Valve status, application and development
Infrastructure construction is the focus of China’s economic construction. Water supply and drainage projects, especially sewage treatment projects, are large in scale. WTO has entered, and the development of the western region has started. The valve market prospects are promising. Although China’s tap water industry has a history of nearly 120 years, 7 years earlier than Japan, there is still a big gap and need to be built. The current situation is that half of China’s 700 cities are short of water, and 100 cities are seriously short of water. The average annual water consumption will increase by 7 million m3/d; especially during the “Ninth Five-Year Plan” period, one of the key to the treatment of environmental pollution is sewage treatment, focusing on the treatment of the Three Lakes (Taihu Lake, Chaohu Lake, Dianchi Lake) and Sanhe River (Huaihai Lake). , Liaohe, Haihe), and at the same time require all 640 county-level cities with a population of more than 500,000 to build at least one sewage treatment plant in one city before 2003, with a sewage treatment rate of 25% during the Ninth Five-Year Plan period and 46% in 2010. That is, 1,000 factories. At present, there are only more than 200 prefecture-level cities in the country with sewage treatment plants. In the next few years, more than 400 cities will build sewage treatment plants. By 2003, 180 billion yuan will be invested in the treatment of urban sewage. Since 2000, 60 billion yuan will be invested each year, and the annual equipment investment will reach 20 billion. Valves also occupy a large share. A medium-sized sewage treatment plant requires about 600-800 valve investment. ten thousand. In addition, the demand for water supply engineering, hydropower, and chemical industries is also great. By 2008, Beijing will invest 180 billion yuan to host the Olympics, most of which will be used in basic construction, and the number of valves used in basic construction will also be considerable. The above situation is only the domestic market. After entering the WTO, the domestic market we can see is only a small piece, and the larger market is abroad. Now that we have entered globalization, we must make full use of the advantages of China’s valve industry and actively move toward the world economy. platform. It is divided into several topics to briefly describe the general situation of valve design, production status, application and development in our country. 1 Current status? After the reform and opening up, with the vigorous development of China’s economic construction, the valve industry has also shown a thriving vitality. Now there are thousands of large and small valve factories. Although the good and the bad are uneven, the product varieties have reached more than 3,000 models and nearly 30,000 specifications. Great progress has been made in sealing performance, strength requirements, adjustment function, action performance and circulation performance, especially sealing performance. For ten years, an authoritative person from the Urban Water Supply Association has a famous industry saying that “ten valves and nine leaks”, although Some exaggerations are indeed unsatisfactory. Example: From September to December 1989, Chengdu Water Supply Company entered the factory to inspect 145 valves, with a pass rate of 72. 5%. In 1996, the pass rate of 917 inspections was 88.66%. Due to the limitation of inspection methods, the acceptance rate cannot be checked according to national standards. If strictly according to the national standard, the pass rate is even lower. The gate valve mainly has poor sealing performance, and the butterfly valve and the operating mechanism are poor in reliability. Wait, this situation has indeed improved significantly. Nowadays, ductile casting, resin sand casting process, austenitic stainless steel valve shaft, hard sealing valve seat, special valve varieties, application of anti-corrosion spray coating, and sealing structure design and other technologies Many manufacturers have already mastered the problem of materials. Some (a few) manufacturers have some products that have approached and reached the international advanced level, replacing imported products, or exported abroad. The overall level is constantly improving, and the distance from the foreign advanced level is further narrowed. However, because the overall level of China’s machinery manufacturing industry is not high, the quality of valves needs to be further improved. 2 Commonly used valves are widely used in urban, construction, and enterprise water supply and drainage projects, especially low-pressure large-diameter valves (low-pressure valves refer to valves with a nominal pressure of PN ≤ 1.6 MPa), and medium-pressure valves (PN2.5- 6.4MPa) is less, according to the principle and structure classification (international methods) commonly used valves are butterfly valves and gate valves, the others are ball valves, check valves, exhaust valves, plug valves, diaphragm valves, pressure reducing valves, flow regulators Valves, mud drain valves, etc. 2. 1. Current status and development of gate valves. Gate valves have the longest history of use, and are still used in various countries around the world, especially those below DN300mm, which still occupy a dominant position in all types of valves. According to the shape of the valve plate, it is divided into parallel gate valve and wedge gate valve. Wedge gate valve is commonly used internationally. Its characteristic is that the pressure difference between the front and rear of the valve plate makes the valve plate forced to press on the valve seat on the outlet side, with good sealing performance and fully open. When the valve plate is not left in the flow path, the pressure loss is small. Used to cut off the medium in the pipeline. The disadvantage is that the operating torque is large, and the valve body and stem are easily damaged during operation, especially the large-mouth gate valve. The size and weight are also large. The DN1000 gate valve is about 3200mm high and requires four people to operate it. Therefore, the gate valve is generally suitable for small-diameter, that is, the use of