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What valves are used in pneumatic conveying systems?
2015-03-09 Source: wglobalnba.com Author: Rich Machinery read: Times
Pneumatic conveying uses compressed air as the driving force to convey powdered materials from one end to the other end in a closed container. Pneumatic conveying is used in a wide range of industries, such as coal powder, fly ash and furnace bottom slag in power plants, chemical raw materials in the chemical industry, cement and lime in the construction industry, various powder or granular materials in the food, medical and health industries, etc. . The reasonable selection of various valves is vital in pneumatic conveying. The valve should meet the needs of pneumatic conveying in various industries and should have excellent performance in various aspects such as temperature resistance, corrosion resistance and wear resistance.
Valve classification
A typical positive pressure pneumatic conveying system is shown in Figure 1. According to the position and function of the valve in the system, it is divided into feed valve, exhaust valve, intake valve, discharge valve and switching valve.
Two. One, the valve for feeding
The feed valve is a valve used to cut off and connect the material from the transfer point to the transfer container during pneumatic conveying. The proper selection of the diameter of the feed valve determines the flow of material and the transport capacity of the entire system. Depending on the transport medium, there are different requirements for its temperature resistance, corrosion resistance and wear resistance. According to experience, gate valves, ball valves, dome valves, etc. are usually selected as feed valves in different systems.
Two, two, exhaust valve
The exhaust valve is a valve that discharges the air in the conveying system to the feed bin or flue when the pneumatic conveying system is feeding, in order to smoothly discharge and shorten the feeding time. The main conveying medium is air with a small amount of powdery material. Commonly used exhaust valves are gate valves, butterfly valves, pinch valves, ball valves and dome valves.
Two, three, intake valve
Commonly used as intake valves are butterfly valves, ball valves, angle seat valves, etc. The conveying medium of the intake valve is compressed air, so the requirements for abrasion resistance are not high.
Two, four, discharge valve
When the conveying equipment is full of materials, the intake valve and the discharge valve are opened to allow the materials to be conveyed in the conveying pipeline system under the drive of compressed air. Therefore, the medium of the discharge valve is a powdery material mixed with compressed air. Valves used as discharge valves are generally gate valves, ball valves, dome valves, etc.
Two. Five, branch valve
The shunt valve can play the role of diverting and converging, bringing materials from multiple points to one point or dispersing materials from one point to multiple points. As the branch valve, there are ball valves, butterfly valves or combination valves, and there are also branch valves made by various manufacturers. The shunt valve can be used for both pipeline installation and roof installation. Generally used for the installation of the warehouse roof, the inlet of the valve needs to be connected with an elbow, and a small ash dump bin needs to be installed on the warehouse roof to appropriately enlarge the material passage and reduce dust in the ash warehouse.
Third, the valve performance
Three. One, the gate valve
The gate valve (Figure 2) has two gates to cut off the material. The material of the gate can be stainless steel or alloy steel, which is suitable for working conditions such as corrosion resistance and wear resistance. The gate valve has three operation modes: manual, pneumatic and electric. Usually choose two automatic control modes: pneumatic or electric. However, due to its structure, the stroke of the brake is long, the torque requirements of the pneumatic or electric device are large, the action cycle is long, and the use is not flexible. The pneumatic gate valve is driven by the cylinder to open or close the valve.The T-shaped gate seat adopts a special structure, which drives the gate to move linearly while rotating around the center, which has a certain self-grinding effect on the sealing surface of the valve seat. Polishing and cleaning effect, so that the circumference of the sealing sluice plate can evenly withstand the impact and abrasion of the material, avoiding local damage or wear-out of the sluice plate during long-term operation, which greatly prolongs its service life. At the same time, under the static pressure of the spring, the double brake plate can ensure that the two sides of the seal are not leaking. When there is pressure on one side of the brake plate, the other side can be in closer contact with the seal seat through the action of the spring to achieve the sealing effect. The main problem of the valve in use is the damage of the seal and the erosion of the gate. The valve is suitable to work in the fully open or fully closed state. If the gate is operated in a half-switched state for a long time, the sealing surface of the gate will become tight due to the erosion of the medium. Therefore, the valve should be further improved in terms of erosion resistance and wear resistance.
