A control valve of the composition and classification
Control valve control valve is also called, is the main type of actuator, control unit regulating the output by receiving the control signal, using power-operated to change the fluid flow. Control valve and valves are generally composed of the implementing agencies. If you press it with the implementing agencies to use power, control valve can be divided into pneumatic, electric, hydraulic three kinds, namely, the source of compressed air-powered pneumatic control valve, to electric power source for electric control valve in order to liquid medium (eg oil) pressure-powered electric hydraulic control valve addition, according to their functions and features points, as well as solenoid valves, electronic, intelligent, fieldbus-type regulating valve. Many types of valve products, the structure is also diverse, but also continuously updated and changes. Generally speaking valve is a common, either with the pneumatic actuator match, but also with electric actuators, or other implementing agencies match.
The choice of two types of control valve
2.1 The control valve of the valve type selection valve option is to control valve selection of the most important aspects.
Many types of valve body, often there are through single-seat, through two-seater, angular, diaphragm, small flow, three-, eccentric rotation, butterfly, sleeve type, spherical, etc. 10. In selecting the valve prior to the media of the control process, process conditions and parameters to make a careful analysis, to collect enough data to understand the system requirements on the control valve, according to the data collected to determine the type of valve to be used. Specific options are available from the following aspects:
(1) Plug the structure mainly based on the selected shape of the flow characteristics and to consider factors such as unbalanced force.
(2) The wear resistance when the fluid medium that contain high concentrations of abrasive particles in suspension, the valve core, valve seat will be closed joints every serious friction. Therefore, the valve flow path should be smooth, Valve's internal materials to be hard.
(3) anti-corrosion due to corrosive media, in the regulatory function to meet the circumstances, as we choose simple valve.
(4) medium temperature and pressure when the medium temperature and pressure variation in high and when the valve core and valve seat should use the material by the temperature and pressure changes in the small valve.
(5) to prevent flashing and flashing and cavitation cavitation produced only in liquid medium. In the actual production process, the flashing and cavitation not only affect the calculation of flow coefficient, but also the formation of vibration and noise, shorten the life of the valve, so the choice should be to prevent the valve produces flash valve and cavitation .
2.2 The selection of the implementing agencies regulating valve
2.2.1 consideration of output force
No matter what type of executive body, the output power are used to overcome the load effective (mainly refers to the unbalanced force and unbalanced moment together with friction, sealing force, gravity force and other relevant role). Therefore, in order to control valve to work, with executive body used to be able to generate sufficient output force to overcome resistance and ensure a high degree of sealing and the valve opening.
For the double-acting pneumatic, hydraulic, electric actuator, generally do not reset spring. The size of the force direction has nothing to do with its operation, therefore, selecting executing agencies is the key to understand the maximum output power and the motor rotational torque. For single role of the pneumatic actuator, the output force and the opening of the valve, regulating valve on the emergence of the force will also affect the motion characteristics, thus requiring the entire range of control valve opening to establish the force balance.
2.2.2 Determination of types of the implementing agencies
Output force of the implementing agencies to determine, based on the use of environmental requirements, process, select the appropriate enforcement agency. For the scene, where explosion-proof requirements, should use pneumatic actuator, and the junction box for the explosion-proof type electric actuator is not an option. If there is no explosion-proof requirements, pneumatic and electric actuator can be selected, but energy-saving considerations, should try to use electric actuator. For the hydraulic implementing agency, its use than pneumatic, electric actuator broad, but with adjustable high precision, speed and smooth movement characteristics, therefore, in some cases, in order to achieve better conditioning results, we must use hydraulic implementing agencies, such as power plants and transparent machine speed regulation, the refinery's catalytic cracking unit reactor temperature adjustment control.
3 mode of action of regulating valve selection
Mode of action of control valve selection of pneumatic actuator only when they left, their mode of action through the pros and cons of the role of implementing agencies and valve combination of positive and negative role in the formation. There are four kinds of combination that is precisely (gas off type), the pros and cons (gas open type), anyway (gas open type), anti-anti-(gas off type), through the formation of these four combined mode of action of a gas control valve Open off two kinds of gas. For the choice of mode of action of control valve, the main three-pronged approach to consider: a) process production safety; b) characteristics of the medium; c) to ensure product quality, minimize economic losses.
4 regulating valve flow characteristics of the choice of
Regulating valve flow characteristic refers to the medium flowing through the valve flow and the relative displacement (relative valve opening) the relationship between the ideal flow characteristics are mainly straight lines, such as percentage (right number), a parabola, and quick to open, etc. four kinds of , characteristics curves and spool shape as shown in Figure 1 and Figure 2. Good flow characteristics of commonly used only straight lines, such as percentage (right number), quick to open three kinds. Parabolic flow characteristic between the percentage of straight lines and so on between the percentage of the general features can be used to replace the other, while the quick opening feature is mainly used for two-conditioning and process control, so control valve characteristics of choice is a straight line and the percentage of flow characteristics such as choice.
The choice of control valve flow characteristic through theoretical calculations, but the methods and equations used are very complex. At present, many with experience of the criteria, specifically from the following aspects to consider: ① from the conditioning system regulating the quality of analysis and selection; ② be considered from the process piping; ③ load change from the situation analysis.
Choose a good control valve flow characteristics, we can determine the flow characteristics according to their shape and structure of the valve spool, but like the diaphragm valves, butterfly valves, etc., because of their structural characteristics, it is impossible to change the spool of the surface shape to achieve the the required flow characteristics, when, through a change in the valve positioner with feedback cam profile to achieve.
5, the choice of regulating valve diameter
Valve caliber selection and identify key capability that is based on the flow valve Cv. Instrument in a variety of engineering design and selection, we need to calculate Cv regulating valves, and provides control valve design specifications. The Cv calculated from the control valve to the valve diameter to be determined, subject to the following steps:
1) Calculate the flow rate determination. The existing production capacity, equipment, load and medium situation, we decided the calculation flow Qmax and Qmin.
2) The valves before and after the determination of pressure. According to the selected valve flow characteristics and system characteristics of the selected S (drag coefficient), and then determining the calculation of pressure.
3) The calculation of Cv. According to the regulation of the media choose the right formula and charts, obtained Cmax and Cmin.
4) The selection of Cv. According to Cmax, in the standard series of products chosen, select> Cmax and the closest one with C.
5) regulating valve opening checking. Generally require maximum computing flow of the opening ≯ 90%, the minimum calculated flow of the opening ≮ 10%.
6) control valve with adjustable ratio of the actual checking. Generally require more than the actual adjustable ≮ 10.
7) The seat diameter and the determination of the nominal diameter. Verified suitable, according to C determined.
6 Conclusion
The choice of regulating valve is a very painstaking work, not only a solid professional theoretical knowledge, but also has rich practical experience. Is well chosen not only helps to balance control loop tuning of PID parameters, so parameters were adjusted to get better control effect, also control valve life has increased considerably. Regulating valve should be selected according to local conditions, not set in stone, we must constantly sum up the process of practice and innovation, in particular with the mechanical and electrical integration technologies, computers and digital information technology, control valve structure and function has become better, more comprehensive, for the selection of control valve provides a great convenience. ball valve Trunnion Mounted Ball Valve Floating Ball Valve Top Entry Ball Valve Three way ball valve V Type Ball Valve
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