Control valve is also known as control valve. In the field of industrial automation process control, it is the final control element that changes the process parameters such as medium flow, pressure, temperature, liquid level by receiving the control signal output from the regulation control unit with the help of power operation.
As the terminal executive device of industrial process control, control valve has become the key equipment of industrial automation control with the continuous improvement of industrial automation level. The selection of control valve is directly related to the quality of industrial control, even to the safety of industrial production system. Therefore, in the actual production process, it is very necessary to select the appropriate control valve according to the process conditions.
Selection of the structure of the control valve: 1. In general, sleeve valve should be selected in the case of large vibration or noise. For example, when there is viscosity or small particles in the medium, the eccentric rotary valve is more suitable. Generally, the widely used valves are through single seat valve, straight through double seat valve, angle valve, butterfly valve, ball valve, diaphragm valve, etc. When selecting the structure of the valve, the process conditions and fluid characteristics of the regulating medium should also be considered.
The selection of the gas on / off of the control valve and the positive and negative effects of the regulator must be considered from the aspects of process production needs and safety requirements. The principle is that when the signal pressure is interrupted, the safety of process equipment and production should be ensured, that is, when the control valve loses the signal pressure source, the valve will automatically enter the safe state. After selecting the air opening or closing mode of the control valve, the positive and negative actions of the regulator must be determined according to the form of the valve, so that the control valve and the regulator constitute a negative feedback control system. That is to say, for the air on valve, if the controlled parameter is greater than the given parameter, the valve is required to be turned down, then the regulator selects the reaction; for the air closing valve, if the controlled parameter is greater than the given parameter, the valve is required to be turned down, then the regulator selects the positive action. The flow rate of the regulating valve is not only related to the opening of the valve, but also related to the pressure difference at both ends of the valve. The flow characteristics of the valve when the pressure difference between the front and rear of the valve is constant is the ideal characteristic of the valve, and the flow characteristic of the valve with the pressure difference changing at any time is the working characteristic of the valve. There are four typical ideal flow characteristics: (1) linear flow characteristics. When the spool with linear flow characteristics is used, the amplification factor of the regulating valve is constant. It is characterized by relatively large flow change at small opening, strong regulating effect and easy to produce oscillation; at opening, the relative change of flow is small, the regulating effect is weak and the regulation is slow. (2) With equal percentage flow characteristics (logarithmic flow characteristics), the magnification of the control valve increases with the increase of the flow rate, but the relative change value of the flow rate is equal. It has small magnification at small opening, and its regulating effect is gentle and stable; it has large magnification at large opening, and its regulating effect is sensitive, so it is often used. (3) Parabolic flow characteristics (, when using the parabolic flow characteristics of the spool, in small opening, the adjustment performance is good, its characteristic curve is between the straight line and the equal percentage characteristic curve. (4) There is no definite mathematical expression for quick opening flow characteristics. When the valve core with quick opening flow characteristic is adopted, the flow rate will be larger when the opening is small. With the increase of opening, the flow rate will reach the maximum value quickly. It is suitable for quick opening and closing shut-off valve or two position regulating system.