Application and selection of the hottest valve in

The application and selection of valves in pneumatic conveying system

pneumatic conveying takes compressed air source as the conveying power to convey powdery materials from one end to the other end in a closed container. Pneumatic conveying is applied in a wide range of industries, such as pulverized coal, fly ash and furnace bottom slag of power plants, chemical raw materials of chemical industry, cement and lime of construction industry, various powder or granular materials of food, medicine and health and other industries. The reasonable selection of various valves is very important in pneumatic conveying. The valve should meet the needs of pneumatic conveying in various industries, and should have excellent performance in temperature resistance, corrosion resistance, wear resistance and other aspects

2. Valve classification

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 feeding valve, exhaust valve, inlet valve, discharge valve and switching valve

2.1. Feeding valve when starting the experimental machine, the feeding valve is used to cut off and connect the material from the conveying point to the conveying container in pneumatic conveying. The reasonable selection of the diameter of the feed valve determines the flow of materials and the conveying capacity of the whole system. According to the different transmission media, there are different requirements for its temperature resistance, corrosion resistance and wear resistance. According to experience, gate valve, ball valve, dome valve, etc. are usually selected as feeding valves in different systems

2.2. Exhaust valve

exhaust valve is a valve that discharges the air in the conveying system into the feeding bin or flue when the pneumatic conveying system is fed, so as to facilitate smooth feeding and shorten the feeding time. Its main transmission medium is air with a small amount of powdery materials. Commonly used exhaust valves include gate valve, butterfly valve, pinch valve, ball valve and dome valve

2.3. Air inlet valve

commonly used air inlet valves include butterfly valve, ball valve, angle seat valve, etc. The transmission medium of the air inlet valve is compressed air, so the requirements for abrasion resistance are not high

2.4. Discharge valve

when the conveying equipment is full of materials, open the inlet valve and discharge valve to allow the materials to be transported in the conveying pipeline system driven by compressed air. Therefore, the medium of the discharge valve is powdery material mixed with compressed air. As a discharge valve, the valves generally include gate valve, ball valve, dome valve, etc

2.5. Shunt valve

shunt valve can play the role of diversion and confluence, collecting materials from multiple points to one point or dispersing materials from one point to multiple points. As shunt valves, there are ball valves, butterfly valves or combination valves, as well as shunt valves made by various manufacturers. The shunt valve can be used for both pipeline installation and warehouse top installation. Generally, when it is used for installation on the top of the silo, the valve inlet needs to be connected with an elbow, and a small ash unloading silo needs to be installed on the top of the silo, so as to appropriately expand the material channel and reduce the dust in the ash silo

3. Valve performance

3.1. Gate valve

gate valve (Figure 2) has two rams to cut off materials. The materials of the RAMs can be stainless steel or alloy steel, etc., which is suitable for working conditions such as corrosion resistance and wear resistance. The gate valve can be operated manually, pneumatically and electrically. Usually, pneumatic or electric automatic control methods are used. However, due to its structure, the ram has a long stroke, high torque requirements for pneumatic or electric devices, long action cycle and inflexible use. 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 in a straight line and rotates circumferentially along the center, playing a certain role of self grinding and polishing and cleaning on the sealing surface of the valve seat, so that the circumference of the sealing gate can bear the impact and wear of materials evenly, avoiding local damage or wear through of the gate in the long-term operation process, and greatly extending its service life. At the same time, the double ram can ensure no leakage on both sides under the static pressure of the spring. When one side of the ram is under pressure, the other side can be in closer contact with the sealing seat through the action of the spring, so as to achieve the sealing effect. The main problem of the valve in use is the erosion damage of the seal and ram. The valve is suitable for working in the fully open or closed state. If the ram works in the semi open state for a long time, the sealing surface of the RAM will become loose due to the scouring of the medium. Therefore, the valve should be further improved in terms of erosion resistance and wear resistance

3.2 ball valve

ball valve (Figure 3) is usually automatic (pneumatic or electric) control ball valve. There are two kinds of ball valves: soft seal and hard seal. The seat sealing ring material of soft sealing ball valve is polytetrafluoroethylene, which has the characteristics of small friction coefficient, stable performance, not easy to aging and excellent sealing performance. Because polytetrafluoroethylene has high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, when the sealing material hardens, the reliability of the seal will be affected. Moreover, the temperature resistance grade of polytetrafluoroethylene is low, and it can only be used under the condition of 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 ℃

as a feed valve, the hard sealed ball valve is generally used in the ash removal system of power plants with high medium temperature. Because the pneumatic conveying system usually works continuously for 24 hours, the valve action frequency is relatively high, which is generally a few minutes, or even a few seconds. If the service life of the hard sealed ball valve is 20000 ~ 30000 times, assuming that in the pneumatic conveying system, the valve acts once in 5min, the service life of the ball valve is 70 ~ 104 days, and the service life of the valve is relatively short. Therefore, in the ash handling system of power plants, hard sealed ball valves with longer service life are required

