News

Selection principle and maintenance method of imported control valve

First, the principle of the control valve
A, the importance of control valve selection
The regulating valve is an actuator in the automatic control system, and its application quality directly reflects the adjustment quality of the system. As a terminal component in process control, people are more aware of its importance than in the past. In addition to the quality of the product itself, whether the user is correctly installed, used, and maintained, it is important to calculate and select correctly. Due to mistakes in valve calculation and selection, the equipment system is opened and stopped, and some even cannot be used. Therefore, users and system designers should recognize the importance of the valve at the site, and must pay sufficient attention to the selection of the regulating valve. .

B, the principle of control valve selection
1. Select the appropriate structural form and material according to the process conditions.
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 control valve diameter size.
4. Select the required auxiliary device according to the requirements of the process.
5. Reasonably choose the executing agency. The response speed of the actuator should be able to meet the process
Requirements for controlling the travel time: The selected control valve actuator should be able to meet the valve stroke and process requirements for the leak level. In some cases, such as the selection of pressure control valves (including venting valves), the appropriate possible differential pressure should be considered for proper amplification, which requires the actuator to provide a large force. Otherwise, the actual pressure difference between the control valve before and after the control valve may be large when there is an abnormality in the process, and there is a danger that it may not be closed or opened.

Second, the control valve accessories
In the production process, the control system puts a variety of special requirements on the valve. Therefore, the control valve must be equipped with various accessory devices (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 control valve regulating performance and achieve correct positioning.
2. The valve position switch shows the working position of the upper and lower limit stroke of the valve.
3. Pneumatic retaining valve When the gas source of the control valve fails, keep the valve in the opening position before the gas source fails.
4. The electromagnetic valve realizes the electromagnetic switching of the gas path to ensure that the valve is in the desired safe opening position when the power is faulty.
5. Handwheel mechanism When the controller of the control system fails, it can be switched to the manual mode operation 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 gas storage tank ensures that when the air source fails, the springless cylinder operator piston actuator can move the control valve to the fail safe position. Its size depends on the size of the cylinder, the valve operating time requirements and the working conditions of the valve.
In short, the function of the accessory is to make the function of the control valve more complete, more reasonable and more complete.
Third, the maintenance of the control valve
The control valve has the characteristics of simple structure and reliable operation. However, since it directly contacts the process medium, its performance directly affects the system quality and environmental pollution. Therefore, the control valve must be frequently maintained and regularly inspected, especially for the use conditions and important. Occasionally, more attention should be paid to maintenance work. Key inspection site
1, the inner wall of the valve
For control valves used in high pressure differential and corrosive media, the diaphragm inside the valve body and the diaphragm valve are often subjected to the impact and corrosion of the medium. It is necessary to check the pressure and corrosion resistance.
2, valve seat
When the control valve is working, due to the infiltration of the medium, the inner surface of the thread for fixing the valve seat is susceptible to corrosion and the valve seat is loosened, so attention should be paid during inspection. For valves operating under high pressure differentials, check the seal faces of the seat for damage.
3, spool
The valve core is the movable part during the adjustment work. It is most seriously washed and corroded by the medium. During the inspection, it is necessary to carefully check whether the various parts of the valve core are corroded and worn, especially in the case of high pressure difference, the wear of the valve core is more serious. (Because of cavitation), attention should be paid. If the valve core is damaged seriously, it should be replaced. Also, please pay attention to whether the valve stem has a similar phenomenon or loose connection with the valve plug.
4. Diaphragm "O" ring and other gaskets. Check whether the diaphragm and "O" seal in the control valve are aging and cracked.
5, sealing filler
It should be noted that the PTFE filler, the sealing grease are aging, and the mating surface is damaged, and should be replaced if necessary.

PREVIOUS：Analysis of the working principle of importedNEXT：Mechanical seal maintenance instructions