Butterfly Valves
A butterfly valve is a valve that uses a circular butterfly disc as the opening and closing part and rotates with the valve stem to open, close and adjust the fluid. There are many types of butterfly valves with a wide application, and they are also the valves commonly seen on pipelines.
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A butterfly valve has the characteristics of simple structure, small sizes, light weights, low material consumption, small installation space, rapid opening and closing, and small driving torque. It is used to cut off, connect, and adjust the medium flow in the pipeline, which has fluid control and good sealing. The butterfly valve plate has a streamlined design, and the loss of fluid resistance is small. The valve stem of the butterfly valve has a through rod, which has good comprehensive mechanical properties, corrosion resistance and scratch resistance after quenching and tempering treatment. When the butterfly valve is opened and closed, the valve stem only rotates and does not move up and down. The packing of the valve stem is not easy to damage and the seal is reliable.
Butterfly valves are often used in water systems and gas systems, such as the water supply and drainage system of municipal engineering, various water supply systems, heat pipelines and sewage treatment systems of hydroelectric power plants and nuclear power plants, water inlet valves of large turbines in hydropower stations, electric power, chemical industry, petroleum, metallurgy, ships, papermaking, city gas, etc. The media that the butterfly valve can transport and control include water, condensed water, sewage, seawater, and other liquids, air, gas, steam, dry powder, mud, fruit pulp, and mixtures with suspended solids. At present, China has been able to manufacture butterfly valves with PN100 and even higher pressure. The world's largest butterfly valve has a size of 32ft (9.75m).
The butterfly valve is a valve that uses a circular butterfly plate as the opening and closing part to open and close and adjust the medium flow in the pipeline. In the cylindrical channel of the butterfly valve body, the disc-shaped butterfly plate rotates around the axis, and the rotation angle is 0 to 90. When rotating to 90°, the butterfly valve is fully open.
The sealing principle of the valve is achieved by a certain sealing-specific pressure acting on the sealing surface, and the butterfly valve is no exception. There are generally three forms of butterfly valve sealing: (1) The specific pressure is compressed by the magnitude of interference. (2) Press the sealing pair well by direct pressure or torque, such as double eccentric butterfly valve and triple eccentric butterfly valves; the sealing ring is filled with air and liquid. (3) Use the medium to press on the sealing ring by the Poisson effect. The greater the pressure difference between upstream and downstream is, the easier it is to seal.
The axis line of the centerline butterfly valve stem and the center plane of the butterfly plate is in the same plane and perpendicular to the centerline of the valve body pipe. The areas on both sides of the butterfly plate are symmetrical to the axis of the valve stem. The centerline butterfly valve is generally made of rubber lining. Because of the simple structure, central symmetry, the centerline butterfly valve has the same two-way sealing effect, and the flow resistance is small; the opening torque is also small. The shaft head is often in a state of friction. It wears faster than other parts, and it is easy to leak here. Therefore, sometimes the shaft head is lined with PTFE film to reduce friction or add springs to compensate for wear. If the centerline butterfly valve adopts metal to metal, it is difficult to seal. The shaft head of the inclined plate and offset plate butterfly valve has no friction, but their flow resistance and sealing torque are greater than that of the center symmetrical butterfly plate.
The valve shaft of the single eccentric butterfly valve deviates from the center plane of the butterfly plate to produce an eccentricity h, so that the upper and lower ends of the butterfly plate close to the valve shaft no longer become the rotary axis, eliminating leakage due to the friction and wear of the upper and lower shaft ends. The sealing effect is better, but the sealing effect in the two directions is inconsistent. Generally, the positive direction (flowing from the valve shaft to the sealing surface) is easy to seal, and the reverse direction is easy to leak due to the lack of support of the sealing surface. However, the rubber soft seal is used, and its elasticity can be used to achieve two-way sealing. Since its sealing contact mechanism belongs to the "ball in a cone", 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 harm the seal. If the metal seat is adopted and the two positive cones cooperate, when the butterfly plate is rotated, the sealing surface will interfere, resulting in difficult opening and closing.
