Pneumatic conveying is the compressed air source for conveying power, the powder material in a closed container from one end of the delivery to the other end. Pneumatic conveying is used in a wide range of industries such as pulverized coal, fly ash and bottom ash of power plants, chemical raw materials for the chemical industry, cement and lime for the construction industry, various powders or particulate materials for food, medical and health industries . The rational selection of various valves in the pneumatic conveying is crucial. Valve to meet the needs of the industry pneumatic conveying, should have excellent properties of temperature, corrosion resistance and wear resistance and other aspects. 2, the valve classification Typical positive pressure pneumatic conveying system 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. 2.1, feed valve with the valve used in pneumatic conveying is used to cut off and connect the material from the delivery point to the delivery container valve. A reasonable choice of the diameter of the inlet valve determines the material flow and the overall system capacity. According to the different transmission medium, its temperature, corrosion resistance and wear resistance also have varying degrees of requirements. As a rule of thumb, gate valves, ball valves, dome valves, etc., are often used as feed valves in different systems. 2.2, exhaust valve exhaust valve is used when the pneumatic conveying system, the conveying system to the air discharge chamber or the flue to the valve, in order to facilitate smooth feeding, shorten the feeding time. The main conveying medium is air with a small amount of powdery material. Commonly used exhaust valve gate valve, butterfly valve, pinch valve, ball valve and dome valve. 2.3, the intake valve is often used for inlet valve with butterfly valve, ball valve, angle seat valve. Inlet valve with the transmission medium for compressed air, so less demanding on the abrasion resistance. 2.4, the discharge valve When the delivery equipment is full of materials, open the intake valve and the discharge valve, so that the material in the compressed air drive in the delivery pipeline system. Therefore, the discharge valve of the media mixed with compressed air powder material. Valve as a discharge valve generally have gate valves, ball valves, dome valves and so on. 2.5, shunt Valve shunt valve can play the role of diversion and confluence of materials from a few points to bring together or from one point to the multi-point dispersion of the material. As a branch valve ball valve, butterfly valve or a combination of valves, there are manufacturers made of the branch valve. Tap-off valve can be used for both pipe installation and top installation. Generally used for the installation of the top of the valve, the valve inlet needs to be connected to the elbow, and the top of the reservoir needs to be equipped with a small unloading ash storage bin so as to appropriately expand the material access and reduce dust in the ash container. 3, the valve performance 3.1 Gate valve (Figure 2) There are two gate to cut off the material, the gate material can be used stainless steel or alloy steel, etc., for corrosion and wear resistant and other conditions. Gate valve manual, pneumatic and electric 3 kinds of operation. Usually choose pneumatic or electric two kinds of automatic control. However, due to its structural reasons, the stroke of the gate plate is long, the torque requirement for the pneumatic or electric device is large, the operation period is long, and the use is not flexible enough. Pneumatic gate valve is driven by the cylinder under the gate to open or close the valve, T-shaped gate seat with a special structure, while driving the gate linear movement along the center for a circumferential rotation of the valve seat sealing surface has played a certain self-grinding and Polishing cleaning effect, so that the seal gate circumference evenly withstand the impact of material and wear and tear, to avoid the gate in the long run during the local damage or wear, so that its life greatly extended. At the same time, the double gate under the spring static pressure can ensure that both sides seal does not leak, when the pressure on one side of the gate, the other side by the role of the spring and seal seat closer contact, in order to achieve the sealing effect. The main problem with this valve in use is the erosion of the seal and ram. The valve is suitable for full or full off the state of work, if the long-term switch in the state of the shutter, the shutter sealing surface due to media erosion and become less rigorous. Therefore, the valve in the erosion-resistant and wear-resistant aspects should be further improved. 3.2 ball valve Ball valve (Figure 3) is usually the choice of automatic (pneumatic or electric) control valve. Ball valve has two kinds of soft seal and hard seal. Soft sealing ball valve seat ring material is PTFE, which has a small friction coefficient, stable performance, easy aging and excellent sealing performance characteristics. Because PTFE has a high coefficient of expansion, sensitivity to cold flow and poor thermal conductivity, the reliability of the seal is compromised as the seal becomes hardened. Moreover, the low temperature grade of PTFE, can only be used in less than 180 ℃. Beyond this temperature, the sealing material will soften. When used for a long time, the temperature should generally be controlled below 120 ℃. Hard sealing ball valve as a feed valve is generally used for high temperature medium power plant ash removal system, because the pneumatic conveying system usually 24h continuous operation, the valve operating frequency is relatively high, usually a few minutes, short or even tens of seconds will act once . If the service life of a hard-sealed ball valve is 2 to 30,000 times, assuming that the valve operates once for 5 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, ash removal system in power plants, the need to use a longer life of the hard seal ball valve. 