Valve status, application and development

Infrastructure construction is the focus of China's economic construction. The water supply and drainage project, especially the sewage treatment project, is large in size, the WTO has entered, the development of the western region has started, and the valve market has a promising future. Although China's water supply business has a history of nearly 120 years and is 7 years earlier than Japan, it still has a huge gap and needs to be built. The current situation is that half of the 700 cities in China are short of water, and 100 cities are seriously deprived of water. The average annual water consumption will increase by 7 million m3/d. In particular, one of the keys to the treatment of environmental pollution during the “Ninth Five-Year Plan” period in China is the treatment of sewage, focusing on the governance of three lakes (Taihu, Chaohu, Dianchi) and Sanhe (Huaihai , Liaohe, and Haihe), and at the same time requested that 640 county-level cities with a population of 500,000 or more in the country should build at least one city's sewage treatment plant by 2003. The wastewater treatment rate during the period of the 5th Five-Year Plan is 25% and the demand in 2010 is 46%. That is, 1000 plants. At present, there are sewage treatment plants in more than 200 prefecture-level cities nationwide. In the next few years, sewage treatment plants will be built in nearly 400 cities. By 2003, 180 billion yuan will be spent on urban sewage treatment. Since 2000, 60 billion yuan will have been invested each year. The annual investment in equipment will reach 20 billion. The valve also occupies a notable share. A medium-sized sewage treatment plant will require approximately 600-800 investment in the valve. Million. In addition, the demand for water supply projects, hydropower, and chemical industries is also large. By 2008, Beijing will invest 180 billion yuan to host the Olympic Games. Most of these will be used for capital construction. The number of valves used in the capital construction will also be substantial. The above situation is only for the domestic market. After entering the WTO, we can see only a small segment of the domestic market. The bigger market is in foreign countries. Now we have entered globalization to make full use of the advantages of China's valve industry and actively go to the world economy. platform. Several topics briefly describe the status of valve design, production status, application and development in China. 

1 Current situation

After the reform and opening up, along with the vigorous development of China's economic construction, the valve industry has also shown a thriving vitality. There are now thousands of large and small valve plants. Despite the mixed results, the product variety has reached more than 3,000 models and nearly 30,000 specifications. In sealing performance, strength requirements, adjustment functions, action performance and circulation performance, especially sealing performance, there has been considerable progress, and ten years back, the authoritative sources of the Town Water Supply Association have a saying in the industry that “Ten Valves and Nine Leaks”, Although somewhat exaggerated, it is indeed not satisfactory. Example: In September 1989-December 1989, Chengdu Water Supply Company entered the factory and tested 145 valves with a qualification rate of 72.5%. In 1996, the 917 qualification rate was 88.66%. Due to the limitations of inspection methods, it cannot be accepted according to national standards, such as strictly following national standards. The pass rate is even lower, the gate valve is mainly poor in sealing performance, and the reliability of the butterfly valve and the operating mechanism are poor. This situation is obviously improved. Now the ball-casting enamel, resin sand casting process, austenitic stainless steel valve shaft, and hard Sealed valve seats, special types of special valves, anti-corrosion spray coating applications, sealing structure design and other technologies, material issues, many manufacturers have mastered, some (a few) manufacturers of some products have approached and reached the international advanced level, replacing imported products , Or export to foreign countries, the overall level has also been continuously improved, and the distance from advanced foreign countries has further narrowed. However, as the overall level of China's machinery manufacturing industry is not high, so the quality of the valve needs further improvement. 

2 commonly used valves

In cities, buildings, 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 nominal pressure PN≤1.6MPa), and medium pressure valves (PN2.5-6.4MPa) Less, according to the principle and structure classification (International commonly used 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 valves, sludge Valves and so on. 

2.1 Status Quo and Development of Gate Valves

Gate valves have the longest history of use, and they are still in use in all countries of the world. Especially below DN300mm, they still dominate the various types of valves. According to the shape of the valve plate, the valve is divided into parallel gate valve and wedge gate valve. The wedge gate valve is commonly used in the world. 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 outlet side, and the sealing performance is better and fully opened. When the valve plate is not left in the flow path, the pressure loss is small. Used to cut 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 large-sized gate valves. 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 suitable for small diameter, that is, the following DN500. 

