(1) orifice flow meter - pilot valve - the principle of the main valve. The main valve in front of a flow orifice plate, pilot sensing, comparison before and after the orifice pressure difference, such as the pressure difference greater than the set pressure, means that the flow exceeds the set flow, the pilot valve to control the main valve to close the valve action. If the sensing pressure difference is less than the set pressure difference, it means that the flow rate is less than the set flow rate, and the pilot valve controls the main valve to open the valve. Pilot valve set the pressure difference adjustable, adjust the small set small pressure difference, you can adjust the small adjustable small flow. As the orifice flowmeter flow pressure corresponding to the relationship between the former flow influence is extremely heavy, if the required flow rate accuracy of 10%, you must have 10d before the valve above the straight pipe, and this project is very difficult to protect in practice. In addition this valve factory flow adjustable range is very small, to maintain the accuracy of flow under the premise of the flow ratio can not exceed 2: 1 adjustable. (2) "Kv · = constant" principle, the free spring and the pressure sensitive membrane form the valve switch power system △ P / S = εL S-sense membrane working area, ε-spring Hook coefficient, L- valve stroke It can be seen Each stroke of the valve determines the value of ΔP. If Kv of the valve travel is inversely proportional to, then K = Kv · is a constant value. The principle of the valve initially made flow rate can not be adjusted flow limiter, adjustable flow in recent years, the production is made of a multi-tube channel, set the flow through the plugging adjustment; the other is to use a manual valve to change self-valve Kv and the relationship between the trip, but this method is difficult to guarantee Kv and the inverse relationship between the position of each adjustment, resulting in adjustment bit flow control accuracy is not high. In addition, some products use corrugated pipe to make pressure sensitive film and free spring integrated products. Because stainless steel corrugated pipe is in flowing dead zone, when chloride ion content in water is high, it is easy to produce corrosion. (3) Self-operated differential pressure control valve and manual control valve principle. This principle is now the most widely used flow control valve made. Manual control valve position of each opening corresponds to a Kv value by the self-operated pressure control valve to control the pressure difference before the manual control valve unchanged, then G = Kv · unchanged, only need to adjust the flow control valve manual adjustment Kv value. Flow control accuracy of this valve depends on the accuracy of the differential pressure control valve, the pressure difference △ P = N / SN - spring force S - pressure sensitive membrane working area spring force in the valve stroke will change, but H / △ L = 1/10 H-- maximum self-displacement valve displacement △ L - spring pre-compression △ P changes only ± 5%, flow accuracy of up to 3%. This self-flow control valve has the drawback of the valve has the minimum working pressure requirements, the general requirements of the product minimum working pressure 20KPa, if installed in the most adverse circuit, will inevitably require circulation pump to increase the working lift of 2 m water column, so Near-end installation should be taken, distal uneasy approach. Users from the heat source is greater than the heating radius of 80% should not install this self-flow control valve. (4) With the self-operated pressure control valve directly control the indoor flow resistance coefficient S, the average heating in the premise of the same value, indoor design flow G, △ P = SG2, by controlling indoor supply and return pressure Poor, the same can control the circulating flow, adjust the control pressure can adjust the circulating flow. Use this approach to regulate traffic, but must rely on portable flow test equipment such as ultrasonic flowmeter. This way for the remote user, the valve does not increase the pressure head. This article Keywords: control valve
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