1 The composition and classification of the regulating valve The regulating valve, also known as the control valve, is the main type of actuator. By receiving the control signal output by the regulating control unit, the fluid flow can be changed by power operation. The regulating valve is generally composed of an actuator and a valve. According to the power used by the actuator, the regulating valve can be divided into three types: pneumatic, electric and hydraulic, namely pneumatic regulating valve with compressed air as power source, electric regulating valve with electricity as power source, and liquid medium (Such as oil, etc.) Pressure-driven electro-hydraulic control valves. In addition, according to their functions and characteristics, there are solenoid valves, electronic, intelligent, and fieldbus control valves. There are many types of regulating valves, and their structures are also diverse, and they are constantly being updated and changed. Generally speaking, the valve is universal, which can be matched with pneumatic actuators, electric actuators or other actuators.
2 Selection of control valve type
2.1 Selection of valve body type of control valve The choice of valve body is the most important link in the selection of control valve.
There are many types of regulating valve bodies. There are 10 types commonly used, such as straight single-seat, straight double-seat, angle, diaphragm, small flow, three-way, eccentric rotation, butterfly, sleeve, and spherical. Before selecting a valve, carefully analyze the medium, process conditions and parameters of the control process, collect enough data, understand the system’s requirements for the regulating valve, and determine the type of valve to be used based on the collected data. In the specific selection, the following aspects can be considered:
(1) The shape and structure of the valve core are mainly considered according to the selected flow characteristics and unbalanced force.
(2) Wear resistance When the fluid medium is a suspension containing a high concentration of abrasive particles, the valve core and valve seat joint surface will be severely rubbed every time it is closed. Therefore, the flow path of the valve must be smooth, and the internal material of the valve must be hard.
(3) Corrosion resistance As the medium is corrosive, if the regulating function can be satisfied, try to choose a valve with a simple structure.
(4) The temperature and pressure of the medium. When the temperature and pressure of the medium are high and the change is large, the valve core and seat material should be selected for small changes in temperature and pressure.
(5) Prevent flash evaporation and cavitation Flash evaporation and cavitation only occur in liquid media. In the actual production process, flashing and cavitation not only affect the calculation of the flow coefficient, but also cause vibration and noise, which will shorten the service life of the valve. Therefore, the valve should be prevented from flashing and cavitation when selecting the valve.
2.2 The selection of the actuator of the regulating valve
2.2.1 The consideration of the output force
Regardless of the type of actuator, its output force is the effective force used to overcome the load (mainly refers to the effect of unbalanced force and unbalanced moment plus friction, sealing force, gravity and other related forces). Therefore, in order to make the regulating valve work normally, the equipped actuator must 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 and electric actuators, there is generally no return spring. The magnitude of the force has nothing to do with its running direction. Therefore, the key to selecting an actuator is to understand the maximum output force and the rotating torque of the motor. For single-acting pneumatic actuators, the output force is related to the opening of the valve, and the force appearing on the regulating valve will also affect the motion characteristics, so it is required to establish a force balance across the opening range of the regulating valve.
2.2.2 determining the type of the actuator
after performing the determination of the force output means, using the process according to environmental requirements, select the appropriate enforcement agency. For explosion-proof requirements on site, pneumatic actuators should be selected, and the junction box is explosion-proof, and electric actuators cannot be selected. If there is no explosion-proof requirement, both pneumatic and electric actuators can be used, but in terms of energy saving, electric actuators should be used as much as possible. For hydraulic actuators, they are not as widely used as pneumatic and electric actuators, but they have the characteristics of high adjustment accuracy, fast action speed and smoothness. Therefore, in some cases, in order to achieve better adjustment results, hydraulic actuators must be selected Institutions, such as the speed adjustment of the transparent machine in the power plant, the temperature adjustment control of the catalytic device reactor of the oil refinery, etc. The mode of action of the regulating valve is only available when the pneumatic actuator is selected, and its mode of action is formed by the combination of the positive and negative effects of the actuator and the positive and negative effects of the valve. There are 4 types of combinations: positive (air-to-close type), positive and negative (air-to-open type), anyway (air-to-open type), and reverse (air-to-close type). The regulating valve action mode formed by these four combinations has air There are two types: on and off. Regarding the choice of the regulating valve’s action mode, it is mainly considered from three aspects: a) process production safety; b) characteristics of the medium; c) ensuring product quality with minimal economic loss. 4Adjust valve flow, selection of characteristics
3 Selection of the mode of action of the regulating valve
control valve flow characteristics refers to the relationship between the flow and the relative displacement (relative to the valve opening) the medium flows through the valve, the ideal flow characteristics are mainly linear, equal percentage (logarithmic), parabolic 4 types such as quick opening and quick opening. The characteristic curve and spool shape are shown in Figure 1 and Figure 2. There are only three commonly used ideal flow characteristics: straight line, equal percentage (logarithmic), and quick opening. Parabolic flow characteristics are between straight line and equal percentage. Generally, equal percentage characteristics can be used instead. Quick opening characteristics are mainly used in two-position adjustment and program control. Therefore, the selection of control valve characteristics is actually linear and equal percentage flow characteristics. s Choice.
The selection of the flow characteristics of the regulating valve can be calculated theoretically, but the methods and equations used are very complicated. At present, empirical guidelines are mostly used, which are specifically considered from the following aspects: ①Analysis and selection from the adjustment quality of the regulation system; ②Consideration from the process piping situation; ③Analysis from the load change situation.
By selecting the flow characteristics of the regulating valve, the shape and structure of the valve core can be determined according to its flow characteristics. However, for diaphragm valves, butterfly valves, etc., due to their structural characteristics, it is impossible to change the curved surface shape of the valve core to achieve the desired result. The required flow characteristics can be achieved by changing the shape of the feedback cam of the valve positioner.
5 control valve bore selection
control valve selection and determination of the diameter of the flow capacity of the valve, i.e. mainly based Cv. In the instrument design and selection of various projects, the Cv calculation of the control valve must be carried out, and the design manual of the control valve shall be provided. From the calculation of the Cv of the control valve to the determination of the valve diameter, the following steps are generally required:
1) Calculate the determination of the flow rate. The existing production capacity, equipment load and media conditions determine the Qmax and Qmin of the calculated flow.
2) The determination of the pressure difference before and after the valve. According to the selected valve flow characteristics and system characteristics, select S (resistance coefficient), and then determine and calculate the pressure difference.
3) Calculate Cv. Choose the appropriate calculation formula and chart according to the adjusted medium to obtain Cmax and Cmin.
4) Choose Cv. According to Cmax, select the first level C> Cmax and the closest to it in the selected product standard series.
5) Checking the opening of the regulating valve. Generally, it is required that the opening for the maximum calculation flow is ≯90%, and the opening for the minimum calculation flow is ≮10%.
6) Checking calculation of actual adjustable ratio of control valve. Generally, the actual adjustable ratio is required to be ≮10.
7) Determination of valve seat diameter and nominal diameter. After verification is appropriate, determine according to C.
regulating valve choice is very painstaking work, not only have a solid theoretical knowledge, but also has extensive practical experience. A good choice is not only conducive to the adjustment of the PID parameters of the control loop, so that the adjusted parameters can be better controlled, but also the service life of the control valve is greatly increased. The choice of regulating valve should be tailored to local conditions, not static. It should be continuously summarized and innovated in the course of practice. Especially with the application of mechatronics technology, computer and digital information technology, the structural function of the regulating valve has become better and more comprehensive. It provides great convenience for selecting the regulating valve.
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