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 violent vibration noise, which will cause the metal of the valve body and the gate to occur when the throttle is operated for a long time. 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, even in a region where the incidence of cavitation is low, if the valve is throttled for a long time, the guide rail may be deformed, worn, or damaged. Since the valve seat is not in full contact, the surface pressure of the contact portion is high, and this tends to occur. Erosion or adhesion, therefore, should not be used 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 is increased sharply or a negative pressure is generated, especially when the water hammer is closed. 

Based on metal-sealed gate valves, in recent years, rubber soft-seal gate valves have been developed, which are characterized by:

(1) There is no recess at the bottom of the valve body, and the fluid passes through the cross section in a straight line, which is not easy to accumulate and inclusion of 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. 

Thanks to its superior functions, the international demand (especially in Europe) has rapidly increased. The soft-seal gate valve was invented by Austrians 30 years ago. It produces more than 500,000 units in Europe and uses almost 100% per year. It is an alternative to hard-sealed gate valves. The manufacturing caliber is DN500 in Europe and Japan, DN400 in the United States, and DN1200 in Italy. 

2.2 Status and Development of Butterfly Valves

In the 30's, the United States invented the butterfly valve. It was introduced to Japan in the 1950s. It was generally adopted in Japan in the 1960s, and promotion in China was after the 1970s. At present, in the world, butterfly valves with DN300 mm or more have gradually replaced gate valves. Compared with the gate valve, the butterfly valve has a short opening and closing time, a small operation, a small installation space and a light weight. Taking DN1000 as an example, the butterfly valve is about 2T and the gate valve is about 3.5T. The butterfly valve is easy to be combined with various driving 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, the metal seal butterfly valve is now being developed internationally, and the cavitation area has been reduced. In recent years, China has also developed a metal seal butterfly valve. In recent years, Japan has also developed a comb-shaped butterfly valve that is resistant to cavitation erosion, low vibration, and low noise. 

Generally the life of the seal seat is under normal conditions, rubber 15 years - 20 years, metal 80 years - 90 years. 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 it is used to control the flow, its flow characteristics are also closely related to the flow resistance of the piping. If the diameters and forms of the two pipeline installation valves are the same, and the pipeline loss coefficient is different, the flow rate difference of the valves will also be very large. 

If the valve is in a large throttling state, the back of the valve plate is prone to cavitation and may damage the valve. Generally, it is used outside 15°. 

When the butterfly valve is in the middle opening degree, the shape of the opening formed by the front end of the valve body and the butterfly plate is centered on the valve axis, and the two sides form different states. The front end of the butterfly plate moves in the direction of the flowing water and the other side flows in the direction of the flowing water. Moving, therefore, one side of the valve body and the valve plate form a nozzle-like opening, the other side similar to the orifice-shaped opening, the nozzle side faster than the throttling side of the flow rate, and the throttle valve will produce negative pressure, often Rubber seals will come 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. 

When the valve is in the middle opening, the operating mechanism needs to be self-locking due to the effect of the water flow torque. 

The figure shows the flow characteristics of the butterfly valve installed in the pipe. K is the loss coefficient of the whole pipe except the butterfly valve. The characteristic when the K is zero is the inherent characteristic of the valve. When the value of K is large, if K is 100, the valve is closed even if it is 50. %, it is also the same as when it is fully open.

2.3 Improvement of exhaust valve

Older dual-port exhaust valves are gradually being replaced by new high-speed intake and exhaust valves because of their small discharge capacity. This type of ball valve was successfully developed by the Beijing Municipal Institute of Design and Design in the early 1980s, and passed the appraisal. At the same time, Japan was also successfully developed. And China's own development of a single hole large, small hole exhaust valve, respectively, according to a certain way installed on the pipeline to eliminate the role of water hammer pipe. The exhaust capacity is several times that of the old exhaust valve. The symmetry of the internal exhaust passage of the valve is very high, so the installation has higher requirements and the inclination angle should not exceed 1.5 degrees.

