Selection of valves for water supply and drainage pipelines and advantages and disadvantages of various valves

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This post was last edited by Sammy_Wang on July 10, 2015, at 17:17. I. Selection and installation locations of valves: (1) Valves used in water supply pipelines are generally selected according to the following principles: 1. When the pipe diameter is no more than 50 mm, globe valves are recommended; when the pipe diameter exceeds 50 mm, gate valves or butterfly valves should be used. 2. Regulating valves and globe valves are suitable when it is necessary to adjust flow rate and water pressure. 3. In areas where low water flow resistance is required (such as on the suction pipe of a water pump), gate valves are advisable. 4. Gate valves and butterfly valves should be used in pipe sections where the water flow needs to be bidirectional; globe valves must not be used. 5. Butterfly valves and ball valves are suitable for areas with limited installation space. 6. Globe valves are recommended for pipe sections that are frequently opened and closed. 7. Multifunctional valves are advisable to be used on the outlet pipes of water pumps with larger diameters. (II) Valves shall be installed at the following locations on the water supply pipes: 1. At the section where the water supply pipes in residential areas connect to the municipal water supply pipes. 2. At the nodes of the outdoor ring pipeline network in residential communities, they should be installed in accordance with the separation requirements. When the length of the annular pipe section is too large, it is advisable to install sectional valves. 3. The starting point of a branch pipe extending from the main water supply pipe in a residential complex, or the starting point of a service pipe leading to a household. 4. Inlet pipes, water meters, and various branch risers (at the bottom of the risers, as well as at the upper and lower ends of the vertical ring-shaped pipe network risers). 5. Branch pipes of circular pipe networks, and connection pipes that link branched pipe networks. 6. At the starting point of the distribution pipes that lead from the indoor water supply pipes to households, public restrooms, etc., installation is required when there are 3 or more water distribution points on the branch distribution pipes. 7. The outlet pipe of the water pump, and the suction pump of the self-priming water pump. 8. The inlet and outlet pipes, as well as the drain pipe of the water tank. 9. Water supply pipes for equipment (such as heaters, cooling towers, etc.). 10. Plumbing pipes for sanitary fixtures (such as toilets, urinals, washbasins, showers, etc.). 11. Certain attachments, such as automatic exhaust valves, pressure relief valves, water hammer arresters, pressure gauges, sprinkler heads, etc., as well as those located before and after pressure reducers and check valves. 12. A drain valve should be installed at the lowest point of the water supply network. (III) Check valves should generally be selected based on factors such as their installation location, the water pressure before the valve, the requirements regarding sealing performance when closed, and the magnitude of water hammer generated upon closure: 1. When the water pressure before the valve is low, swing check valves, ball check valves, and plug check valves are suitable choices. 2. When strict sealing performance is required after closure, a check valve equipped with a closing spring is advisable. 3. To reduce the impact of water hammer during shutdown, it is advisable to use quick-closing noise-suppressing check valves or slowly-closing check valves equipped with damping devices. 4. The valve disc or plug of the check valve should be able to close automatically under the action of gravity or spring force. (IV) Check valves shall be installed on the following sections of water supply pipes: on the service pipes ； On the inlet pipe of a sealed water heater or water-using device ； On the water pump outlet pipe ； On the outlet pipe section of water tanks, water towers, and elevated water reservoirs that use a single pipe for both inlet and outlet water. Note: Pipe sections equipped with pipe check valves do not require a check valve to be installed. (V) Exhaust devices should be installed at the following locations of the water supply pipes: 1. In water supply networks that are used intermittently, automatic exhaust valves should be installed at the end of the network and at its highest point. 2. In sections of the water supply network where air accumulates due to significant fluctuations, automatic exhaust valves or manual valves have been installed at the peaks of these sections for exhaust. 3. For pneumatic water supply systems, when using an automatically air-supplementing pressure tank, an automatic air vent valve shall be installed at the highest point of the distribution piping network. II. Advantages and disadvantages of various valves: 1. Gate valve: A gate valve is a valve in which the closing element (gate) moves perpendicular to the axis of the passage. On pipelines, it is primarily used to cut off the flow of media, meaning it operates in either fully open or fully closed positions. Generally, gate valves cannot be used to regulate flow. It can be used under low-temperature and low-pressure conditions as well as high-temperature and high-pressure conditions, depending on the material of the valve. However, gate valves are generally not used in pipelines for transporting media such as slurry. Advantages: ① Low fluid resistance ； ②The torque required for opening and closing is relatively small ； ③It can be used in ring piping systems where the medium flows in both directions; in other words, the flow direction of the medium is not restricted ； ④When fully open, the erosion of the sealing surface by the working medium is less than that in globe valves ； ⑤Its structural design is relatively simple, and it is easy to manufacture ； ⑥The structural length is relatively short. Disadvantages: ① The external dimensions and opening height are relatively large, requiring a considerable amount of installation space ； ②During the opening and closing process, there is relative friction between the sealing surfaces, resulting in significant wear; under high temperatures, scuffing is also likely to occur ； ③Generally, gate valves have two sealing surfaces, which adds some difficulties to processing, grinding, and maintenance ； ④The opening and closing time is long. 2. Butterfly valve: A butterfly valve is a type of valve that uses a disc-shaped closing element to rotate back and forth by about 90° in order to open, close, and regulate the flow of fluid. Advantages: ① Simple structure, small size, light weight, and low material consumption; suitable for use in large-diameter valves ； ②Fast opening and closing, low flow resistance ； ③It can be used in media containing suspended solid particles, and depending on the strength of the sealing surface, it can also be used for powdered and granular media. It can be used for two-way opening, closing, and adjustment in ventilation and dust removal pipelines, and is widely applied to gas pipes and water pipelines in metallurgy, light industry, power, and petrochemical systems. Disadvantages: ① The flow rate adjustment range is limited; when it is set to 30%, the flow rate already exceeds 95%. ②Due to the structure of butterfly valves and the limitations of their sealing materials, they are not suitable for use in high-temperature and high-pressure pipeline systems. The normal operating temperature is below 300°C, and the pressure rating is below PN40. ③Its sealing performance is inferior to that of ball valves and globe valves, so it is used in applications where high sealing requirements are not necessary. 