The current status of the valve industry
There are currently about 5000 valve factories of various sizes, including 1000 in Wenzhou, nearly 500 in Zhengzhou, over 1000 in Shanghai, 250 in Liaoning, and over 500 each in Jiangsu and Fujian. There are also small parts in other regions, although the quality varies, the product variety has reached more than 3000 models and nearly 30000 specifications. There has been significant progress in terms of variety, sealing performance, strength requirements, regulating function, action performance, and flow performance, especially in terms of sealing performance. Ten years ago, there was an industry saying from the Urban Water Supply Association that "ten valves and nine leaks". Although it may be exaggerated, it is indeed a fact.

The numerous problems in the valve industry are mainly caused by the rapid expansion of the market after the reform and opening up, and the closure and transformation of existing state-owned valve enterprises. The level of mechanical industry was already not high, so township enterprises (private enterprises) quickly developed like mushrooms after rain, with a low starting point, weak technical strength, rudimentary equipment, and most products being imitated for production. In addition, the market is chaotic, especially for low-pressure valves used in water supply and drainage, the problem is serious. In the past decade, driven by a few valve companies, there has been significant progress, with some varieties even reaching a certain level, and many enterprises engaging in secondary entrepreneurship. But overall, there is still a big gap and a lot of work to be done. Therefore, it is of great significance to make separate plans for technological progress and development in 2010 and 2020.

Commonly used 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 relatively rare. Gate valves and butterfly valves are the most widely used leading products.

The current situation and development of gate valves

Gate valves have a long history of use and are still used in various countries around the world, especially for valves below DN300mm, which still dominate in various types of valves. Its characteristic is that the pressure difference between the front and rear of the valve plate forces the valve plate to press against the valve seat on the right side of the outlet, which has good sealing performance. When 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.

Its disadvantage is that it has a large operating torque, which can easily damage the valve body and stem during operation, especially for large-diameter gate valves. The external dimensions and weight are also large. The DN1000 caliber gate valve is about 3200mm high and requires four people to operate, so gate valves are generally used for small diameters, i.e. below DN500.

When the gate valve is fully opened to the middle opening, the flow resistance of the valve is relatively small, and when it approaches full closure, the flow resistance increases sharply. Within a larger range, the change in flow resistance is relatively small, and when approaching full closure, the flow rate changes significantly. It is important not to use it when the valve is approaching closure, as it is difficult to make small adjustments to the flow rate. If the opening of the valve is extremely small and the flow rate at the opening is extremely fast, it will cause cavitation on the lower side of the gate, accompanied by intense vibration and noise. Long term throttling operation often leads to metal cavitation on the valve body and gate.

If the gate valve is in the middle opening position, the valve plate is supported by the guide rails on both sides and fits tightly when fully closed. However, when in the middle position, the gap between the valve plate and the guide rail is large, causing vibration. Therefore, even in areas with low incidence of cavitation, if the valve is throttled for a long time, the guide rail will still be deformed, worn, or damaged. Moreover, due to the incomplete contact of the valve seat and the high surface pressure at the contact area, it often leads to biting or adhesion. Therefore, it is not suitable for flow control. It should also be noted that when the gate valve has a small opening, the flow rate changes greatly. If the valve is opened or closed urgently, it will cause a sharp increase in pressure or negative pressure, especially the generation of water hammer when closing the valve. How to improve the service life of valve packing
In valve design or usage experience, the packing of lift valves is mostly asbestos packing, graphite packing, or PTFE V-type packing. However, these types of packing are limited by the number of valve opening and closing times. As the number of valve opening and closing times increases, the packing will wear out and the gap will gradually increase. At a certain number of times (according to statistics, it will not exceed 2000 times), the valve will leak from the packing. After leakage, it is necessary to re tighten the packing gland. This scheme is suitable for situations where the valve opening and closing frequency is not high.
If the valve is opened and closed frequently (such as 100000 to 300000 times a year), it undoubtedly increases the cost of using and maintaining the valve.