3.3, dome valve dome valve (Figure 4) because of its particularity of the structure, usually pneumatic conveying the preferred valve. Domed valve shell and dome valve material often used nodular cast iron, stainless steel, special wear-resistant materials and steel lined with fluorine plastic. Ring materials are often used neoprene, silicone rubber, fluorine rubber and EPDM rubber. Domed valves rely on compressed air to achieve a tight sealing effect of the special valve for the treatment of dust-laden gas, erosive bulk materials, such as for the discharge valve warehouse / warehouse, process valves, valves, reactor valves, Fluidized bed combustion chamber valves, high pressure gas reactors and pressure relief devices. When the dome valve is closed (Figure 5), the seal is fully relaxed (not inflated) and there is some clearance between the valve plate and the valve seat. Valve rotation, the material with the rotation of the valve, and some through the gap into the container, and some remain in the gap. When the flap is rotated to the closed position, the ring begins to inflate and tightly wrap the various shapes of particles in the gap to the edge of the flap. The dome valve is fully closed. Before the dome valve is opened, the seal is degassed until completely relaxed. Between the disc and the valve seat to restore the original gap, the disc began to rotate to open position. Depending on the material used, dome valves with scraping ring (residual material adhering to the dome surface for adhesive material) or without scraping ring (for fluid material) may be used. The dome valve must be used to inspect all fasteners, especially to ensure that the bolts securing the dome to the shaft in the valve are tightened. When the dome valve is opened or closed, there should be no pressure difference on both sides, otherwise the gas and material on the high pressure side will pass through the gap between the valve disc and the seal ring at high speed, causing the valve disc and the seal ring to wear out. The inflatable seal should fully retract before the valve is opened or closed, otherwise it will cause the valve disc to cut and damage the seal. Inflatable ring expansion pressure must be higher than the valve medium pressure, or the ring will not be fully expanded, will result in leakage of air and material between the seal and the valve flap, will wash the seal ring, the valve disc and other components. Due to its unique structure and processing technology, the dome valve can be operated continuously and stably under special operating conditions such as abrasion, high temperature, corrosion or stickiness. The performance of the dome valve is reliable. Under normal operating conditions, it needs only 1 million operations Maintenance time. The seal is the only wear piece. Because the seal is a flexible part, located between the carrier ring and the top plate, fastened by fasteners, easy to disassemble and overhaul. Depending on the material used, dome valves with scraping ring (residual material adhering to the dome surface for adhesive material) or without scraping ring (for fluid material) may be used. The dome valve must be used to inspect all fasteners, especially to ensure that the bolts securing the dome to the shaft in the valve are tightened. When the dome valve is opened or closed, there should be no pressure difference on both sides, otherwise the gas and material on the high pressure side will pass through the gap between the valve disc and the seal ring at high speed, causing the valve disc and the seal ring to wear out. The inflatable seal should fully retract before the valve is opened or closed, otherwise it will cause the valve disc to cut and damage the seal. Inflatable ring expansion pressure must be higher than the valve medium pressure, or the ring will not be fully expanded, will result in leakage of air and material between the seal and the valve flap, will wash the seal ring, the valve disc and other components. 3.4, butterfly valve (Figure 6) has the characteristics of simple structure, small size, light weight, material consumption, small installation size, rapid switching, 90 ° reciprocating rotation, small driving torque, etc., for the cut off, connect and regulate the tube Medium in the road, with good fluid control features and close the sealing performance. 1. Valve body 2. Gate 3. Valve stem 4. Multi-layered seal 5. Packing 6. Bracket 7. Electric device Butterfly streamline design, the fluid resistance becomes smaller, is an energy-saving products. With the application of high temperature resistant, low temperature resistant, strong corrosion resistant and high strength alloy material in butterfly valves, the metal sealed butterfly valves are widely used under the conditions of high temperature, low temperature and strong erosion, and partially replace the stop valves, Gate valve and ball valve. Because of butterfly valve disc movement with wiping, so most of the butterfly valve can be used for medium with suspended particles, depending on the strength of the seal can also be used for powder and granular media, but also for sensitive flow control, butterfly valve structure Principle suitable for the production of large-caliber valves. In the requirements of throttling and regulation of control flow, mud medium and medium containing solid particles, requiring a short length of the valve structure, opening and closing speed requirements and pressure conditions smaller, the recommended choice of butterfly valve. In the two-bit regulation, narrow mouth channel, low noise, cavitation and gasification phenomenon, a small amount of leakage to the atmosphere and media wear, you can choose butterfly valve. Under special operating conditions, the throttling regulation or strict seal, over wear and tear, cryogenic (cryogenic) conditions such as the use of butterfly valve, the seal should be specially designed metal seal with three eccentric or double eccentric butterfly valve . 3.5, pinch valve Pinch valve with flange and thread 2 kinds of connections (Figure 7). DN10 ~ 50 pinch valve can be threaded connection, DN40 ~ 250 pinch valve can be used flange connection. Pinch valve without obstruction flow, low flow resistance, non-blocking, 100% off the seal, low gas consumption, light weight, widely used in chemical products such as granules, powder, fiber, , Food, electricity, recycling, water treatment and mineral processing industries. Pinch valve control is simple, more economical, easy to replace pinch, pinch valve is usually used as exhaust valve. As the clip for the rubber products, its wear resistance and temperature resistance are not dome valves. Exhaust valve can be selected according to different operating conditions, the use of temperature is not high, material wear and tear is not strong case, the choice of pipe valve price is better. However, the relatively short life span of ball valve and butterfly valve, large gate valve, inflexible action, the longest service life of dome valve, the high service life of dome valve, Easy maintenance, but higher prices, but the overall cost is still very good. 3.6, DC pneumatic control valve (angle seat valve) Angle seat valve (Figure 8) commonly used specifications for the DN10 ~ 65, according to the connection can be divided into general thread and welded two, according to the control form can be divided into single-acting corner seat Valve and double-acting angle seat valve. Valve body made of stainless steel, seat sealing material is mainly PTFE, suitable medium is water, neutral gas or liquid, alcohol, oil, organic solvent, water vapor, weak acid or weak base solution, applicable temperature range -100 ~ 180 ℃ , The applicable pressure range is ≤116MPa. In the pneumatic conveying system generally choose threaded connection single-acting angle seat valve. For different delivery system can be selected according to need. 4 Conclusion In order to ensure the stable operation of pneumatic conveying system, the type of valve should be properly selected and well maintained.
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