First, the flow coefficient of the valve The flow coefficient of the valve is a measure of the ability of the valve flow index, the larger the value of the flow coefficient shows the pressure loss of the fluid flow through the valve smaller. Valve manufacturers in most industrialized countries in foreign countries put the flow coefficient values ​​of different pressure levels, different types and different nominal diameter valves into product samples for selection by design departments and users. Flow coefficient value with the valve size, form, structure and change, different types and different specifications of the valve to be tested separately in order to determine the type of valve flow coefficient values. 1. The definition of flow coefficient The flow coefficient indicates the fluid flow through the valve when the unit pressure loss occurs. Due to the different units, the flow coefficient has several different codes and values. 2. Valve flow coefficient calculation 3. Flow coefficient of the typical data and factors that affect the flow coefficient Nominal diameter DN50mm various types of typical flow coefficient valve shown in the table. Flow coefficient value with the valve size, form, structure and change. Several typical valve flow coefficient with the diameter changes as shown in Figure 1-9. For the same structure of the valve, the direction of fluid flow through the valve is different. Flow coefficient values ​​have also changed. This change is generally due to pressure recovery caused by different. If the fluid flows through the valve so that the valve tends to open, then the valve and the valve body formed by the annular diffusion channel can make the pressure recovery. When the fluid flows through the valve so that the valve tends to close, the seat pressure on the recovery of great influence. When the disc opening is + or less, the diffusion angle downstream of the disc causes some pressure recovery in both directions of flow. The flow coefficient is higher as the flow of the fluid tends to close the valve, as the diffuser cone of the seat recovers the pressure of the fluid. Different internal valve geometry, flow coefficient curve is also different. The mechanism of pressure recovery inside the valve is the same as the pressure loss mechanism caused by the contraction and diffusion of the Venturi tube. When the same pressure drop inside the valve, if the valve pressure can be restored, the flow coefficient value will be larger, the flow will be larger. Pressure recovery and the valve cavity geometry, but more importantly depends on the disc, valve seat structure. Second, the valve flow resistance coefficient Fluid through the valve, the fluid resistance loss to the valve before and after the fluid pressure drop △ p said. 1. Valve element fluid resistance Valve flow resistance coefficient! Depending on the size of the valve product, structure and shape of the cavity. It can be assumed that each element in the body cavity of the valve can be considered as a component system that generates a resistance (fluid turns, expands, contracts, turns, etc.). So the pressure loss within the valve is equal to the sum of the pressure loss of the various components of the valve. It should be noted that changes in the resistance of a component in a system can cause changes or redistribution of resistance throughout the system, meaning that the media flow affects each pipe section. To assess the impact of each component on valve resistance, reference is made to the resistance data for some common valve components that reflect the shape and size of the valve components as a function of fluid resistance. (1) sudden expansion will have a great pressure loss. At this time, the fluid part of the speed consumed in the formation of vortex, fluid agitation and fever and so on. The approximate relationship between the local resistance coefficient and the ratio of the cross-sectional area A1 of the pipeline before expansion and the cross-sectional area A2 of the pipeline after expansion can be expressed by Eqs. (1-9) and (1-10); the resistance coefficient is shown in Table (2) When θ <40 ℃, the resistance coefficient of the tube gradually enlarged is smaller than that of the sudden expansion. However, when θ = 50-90 ℃, the resistance coefficient increases 15% -20% more than the sudden expansion. Gradual expansion of the best angle of expansion θ: circular tube θ = 5-6.5 ℃, square tube θ = 7-8 ℃, rectangular tube 10-12 ℃. (3) Shrinking (4) Shrinking (5) Smooth and Uniform Turning (6) Corner Turning Corner turning mainly occurs in forged valves because the media passages for forged valves are machined by drilling. In the welding valve will have a sharp turn. (7) Symmetrical tapered joint Symmetrical tapered joint similar to the valve necked-in channel. 2. Valve fluid resistance Valve flow resistance coefficient with the valve type, model, size and structure of the different. Third, the pressure loss of the valve As the butterfly valve in the pipeline pressure loss is relatively large, about three times the gate valve, so the choice of butterfly valve, piping system should be fully taken into account the impact of pressure loss.
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