3.3. Dome valve

dome valve (Figure 4) is usually the preferred valve in pneumatic conveying due to its special structure. Nodular cast iron, stainless steel, special wear-resistant materials and steel lined fluoroplastics are often used for the shell and dome valve disc of dome valve. Neoprene, silicone rubber, fluororubber and EPDM are often used as sealing ring materials. Dome valve is a special valve that relies on compressed air to achieve tight sealing effect. It is suitable for handling dusty gas and erosive bulk materials, such as discharge valve, process flow valve, reversing valve, reactor valve, fluidized bed combustion chamber valve, high-pressure gas reactor and pressure reducing device for silo/silo

when the dome valve is closed (Fig. 5), the sealing ring is in a completely relaxed (non inflated) state, and there is a certain gap between the valve disc and the valve seat. As the valve disc rotates, some materials enter the container through the gap or stay at the gap with the rotation of the valve disc. When the valve disc rotates to the closed position, the sealing ring begins to inflate and expand, tightly wraps the particles of various shapes in the gap, and adheres to the edge of the valve disc, and the dome valve is completely closed. Before the dome valve is opened, the sealing ring is vented to complete relaxation. The clearance between the valve clack and the valve seat is restored to the original clearance, and the valve clack begins to rotate to the open position

according to the applicable materials, the dome valve with scraper ring (to clean the residual materials adhered to the surface of the dome for viscous materials) or without scraper ring (for materials with good fluidity) can be selected. When the dome valve is used, all fasteners must be checked, especially to ensure that the bolts fixing 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 wear of the valve disc and the sealing ring. The expandable seal ring should be fully contracted before the valve is opened or closed, otherwise the valve disc will be cut and the seal ring will be damaged. The expansion pressure of the expandable seal ring must be higher than the medium pressure of the valve, otherwise the seal ring will not fully expand, which will lead to air and material leakage between the seal ring and the valve disc, and will scour the seal ring, valve disc and other parts

due to its unique structure and processing technology, the dome valve can operate continuously and stably under special working conditions such as wear, high temperature, corrosion or viscosity, with reliable performance. Under normal working conditions, it needs to be overhauled every 1million times of operation. The seal ring is the only wear part. As the sealing ring is a flexible part, it is located between the support ring and the top plate and is fixed by fasteners, which is convenient for disassembly and maintenance

according to the applicable materials, the dome valve with scraper ring (to clean the residual materials adhered to the surface of the dome, for materials with stable and reliable viscosity) or without scraper ring (for materials with good fluidity) can be selected. When the dome valve is used, all fasteners must be checked, especially to ensure that the bolts fixing 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 wear of the valve disc and the sealing ring. The expandable seal ring should be fully contracted before the valve is opened or closed, otherwise the valve disc will be cut and the seal ring will be damaged. The expansion pressure of the expandable seal ring must be higher than the medium pressure of the valve, otherwise the seal ring will not fully expand, which will lead to air and material leakage between the seal ring and the valve disc, and will scour the seal ring, valve disc and other parts

3.4. Butterfly valve

butterfly valve (Figure 6) has the characteristics of simple structure, small volume, light weight, low material consumption, small installation size, fast switching, 90 ° reciprocating rotation, small driving torque, etc. it is used to cut off, connect and adjust the medium in the pipeline, and has good fluid control characteristics and closing sealing performance

1. Valve body 2 Ram 3 Valve stem 4 Multi level seal ring 5 Filler 6 Support 7 The streamlined design of the electric device

butterfly plate reduces the fluid resistance, which 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 are widely used under high temperature, low temperature, strong erosion and other working conditions, and partially replace stop valves, gate valves and ball valves. Because the movement of the butterfly plate of the butterfly valve is wiping, most butterfly valves can be used for media with suspended particles. According to the strength of the seal, they can also be used for powdery and granular media, and can also carry out sensitive flow control. The structural principle of the butterfly valve is suitable for making large-diameter valves. Butterfly valve is recommended under the working conditions of throttling and regulating control flow, mud medium and medium containing solid particles, short valve structure length, fast opening and closing speed and small differential pressure. Butterfly valve can be selected in case of double position adjustment, necked channel, low noise, cavitation and gasification, a small amount of leakage to the atmosphere and abrasion of medium. When the butterfly valve is used under special working conditions, such as throttling regulation or strict sealing requirements, severe over wear, low temperature (cryogenic) and so on, the special butterfly valve with three eccentric or double eccentric of special design metal sealing belt regulating device should be selected

3.5. Pinch valve

pinch valve has two connection forms: flange and thread (Fig. 7). Threaded connection can be used for dn10 ~ 50 pinch valve, and flange connection can be used for Dn40 ~ 250 pinch valve. The pinch valve has no elongation at break to hinder the flow passage, low flow resistance, no blockage, 100% off sealing, low air consumption, light weight, and is widely used in chemical, pulp, paper making, food, power, recycling, water treatment, mineral processing and other industries with abrasive substances such as particles, powders, fibers, viscose and so on. The pinch valve is easy to control, economical and convenient to replace. It is usually used as the exhaust valve. Because the clamp pipe is made of rubber, its wear resistance and temperature resistance are not as good as dome valve. The exhaust valve can be selected according to different operating conditions. Under the condition of low operating temperature and low abrasion of materials, the pinch valve has better cost performance. Under high temperature or strong abrasion of materials, gate valve, ball valve, butterfly valve and dome valve can be selected, but the relative life of ball valve and butterfly valve is short, the volume of gate valve is large, and the action is not flexible. The dome valve has the longest service life and convenient maintenance, but the price is high, but the overall price is high