A, the importance of control valve selection Valve is a controlled system actuator, its application quality directly reflects the quality of the system adjustment. As a terminal element in process control, people are more aware of its importance than the past. Control valve application is good or bad, in addition to the quality of the product itself, the user is correctly installed, use, maintenance, the correct calculation, selection is very important. Due to the mistakes in calculating the selection, resulting in the system open stop, and some even can not be put into use, so for users and system designers should recognize the importance of the valve at the scene, the selection of the control valve must be given sufficient attention. B, the principle of control valve selection 1, according to process conditions, select the appropriate structure and materials. 2, according to the characteristics of the process object, select the control valve flow characteristics. 3, according to process operating parameters, select the appropriate control valve caliber size. 4, according to the process requirements, select the necessary auxiliary device. 5, a reasonable choice of implementing agencies. The response speed of the actuator should be able to meet the process requirements of the control travel time: the selected control valve actuator should be able to meet the full valve stroke and the process of leakage level requirements. In some cases, such as the use of pressure control valves (including vent valves), the actual pressure differential should be considered for proper amplification, which requires the actuator to provide a greater force. Otherwise, when the process may be abnormal, the actual pressure drop before and after the control valve larger, there will be no danger of closing or opening. Second, the control valve accessories In the production process, the control system on the valve made a wide range of special requirements, therefore, the control valve must be equipped with a variety of ancillary equipment (Annex) to meet the needs of the production process. Control valve accessories include: 1, the valve positioner is used to improve the control valve regulating performance of the working characteristics, to achieve the correct positioning. 2, the valve position switch shows the upper and lower limits of the valve work position. 3, the pneumatic valve When the control valve of the gas source fails, keep the valve in the gas source before the opening position. 4, electromagnetic solenoid valve to achieve gas switch to ensure that the valve in the power failure valve at the desired safety opening position. 5, hand wheel mechanism When the control system controller fails, you can switch to manually operate the valve. 6, pneumatic actuator to the implementing agencies to speed up the movement, reducing transmission time. 7, air filter pressure reducer for purifying gas source, regulating pressure. 8, the gas tank to ensure that when the gas source failure, so that non-spring cylinder piston actuator can move the control valve to fail-safe position. Its size depends on the size of the cylinder, the valve action time requirements and the working conditions of the valve. In short, the role of the annex is to make the control valve function more perfect, more reasonable and more complete. Third, the control valve maintenance The control valve has the characteristics of simple structure and reliable operation, but because of its direct contact with the process medium, the performance of a direct impact on system quality and environmental pollution, so the control valve must be regularly maintained and regularly overhaul, especially on Conditions of use harsh and important occasions, should pay more attention to maintenance work. Key parts of the inspection 1, the valve body wall For the use of high pressure and corrosive media in the case of control valves, valve body wall, diaphragm valve diaphragm often subject to media impact and corrosion, we must focus on checking pressure and decay conditions. 2, the valve seat control valve work, due to media infiltration, fixed the inner surface of the threaded seat with the valve seat susceptible to corrosion loosening, inspection should be noted. For high pressure differential valve work, check the seat sealing surface should be washed away. 3, the spool valve plug is to adjust the work of the moving parts, subject to erosion of the media, the most serious corrosion, valve repair check carefully when each part of the corrosion, wear, especially in the case of high pressure valve spool Wear more serious (due to cavitation), should be noted. Spool damage should be replaced when serious, in addition should also pay attention to whether the stem has a similar phenomenon, or loosening and spool connections. 4, diaphragm "O" ring and other gaskets. Should check valve in the diaphragm, "O" -shaped gasket is aging, cracking. 5, sealing filler should pay attention to PTFE packing, sealing grease is aging, with the surface is damaged, should be replaced if necessary.
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