Overview, classification, sealing, testing, installation, and common faults of butterfly valves
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Overview of butterfly valves: The history of development of butterfly valves. Butterfly valves were invented in the United States in the 1930s; they were introduced to Japan in the 1950s, and it was not until the 1960s that they became widely used there. In China, their adoption started after the 1970s. Currently, in the world, butterfly valves with a diameter of DN300 millimeters or more are gradually replacing gate valves. Compared to gate valves, butterfly valves have a shorter opening and closing time, require less force for operation, need less installation space, and are lighter in weight. Taking DN1000 as an example, the butterfly valve weighs about 2 tons, while the gate valve weighs about 3.5 tons. Moreover, the butterfly valve can be easily combined with various driving devices, and it boasts good durability and versatility. The disadvantage of rubber-sealed butterfly valves is that when used for throttling, improper use can lead to cavitation, causing the rubber seat to peel off or get damaged. To this end, metal-sealed butterfly valves have been developed internationally, reducing the cavitation area; in recent years, China has also developed metal-sealed butterfly valves. In Japan, comb-toothed butterfly valves that are resistant to cavitation and feature low vibration and low noise have been developed in recent years. ? Under normal conditions, the service life of a standard sealing seat is 15 to 20 years for rubber, and 80 to 90 years for metal. However, the proper selection depends on the operational requirements. ? The relationship between the opening degree of the butterfly valve and the flow rate is essentially linear. If used for flow control, its flow characteristics are closely related to the flow resistance of the piping as well. For example, if two pipes have identical valve sizes and types, but different pipe loss coefficients, the flow rates through the valves will also differ significantly. ? If the valve is in a state with a large throttling range, cavitation is likely to occur on the back side of the valve disc, which may cause damage to the valve; it is generally used at angles other than 15°. ? When the butterfly valve is at a medium opening position, the shape of the opening formed between the valve body and the front end of the butterfly plate is centered around the valve shaft, with different conditions existing on each side. The front end of the butterfly plate on one side moves in the direction of water flow, while it moves against the direction of water flow on the other side. As a result, one side forms an opening similar to a nozzle, while the other side forms an opening similar to a throttle orifice. The flow velocity is much higher on the nozzle side than on the throttle side, and negative pressure is generated below the valve on the throttle side, which often leads to the detachment of the rubber seals. ? The operating torque of a butterfly valve varies depending on the degree of opening and the direction in which the valve is opened or closed. For horizontal butterfly valves, especially those with large diameters, the torque generated by the difference in water pressure above and below the valve shaft due to the depth of water cannot be ignored either. Additionally, when a elbow is installed on the inlet side of the valve, flow deviation occurs, resulting in an increase in torque. When the valve is at an intermediate opening, the operating mechanism needs to self-lock due to the hydrodynamic torque. A butterfly valve is a type of valve that uses a circular butterfly disc as the closing element, which rotates along with the valve stem to open, close, and regulate the flow of fluid. The disc of a butterfly valve is installed in the diameter direction of the pipeline. Within the cylindrical passage of the butterfly valve body, the disc-shaped butterfly plate rotates around its axis by an angle ranging from 0° to 90°; when it reaches 90°, the valve is in its fully open position. Butterfly valves have a simple structure, consisting of only a few components, which results in reduced material usage ； Small in size and light in weight, with compact installation dimensions; it features low driving torque, is easy and quick to operate – just a 90° rotation is sufficient to open or close it rapidly ； It also possesses excellent flow regulation capabilities and sealing properties; in applications involving large to medium diameters and medium to low pressures, the butterfly valve is the dominant type of valve. When the butterfly valve is in its fully open position, the thickness of the butterfly disc is the only resistance to the flow of the medium through the valve body; as a result, the pressure drop across this valve is very small, giving it excellent flow control properties. Butterfly valves come in two types of sealing: elastic sealing and metal sealing. Elastic-sealing valves, where the sealing ring can be embedded in the valve body or attached around the butterfly disc. Valves with metal seals generally have a longer lifespan than those with elastic seals, but it is difficult to achieve a complete seal. Metal seals can withstand higher operating temperatures, while elastic seals have the drawback of being limited by temperature. If a butterfly