How to select the flow rate for electric control valves

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The flow characteristics of electric control valves directly affect the control quality and stability of the system, so it is necessary to select them properly. The flow characteristics of electric control valves are divided into ideal flow characteristics and operating flow characteristics. The flow characteristics provided by general manufacturing plants are ideal flow characteristics, whereas what is required in practical applications are operating flow characteristics. Since the pressure drop ratio S is less than 1, the operating flow rate characteristic is convex. Therefore, when selecting the flow characteristic of a control valve, the operating flow characteristic should be considered first. Then, select the ideal flow characteristic based on practical applications. The ideal flow characteristics commonly used in production are linear, equal percentage, and quick-open characteristics. The quick-opening characteristic is mainly used for two-position control and program control; therefore, the selection of the flow characteristic for control valves generally involves determining how to appropriately choose between linear and equal percentage ideal flow characteristics. In actual use, control valves are always installed in process piping systems, and the pressure difference before and after the control valve changes depending on the resistance of the piping system. Therefore, when selecting the flow characteristic of a control valve, it is necessary to consider not only the process characteristics but also the piping layout of the system. (1) Select the operating flow characteristic of the valve based on the process characteristics. The control law of a conventional controller is linear; once its parameters are tuned, it is expected to function within a certain operating range without the need for frequent adjustments. This requires the generalized object to be linear, meaning that its characteristics remain essentially unchanged in the face of load changes, pressure variations before the valve, or changes in setpoints. Therefore, from the perspective of automatic control systems, the principle for selecting the operating characteristics of control valves is to ensure that the entire generalized plant exhibits linear behavior. That is, when the characteristics of all components in the generalized plant other than the control valve itself (such as transmitter characteristics and process characteristics) are linear, a control valve with linear flow-rate characteristics should be chosen (i.e., one for which Kv is a constant) ； If the transmitter characteristics are linear, and the amplification factor α of the process characteristics decreases as the control variable increases, then a control valve with an equal percentage flow characteristic should be selected. In short, when general objects (other than control valves) exhibit nonlinear characteristics, the control valve should be able to overcome these nonlinear effects and bring the general object close to linear behavior. (2) Based on the piping conditions and starting from the required operating flow characteristics, the ideal flow characteristics are inferred. At the production site, control valves are always used in conjunction with equipment such as pipes, and therefore piping resistance inevitably exists, causing a certain difference between the flow characteristic of the control valve and its ideal flow characteristic. Therefore, when selecting the characteristics of the control valve, the system’s process piping conditions should also be taken into consideration. After determining the pressure drop ratio S value based on the process piping conditions, the ideal flow characteristic can be established starting from the selected operating flow characteristic.