2.4 Development of Multifunctional Control Valves

The valve is based on the medium pressure of the pipeline itself as the power source for opening and closing, adjustment, and different combinations of the pilot valve and the system of small pipelines, it can have nearly 30 kinds of functions, and now it has gradually been more commonly used. 

The pilot valve operates by changing the water level and pressure as the control object. Because there are many kinds of pilot valves, they can be used alone or in combination, so that the main valve can be adjusted individually or in combination with the water level, water pressure, and flow rate. Features. However, the main valve is similar to the cut-off valve. When the valve is fully opened, the pressure loss is much larger than that of other valves. The closer the loss coefficient of each opening is to full closure, the larger the valve diameter is, the larger the valve diameter becomes. 

Valves with the above characteristics, when approaching full closure, the action of the valve flap is accelerated, and water hammer (water impact pressure) is prone to occur. When close to full closure, the slower the valve movement, the better, so throttling can be provided on the valve flap. mechanism. In addition, the throttling and moving parts of the pilot valve should be as small as possible to avoid setting a small orifice to avoid clogging. When necessary, filter screens should be added, and regular inspections and bypass lines should be provided. This kind of valve has good prospects for development and use.

3 Two important parameters need to further deepen understanding

The valve's flow coefficient and cavitation coefficient are important parameters of the valve, which can generally be provided in the valve data produced in advanced industrial countries, and even printed in the sample. Valves produced in China have basically no information in this regard, because the information obtained in this respect needs to be tested before it is proposed. This is one of the important manifestations of the valve gap between China and the world. 

3.1 valve flow coefficient 

The flow coefficient of the valve is an indicator of 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. 

Calculated according to KV value: KV-flow coefficient

Q—volume flow m3/h

ΔP—pressure loss of the valve

P-fluid density kg/m3

3.2 Cavitation coefficient of valve

Use the cavitation coefficient δ value to select what valve structure type to use when controlling the flow rate.

In the formula: H1—after the valve (outlet) pressure m

H2—the difference between saturated vapor pressure and atmospheric pressure corresponding to atmospheric pressure

ΔP—pressure difference before and after the valve

Because of the different configurations 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 is available and no cavitation will occur. If the butterfly valve allows a cavitation coefficient of 2.5, then:

If δ>2.5, cavitation does not occur. 

When 2.5>δ>1.5, slight cavitation occurs. 

When δ<1.5, vibration occurs. 

When δ < 0.5 continues to be used, it will damage the valve and downstream piping.

The basic characteristic curve and operating characteristic curve of the valve are invisible to the valve when cavitation occurs, and it is impossible to reach the operating limit at that point. Through the above calculations at a glance. The reason why cavitation occurs is that when a liquid passes through a tapering cross section during accelerated flow, part of the liquid is vaporized, and the generated bubbles subsequently burst in the open section of the valve, and its performance is threefold:

(1) Noise occurs

(2) Vibration (when severe can cause damage to the foundation and related structures, resulting in fatigue fracture)

(3) Destruction of materials (erosion of valve bodies and pipes)

From the above calculation, it is not difficult to see that there is a great relationship between cavitation and the pressure H1 after the valve. Increasing H1 will obviously change the situation. The improvement method is:

a. Install the valve at the lower point of the pipe. 

b. The orifice plates on the rear of the valve increase the resistance. 

c. The valve outlet is open and a direct reservoir is provided to increase the space for bursting bubbles and reduce cavitation. 

The above four aspects of the analysis and discussion, summed up the gate valve, butterfly valve main features and parameter list for easy selection.

4 valve standards must pay attention to

Now the State Council has established a standardization management committee and a standards bureau. The standardization issue has been highly valued by the country and is an important means to improve our products. It is also an important response measure for the entry of the WTO. 

At present, China has more than 19,000 national standards, more than 23,000 local standards, and more than 80,000 corporate standards. Among them, 43.5% use international standards and only 25% equivalent to international standards. 

