1 control valve composition and classification Regulating valve, also known as control valve, is the main type of actuator, by receiving the control signal output of the control unit, with the power to change the fluid flow. Regulating valve generally by the implementing agencies and valves. If you press the actuator used by its power, the control valve can be divided into pneumatic, electric, hydraulic three, that is, compressed air as the power source of the pneumatic control valve, electric power to the electric control valve to liquid medium (Such as oil) pressure-powered electro-hydraulic control valve, in addition, according to their functions and characteristics of points, as well as solenoid valve, electronic, intelligent, field bus type regulating valve. Control valve of many types of products, the structure is also diverse, but also constantly updated and changing. Generally speaking, the valve is universal, it can be matched with the pneumatic actuator, but also with the electric actuator or other actuator. 2 control valve type selection 2.1 valve body type selection valve body selection is the most important part of the control valve selection. There are many types of regulating valve body, commonly used through a single seat, through two-seater, angular, diaphragm, small flow, tee, eccentric rotation, butterfly, sleeve, spherical and other 10 species. Before selecting a valve, carefully analyze the medium, process conditions, and parameters of the control process, collect enough data to understand the system requirements for the regulator, and determine the type of valve to be used based on the data collected. In the specific choice, the following aspects can be considered: (1) spool shape and structure based on the selected flow characteristics and unbalanced forces and other factors to consider. (2) Wear resistance When the fluid medium is a suspension containing high concentration of abrasive particles, the valve core, the valve seat joint surface will be severely rubbed each time it is closed. Therefore, the flow of the valve should be smooth, the valve's internal material to be hard. (3) Corrosion resistance Due to the corrosive medium, try to choose a simple structure of the valve in the case of regulatory function. (4) Medium temperature, pressure When the medium temperature, high pressure and changes, should use the valve core and valve seat material by temperature, pressure changes in small valves. (5) to prevent flash and cavitation flash and cavitation only produce liquid medium. In the actual production process, flash and cavitation not only affect the calculation of flow coefficient, but also the formation of vibration and noise, so that the valve life is shorter, so the valve should be selected to prevent flash and cavitation. 2.2 Control Valve Actuator Selection 2.2.1 Output Force Considerations Regardless of the type of actuator, its output force is used to overcome the effective load (mainly refers to unbalanced and unbalanced torque plus friction, Sealing force, gravity and other related forces). Therefore, in order for the regulating valve to work normally, the matched actuator should be able to generate enough output force to overcome various resistances and ensure a high degree of sealing and valve opening. For double-acting pneumatic, hydraulic, electric actuator, there is usually no return spring. The size of the force has nothing to do with its direction of operation, so the key to choosing an actuator lies in knowing the maximum output force and the torque of the motor. For single-acting pneumatic actuators, the output force is related to the opening of the valve, and the presence of force on the regulating valve will also affect the kinematic behavior, thus requiring a force balance across the opening of the regulating valve. 2.2.2 Determination of the type of implementing agencies After determining the output of implementing agencies, according to the requirements of the process environment, select the appropriate implementing agencies. For on-site explosion-proof requirements, pneumatic actuators should be used, and the junction box for the explosion-proof, you can not choose the electric actuator. If there is no explosion-proof requirements, the pneumatic and electric actuators can be used, but from the energy-saving aspects, should try to use electric actuators. For the hydraulic actuator, its use is not as good as the pneumatic, electric actuators a wide range, but with high precision, fast speed and smooth characteristics, therefore, in some cases, in order to achieve better regulation effect, you must use hydraulic actuator Institutions, such as power plant transparent speed control, refinery catalytic reactor temperature control and so on. 3 control valve mode of action The role of the control valve is selected only when the pneumatic actuator only, the role of the positive and negative through the implementing agencies and the positive and negative valve action combination. There are four types of combination that is positive (air-off type), positive and negative (air-open type), anyway (air-open type), anti-anti (anti-off type) Open and Qi off two. For the control valve mode of action, mainly from three aspects: a) process safety; b) the characteristics of the media; c) to ensure product quality, economic loss is minimal. 4 regulating valve flow, the choice of characteristics Flow characteristics of the control valve is the relationship between the relative flow through the valve medium and the displacement (relative opening of the valve) relationship between the ideal flow characteristics are straight, equal percentage (logarithm), parabolic And fast open, etc. 4, the characteristic curve and spool shape shown in Figure 1 and Figure 2. The only commonly used ideal flow characteristics, such as straight line, such as the percentage (logarithm), quickly open three. Parabolic flow characteristics between the straight line and equal percentage, the general percentage of the characteristics can be used instead of the other, while the quick opening characteristics are mainly used for two-bit adjustment and program control, so the choice of control valve characteristics is actually straight and equal percentage flow characteristics s Choice. The choice of flow characteristics of the control valve can be calculated theoretically, but the methods and equations used are complicated. At present, the use of more experience guidelines, specifically from the following aspects: ①from the regulatory system to adjust the quality of analysis and selection; ② from process piping considerations; ③ from the load changes. Select the flow characteristics of the control valve, you can determine the valve spool according to its flow characteristics of the shape and structure, but for such as diaphragm valves, butterfly valves, due to their structural features, it is impossible to change the shape of the valve spool to achieve The required flow characteristics can be achieved by changing the feedback cam profile of the valve positioner fitted. 5 control valve diameter selection Valve diameter selection and to determine the main basis for the valve flow capacity Cv. In a variety of engineering design and selection of instruments, the valve must be calculated Cv, and provide control valve design specifications. Cv from the control valve to determine the caliber of the valve to determine the general subject to the following steps: 1) to determine the flow rate. Existing production capacity, equipment load and media status, the decision flow Qmax and Qmin.2) The determination of the pressure before and after the valve. According to the selected valve flow characteristics and system characteristics selected S (resistance coefficient), and then determine the calculation of pressure. 3) Calculate Cv. According to the adjusted medium to choose the appropriate formula and chart, find Cmax and Cmin.4) choose Cv. According to Cmax, in the selected product standard series select> Cmax and its closest level C.5) regulator valve opening checking. The general requirements of the maximum flow when the calculation of the opening ≯ 90%, the minimum calculation of flow when the opening ≮ 10%. 6) Adjustable valve actual adjustable ratio checking. General requirements of the actual adjustable ratio ≮ 10.7) Determination of seat diameter and nominal diameter. After verification, according to C to determine. 6 Conclusion The choice of control valve is a very meticulous work, not only have a solid professional theoretical knowledge, but also have rich practical experience. Good choice not only conducive to adjusting the control loop PID parameter setting, so that the parameters have been controlled to better control effect, but also greatly increase the life of the control valve. Regulating valve selection should be adapted to local conditions, not static, to keep in the process of practice and innovation, especially with the mechatronics, computer and digital information technology applications, the structural function of the control valve to become better and more comprehensive, For the choice of regulating valve provides a great convenience.
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