and sealing surface wear, ensuring the sealing in the system. When the pressure continues to increase, the pressure difference between the two sealing pairs will also continue to increase. The tighter the two contact sealing surfaces are, the better the sealing effect will become. This sealing form has excellent sealing properties such as high-temperature resistance, high-pressure resistance, corrosion resistance and wear resistance in the working pipeline system.
 
Problems in use
1. The front side of the medium scouring the sealing surface
The laminated sealing surface is made of soft and hard laminated sealing ring stainless steel plate and high-temperature graphite. The sealing ring is fixed on the butterfly plate.
 
When the plate is in the normally open state, the medium in the piping system forms a frontal scouring on its sealing surface. The sealing form of the triple eccentric butterfly valve is based on the micro-deformation of the metal stainless steel plate and the high-temperature graphite soft laminate for processing accuracy and wear compensation to complete the sealing. Therefore, when the valve is in the normally open state, the stainless steel plate and high-temperature graphite on the laminated sealing ring are directly scoured by the medium in the pipeline system, which directly affects the sealing between the sealing pairs, which will lead to leakages of valves.
 
2. Low reverse pressure
The general metal seated butterfly valve has a double eccentric or triple eccentric structure. The geometric shape of the plate seal is often conical or spherical. The elasticity between the sealing pair is produced by the radial compression of the plate and the thrust of the positive medium to produce a block effect. Make a flexible contact between the sealing pairs to achieve positive sealing, but the reverse effect is poor. Therefore, it can only be used in pipelines that require one-way sealing.