valve is to be used for flow control, the key is to select the correct size and type of valve. The structural principle of butterfly valves makes them particularly suitable for manufacturing large-diameter valves. Butterfly valves are widely used not only in general industries such as petroleum, gas, chemicals, and water treatment, but also in the cooling water systems of thermal power plants. The commonly used butterfly valves are wafer-type butterfly valves and flanged butterfly valves. A wafer butterfly valve is connected between two pipe flanges using bolts, while a flanged butterfly valve has flanges on it, with the flanges at both ends of the valve being secured to the pipe flanges using bolts. The strength performance of a valve refers to its ability to withstand the pressure of the medium. Valves are mechanical components that are subjected to internal pressure; therefore, they must possess sufficient strength and stiffness to ensure they do not crack or deform over time. With the use of anticorrosive synthetic rubber and polytetrafluoroethylene, the performance of butterfly valves has been improved, enabling them to meet various operating conditions. Over the past decade or so, metal-sealed butterfly valves have seen rapid development. With the use of alloy materials that are resistant to high and low temperatures, severe corrosion, and intense erosion, these valves can be used in conditions involving high temperatures, low temperatures, and intense erosion. As a result, they have been widely employed in such applications and have partially replaced globe valves, gate valves, and ball valves. Structural features of butterfly valves: Butterfly valves feature a simple structure, small size, light weight, reduced material consumption, compact installation dimensions, rapid opening and closing, 90° reciprocating rotation, and low driving torque. They are used to shut off, connect, or regulate the flow of media in pipelines, offering excellent fluid control capabilities and good sealing performance when closed. The streamlined design of the butterfly valve results in low fluid resistance losses, making it an energy-saving product. The valve stem features a through-shaft design and has been heat-treated, granting it excellent comprehensive mechanical properties as well as resistance to corrosion and abrasion. When a butterfly valve is opened or closed, the valve stem only moves in a rotational motion and does not move up or down; as a result, the packing on the valve stem is not easily damaged, ensuring reliable sealing. It is fixed to the butterfly plate via a taper pin, and its extended end is designed to prevent extrusion, so as to avoid the valve stem breaking free in case of an accidental breakage at the connection between the valve stem and the butterfly plate. The connection methods include flange connection, clamp connection, butt welding connection, and lug clamp connection. The drive types include manual, worm gear transmission, electric, pneumatic, hydraulic, electro-hydraulic actuators, etc., enabling remote control and automated operation. Application scenarios of butterfly valves: Due to the wiping action of the valve disc in butterfly valves, most of them can be used with media containing suspended solid particles; depending on the strength of the sealing elements, they can also be used with powdered and granular media. Butterfly valves have a small structural length and overall height, operate quickly when opened and closed, present low fluid resistance when fully open, and allow for precise flow control when opened at angles between approximately 15° and 70°. Their structural principles make them particularly suitable for manufacturing valves of large diameter. Butterfly valves are recommended for the following operating conditions: ① When throttling and regulating flow rate is required ； ② Mud media and solid-particle-containing media ； ③ Applications requiring a short valve structure length ； ④ Applications requiring fast opening and closing speeds ； ⑤ Applications with low pressure differences. Butterfly valves can be chosen in applications involving two-position control, narrowed channels, low noise, cavitation and vaporization phenomena, slight leakage to the atmosphere, and abrasive media. In situations with special operating conditions where throttling regulation is required, strict sealing is necessary, or where there is severe wear or low-temperature cryogenic environments, special butterfly valves with a three-eccentric or double-eccentric design and a specially designed metal seal strip for adjustment are needed. Soft-sealed butterfly valves are suitable for two-way opening, closing, and regulation in ventilation and dust removal pipelines, as well as in gas pipes and water conduits in metallurgy, light industry, power generation, and petrochemical systems. The metal-to-metal wire-sealed double-eccentric butterfly valve is suitable for use in pipelines for heating, steam supply, water supply, as well as gas, oils, acids, and alkalis, serving as a regulating and shut-off device. The metal-to-metal face-sealed triple-eccentric butterfly valve can be used not only as large-scale pressure swing adsorption gas separation control valves, but also in various fields such as petroleum, chemicals, metallurgy, power generation, food processing, pharmaceuticals, water supply and drainage, and gas transportation.
