Pneumatic control valve is the compressed air as the power source to the cylinder as the actuator, and with the help of electrical valve positioners, converters, solenoid valves, retention valves and other accessories to drive the valve to achieve the switch or the proportion of regulation, the receiving industry Automatic control system to complete the control signal to adjust the pipeline media flow, pressure, temperature and other process parameters. Pneumatic control valve is characterized by simple control, rapid response, and intrinsically safe, without the need to take another explosion-proof measures. Pneumatic control valve classification and application Pneumatic control valve action open air and air off two. Air to Open When the air pressure on the membrane head increases, the valve moves in the direction of increasing opening. When the upper limit of input air pressure is reached, the valve is fully open. In turn, when the air pressure is reduced, the valve moves in the closing direction and in the absence of air, the valve is fully closed. Therefore, the gas-type valve is sometimes called fail-closed (Fail to Close FC). Air to Close The direction of action is exactly the opposite of the air-to-air type. When the air pressure increases, the valve moves to close the direction; air pressure decreases or not, the valve to the opening direction or fully open so far. It is sometimes called Fail to Open FO. Pneumatic valve opening or closing the air, usually through the implementation of the positive and negative agency and valve structure of the different ways to achieve assembly. The choice of open air is based on the safety of the production process to consider. When the air supply is cut off, is the regulating valve in the closed position safe or open position safe? For example, the combustion control of a heating furnace, the control valve installed in the fuel gas pipeline, according to the temperature of the furnace or heated material at the outlet of the heating furnace to control the supply of fuel. At this time, should choose to open the valve more secure gas, because once the gas supply is stopped, the valve is closed more than the valve is fully open more appropriate. If the gas supply is interrupted, the fuel valve is fully open, dangerous overheating can occur. Another example is a heat exchanger with cooling water cooling equipment, hot materials in the heat exchanger and cooling water for heat exchange is cooled, the control valve is installed in the cooling water pipe, with the heat exchanger material temperature to control the amount of cooling water in the Gas source is interrupted, the control valve should be in the open position safer, should choose gas-off (or FO) control valve. Air-open to gas-off or gas-off to open the air-open, such as the valve installed intelligent valve positioner, in the field can easily switch between each other. But there are some occasions, the failure does not want the valve is fully open or fully closed position, the operation is not allowed, but rather to maintain the fault in the original position before the rupture. At this time, some other measures may be taken, such as using a retention valve or setting up a dedicated air storage tank facility for the accident. Pneumatic control valve repair: Pneumatic control valve to work accurately and correctly to ensure the normal operation of process equipment and safety of production is of great significance. Therefore, to strengthen the maintenance of pneumatic control valve is necessary. First, check the focus of the inspection site when the body wall: in the high pressure and corrosive media occasions, the valve body wall, diaphragm valve diaphragm often subject to media impact and corrosion, we must focus on inspection of pressure decay; check Valve seat: medium infiltration due to work, the fixed valve seat thread inner surface vulnerable to corrosion and make the valve seat loose; check the valve: the valve spool is one of the movable parts of the regulating valve, the erosion of the media is more serious, Maintenance should carefully check the various parts of the valve core is corroded, worn, especially in the case of high pressure, valve core wear due to cavitation caused cavitation is more serious. Check the sealing packing: Check the packing asbestos rope is dry, such as the use of PTFE packing, should be checked to see if aging and its mating surface is damaged; Check the actuator in the aging of the rubber film , Whether there is cracking phenomenon. Second, the pneumatic control valve with the routine maintenance When the control valve graphite asbestos as a filler, about three months should be added to the filler on a lubricant to ensure flexible and easy to use control valve. If it is found that the packing pressure cap is very low, it should be filled with filler. If it is found that the PTFE packing is hardened, it should be promptly replaced. Attention should be paid to the operation of the regulating valve during the inspection, check the position indicator and the regulator The output is consistent with; regulator valve positioner should always check the gas source and found the problem promptly handled; should always maintain the health of the valve and the complete and easy-to-use components. Third, the common faults and causes (a) control valve does not move. Symptoms and causes are as follows: 1. No signal, no air source. ①gas source is not open, ② due to gas source water in the winter icing, resulting in duct blockage or filter, pressure relief valve plug failure, ③ compressor failure; ④ gas main leakage. 2. A gas source, no signal. ①regulator failure; ③ bellows leak locator; ④ regulatory mesh membrane damage. 3. Locator airless. ①filter blockage; ② valve failure I ③ pipeline leakage or blockage. 4. Locator gas source, no output. The throttle of the positioner is blocked. 5. A signal, no action. ①spool off, ② valve core and the community or with the seat stuck; ③ stem bent or broken; ④ seat valve core block or block of dirt; ⑤ actuator spring rust due to long-term useless. (B) control valve instability. Symptoms and causes are as follows: 1. Gas pressure instability. ①compressor capacity is too small; ② valve failure. 2. Signal pressure is not stable. ①The time constant of the control system (T = RC) is not appropriate; ② The regulator output is not stable. 3. Gas pressure is stable, the signal pressure is stable, but the control valve action is still unstable. ①locator amplifier ball valve dirt worn lax, air consumption will increase when the output shock will produce; ② locator amplifier nozzle baffle is not parallel, the baffle cover nozzle; ③ output tube , Line air leakage; ④ actuator stiffness is too small; ⑤ stem friction in the movement of large, with the contact parts of the block phenomenon. (C) regulating valve vibration. Symptoms and causes are as follows: 1. The regulating valve vibrates at any opening. ①unsteady support; ② nearby vibration source; ③ valve core and bush wear serious. 2. Valve close to the fully closed position when the vibration. ①control valve selected, often used in a small opening; ② single-seat valve flow direction and closed the opposite direction. (D) regulating valve slow action. The phenomenon of dullness and the reasons are as follows: 1. Stem only in unilateral action slow. ①Pneumatic diaphragm actuator diaphragm rupture leakage; ② "O" -type seal leakage in the actuator. 2. Stem in the reciprocating action are slow phenomenon. ①blockage of viscous substances in the valve body; ② polytetrafluoroethylene filler hardening or graphite asbestos filler lubricant drying; ③ filler plus too tight, frictional resistance increases; ④ stem because the valve does not lead to direct friction resistance; ⑤ no Pneumatic positioner valve will lead to slow action. (E) the amount of leakage control valve increases. The reasons for the leakage are as follows: 1. Leakage when the valve is closed. ①spool is worn, serious leakage, ② valve not adjusted off is not strict. 2. Valve can not reach the fully closed position. ①medium pressure is too large, the actuator rigidity is small, the valve is not strict; ② valve foreign body; ③ bushing sintering. (F) adjustable flow rate smaller. The main reason is that the valve core is corroded and smaller, so that the adjustable minimum flow rate becomes larger. Understand the phenomenon of pneumatic control valve failure and the reasons that symptomatic measures can be taken to resolve.
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