To improve the situation of single eccentricity, the valve shaft is offset by a distance e, so that the connection line between the shaft center and the sealing point and the sealing surface form an obtuse angle (greater than 900), so that it is easy to seal. There is no interference, and the closer the seal is, the tighter it is, resulting in a greater pressing force on the sealing surface. The characteristic of double eccentricity is that when the valve is opened, the sealing surface of the butterfly plate can quickly disengage from the valve seat.
Triple eccentric butterfly valves replace the positive cone angle with the oblique cone angle, so that the eccentricity e can be reduced, and the opening torque will also be reduced accordingly.
Butterfly valves are widely used in nuclear power plants due to their characteristics of compact structure, simple structure, lightweight, fast opening and closing and high flow coefficient.
To prevent the temperature from being transmitted to the packing, the butterfly valve has a long-necked structure. There are fins on the long neck of the high-temperature butterfly valve, while the low-temperature butterfly valve is equipped with a cold plate to reduce cold loss. Both high-temperature and low-temperature valves have strict requirements for materials.
The process coefficient in the containment mostly adopts rubber-lined butterfly valves. They have a size of less than or equal to DN600, pressure less than or equal to PN16, and temperature less than or equal to 80°C.
Large-diameter butterfly valves for circulating water systems. They have a size of less than or equal to DN1200, pressure less than or equal to PN10, and temperature less than or equal to 80°C.
Generally, to prevent corrosion caused by seawater and salt spray, internal rubber lining, epoxy painting or other anti-corrosion coatings are required, and two-way stainless steel and titanium alloy can also be used.
The have a size of less than or equal to DN2400.
Regulating butterfly valves are used for steam turbine reheater medium and low-pressure connecting pipes. Generally, they have a size of DN1200 to 1600.
PVC butterfly valves can regulate the flow in the system, which are used for food processing, irrigation and plumbing systems.
Gray cast iron is suitable for nominal pressure less than and equal to 1.0MPa, media such as water, steam, air, gas and oil with a temperature of -10°C to 200°C.
Malleable cast iron butterfly valves are suitable for water, steam, air and oil media with nominal pressure less than and equal to 2.5MPa and a temperature of -30℃ to 300℃.
They are suitable for corrosive media with nominal pressure less than and equal to 0.25MPa and temperatures lower than 120°C.
They are suitable for media such as water, steam, air, hydrogen, ammonia, nitrogen and petroleum products with a nominal pressure less than and equal to 32.0MPa and a temperature of -30 to 425°C. The commonly used grades are WC1, WCB, and low alloy structural steel 16Mn.
They are suitable for water, seawater, oxygen, air, oil and other media with a pressure less than and equal to 2.5MPa, as well as steam with a temperature of -40℃ to 250℃. High-temperature copper butterfly valves are suitable for steam and petroleum products with nominal pressure less than and equal to 17.0Mpa and a temperature less than and equal to 570℃. The commonly used grades are WC6, WC9 and other grades.
They are suitable for media such as ethylene, propylene, liquid natural gas, liquid nitrogen, etc., with nominal pressure less than and equal to 6.4Mpa and a temperature greater than and equal to -196°C.
They are suitable for nitric acid, acetic acid and other media with nominal pressure less than and equal to 6.4Mpa and a temperature less than and equal to 200℃.
The electric butterfly valve consists of an electric actuator and a butterfly valve. The opening and closing of the butterfly valve are driven by the rotation of the electric actuator. At the same time, the electric butterfly valve also has a manual operation function, which is convenient in the event of a power failure and can be operated by manually operation to open and closed the butterfly valve. Electric butterfly valves are used for canning and bottle systems, beer brewing and beverage technology, the chemical industry, fluid mixing devices, food, industrial concrete and cement industry, vacuum technology, water treatment device, laundry, medical equipment, boiler feed water, etc.