Three. Two ball valve
Ball valves (Figure 3) are usually automatic (pneumatic or electric) control ball valves. Ball valves are available in soft and hard seals. The material of the seat ring of the soft seal ball valve is polytetrafluoroethylene, which has the characteristics of small friction coefficient, stable performance, not easy aging and excellent sealing performance. Because PTFE has a high expansion coefficient, sensitivity to cold flow, and poor thermal conductivity, when the sealing material becomes hard, the reliability of the seal is affected. Moreover, the temperature resistance grade of PTFE is relatively low, and it can only be used at a temperature lower than 180 ° C. Above this temperature, the sealing material will soften. When used for a long time, the temperature should generally be controlled below -20 ° C.
Hard-sealed ball valves as feed valves are generally used in power plant ash removal systems with high media temperatures. Because pneumatic conveying systems usually work continuously for two to four hours, the valve's operating frequency is relatively high, usually a few minutes, as short as tens of seconds. Move once. If the service life of a hard-sealed ball valve is 20,000 to 30,000 times, assuming that the valve operates once every five minutes in a pneumatic conveying system, the service life of the ball valve is 70 to 104 days, and the service life of the valve is short. Therefore, in power plant ash removal systems, hard-sealed ball valves with longer service life are required.
Three, three, dome valve
Dome valve (Figure 4) is usually the first choice for pneumatic conveying due to its special structure. Dome valve housing and dome valve disc materials are usually made of ductile iron, stainless steel, special wear-resistant materials and steel-lined fluorine plastic. Sealing ring materials are often selected from neoprene, silicone rubber, fluorine rubber and EPDM rubber. Dome valve is a special valve that relies on compressed air to achieve a tight sealing effect.It is suitable for handling dusty gases and erosive bulk materials, such as discharge valves for warehouses / warehouses, process valves, reversing valves, reactor valves, Fluidized bed combustion chamber valves, high-pressure gas reactors, and pressure-reducing devices.
When the dome valve is closed (figure 5), the sealing ring is completely relaxed (non-inflated), and there is a certain gap between the valve disc and the valve seat. The valve disc rotates, and as the valve disc rotates, some enter the container through the gap, and some stay at the gap. When the valve flap rotates to the closed position, the sealing ring begins to inflate and expand, and tightly wraps particles of various shapes in the gap to adhere to the edge of the valve flap, and the dome valve is completely closed. Before the dome valve opens, the seal is vented until it is fully relaxed. The original clearance is restored between the valve disc and the valve seat, and the valve disc starts to rotate to the open position.
According to the applicable materials, you can choose a dome valve with a scraper ring (to clean the residual material adhered to the surface of the dome, for viscous materials) or without a scraper ring (suitable for materials with good fluidity). All fasteners must be inspected when the dome valve is in use, especially to ensure that the bolts that fix the dome and shaft in the valve have been tightened. When the dome valve is opened or closed, there should be no pressure difference on both sides, otherwise the gas and materials on the high pressure side will pass through the gap between the valve disc and the sealing ring at high speed, causing the valve disc and the sealing ring to wear. The expandable sealing ring should be fully contracted before the valve is opened or closed, otherwise the cutting of the valve flap will damage the sealing ring. The expansion pressure of the expandable sealing ring must be higher than the medium pressure of the valve, otherwise the sealing ring will not expand completely, which will cause air and material leakage between the sealing ring and the valve disc, which will wash away the sealing ring, valve disc and other components.
Due to its unique structure and processing technology, the dome valve can be continuously and stably operated under special conditions such as wear, high temperature, corrosion or viscosity, and its performance is reliable. Under normal operating conditions, each time it is operated for one million times Only need to be overhauled once. The seal is the only wear part. Since the sealing ring is a flexible part, it is located between the supporting ring and the top plate and is fixed by fasteners, which is convenient for disassembly and maintenance.
