Regulating valve in the role of process control is to accept or adjust the computer's control signal, changing the flow of medium was transferred to be transferred within the required parameters to maintain the framework of the production process so as to achieve the automation. If the automatic adjustment system and the manual adjustment process, compared to detection unit is the eyes, and adjust the control unit is the brain, then the unit - one control valve is the hands and feet. Process to achieve a certain parameters such as temperature, pressure, flow, level of regulation, such as control, are inseparable from the control valve. Therefore the correct choice of control valve automation in the process of great significance.
The composition of a control valve and classification
Also known as the control valve control valve is the main type of actuator, control unit through the acceptance of adjustment of the control signal output, with power to change the fluid flow operation. Regulating valve is usually composed of the implementing agencies and valves. If the use of its executive body with the power to regulate the valve can be divided into pneumatic, electric, hydraulic three, that is the source of compressed air as a driving force of the pneumatic valve to electrical power source for electric control valve to liquid medium (such as oil) pressure-driven dynamic electro-hydraulic control valve, while, according to their functions and characteristics of sub-, and solenoid valve, electronic, smart, on-site, such as bus-type regulating valve. Control valve are many types of products, a variety of structure, but also constantly updated and changes. Generally speaking valve is common, both implementing agencies and pneumatic matching can also be the executing agency and the electrical or other executive body to match.
2 Select the type of control valve
2.1 Control Valve Valve Valve type control valve of choice is to choose the most important aspects. Many different types of control valve body, are commonly used straight-through single-seater, two-seater straight, angular, diaphragm, small traffic, links, eccentric rotation, butterfly, sleeve, spherical, such as 10. Prior to the selection of valves, to control the course of the medium, the process conditions and parameters of careful analysis, to collect sufficient data to understand the system requirements on the control valve, according to the data collected to be used to determine the type of valve. Specific options, available from the following considerations:
(1) spool shape chosen primarily on the basis of the structure of the flow characteristics and to consider factors such as unbalanced force.
(2) abrasion-resistant when the fluid medium containing high concentrations of wear particles of suspension, the valve core, valve seat joints will be shut down every time a serious friction. Thus the valve flow path to smooth, the internal valve to a hard material.
(3) as a result of corrosion-resistant medium is corrosive, can satisfy the regulatory function in the circumstances, the valve structure is simple as we choose.
(4) medium temperature and pressure when the medium temperature, pressure and changes in the high when the valve core and valve seat should be used by the material temperature, pressure changes in the small valve.
(5) to prevent the flash and the flash cavitation and cavitation generated in the liquid medium only. In the actual production process, the flash and the air flow of not only affect the calculation of coefficients, but also the formation of vibration and noise, so that shorter service life of valves, the valve should be in the selection of flash and have a valve to prevent cavitation .
2.2 Control Valve Selection of implementing agencies
2.2.1 consider the output power
No matter what kind of executive body type, the output power are used to overcome the effective load (mainly refers to the imbalance and the imbalance torque plus friction, sealing force, gravity force and so on). Therefore, in order to enable the valve to work with the implementing agencies to use to be able to generate sufficient output power to overcome all kinds of resistance, to ensure a high degree of sealing and the valve open.
Dual role for pneumatic, hydraulic, electric actuator, generally did not reset the spring. The size of force and its direction has nothing to do, therefore, to choose the key implementing agencies to ascertain the maximum output force and torque motor rotation. The role of the single executive body of the pneumatic, the output force and the opening of the valve, regulating valve on the emergence of the power will also affect the motion characteristics, it requires the opening of the regulating valve to establish equilibrium range.
