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The flow characteristic of a control valve is the relationship between the relative flow rate of the medium flowing through the control valve and the relative displacement (relative opening of the control valve). Generally speaking, the flow rate can be controlled by changing the flow section of the valve core and valve seat of the control valve. But in fact, due to the influence of many factors, for example, when the cross-sectional area changes, the pressure difference before and after the valve also changes, and the change in pressure difference will cause the flow rate to change. When the pressure difference between the front and rear of the valve remains unchanged, the flow characteristics of the control valve are called ideal flow characteristics. ； The structural characteristics of the control valve refer to the relationship between the valve core displacement and the fluid flow cross-sectional area. It is purely determined by the size and geometry of the valve core. In addition to the geometry of the control valve, the influence of the flow coefficient is also considered when the pressure difference remains unchanged. Therefore, the ideal flow characteristics and structural characteristics of the control valve are different. There are four main types of rational flow characteristics: linear, equal percentage, parabola and quick opening. In the actual production application process, the pressure difference before and after the control valve always changes. The flow characteristics at this time are called working flow characteristics. Because control valves are often used in series or parallel with process equipment, the flow rate changes due to changes in resistance loss. In actual work, the ideal flow characteristics are distorted into working characteristics due to changes in the pressure difference before and after the valve. The ideal flow characteristics of control valves are commonly used in production: straight line, equal percentage, and quick opening. Parabolic flow characteristics are between straight line and equal percentage. Generally, equal percentage can be used instead, while quick opening characteristics are mainly used in two-position adjustment and program control. Therefore, the characteristic selection of control valve refers to how to select the linear and equal percentage flow characteristics. At present, the selection of control valve flow characteristics mostly adopts empirical criteria, which can be considered from the following aspects:: 1. Quality analysis of the regulating system. The figure below shows an automatic regulating system of a heat exchanger. It consists of regulating objects, transmitters, regulating instruments and control valves. The amplification factor of K1 transmitter, the amplification factor of K2 adjustment instrument, the amplification factor of K3 actuator, the amplification factor of K4 control valve, and the amplification factor of K5 adjustment object. It is obvious that the total amplification factor K of the system is: K=K1 * K2 * K3 * K4 * K5 K1, K2, K3, K4, and K5 are the amplification coefficients of transmitters, regulating instruments, actuators, control valves, and regulating objects respectively. In order to ensure that the regulating system can still maintain the predetermined quality index when the load changes. ； It is then expected that the overall amplification factor remains constant over the entire operating range of the regulating system. Usually, the amplification factor of the transmitter, regulator (which has been adjusted) and the actuator is a constant, but the amplification factor of the regulated object always changes as the operating conditions change, so the characteristics of the object are often non-linear. Therefore, the characteristics of the control valve should be appropriately selected, and the change in the amplification coefficient of the valve should be used to compensate for the change in the amplification coefficient of the regulating object, so that the total amplification coefficient of the system remains unchanged or approximately unchanged, thereby improving the quality of the regulating system. Therefore, the selection of control valve flow characteristics should comply with: K4 * K5=constant Regarding the phenomenon that the amplification coefficient becomes smaller as the load increases, if an equal-percent characteristic control valve whose amplification coefficient becomes larger as the load increases is selected, the two can cancel each other out, and the combined result will keep the total amplification coefficient unchanged, which is approximately linear. When the amplification factor of the adjustment object is linear, the linear flow characteristics should be adopted to keep the total amplification factor unchanged. 2. Considering the process piping conditions, control valves are always used together with pipelines, equipment, etc. Due to different system piping conditions, the existence of piping resistance causes changes in the pressure drop on the control valve. Therefore, the working flow characteristics of the valve are also different from the ideal flow characteristics of the valve. The desired operating characteristics must be selected based on the system characteristics, and then the corresponding ideal characteristics must be selected considering the piping conditions. The pressure difference across the control valve, all process equipment and piping systems is called the total system pressure difference. When the control valve is fully open, the ratio of the pressure difference ΔPmin before and after the valve to the total pressure difference ΔP of the system is called the S value, that is: S= ΔPmin/ΔP Considering the process piping situation, you can refer to the table below to select the corresponding ideal characteristics. Table 1 Consider process piping situation Table piping situation S= 1-0.6 S= 0.6-0.3 S < 0.3 The working characteristics of the valve are straight lines and equal percentages. Straight lines and equal percentages are not suitable for control. Ideal characteristics of the valve are straight lines and equal percentages. Equal percentages are not suitable for control. As can be seen from the above table, when S=1-0.6, the selected ideal characteristics are consistent with the working characteristics. When S=0.6-0.3, if the operating characteristics are required to be linear, equal percentage should be selected. This is because the ideal characteristics of the valve are equal percentage characteristics. When S=0.6-0.3, the distorted operating characteristics are already close to linear. ； When the required operating characteristics are equal percentages, the ideal curve should be more concave than it is. In this case, it can be compensated by the valve positioner cam profile curve or solved by using hyperbolic characteristics. When S