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The flow resistance coefficient of a valve depends on factors such as the size, structure, and internal cavity shape of the valve product. It can be considered that each component within the valve chamber can be viewed as a system of components that generate resistance (such as fluid turning, expanding, contracting, turning again, etc.). Therefore, the pressure loss within the valve is approximately equal to the sum of the pressure losses of all the components of the valve. It should be noted that a change in the resistance of one component within the system will cause changes or redistributions in the resistances throughout the entire system; in other words, the flow of the medium affects each pipe section mutually. To evaluate the impact of various components on the valve resistance, resistance data for some common valve components are presented here; these data reflect the relationship between the shape and size of the valve components and the fluid resistance. (1) Sudden expansion will cause a large pressure loss. At this point, part of the fluid’s velocity is consumed in forming vortices, stirring the fluid, and generating heat. The approximate relationship between the local resistance coefficient and the ratio of the pipe cross-sectional area A1 before expansion to the pipe cross-sectional area A2 after expansion can be expressed by equations (1-9) and (1-10) ； The resistance coefficient is shown in Table (2) and increases gradually. When θ < 40°C, the resistance coefficient of a gradually expanding circular tube is lower than that of one with a sudden expansion; however, when θ = 50–90°C, the resistance coefficient is 15%–20% higher than in the case of a sudden expansion. The optimal expansion angle θ that increases gradually: for circular tubes, θ=5-6.5°C; for square tubes, θ=7-8°C; for rectangular tubes, θ=10-12°C. (3) Sudden reduction (4) Gradual reduction (5) Smooth and uniform turning (6) Angular turning. Angular turning occurs mainly in forged valves, as the flow channels in such valves are created by drilling. Sharp turns also occur in welded valves. (7) Symmetric conical joint: A symmetric conical joint is similar to the throat section of a valve.