1 Current situation 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 inspected valves was 88.66%. Due to the limitation of inspection methods, it cannot be accepted according to national standards, such as strictly according to national standards. The qualification rate is even lower. Gate valves are mainly due to poor sealing performance, butterfly valves and poor operating mechanism reliability, etc. This situation has indeed been improved significantly. Nowadays, ductile casting, resin sand casting process, austenitic stainless steel valve shaft, Many manufacturers have mastered the technical and material issues of sealing valve seats, special valve varieties, anti-corrosion spray coatings, and sealing structure design. Some (a few) manufacturers have some products that have approached and reached the international advanced level, replacing imported products. , Or export to foreign countries, the overall level is constantly improving, and the distance from the advanced level of foreign countries is further reduced. 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 In urban, construction, and enterprise water supply and drainage projects, medium and low pressure valves are widely used, especially low pressure large diameter valves (low pressure valves are 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, others are ball valves, check valves, exhaust valves, plug valves, diaphragm valves, pressure reducing valves, Flow regulating valve, sludge discharge valve, etc.
2. 1 The status quo 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. Its disadvantage is that the operating torque is large, and it is easy to damage the valve body and stem 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 from fully open to middle opening is small, 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.
On the basis of metal seal gate valves, rubber soft seal gate valves have been developed in recent years. 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 will not be 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 while 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 status quo and development of butterfly valves In the 1930s, the United States invented the butterfly valve, and it was introduced to Japan in the 1950s. It was only widely used in Japan in the 1960s, and it was promoted in China 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, 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 are formed to complete different states. The front end of the butterfly plate on one side moves in the direction of the flowing water, and the other side is counter to the direction of the flowing water. 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. Rubber seals often 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. 50% is also the same as the flow when fully opened. 2. 3 Improvement of the exhaust valve The old double-hole exhaust valve is gradually being replaced by a 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 Development of multi-function control valve This valve uses the medium pressure of the pipeline itself as the power source to open, close, adjust, and use the pilot valve and the system<
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