DN500 or less. The flow resistance of the gate valve is small from fully open to middle opening, and when it is close to fully closed, the flow resistance increases sharply. In a larger range, the flow resistance changes less. When the valve is close to full closure, the flow change is large. Do not use it when the valve is close to closed. At this time, it is difficult to fine-tune the flow. If the valve opening is extremely small, the flow velocity at the opening is extremely fast, which will cause cavitation on the underside of the gate, accompanied by intense vibration noise, and long-term throttling operation will often cause the metal of the valve body and gate to occur. Cavitation. If the gate valve is in the middle opening, because the valve plate is supported by the guide rails on both sides, it fits tightly when fully closed. In the middle position, the gap between the valve plate and the guide rail is large and vibration occurs. Therefore, even in areas where the incidence of cavitation is low, if the valve is throttled for a long time, the guide rail will be deformed, worn or damaged, and because the valve seat is not in full contact, and the surface pressure of the contact part is very high, it will often produce Seizure or adhesion, therefore, it is not suitable for flow control. Also note that when the gate valve has a small opening, the flow changes greatly. If the valve is opened and closed in an emergency, it will cause a sharp rise in pressure or negative pressure, especially when water hammer occurs when the valve is closed. Based on the metal seal gate valve, in recent years, the rubber soft seal gate valve has been developed. Its characteristics are: (1) There is no recess at the bottom of the valve body, and the fluid passage section is linear, which is not easy to accumulate and foreign matter. (2) The sealing surface of the gate is pressed on the valve body to stop water, and the valve closing torque is small. (3) The sealing performance is not affected by the bending and stretching of the piping. (4) The entire valve plate is lined with rubber, which is corrosion-resistant and does not produce rusty water. (5) The valve can replace the sealing material and O-ring when the water is fully opened. Due to its superior functions, the international demand (especially in Europe) is increasing rapidly. The soft-seal gate valve was invented by the Austrian 30 years ago. It produces more than 500,000 units in Europe every year, and it is almost 100% used. It is an alternative to the hard-seal gate valve. The manufacturing caliber is DN500 in Europe and Japan, DN400 is generally used in the United States, and DN1200 can be produced in Italy. 2.2 The current situation and development of butterfly valves In the 1930s, the United States invented butterfly valves. They were introduced to Japan in the 1950s and were only widely used in Japan in the 1960s. However, their promotion in China was only after the 1970s. At present, butterfly valves above DN300 mm have gradually replaced gate valves in the world. Compared with gate valves, butterfly valves have shorter opening and closing times, small operating moments, small installation space and light weight. Take DN1000 as an example, the butterfly valve is about 2T, and the gate valve is about 3. 5T, and the butterfly valve is easy to combine with various driving devices, with good durability and reliability. The disadvantage of the rubber seal butterfly valve is that when it is used for throttling, cavitation will occur due to improper use, causing the rubber seat to peel off and damage. For this reason, metal-sealed butterfly valves have been developed internationally, and the cavitation zone has been reduced. In recent years, China has also developed metal-sealed butterfly valves. In Japan, comb-shaped butterfly valves with cavitation resistance, low vibration and low noise have also been developed in recent years. Generally, the life of the seal seat is 15-20 years for rubber and 80-90 years for metal under normal conditions. However, the correct selection depends on the requirements of the working conditions. The relationship between the opening of the butterfly valve and the flow rate basically changes linearly. If it is used to control flow, its flow characteristics are also closely related to the flow resistance of the piping. For example, two pipelines are installed with the same valve diameter and form, but the pipeline loss coefficient is different, and the flow rate of the valve will also be very different. If the valve is in a state with a large throttle range, the back of the valve plate is prone to cavitation, which may damage the valve. Generally, it is used outside 15°. When the butterfly valve is in the middle opening degree, the opening shape formed by the valve body and the front end of the butterfly plate is centered on the valve shaft, and the two sides form different states. Therefore, one side of the valve body and the valve plate form a nozzle-like opening, and the other side is similar to a throttle opening. The nozzle side has a much faster flow rate than the throttle side, and negative pressure will be generated under the throttle side valve. The rubber seal may fall off. The operating torque of the butterfly valve has different values due to the different opening and closing directions of the valve. The horizontal butterfly valve, especially the large-diameter valve, due to the depth of water, the torque produced by the difference between the upper and lower water heads of the valve shaft cannot be ignored. In addition, when an elbow is installed on the inlet side of the valve, a bias flow is formed and the torque will increase. When the valve is in the middle opening, the operating mechanism needs to be self-locking due to the action of the water flow torque. The figure shows the flow characteristics of the butterfly valve installed in the pipeline. K is the loss coefficient of the entire pipeline except the butterfly valve. The characteristic when K is zero is the inherent characteristic of the valve. It means that when the K value is large, if K is 100, the valve will be closed even if it is 50. %, also the same as the flow when fully open.
