Introduction, performance, and applications of exhaust valves

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This post was last edited by feng_200108047 on 2011-9-30 at 13:04. Introduction, performance, and applications of vent valves: In pipeline transportation systems, valves that are used to remove air accumulated in the pipes and thereby improve the efficiency of medium transportation are known as vent valves. When the transfer pump stops and negative pressure is created as the pipe is emptied, the exhaust valve plug (valve disc) opens rapidly to draw in air, thereby preventing the pipe from being damaged by the negative pressure. Such a valve is called a composite exhaust valve, as shown in Figure 2-100. http://www.sn-v.com/images/pqvalve.GIF Exhaust valves are widely used in modern water supply and drainage systems, and are generally installed at the outlet of pumps or within the water supply and drainage pipes. Under normal conditions, water contains about 2 VOL% dissolved air. During water transfer, this air is continuously released from the water and accumulates at the highest points of the pipelines, forming air pockets that make water transfer difficult; as a result, the system’s water transfer capacity decreases by about 5–15%. The main function of the exhaust valve is to remove this dissolved air, thereby improving water transfer efficiency and saving energy. (1) Structure and working principle of the exhaust valve. The exhaust valve is generally cylindrical, with some being elliptical in shape (Figure 2-100). It is composed of components such as the valve body, valve cover, valve stem, lever, valve disc, valve seat, and float ball. The valve body and valve cover are generally made of ductile iron ； The valve stem, valve disc (plug), and float are generally made of stainless steel ； The valve seat is made of materials such as aluminum bronze, synthetic rubber, or F4. In water supply and drainage systems, when water begins to flow into the pipes, the valve disc (plug) is in the open position to allow for extensive air expulsion. Once all the air has been removed, water is poured into the valve; the float rises, causing the valve disc to close and thus stopping the air expulsion. When water is flowing normally inside the pipe, if a small amount of air accumulates inside the valve to a significant degree, the water level inside the valve drops, causing the float to descend as well; at this point, the air is expelled through the small holes. When the water pump stops, the water flow in the pipe is exhausted or a negative pressure is created within the pipe; at this point, the valve disc (plug) opens quickly to allow air to enter, thereby ensuring the safety of the pipeline. (2) Parameters and selection of the exhaust valve: Nominal pressure: PN≤1.6Mpa ； Nominal diameter: DN≤400mm ； Applicable temperature: t≤80℃ ； Applicable media: clean water, wastewater. In water supply and drainage networks, vent valves are generally installed at the highest point of the pipeline. When selecting vent valves with good performance, the following factors should be taken into consideration: ① The volume of air to be vented: It often takes a long time for water supply to return to its normal level after a shutdown, whereas valves with a larger venting capacity enable the water supply to return to normal levels in a much shorter time. ② Air shut-off pressure: This is the most important factor when selecting a good exhaust valve. If the air shut-off pressure is too low (such as 0.03 Mpa), the air has not yet started to be discharged, but the float inside the exhaust valve is already lifted by the air and closes the exhaust opening. Therefore, a good exhaust valve should have an air shut-off pressure of around 0.07 Mpa, which is sufficient to quickly discharge all the air from the pipe. When there is at least 0.05 Mpa of air inside the valve, the speed of the air can reach up to 90 meters per second – almost the speed of sound – resulting in a loud sonic boom and significant noise. ③ Water shut-off pressure range: Some exhaust valve devices are installed at the highest point of the pipeline. Since the water pressure in the pipe at this point can sometimes be very low, some of these exhaust valves require a water pressure of at least 0.05 Mpa to close completely; if the pressure is lower than this, leakage will occur. Therefore, the higher the water shut-off pressure range, the better. Generally, a range of 0.02 to 1 Mpa is the most commonly used. If the risk of pipeline rupture due to negative pressure is not considered, the appropriate diameter of exhaust valve can be selected directly from the table below; moreover, when the water flow velocity in the pipe is between 1.2 and 2.4 m/sec, a larger diameter should be chosen. Generally, tube rupture is possible only when negative pressure is generated within the tube and exceeds 0.028 Mpa. Nominal diameter DN (mm): 25, 50, 80, 100, 150, 200, 250, 300, 350, 400
CM3/h – Cubic meters per hour: 0–350, 220–740, 650–1600, 1300–3100, 3000–7500, 7300–15000, 11000–21000, 14000–31000, 19000–42000, 27000–59000
http://www.sn-v.com/images/dxpqf.GIF