The flow characteristic of electric control valve includes ideal flow characteristics and working flow characteristics. The flow characteristic provided by the general manufacturing factory is the ideal flow characteristic, but the practical application needs is the workflow characteristic. As the pressure drop ratio is less than 1, the workflow characteristic increases. Therefore, in the selection of the flow characteristics of the control valve, one should first consider the selection of the working flow characteristics. Then, the ideal flow characteristic is selected according to the practical application. The ideal flow characteristics commonly used in production are linear, equal percentage and fast-open characteristics. The quick-open characteristic is mainly used in two-position control and program control, so the choice of flow characteristic of control valve usually means how to choose linear and equal percentage of ideal flow characteristic reasonably.
In actual use, the control valve is always installed in the process pipeline system, the pressure difference before and after the control valve changes with the resistance of the pipeline system. Therefore, the choice of the flow characteristics of the control valve is not only based on the process characteristics, but also should be considered in conjunction with the piping system.
(1) According to the process characteristic, select the working flow characteristic of the valve. The control Law of the conventional controller is linear. The parameters of the controller can be adjusted to a certain working range without frequent adjustment. This requires that the generalized object is linear, that is, in the face of load, pressure changes before the valve or set value changes, the characteristics of the generalized object remain basically unchanged. Therefore, from the point of view of the automatic control system, the principle of selecting the operating characteristics of the control valve is to make the whole generalized object have linear characteristics, that is, in the generalized object, in addition to the control valve other parts of the characteristics (transmitter characteristics, process characteristics) for linear, should choose linear flow characteristics of the Control Valve (that is, Kv is a constant) ; if the characteristics of linear transmitter, and the process characteristics of the amplification factor watts is with the control variable increases and decreases, then the control valve should choose the same percentage of working flow characteristics. In a word, when the generalized object (except the Control Valve) has nonlinear characteristics, the control valve should be able to overcome its nonlinear influence and make the generalized object nearly linear.
(2) According to the piping condition, the ideal flow characteristic can be deduced from the required flow characteristic. In the production site, the control valve is always connected with the pipeline and other equipment, so there must be pipe resistance so that the flow characteristics of the control valve and the ideal flow characteristics of a certain difference. Therefore, in the selection of control valve characteristics, the system should also be considered in conjunction with the process piping. After the pressure drop ratio s value is determined according to the process piping condition, the ideal flow characteristic can be determined according to the selected flow characteristic.
Post time: Jul-28-2021