The valve professional has about 60 national standards and more than 180 industry standards. Many special and special valves do not have national or industrial standards and urgently need to be formulated. In terms of standard setting, the state has formulated guidelines and policies for reforms, starting from the four aspects of the management system, system, operation mechanism, and work mode. In the future, the formulation of standards should be based on companies, be open, transparent, and widely involved, and should increase the attention of the leaders of enterprises and the society. 

5 problems that should be paid attention to in the valve for water supply and drainage engineering

5.1 Commonly used valves for water supply and drainage works are the same as those for other industries, such as butterfly valves, gate valves, and check valves, but special valves with special features are also required. Such as the exhaust valve, slow closing check valve, flow regulating energy dissipation valve, control valve, fire signal valve, stop pump water hammer elimination valve (water hammer eliminator), anti-pollution check valve, muffler valve and so on. Compared with foreign countries, the variety of general-purpose valves is relatively complete, but there is a large gap in the variety of special valves that need to be filled. 

5.2 The inner wall paint used for water supply engineering valves must be non-toxic and can only be used after being approved by the provincial and epidemic prevention departments. 

Valves used for sewage works should be resistant to corrosion and not easy to block, while for mud discharge is more quickly open and close the straight channel structure type, such as soft seal gate valve; when using the butterfly valve fastening bolts applied austenitic stainless steel, convex and concave part easy Hold the fiber, and should be horizontal. 

5.3 The operating mechanism should be worm gear drive, and designed to be self-locking, that is, the indexing angle λ of the indexing circle is smaller than the equivalent friction angle φ between the teeth, and the backlash of the tooth face is smaller, otherwise the head is shaken. 

5.4 When the butterfly valve inlet pipe is equipped with a regulating valve or equipped with a water pump, it cannot be too close, because the valve will vibrate when the flow of water is turbulent. 

5.5 When the front or rear valve bends too close, because the force is unbalanced, the tightness of the valve seat is affected. 

5.6 The pump suction pipe should be fitted with a butterfly valve with a centerline shape. When installing an eccentric butterfly valve, it should be installed horizontally. Otherwise, the suction water of the double-suction centrifugal pump is unbalanced. 

5.7 When the large-diameter butterfly valve is horizontally mounted, the torque should be 0.2 times larger. Attention should be paid when configuring the valve operating mechanism. The media should not be net and should be horizontally mounted. When the valve is horizontally mounted, the bearing condition at both ends of the valve shaft is good. 

5.8 When the butterfly valve is used as a diverter valve, it should not be used within a 15° degree of opening, which will cause vibration and cavitation. When the gate valve is at a small opening, the closing speed must be slow, otherwise it will produce a water hammer that closes the valve.

5.9 List of main performance parameters of gate valve and butterfly valve for reference.
Type of Valve Gate Valve Butterfly Valve Metal Valve Seat Rectification, Comb Shape Main Use Cut-Off Control, Cut-off, Control Permissible Flow Velocity (m/s) 3 (6) 3 (6) 6 6 Seat tightness is good with a small amount of leakage Excellent durability of valve seats Excellent rubber peeling Disadvantages Excellent loss coefficient at full opening 0.1-0.2 0.3 0.3 0.8 Allowable cavitation system 3 2.5-4.0 1.5-3.5 0.9-3.0 Flow control characteristics Unsuitable for cavitation outside rubber seats For example, the metal seat is a good driving method manual, electric, hydraulic manual, electric, pneumatic and hydraulic with the left and left features 1. Use a small caliber appropriate 1. Simple structure 1. Simple structure 1. Good control characteristics 2. Fully closed water well 2. Full-time pressure loss is small 2. Fully-closed water-retaining time 2. Good control characteristics 3. Short open-close time 3. Small pressure loss when fully closed 3. Short-time of opening and closing 3. Short-time of opening and closing 4. Small operating torque 5. Small size 4. Valve groove is easy to accumulate debris and is not strict 4. Small operating torque 4. Small operating torque 6. Slightly complicated structure 5. Long opening and closing time 5. Small size and light weight 5. Small volume and weight Light 7. Resistant to 6. Operating torque is large 6. Cavitation will cause the rubber seat to flake off 6. Seat seat durability 7. Small amount of leakage