The regulating valve, also known as the control valve, is the main type of actuator. By accepting the control signal output from the regulating control unit, the fluid flow is changed by the power operation. The regulating valve is generally composed of an actuator and a valve. If the power is used according to the actuators it is equipped with, the regulating valve can be divided into three types: pneumatic, electric and hydraulic. The pneumatic regulating valve with compressed air as the power source and the electric regulating valve with electric power as the liquid medium (such as oil, etc.) The pressure-powered electro-hydraulic control valve, in addition to its function and characteristics, there are solenoid valves, electronic, intelligent, fieldbus-type regulating valves. The type of control valve is very The structure is also diverse and constantly updated and changed. Valves are generally versatile and can be matched to pneumatic actuators or to electric actuators or other actuators. Control valve type selection Valve body type selection of the valve The choice of valve body is the most important part of the valve selection. There are many types of regulating valve bodies. Commonly used are 10 types of straight through single seat, straight through double seat, angle, diaphragm, small flow, three-way, eccentric rotation, butterfly, sleeve and spherical. Before selecting the valve, carefully analyze the media, process conditions and parameters of the control process, collect sufficient data, understand the requirements of the system for the regulating valve, and determine the type of valve to be used based on the collected data. In the specific choice, you can consider the following aspects: (1) The shape of the spool is mainly based on factors such as the selected flow characteristics and unbalanced forces. (2) Abrasion resistance When the fluid medium is a suspension containing a high concentration of abrasive particles, the valve core and the valve seat joint face are severely rubbed each time they are closed. Therefore, the flow path of the valve should be smooth and the internal material of the valve should be hard. (3) Corrosion resistance Because the medium is corrosive, in the case of meeting the adjustment function, try to choose a simple valve. (4) Temperature and pressure of the medium When the temperature and pressure of the medium are high and the change is large, the valve and the material of the valve seat should be selected to be small in temperature and pressure. (5) Prevention of flashing and cavitation Flashing and cavitation are only produced in liquid media. In the actual production process, flashing and cavitation not only affect the calculation of the flow coefficient, but also generate vibration and noise, so that the service life of the valve is shortened, so the valve should be prevented from flashing and cavitation when selecting the valve. Control valve actuator selection 1 output force considerations Regardless of the type of actuator, its output force is the effective force to overcome the load (mainly refers to the unbalanced force and unbalanced torque plus the friction, sealing force, gravity and other related forces). Therefore, in order for the regulating valve to work properly, the actuator used should be able to generate sufficient output force to overcome various resistances, ensuring high sealing and opening of the valve. For double-acting pneumatic, hydraulic, and electric actuators, there is generally no return spring. The magnitude of the force is independent of its direction of travel. Therefore, the key to selecting an actuator is to clarify the maximum output force and the rotational torque of the motor. For a single-acting pneumatic actuator, the output force is related to the opening of the valve. The force appearing on the regulating valve will also affect the motion characteristics. Therefore, it is required to establish a force balance across the opening range of the regulating valve. 2 Determination of the type of actuator After determining the output force of the actuator, select the corresponding actuator according to the requirements of the process environment. For explosion-proof requirements on site, pneumatic actuators should be used, and the junction box is explosion-proof, and electric actuators cannot be selected. If there is no explosion-proof requirement, pneumatic and electric actuators can be used, but in terms of energy saving, electric actuators should be selected as much as possible. For hydraulic actuators, the use is not as extensive as pneumatic and electric actuators, but it has the characteristics of high adjustment precision, fast and stable movement. Therefore, in some cases, in order to achieve better adjustment effect, hydraulic actuation must be selected. Institutions, such as speed regulation of power plant transparent machines, temperature regulation control of refinery catalytic converters, etc. The mode of action of the regulating valve The mode of action of the regulating valve is only when the pneumatic actuator is selected, and its action mode is formed by the combination of the positive and negative action of the actuator and the positive and negative action of the valve. There are 4 kinds of combination forms, namely, positive (gas-off type), positive and negative (air-open type), anyway (air-open type), anti-reverse (air-closed type), and the regulating valve formed by these four combinations has a gas function. Both open and closed. The choice of the mode of operation of the regulating valve is mainly considered from three aspects: a) process production safety; b) medium characteristics; c) product quality and economic loss. Regulating valve flow, selection of characteristics The flow characteristic of the regulating valve refers to the relationship between the relative flow rate of the medium flowing through the valve and the displacement (relative opening of the valve). The ideal flow characteristics mainly include four types: straight line, equal percentage (logarithm), parabola and quick opening. The curve and spool shape are shown in Figures 1 and 2. The commonly used ideal flow characteristics are only straight lines, equal percentages (logarithmic), and quick opening. The parabolic flow characteristics are between straight and equal percentages. They can generally be replaced by equal percentage characteristics. The fast opening characteristics are mainly used in two-position adjustment and program control. Therefore, the selection of the regulating valve characteristics is actually linear and equal percentage flow characteristics. s Choice. The choice of the flow characteristics of the regulating valve can be calculated theoretically, but the methods and equations used are complex. At present, the experience criteria are often used, and the following aspects are considered: 1 from the adjustment quality of the adjustment system and selected; 2 from the process piping; 3 from the load change analysis. By selecting the flow characteristics of the regulating valve, the shape and structure of the valve spool can be determined according to the flow characteristics. However, for diaphragm valves, butterfly valves, etc., due to their structural characteristics, it is impossible to change the shape of the valve core to achieve the desired shape. The required flow characteristics can be achieved by changing the shape of the feedback cam of the valve positioner. Adjustment valve diameter selection The selection and determination of the valve diameter is mainly based on the valve's flow capacity, Cv. In the design and selection of instruments for various projects, the Cv calculation of the regulating valve is carried out, and the design specification of the regulating valve is provided. From the calculation of the Cv of the regulating valve to the determination of the diameter of the valve, the following steps are generally required: 1) Calculate the calculation of the flow. The existing production capacity, equipment load and the condition of the medium determine the Qmax and Qmin of the calculated flow. 2) Determination of the differential pressure before and after the valve. S (resistance coefficient) is selected according to the selected valve flow characteristics and system characteristics, and then the calculated differential pressure is determined. 3) Calculate Cv. Select the appropriate calculation formula and graph according to the adjusted medium to find Cmax and Cmin. 4) Select Cv. According to Cmax, select >Cmax and the closest level C in the selected product standard series. 5) Check valve opening degree check. Generally, the opening degree of the maximum calculation flow rate is ≯90%, and the opening degree when calculating the minimum flow rate is ≮10%. 6) Check the actual adjustable ratio of the regulating valve. Generally, the actual adjustable ratio ≮10 is required. 7) Determination of seat diameter and nominal diameter. After verification is appropriate, it is determined according to C. 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