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Section 1 Flow Coefficient of Valve The flow coefficient of the valve is an indicator of the valve's flow capacity. The larger the flow coefficient value, the smaller the pressure loss when the fluid flows through the valve. Developed foreign industries * * Most of the valve manufacturers include the flow coefficient values of valves with different pressure levels, different types and different nominal diameters in their product samples for selection by the design department and user units. The flow coefficient value changes with the size, form, and structure of the valve. Valves of different types and specifications must be tested separately to determine the flow coefficient value of the valve. 1. Definition of flow coefficient The flow coefficient represents the flow rate of the fluid when the fluid flows through the valve and produces unit pressure loss. Due to different units, the flow coefficient has several different codes and values. 2. Calculation of valve flow coefficient 3. Typical data of flow coefficient and factors affecting flow coefficient. The typical flow coefficients of various types of valves with a nominal diameter of DN50mm are shown in the table. The flow coefficient value changes with the size, form, and structure of the valve. The change of flow coefficient with diameter of several typical valves is shown in Figure 1-9. For valves with the same structure, the direction of fluid flow through the valve is different. The flow coefficient value also changes. This change is generally due to differences in pressure recovery. If the fluid flowing through the valve causes the valve disc to open, the annular diffusion channel formed by the valve disc and valve body can restore the pressure. When fluid flows through the valve causing the valve disc to close, the valve seat has a great influence on pressure recovery. When the disc opening is &#+ or less, the divergence angle downstream of the disc allows some pressure recovery in both flow directions. For the high-pressure angle valve shown in Figure 1-11, the flow coefficient is higher when the fluid flow causes the valve to close, because the diffusion cone of the valve seat restores the fluid pressure at this time. The internal geometry of the valve is different and the flow coefficient curve is also different. The mechanism of pressure recovery inside the valve is the same as the mechanism of pressure loss caused by the contraction and diffusion of the venturi tube. When the pressure drop inside the valve is the same, if the internal pressure of the valve can be restored, the flow coefficient value will be larger and the flow rate will be larger. Pressure recovery is related to the geometry of the valve cavity, but more importantly, it depends on the structure of the valve disc and valve seat. Section 2: Flow resistance coefficient of valve When fluid passes through the valve, its fluid resistance loss is represented by the fluid pressure drop Δp before and after the valve. 1. Fluid resistance of valve components The flow resistance coefficient of the valve depends on the size, structure and inner cavity shape of the valve product. It can be considered that each element in the valve body cavity can be regarded as a system of elements that generate resistance (fluid turns, expands, shrinks, turns again, etc.). Therefore, the pressure loss within the valve is approximately equal to the sum of the pressure losses of each component of the valve. It should be pointed out that changes in the resistance of one element in the system will cause changes or redistribution of resistance in the entire system, which means that the medium flow affects each other in each pipe section. In order to evaluate the influence of each component on the valve resistance, the resistance data of some common valve components are quoted. These data reflect the relationship between the shape and size of the valve components and the fluid resistance. (1) Sudden expansion will cause great pressure loss. At this time, part of the velocity of the fluid is consumed in the formation of vortices, fluid agitation, and heat generation. The approximate relationship between the local resistance coefficient and the ratio of the pipeline cross-sectional area A1 before expansion and the pipeline cross-sectional area A2 after expansion can be expressed by Equation (1-9) and Equation (1-10) ； The resistance coefficient is shown in Table (2). When θ＜40℃, the resistance coefficient of the gradually expanding circular tube is smaller than that of the sudden expansion. However, when θ=50-90℃, the resistance coefficient increases by 15%-20% compared with the sudden expansion. The optimal expansion angle θ that gradually expands: Circular tube θ=5-6.5℃, square tube θ=7-8℃, rectangular tube 10-12℃. (3) Sudden shrinkage (4) Gradual shrinkage (5) Smooth and even turns (6) Angular turns. Angular turns mainly occur in forged valves, because the medium channels of forged valves are processed by drilling methods. Sharp turns can also occur in welded valves. (7) Symmetrical tapered joints Symmetrical tapered joints are similar to valve narrowing channels. 2. Fluid resistance of the valve The flow resistance coefficient of the valve varies with the type, model, size and structure of the valve.