2.2.2 Determination of the type of executing agency
Output force of the implementing agencies are identified, the use of environmental requirements according to process, select the appropriate enforcement agency. For the scene, where explosion-proof requirements, implementing agencies should use pneumatic and explosion-proof junction box for the type, can not choose the electric actuator. If there is no explosion-proof requirements, pneumatic, electric actuator can be selected, but to consider energy conservation should be the executing agency for electric choice. Executing agency for the liquid, its use as pneumatic and electric actuator extensive regulation accuracy but with high speed and smooth movement characteristics, therefore, in some cases, in order to achieve better regulation of the effect of liquid must be selected implementing agencies, such as power plants transparent machine speed regulation, the refinery's catalytic cracking unit reactor control temperature regulation.
3 The role of valve options
The role of regulating valve means only when the optional pneumatic before executing agency, and its executive body adopted the role of positive and negative role and valve combination of positive and negative role in the formation. There are four kinds of combinations that are (gas-off), both positive and negative (open air-type), in any case (open air-type), counter (air-off), formed through the combination of these four modes of control valve gas two open gas clearance. Regulating valve for the choice of the mode of action, mainly in three aspects to consider: a) Process of production safety; b) the characteristics of media; c) to ensure product quality, minimize economic losses.
Flow regulating valve 4, the choice of characteristics
Control valve flow characteristics of the media refers to the relative flow through the valve flow and the displacement (relative valve opening) the relationship between the ideal flow characteristics of the main straight, and the percentage (logarithmic), and soon to open, such as parabolic four kinds of . Ideal flow characteristics of commonly used only a straight line, such as the percentage (logarithmic), quickly open the three. Parabola and a straight line between the flow characteristics, such as the percentage between the percentage of the general characteristics can be used to replace and so on, and soon to open for the two main characteristics of regulation and control, and therefore the choice of control valve characteristics and is actually a straight line features, such as the percentage of flow choice.
Control valve flow characteristics through the choice of theoretical calculations, but the methods and equations is complex. At present, guidelines for the use of the experience of specific aspects from consideration: ① conditioning systems from the analysis of the regulation of quality and choice; ② piping from the process considered; ③ from the analysis of changes in load.
The choice of a good control valve flow characteristics can be determined in accordance with its flow characteristics of the valve spool of the shape and structure, but like valve, butterfly valve and so on, because of their structural characteristics, it is not possible to change the valve core with the surface shape to achieve flow characteristics needed, then, can be allocated by changing the valve positioner feedback form to achieve the cam.
5, the choice of caliber valve
The choice of valve diameter and the main basis for determining the flow capacity of valve that is Cv. In a variety of engineering design and selection of instruments, the valve must be calculated Cv and provide manual control valve design. Cv from the regulating valve to the valve diameter calculated to determine the general subject to the following steps:
1) determine the flow calculation. The existing production capacity, equipment and medium load conditions, decided to calculate flow Qmax and Qmin.
2) the determination of differential pressure valve before and after. Valve has been chosen in accordance with the flow characteristics and system characteristics of the selected S (drag coefficient), and then calculating the pressure difference to determine.
3) calculation of Cv. According to the regulation of the media choose the right formula and charts to obtain Cmax and Cmin.
4) choice of Cv. According to Cmax, in the standard series of products to choose select> Cmax level and its nearest C.
5) check valve opening. General requirements for calculation of the flow at the time of the largest opening ≯ 90%, the smallest at the time of calculation of the flow opening ≮ 10%.
6) regulating valve of the check than the actual adjustable. Generally require more than the actual adjustable ≮ 10.
7) valve seat diameter and the determination of nominal diameter. The right to verify, in accordance with C determined.
6 Conclusion
The choice of valve is very meticulous work, we not only need a solid theoretical knowledge of the profession, but also have rich practical experience. Choice is not only conducive to good-conditioning control loop tuning of PID parameters so that the parameters were adjusted to get a better control effect, but also the life of valve increase greatly. Select regulating valve in line with local conditions, not static, it is necessary to the process in practice and innovation, constantly sum up, in particular with the mechanical-electrical integration technologies, computers and digital information technology, the structure and function regulating valve to get better, more comprehensive, for the selection of control valve provides a great convenience. valve ball valve gate valve check valve globe valve butterfly valve
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