The flow characteristics of electric control valves directly affect the control quality and stability of the system, so it is necessary to choose them correctly. The flow characteristics of electric control valves are divided into ideal flow characteristics and working flow characteristics. The flow characteristics provided by general manufacturing plants are ideal flow characteristics, while practical applications require working flow characteristics. Due to the pressure drop ratio S being less than 1, the working flow characteristics are convex. Therefore, when selecting the flow characteristics of the regulating valve, the selection of the working flow characteristics should be considered first.
Then, select the ideal flow characteristics based on the actual application. The ideal flow characteristics commonly used in production are linear, equal percentage, and fast opening characteristics. The fast opening characteristic is mainly used for dual position control and program control, so the selection of flow characteristics for regulating valves usually refers to how to reasonably choose linear and equal percentage ideal flow characteristics. In actual use, the regulating valve is always installed in the process pipeline system, and the pressure difference before and after the regulating valve changes with the resistance of the pipeline system. Therefore, when selecting the flow characteristics of the regulating valve, it is not only necessary to consider the process characteristics, but also the piping situation of the system.
(1) Select the working flow characteristics of the valve based on the process characteristics. The control law of a conventional controller is linear, and the parameters of the controller are tuned to adapt to a certain working range without the need for frequent adjustment. This requires the generalized object to be linear, that is, when encountering changes in load, valve front pressure, or set value, the characteristics of the generalized object remain basically unchanged. Therefore, from the perspective of automatic control systems, the principle for selecting the working characteristics of the regulating valve is to make the entire generalized object have linear characteristics. That is, in the generalized object, when the characteristics of the other parts (transmitter characteristics, process characteristics) except for the regulating valve are linear, the regulating valve with linear working flow characteristics (i.e. Kv is a constant) should be selected;
If the transmitter characteristics are linear, and the amplification factor of the process characteristics decreases with the increase of the control variable, the regulating valve should choose an equal percentage working flow characteristic. In summary, when a generalized object (excluding a regulating valve) has nonlinear characteristics, the regulating valve should be able to overcome its nonlinear effects and make the generalized object approach linear characteristics.
(2) Based on the piping situation, infer the ideal flow characteristics from the required working flow characteristics. In the production site, regulating valves are always connected to pipelines and other equipment for use, so there must be piping resistance, which leads to certain differences between the working flow characteristics of regulating valves and the ideal flow characteristics. Therefore, when selecting the characteristics of the regulating valve, consideration should also be given to the process piping situation of the system. After determining the pressure drop ratio S value based on the process piping situation, the ideal flow characteristics can be determined from the selected working flow characteristics.