3. Ball valve: It evolved from a plug valve. Its closing element is a sphere; rotating the sphere 90° around the axis of the valve stem serves to open and close the valve. Ball valves are primarily used in pipelines to shut off, distribute, and change the direction of fluid flow. Ball valves designed with a V-shaped opening also possess excellent flow regulation capabilities. Advantages: ① It has the lowest flow resistance (actually 0) ； ②It does not get stuck during operation (without lubricant), allowing it to be reliably used in corrosive media and low-boiling-point liquids ； ③Complete sealing can be achieved over a wide range of pressures and temperatures ； ④Fast opening and closing can be achieved; the opening and closing time for certain structures is as low as 0.05–0.1 seconds, ensuring their suitability for use in automated systems on test benches. When opening and closing the valve quickly, there is no operational shock. ⑤The spherical shut-off element can automatically position itself at the boundary position ； ⑥The working medium is reliably sealed on both sides ； ⑦When fully open or fully closed, the sealing surfaces of the sphere and the valve seat are isolated from the medium; therefore, the high-speed flow of the medium through the valve does not cause erosion of these sealing surfaces ； ⑧With its compact design and low weight, it can be considered the most suitable valve structure for low-temperature medium systems ； ⑨The valve body is symmetrical; especially in the case of welded valve bodies, it can effectively withstand the stresses applied from the pipelines ； ⑩The closing element can withstand the high pressure difference during closure. ⑾The ball valve with a fully welded body can be directly buried underground, protecting its internal components from corrosion. It has a maximum service life of 30 years, making it the ideal valve for oil and natural gas pipelines. Disadvantages: ① Since the main material used for the seat seal ring in ball valves is polytetrafluoroethylene, it is inert to almost all chemicals. It also possesses a number of advantages, including a low friction coefficient, stable performance, resistance to aging, a wide operating temperature range, and excellent sealing properties. However, the physical properties of polytetrafluoroethylene, including its high coefficient of expansion, sensitivity to cold flow, and poor thermal conductivity, require that the design of the valve seat seal take these properties into account. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Moreover, polytetrafluoroethylene has a relatively low temperature resistance and can only be used at temperatures below 180°C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120°C. ②Its regulating performance is somewhat inferior to that of globe valves, especially pneumatic valves (or electric valves). 4. Globe valve: It is a valve in which the closing element (valve disc) moves along the center line of the valve seat. Based on this type of movement of the valve disc, the change in the valve seat opening is directly proportional to the stroke of the valve disc. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat opening is proportional to the stroke of the valve disc, it is very suitable for regulating flow rate. Therefore, this type of valve is very suitable for use in cutting off or regulating flow, as well as for throttling. Advantages: ① During opening and closing, the frictional force between the valve disc and the sealing surface of the valve body is lower than that of gate valves, thus it is wear-resistant. ②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than gate valves ； ③Usually, there is only one sealing surface on the valve body and the valve disc, which makes the manufacturing process simpler and facilitates maintenance. ④Since its filler is generally a mixture of asbestos and graphite, it has a relatively high temperature resistance rating. Gauge valves are generally used for steam valves. Disadvantages: ① Due to the change in the flow direction of the medium as it passes through the valve, the minimum flow resistance of globe valves is also higher than that of most other types of valves ； ②Due to the long travel distance, its opening speed is slower than that of a ball valve. 5. Plug valve: It is a rotary valve whose closing element has a plug shape. By rotating it 90°, the passageway on the valve plug aligns with or separates from the passageway on the valve body, thereby opening or closing the valve. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to that of a ball valve, which was developed from a plug valve. It is primarily used in oil field extraction, as well as in the petrochemical industry. 7. Safety valve: It refers to a device used as an overpressure protection mechanism in pressurized containers, equipment, or pipelines. When the pressure inside a device, container, or pipeline rises above the allowable level, the valve opens automatically, allowing for complete discharge in order to prevent further increase in pressure within that device, container, or pipeline ； When the pressure drops to the specified value, the valve should automatically and promptly close, thereby ensuring the safe operation of equipment, vessels, or pipelines. 8. Steam traps: When conveying media such as steam and compressed air, some condensate water is formed. To ensure the efficiency and safe operation of the equipment, it is necessary to remove this unnecessary and harmful material promptly, so as to maintain proper consumption and use of the equipment. It has the following functions: ① It can quickly remove the condensed water that is generated ； ②Prevent steam leakage ； ③Remove air and other non-condensable gases. 9. Pressure reducing valve: A valve that reduces the inlet pressure to a desired outlet pressure through regulation, and relies on the energy of the medium itself to maintain the outlet pressure stable automatically. 10. Check valve: Also known as a non-return valve, reverse flow valve, backpressure valve, and one-way valve. These valves open and close automatically due to the force generated by the flow of the medium itself within the pipeline; they are a type of automatic valve. Check valves are used in piping systems; their main function is to prevent the backflow of fluid, to stop pumps and driving motors from rotating in reverse, and to prevent the leakage of fluid from containers. Check valves can also be used in pipelines that supply auxiliary systems whose pressure may rise above the system pressure. They can be mainly divided into swing-type (rotating around the center of gravity) and lifting-type.