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Selection Principle and Maintenance of Control Valve (1)

Selection principle and maintenance of control valve (1)
1. Selection of control valve
The importance of control valve selection, the control valve is the actuator in the automatic control system, and its application quality is directly reflected in the adjustment quality of the system. As a terminal element in process control, people have a newer understanding of its importance than in the past. In addition to the quality of the product itself, whether the user has correctly installed, used, and maintained, the correct calculation and selection of the control valve are very important. Due to the error of calculation and selection, the system starts and stops, and some even cannot be put into use. Therefore, users and system designers should recognize the importance of the valve on site, and must pay enough attention to the selection of the control valve.
Principles of control valve selection
(1) According to the process conditions, select the appropriate structural form and material.
(2) According to the characteristics of the process object, select the flow characteristics of the control valve.
(3) According to the process operation parameters, select the appropriate size of the control valve caliber.
(4) According to the requirements of the process, select the required auxiliary devices.
(5) Reasonable selection of implementing agencies. The response speed of the actuator should be able to meet the requirements of the process and the control stroke time: the selected control valve actuator should be able to meet the requirements of the valve stroke and the process for the leakage level. In some occasions, if the pressure control valve is selected, the actual possible pressure difference should be considered for appropriate amplification, that is, the actuator is required to provide a larger force. Otherwise, when there is an abnormal situation in the process, the actual pressure difference before and after the control valve is large, and there is a danger that the valve cannot be closed or opened.
Second, the accessories of the control valve. During the production process, the control system puts forward various special requirements for the valve. Therefore, the Shenyi control valve must be equipped with various accessories (referred to as accessories) to meet the needs of the production process.
Accessories for control valves include
(1) The valve positioner is used to improve the working characteristics of the adjustment performance of the control valve and achieve correct positioning.
(2) The valve position switch shows the working position of the upper and lower limit strokes of the valve.
(3) Pneumatic retaining valve When the air source of the control valve fails, keep the valve at the opening position before the failure of the air source.
(4) The solenoid valve realizes the electromagnetic switching of the gas circuit to ensure that the valve is in the desired safe opening position when the power supply fails.
(5) Handwheel mechanism When the controller of the control system fails, it can be switched to manual mode to operate the valve.
(6) The pneumatic relay accelerates the action of the actuator and reduces the transmission time.
(7) The air filter pressure reducer is used to purify the air source and adjust the air pressure.
(8) The air storage tank ensures that when the air source fails, the springless cylinder and piston actuator can move the control valve to the fail-safe position. Its size depends on the size of the cylinder, the requirements of the valve action time and the working conditions of the valve.
In short, the role of accessories is to make the function of the control valve more complete, more reasonable and more complete.