The pneumatic butterfly valve is composed of the pneumatic actuator and the butterfly valve. Because of the characteristics of the butterfly valve, such as lightweight, simple structure and easy operation, it is often installed and used on the large-diameter low-pressure pipeline. Pneumatic butterfly valves can be divided into single-acting pneumatic butterfly valves and double-acting pneumatic butterfly valves. The execution speed of the pneumatic butterfly valve is relatively fast. It is not easy to be damaged due to jamming in the action, and does not require workers. The pneumatic butterfly valve can not only be used as a shut-off valve, but also can be equipped with a valve positioner to achieve the function of adjusting and controlling the pipeline medium.
Hydraulic butterfly valves are mainly used in high-safety systems such as steam turbine extraction pipelines, blast furnace gas residual pressure power generation units, and industrial pipelines with media such as seawater, sewage, flue gas, air, and oil as emergency shut-off valves.
Manual butterfly valves need to be manually operated to open or close. In view of this mode of operation, manual butterfly valves are not suitable for installation in high altitudes or dangerous working conditions. The most important thing in the installation environment of the manual butterfly valve is to facilitate manual operation. The valve’s installation location must be convenient for personnel’s daily operation and future maintenance. The best location for manual valve installation is about 1.2 meters from the ground. Normally, the manual butterfly valve is suitable for the following occasions: adjusting the flow, mud and medium containing solid particles, requiring a short valve structure length, requiring fast opening and closing speed and small pressure difference.
There are two different soft-seated butterfly valves. One is that the sealing pair is composed of non-metallic soft material to non-metallic soft material, and the other is that the sealing pair is composed of metal and non-metal.
The sealing pair of the metal seated butterfly valve is composed of metal to metal.
Forced-sealed butterfly valves are classified into elastic-sealed butterfly valves and torque-sealed butterfly valves. The sealing specific pressure of the forced sealed butterfly valve is generated by the elasticity of the valve seat or valve plate when the valve plate squeezes the valve seat in a closed state. The sealing specific pressure of the torque-sealed butterfly valve is generated by the torque applied to the valve shaft.
The sealing-specific pressure of the pressure-sealed butterfly valve is generated by the pressure of the elastic sealing element on the valve seat or valve plate.
The sealing specific pressure of the automatic sealed butterfly valve is automatically generated by the medium pressure.
The working pressure of the vacuum butterfly valve is lower than the standard atmospheric pressure.
Low-pressure butterfly valves have nominal pressure of less than 1.6MPa.
Medium-pressure butterfly valves have nominal pressure between 2.5 to 6.4MPa.
High-pressure butterfly valves have nominal pressure between 10.0 to 80.0MPa.
Ultra-high pressure butterfly valves have nominal pressure PN greater than 100MPa.
The butterfly plate of the wafer butterfly valve is installed in the direction of the pipeline’s diameter. Wafer butterfly valves have two sealing types: elastic sealing and metal sealing. For elastic sealing valves, the sealing ring can be embedded in the valve body or attached to the periphery of the butterfly plate.
The flanged butterfly valve has a vertical plate, and the valve stem has an integral metal seal. The sealing ring of the valve has a composite structure of flexible graphite plates and stainless steel plates, installed on the valve body, and the sealing surface of the butterfly plate is surfaced with stainless steel. The sealing ring of the soft seated valve is made of nitrile rubber and is installed on the butterfly plate.
The lug butterfly valve can be driven by handwheels, worm gears, pneumatics and electricity; the connection method can be flange or wafer; the structure can be single eccentric, centerline, double eccentric, triple eccentric; metal seats and soft seats can be adopted. The lug butterfly valve can be made from cast steel or stainless steel.
A welded butterfly valve is widely used in pipelines with medium temperatures less than and equal to 300°C and nominal pressure of 0.1Mpa in the production process of building materials, metallurgy, mining and electric power to open and close the valve or adjust the medium volume.
