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At present, there are about 3,000 large and small-sized valve factories, including 600 in Wenzhou, nearly 200 in Zhengzhou, 150 in Shanghai, 250 in Liaoning, and more than 200 in Jiangsu and Fujian. Although the products are mixed, the product variety has reached more than 3,000. Models, nearly 30,000 specifications. There has been considerable progress in variety of products, sealing performance, strength requirements, adjustment functions, action performance and flow performance, especially sealing performance. Ten years ago, the authoritative source of the Water Supply Association had a saying saying â€œTen Valves Nine Leaks, although somewhat exaggerated, is indeed a fact.
The numerous problems that existed in the valve industry were mainly due to the rapid expansion of the market after the reform and opening up, and the original state-owned valve companies were shut down and switched. The level of the mechanical industry was not high, and township and village enterprises (private enterprises) developed rapidly. , The starting point is low, the technical force is very weak, the equipment is simple, most products are imitating production, coupled with market chaos, especially the low-pressure valve for water supply and drainage, the problem is serious. In the past decade, driven by a small number of valve companies, there have been notable advances. Hong Feng concluded that some varieties even reached the international advanced level, and many companies have started their second ventures. But on the whole, there is still a big gap and there is a lot of work to be done. Therefore, it is of great significance to make technical progress and development plans for 2010 and 2020 respectively.
Commonly used valves are widely used in water supply and drainage projects. Low and medium-pressure valves are widely used, especially low-pressure and large-diameter valves (low-pressure valves refer to valves with a nominal pressure of PN â‰¤ 1.6MPa), and medium-pressure valves (PN2.5-6.4MPa) are less. Gate valves and butterfly valves are the most basic and widely used leading products.
Gate valve status and development The history of gate valve use is the longest, and it is still in use in all countries in the world. Especially below DN300mm, it still occupies a dominant position in various types of valves. Its characteristic is that the pressure difference between the front and the back of the valve plate makes the valve plate forcedly press on the valve seat on the right side of the outlet, and the sealing performance is good. When the valve is fully opened, the pressure loss is small because the valve plate is not left in the flow path. Used to cut off the medium in the pipeline.
The disadvantage is that the operating torque is large, and the valve body and the valve stem are easily damaged during operation, especially the large-diameter gate valve. Dimensions and weight are also large, DN1000 caliber gate valve is about 3200mm high, requires four people to operate, so the gate valve is generally used in a small diameter, that is, DN500 below.
Gate valve from full open to the middle opening, the valve flow resistance is small, close to fully closed, the flow resistance increased dramatically. In the larger range, the flow resistance changes little. When the flow is close to full closure, the flow rate changes greatly and must not be used when the valve is closed. It is difficult to fine tune the flow rate at this time. If the opening of the valve is extremely small and the flow rate at the opening is extremely fast, cavitation will be generated at the lower side of the gate, accompanied by intense vibration and noise. The long throttle operation will often cause the valve body and gate metal to occur. Cavitation. If the working condition of the gate valve is in the middle opening, the valve plate is supported by the guide rails on both sides. When the valve plate is fully closed, the valve plate is tightly fitted. In the intermediate position, the valve plate and the guide rail have a large gap and generate vibration. Therefore, Hong Feng believes that even in areas with low incidence of cavitation, long-term operation of the valve throttling, the guide rails will be deformed, worn or damaged, and the contact surface pressure is high because the valve seat is not in full contact. Cramping or sticking tends to occur, so it is not suitable for flow control. Also note that when the gate valve is at a small opening, the flow rate changes greatly. If the valve is opened and closed in an emergency, the pressure will rise sharply or generate negative pressure, especially when the valve is closed.
On the basis of metal-sealed gate valves, rubber soft-seal gate valves have been developed in recent years. Their characteristics are:
(1) There is no recess at the bottom of the body of the body, and the fluid passes through the cross section in a straight line, and it is not easy to accumulate and inclusion foreign matter.
(2) The sealing surface of the gate is pressed against 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 lined with rubber, corrosion resistance, no rust.
(5) The valve can replace the sealing material and the O-ring while fully opening the water.
(6) Gates are interchangeable.
Due to its superior functions, Hongfeng sees a rapid increase in international demand (especially in Europe). The soft seal gate valve was invented by Austrians 30 years ago. It produces more than 500,000 units per year in Europe and is used almost 100%. It is a hard-sealed gate valve. As an alternative product, its manufacturing caliber is DN500 in Europe and Japan, DN400 in the United States, and DN1200 in Italy. However, there are also deficiencies. The soft seal gate valve is compressed by the rubber seat. If it is not closed properly, it must be avoided. To avoid closing the valve too tightly, do not exceed the closing valve torque (the torque is given on the standard).
Butterfly Valve Status and Development In the 1930s, the United States invented the butterfly valve, which was introduced to China in the 1970s and developed and used. At present, the butterfly valves in the world generally have replaced the gate valves above DN300mm. Compared with the gate valve, the butterfly valve has the advantages of short opening and closing time, small operating torque, 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. The butterfly valve is easy to combine with various drive devices and has good durability and reliability.
The disadvantage of the rubber seal butterfly valve is that if it is used for throttling, cavitation may occur due to improper use, and the rubber seat may peel off or be damaged. For this reason, in the seventies and eighties of the last century, metal-sealed butterfly valves were developed in the world, which have cavitation resistance and long life. In recent years, China has also developed metal-sealed butterfly valves, but pay attention to the valve body, valve seat, and valve plate. If the material of the valve seat is not suitable for combination, there will be a risk of hot engagement between the valve seats, and they cannot be opened or closed. In addition, cavitation-resistant, low-vibration, low-noise comb-tooth butterfly valves have also been developed in foreign countries. Due to the improvement of casting technology, cast iron valve bodies, valve plates and stainless steel valve seats can be cast together, so that the structure of the valve seat part is simple, the guide material is not accumulated, and durability and reliability are greatly improved. The life of the general seal seat under normal conditions, rubber more than 15 years, stainless steel metal about 80 years, comprehensive comparison of different. However, how to use it correctly must be based on working conditions. The relationship between the opening and the flow of the butterfly valve is basically a linear proportional change. If the valve is in a large throttle state, the back of the valve plate is prone to cavitation, which may damage the valve. Generally, it is used outside 15 degrees. Below the throttle valve will produce negative pressure, rubber seals often appear off. Butterfly valve operating torque, due to opening and valve opening and closing direction of different values, horizontal butterfly valve, especially large-diameter valve, due to water depth, the shaft shaft, the head of the water generated by the torque can not be ignored. In addition, when the elbow of the inlet side of the valve is deflected, the bias flow will increase and the torque will increase.
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