Infrastructure construction is the focus of China's economic construction. Water supply and drainage projects, especially the large-scale sewage treatment projects, including the manufacturing of water treatment equipment such as pipes and valves, have formed a strong aquaculture industry. It provides a good opportunity for the development of the valve industry. China has entered WTO, and the global demand for valves is about 3 million tons every year. It is necessary to make full use of the advantages of labor-intensive valve industry, poor labor conditions, low per capita wages in China, and production transfer from developed countries to developing countries to actively move towards the platform of the world economy. The present situation and development of water supply and drainage valves are briefly described in several topics.
Current situation
At present, there are about 3000 large and small valve factories, including 600 in Wenzhou, nearly 200 in Zhengzhou, more than 150 in Shanghai, 250 in Liaoning, and more than 200 in Jiangsu and Fujian. Although the good and bad are mixed, the product varieties have reached more than 3000 models and nearly 30000 specifications. In terms of variety, sealing performance, strength requirements, regulating function, movement performance and circulation performance, especially sealing performance, great progress has been made. Ten years ago, an authoritative person of the urban water supply association said that "ten valves and nine leaks", which is a bit exaggerated, but it is true.
Many problems in the valve industry are mainly caused by the rapid expansion of the market after the reform and opening up, while the original state-owned valve enterprises have been closed and operated, and the level of the machinery industry is not high, so township enterprises (private enterprises) have mushroomed up rapidly, with a low starting point, very weak technical strength, simple equipment, and most of the products are imitation production, In addition, the market is in chaos, especially the low-pressure valve for water supply and drainage, which is a serious problem. In the past decade, driven by a small number of valve enterprises, there has been significant progress. Some varieties have even reached the international advanced level, and many enterprises have started their second business. But on the whole, there is still a big gap and there is much work to be done. Therefore, it is of great significance to make technological progress and development plans for 2010 and 2020 respectively.
Common valves
In water supply and drainage engineering, medium and low pressure valves are widely used, especially low pressure large diameter valves (low pressure valves refer to valves with nominal pressure PN ≤ 1.6MPa), while medium pressure valves (PN2.5-6.4MPa) are few. Gate valves and butterfly valves are the most basic and widely used leading products.
2.1 Current situation and development of gate valve
Gate valves have been used for the longest time, and they are still used all over the world, especially those below DN300mm, which still play a leading role in all kinds of valves. It is characterized by that the pressure difference between the front and rear of the valve plate forces the valve plate to press on the valve seat on the right side of the outlet, which has good sealing performance. When the valve plate is fully opened, because the valve plate does not remain in the flow path, the pressure
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The disadvantage is that the operating torque is large, and the valve body and valve stem are easy to be damaged during operation, especially for large diameter gate valves. The overall dimension and weight are also large. The DN1000 caliber gate valve is about 3200mm high and needs four people to operate, so the gate valve is generally used in small caliber, that is, below DN500.
When the gate valve is fully opened to the middle opening, the flow resistance of the valve is small. When it is close to full closing, the flow resistance increases sharply. In a large range, the flow resistance changes slightly. When the valve is close to full closure, the flow changes greatly. You must not use it when the valve is close to closure. It is difficult to fine tune the flow at this time. If the opening of the valve is extremely small and the flow rate at the opening is extremely fast, cavitation will occur at the lower side of the ram, accompanied by intense vibration and noise. Long time throttling operation will often cause cavitation of the valve body and the metal of the ram.
If the gate valve is in the middle of the opening, the valve plate is supported by the guide rails on both sides. When it is fully closed, it is closely matched. When it is in the middle, the gap between the valve plate and the guide rail is large, causing vibration. Therefore, even in areas with low cavitation rate, if the valve is operated through throttling for a long time, the guide rail will be deformed, worn or damaged, and since the valve seat is not in full contact, and the surface pressure of the contact part is high, it will often cause erosion or adhesion, so it is not suitable for flow control. Also note that when the gate valve is in a small opening, the flow changes greatly. If the valve is opened or closed in an emergency, the pressure will rise sharply or negative pressure will occur, especially when the valve is closed, water hammer will occur.
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