According to the applicable materials, you can choose a dome valve with a scraper ring (to clean the residual material adhered to the surface of the dome, for viscous materials) or without a scraper ring (suitable for materials with good fluidity). All fasteners must be inspected when the dome valve is in use, especially to ensure that the bolts that fix the dome and shaft in the valve have been tightened. When the dome valve is opened or closed, there should be no pressure difference on both sides, otherwise the gas and materials on the high pressure side will pass through the gap between the valve disc and the sealing ring at high speed, causing the valve disc and the sealing ring to wear. The expandable sealing ring should be fully contracted before the valve is opened or closed, otherwise the cutting of the valve flap will damage the sealing ring. The expansion pressure of the expandable sealing ring must be higher than the medium pressure of the valve, otherwise the sealing ring will not expand completely, which will cause air and material leakage between the sealing ring and the valve disc, which will wash away the sealing ring, valve disc and other components.
Three, four, butterfly valve
Butterfly valve (Figure 6) has the characteristics of simple structure, small size, light weight, low material consumption, small installation size, fast switching, 90 ° reciprocating rotation, and low driving torque.It is used to cut off, connect and adjust the pipeline. Medium with good fluid control characteristics and closed sealing performance.
1. Valve body 2. Gate 3. Stem 4. Multi-layered sealing ring 5. Packing 6. Support 7. Electric device
The streamlined design of the butterfly plate reduces the fluid resistance and is an energy-saving product. With the application of high temperature resistance, low temperature resistance, strong corrosion resistance, and high-strength alloy materials in butterfly valves, metal-sealed butterfly valves have been widely used under high temperature, low temperature, and strong erosion conditions, and have partially replaced globe valves, Gate and ball valves. Due to the wipeability of the butterfly valve butterfly plate, most butterfly valves can be used for media with suspended particles. Depending on the strength of the seal, it can also be used for powdery and granular media, and it can also perform sensitive flow control.The structure of the butterfly valve The principle is suitable for making large-caliber valves. Under the conditions of throttling and regulating control flow, mud medium and solid particle-containing medium, short valve structure length, fast opening and closing speed and small pressure difference, butterfly valve is recommended. The butterfly valve can be used when the two-position adjustment, the necked passage, low noise, cavitation and gasification, a small amount of leakage to the atmosphere, and the media are abrasive. Under special working conditions, throttling adjustment or strict sealing requirements, severe over-wear, low temperature (deep cold) and other working conditions should be used. Specially designed three-eccentric or double-eccentric butterfly valves with metal seals and adjusting devices should be used. .
Three. Five, pinch valve
The pinch valve has two connection types: flange and thread (Figure 7). DN one zero to five zero pinch valve can choose thread connection, DN four zero to two five zero pinch valve can choose flange connection. The pinch valve has no obstructed flow path, low flow resistance, no blockage, 100% shut-off seal, low air consumption, light weight, and is widely used in chemical and pulp with abrasive materials such as particles, powder, fiber, and slurry. , Paper, food, power, recycling, water treatment and mineral processing industries. The pinch valve is simple to control, relatively economical, and easy to replace the pinch valve. Generally, the pinch valve is used as the exhaust valve. Because the pinch tube is a rubber product, its wear resistance and temperature resistance are not as good as those of the dome valve. The exhaust valve can be selected according to different operating conditions. When the use temperature is not high and the material is not abrasive, it is better to choose a pinch valve. In the case of high temperature or material abrasion, gate valves, ball valves, butterfly valves, and dome valves can be used.However, the relative life of ball valves and butterfly valves is short, the size of the gate valve is large, the action is not flexible, and the service life of the dome valve is the longest. Easy to maintain, only the price is higher, but the overall price is still good.
Three. Six, DC pneumatic control valve (angle seat valve)
Angle seat valve (Figure 8) commonly used specifications are DN 10 to 65, according to the connection type can be divided into two types of thread and welding, according to the control type can be divided into single-acting angle seat valve and double-acting angle seat valve. The material of the valve body is stainless steel, and the sealing material of the valve seat is mainly PTFE. The applicable medium is water, neutral gas or liquid, alcohol, oil, organic solvent, water vapor, weak acid or weak alkali solution, etc., and the applicable temperature range is -100 ~ 180 ° C, the applicable pressure range is ≤ 116 MPa. In pneumatic conveying system, single-acting angle seat valve with screw connection is generally used. For different conveying systems, you can choose according to your needs.
Fourth, the conclusion
In order to ensure the stable operation of the pneumatic conveying system, the valve type should be selected reasonably, and the maintenance work should be done well.
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