I. Concentric butterfly valve: The structural feature of this type of butterfly valve is that the axis of the valve stem, the center of the butterfly disc, and the center of the valve body are all in the same position. It has a simple structure and is easy to manufacture. Common rubber-lined butterfly valves fall into this category. The disadvantage is that the butterfly plate and the valve seat are constantly under compression and scraping, resulting in a large flow resistance and rapid wear. To overcome compression and scraping and ensure sealing performance, the valve seat is generally made of elastic materials such as rubber or polytetrafluoroethylene; however, this also imposes temperature limitations on its use, which is why butterfly valves are traditionally considered unsuitable for high temperatures. II. Single-eccentric butterfly valve: To address the problem of compression between the butterfly disc and the valve seat in concentric butterfly valves, the single-eccentric butterfly valve was developed. Its structural feature is that the axis of the valve stem is offset from the center of the butterfly disc, thereby preventing the upper and lower ends of the butterfly disc from serving as the rotation axis; this reduces the excessive compression between those ends and the valve seat. However, due to the fact that the scraping between the butterfly disc and the valve seat does not disappear throughout the entire opening and closing process of the valve in a single-eccentric design, and its application range is similar to that of concentric butterfly valves, it is not widely used. III. Double-eccentric butterfly valve: The double-eccentric butterfly valve, which is further improved upon the basis of the single-eccentric butterfly valve, is the most widely used type in use today. Its structural feature is that the axis of the valve stem is offset from both the center of the butterfly plate and the center of the body. The effect of dual eccentricity allows the butterfly disc to quickly disengage from the valve seat once the valve is opened, significantly reducing unnecessary excessive compression and scraping between the butterfly disc and the valve seat, decreasing the opening force required, minimizing wear, and extending the lifespan of the valve seat. Scratching is significantly reduced, and this also allows double-eccentric butterfly valves to use metal seat inserts, thereby expanding the application of butterfly valves in high-temperature environments. However, because its sealing principle relies on a position-based sealing mechanism – that is, the sealing surfaces of the butterfly plate and the valve seat are in line contact, and sealing is achieved through the elastic deformation of the valve seat caused by the pressure exerted by the butterfly plate – high requirements are placed on its closed position (especially for metal valve seats), and its pressure resistance is low. This is why it is traditionally believed that butterfly valves cannot withstand high pressures and suffer from high leakage rates. IV. Three-eccentric butterfly valves: They need to be able to withstand high temperatures, thus require hard seals; however, they have high leakage rates ； To achieve zero leakage, soft seals must be used, but they cannot withstand high temperatures. To overcome the contradiction of the double-eccentric butterfly valve, a third eccentricity was applied to the butterfly valve. Its structural feature is that, in addition to the double eccentricity of the valve stem axis, the conical axis of the butterfly valve’s sealing surface is also tilted relative to the cylinder axis of the body; in other words, after the third level of eccentricity, the sealing cross-section of the butterfly valve is no longer circular but elliptical, and as a result its sealing surface becomes asymmetric – one side being inclined relative to the body’s center line while the other side remains parallel to it. The most significant feature of this third type of eccentric design is that it fundamentally changes the sealing mechanism: it is no longer a position-based seal but rather a torque-based seal. In other words, sealing is achieved not by relying on the elastic deformation of the valve seat, but entirely through the pressure at the contact surface of the valve seat. As a result, zero leakage from metal valve seats is achieved.
The sealing structure types of the butterfly valves produced include: single-eccentric sealing, double-eccentric sealing, triple-eccentric sealing, and variable-eccentric sealing. The sealing principles of these different types of butterfly valves are described as follows: 1. Sealing principle of single-eccentric butterfly valves: http://www.shvalves.com/upload/20080304122047.jpg 2. Sealing principle of double-eccentric butterfly valves: http://www.shvalves.com/upload/20080304122839.jpg By offsetting the rotation center of the butterfly disc (i.e., the center of the valve shaft) from the centerline of the valve body by a distance b, based on the single-eccentric butterfly valve design, the sealing surface of the butterfly disc separates from the sealing surface of the valve seat more quickly during operation. When the butterfly disc rotates by 8°–12°, its sealing surface is completely separated from that of the valve seat; at full opening, a larger gap y forms between the two sealing surfaces. This design reduces mechanical wear and compression deformation between the sealing surfaces, thereby improving the sealing performance of the butterfly valve. 3. Sealing principle of triple-eccentric sealed butterfly valves: http://www.shvalves.com/upload/20080304123126.jpg. By offsetting the center line of the valve seat by an angle β relative to the center line of the valve body, based on the design of double-eccentric butterfly valves, the sealing surface of the butterfly disc separates from the sealing surface of the valve seat immediately at the moment of opening, and it comes into contact with and presses against the valve seat sealing surface only at the moment of closing. When fully open, a gap y is formed between the two sealing surfaces, similar to that in double-eccentric sealed butterfly valves. The design of such butterfly valves eliminates mechanical wear and scratching between the two sealing surfaces, thereby **improving** both the sealing performance and service life of the butterfly valve. 4. Sealing principle of the variable-eccentric sealed butterfly valve: http://www.shvalves.com/upload/20080304123427.jpg. The unique feature of the variable-eccentric butterfly valve is that the shaft on which the butterfly plate is mounted has a three-section structure. The two outer sections of this shaft are concentric, while the center line of the middle section is offset from the axes of the two ends by a certain distance; the butterfly plate is mounted on the middle section of the shaft. Such an eccentric structure causes the butterfly valve to be in a double-eccentric state when it is in the fully open position, and to be in a single-eccentric state when it rotates to the closed position. Due to the effect of the eccentric shaft, as it approaches the closed position, the butterfly disc moves inward a certain distance into the sealing conical surface of the valve seat, allowing its sealing surface to fit against that of the valve seat and thus achieving reliable sealing. Since the rotation center of the butterfly plate (i.e., the center of the valve shaft) is offset from the sealing surface of the butterfly plate by a distance a, during the opening process of the butterfly valve, the sealing surface of the butterfly plate gradually moves away from the sealing surface of the valve seat. When the butterfly plate rotates to 20°–25°, its sealing surface is completely separated from that of the valve seat; at full opening, a gap x forms between the two sealing surfaces. This arrangement reduces mechanical wear and compression between the two sealing surfaces during the opening and closing process of the butterfly valve, thereby ensuring its sealing performance.