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Small flow control valve at home and abroad overview
First, the characteristics of small flow control valve The so-called small flow control valve, as its name implies, is the ability to control the flow of small valves. Valve flow capacity is the valve capacity under the unified conditions of indicators. Our country uses C value to express. It is defined as: When the valve is fully open, when the valve before and after the pressure drop of 1 kg / cm 2, medium weight of 1 g / cm 3, the hourly flow through the valve medium (m 3 / hour). For incompressible fluids, the flow capacity of the valve depends only on the structure of the valve itself in a sufficiently turbulent regime (Re> 105 for water Re> 105 for Re> 5x10 4 for Reynolds numbers). Valve flow in the calculation of the required capacity, should pay attention to different media or flow conditions are different, the valve flow state will be very different. At low flow rates, especially viscous fluids and at low pressures, the primary constraint of the fluid is often the laminar or mixed laminar and turbulent regimes. Laminar flow, the valve through the media flow and the valve before and after the pressure was linear. In the mixed laminar and turbulent flow, with the Reynolds number increases, even if the pressure is constant, the amount of medium flowing through the valve will increase. At full turbulence, the flow does not change with the Reynolds number. However, the choice of small flow control valve, still using the traditional methods and calculations. However, the calculated value and the actual value deviate greatly, according to the information introduced in Cv = O. 01 below, it is only as a capacity indicator, a reference only. The actual liquidity should be determined empirically. As flow capacity decreases, the valve's adjustable ratio will decrease. But at least 10: 1 to 15: 1 can be guaranteed, if the adjustable ratio is smaller, it is difficult to regulate the flow. When the valve is used in series, as the opening degree changes, the pressure difference between the front and the back of the valve also changes, thus deviating the working characteristic curve of the valve from the ideal characteristic. If the pipe resistance, linearity will become a quick opening characteristics, and loss of regulatory capacity. Equal percentage characteristics will become linear characteristics. In the case of small flow, due to the lack of pipeline resistance, the above characteristic distortion is not big, and the percentage equivalence is practically unnecessary. From a manufacturing point of view, Cv = O. 05 below, it is impossible to produce equal percentage of the side shape. Therefore, the main problem for small flow valve is how to control the flow within the required range. From the economic point of view, users want a valve can be used both for closure and regulation, and now it can be done. However, for the control valve, the main flow control is achieved, the closure is secondary. That the small flow valve itself is very small flow, it is easy to achieve shut-off when closed, it is wrong. Foreign leakage of small flow control valve generally also made the provisions. For example, Shanghai Petrochemical Plant Plastic Factory introduced the United States Bodeger (Bodger) research pneumatic diaphragm adjustment threshold, the Cv value of 10, the valve leakage is defined as: 5 kg / cm. Under pressure, the amount of leakage is less than 1% of the maximum flow.