2.3 Improvement of the exhaust valve The old two-hole exhaust valve is gradually replaced by the new high-speed intake and exhaust valve due to the small exhaust capacity and other reasons. This floating ball valve was successfully developed by Beijing Municipal Design and Research Institute in the early 1980s and passed the appraisal. At the same time, Japan also developed it in about the same time. In addition, the single orifice large and small orifice exhaust valves developed by our country can be installed on the pipeline in a certain way to eliminate pipeline water hammer. The exhaust capacity is several times that of the old exhaust valve. The internal exhaust passage of the valve requires high symmetry, so there are high requirements for installation, and the inclination angle should not be greater than 1.5 degrees. 2.4 The development of multifunctional control valve The valve uses the medium pressure of the pipeline itself as the power source for opening, closing, and adjustment. The pilot valve and the small system pipeline can be combined to have nearly 30 functions, and it is gradually being used more commonly. . The pilot valve acts on the change of the water level and pressure as the control object. Since there are many types of pilot valves, they can be used alone or in combination, so that the main valve can be individually or compounded to adjust the water level, water pressure and flow rate, etc. Function. However, the main valve is similar to a stop valve. When the valve is fully opened, its pressure loss is much greater than that of other valves, and the closer the opening loss coefficient is to the fully closed, the sharper the increase, and the larger the valve diameter, the more significant it is. The valve with the above characteristics will accelerate the action of the disc when it is close to fully closed, which is prone to water hammer (water impact pressure). When it is close to fully closed, the slower the valve action, the better, so you can set a throttle on the disc mechanism. In addition, the throttling and action parts of the pilot valve should be avoided as far as possible to set up extra-small diameter orifices to avoid blockage. If necessary, filter screens should be added, regular maintenance and bypass pipelines should be installed. The development and use prospects of this kind of valve are promising.
3 Two important parameters need to be further understood that the flow coefficient and cavitation coefficient of the valve are important parameters of the valve, which can generally be provided in the valve materials produced in advanced industrial countries, and even printed in the samples. Basically, the valves produced in our country do not have this information, because obtaining this information requires experiments to be put forward. This is one of the important manifestations of the gap between China and the world’s advanced level of valves. 3.1 The flow coefficient of the valve The flow coefficient of the valve is an index to measure the flow capacity of the valve. The larger the value of the flow coefficient, the smaller the pressure loss when the fluid flows through the valve. According to KV value calculation formula
Where: KV—flow coefficient Q—volume flow m3/h ΔP—valve pressure loss bar P—fluid density kg/m3 3.2 The cavitation coefficient of the valve uses the cavitation coefficient δ value to select when it is used to control the flow rate. What kind of valve structure to choose.
Where: H1—the pressure behind the valve (outlet) m H2—the difference between the atmospheric pressure and the saturated vapor pressure corresponding to its temperature m ΔP—the pressure difference before and after the valve m. Because of the different structures of various valves, the allowable cavitation coefficient δ is also different. as the picture shows. If the calculated cavitation coefficient is greater than the allowable cavitation coefficient, it means it is available and cavitation will not occur. If the butterfly valve allows air<
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