First, the application of pneumatic control valve in modern factories Pneumatic regulating valve is used together with other instruments to realize the automatic adjustment and remote control functions of process parameters such as flow, liquid level, pressure, temperature and other media such as liquid, gas and steam in the production process. Among them, the pneumatic diaphragm control valve plays an important role as the final actuator of the automatic adjustment system. The application of the control valve is directly related to the quality and safety of production. Second, the common failure of pneumatic control valve, analysis of the causes and solutions (1) The regulating valve does not operate. Causes: There is no signal pressure or diaphragm cracking, diaphragm leakage, diaphragm thrust reduction; valve plug and valve seat or bushing stuck, stem bent and other reasons to make the regulator valve can not move. Solution: Connect the signal cable and gas source connector firmly or replace the diaphragm; straighten the valve stem or replace the valve stem. (2) The regulating valve operates normally but does not function as a regulator. Cause: The valve plug is detached or the pipe is clogged. Solution: Connect the valve plug to the stem firmly when the valve core comes off, and contact the competent authority to clear the pipeline. (3) The valve is slow or bouncing. Reasons: Aging or dry seal packing increases the friction between the valve stem and the packing; the valve body contains highly viscous contaminants, as well as plugging, coking, etc., and the diaphragm and “O†seal leaks. Solution: Replace the packing and re-adjust; remove dirt, coke, etc.; fasten the membrane head, or replace the diaphragm, O-ring. (4) The regulator valve is unstable or oscillates. Cause: The valve diameter selection is too large; single-seat valve media flows in the same direction as the closing direction. The valve plug and guide sleeve wear badly. Solution: Replace valve with proper valve diameter; adjust valve direction; improve joint surface (grinding) or replace spool and guide sleeve. Adjustment and Maintenance of Pneumatic Control Valve in Production Process Third, pneumatic control valve typical fault maintenance program Take the example of “the normal operation of the regulating valve, but not the adjustment function†as an example to explain the maintenance plan: A chemical plant's vinyl chloride workshop uses the valve as a single-seat regulating valve of a well-known domestic enterprise. This line produces 100,000 tons of plastic pellets per year. There are two production lines in the compression, conversion, and fractionation sections of the vinyl chloride plant. In addition to compression, a large number of pneumatic control valves are used in the conversion and fractionation sections. The operating environment of each regulating valve is completely different. Each pipe is usually provided with only one regulating valve, and some bypass valves are provided next to some regulating valves. Through manual valve control. If the regulator valve fails, the bypass valve can be opened to ensure normal production. Therefore, before the dismantling, it is necessary to carefully analyze the cause of the malfunction of the regulating valve and formulate a sound maintenance plan. 1. Maintenance methods and standards. Maintenance methods: (1) According to the operating procedures to hand over, requiring a qualified replacement, maintenance workers carrying toxic canisters, vinyl chloride odor detector, and gas defense station staff monitoring, before the valve can be dismantled. (2) Turn off the gas source, remove the positioner and gas source tube, and disassemble the valve (3) Valve stem connection, check the valve stem connection, check whether the connection pin can be removed, can not be taken Use a special tool to remove or change the hole. 2. After the maintenance of the valve assembly. (1) Before the assembly, all components of the valve shall be cleaned and checked. The assembly sequence shall be from bottom to top. The added gaskets shall be greased, and the added packing shall be even and full. (2) Grease should be applied to all fastening bolts and fillers before assembly to facilitate the next inspection and lubrication. 3. Adjustment of the valve. After the valve assembly is completed, the positioner is installed and the air source tube is adjusted. The calibration point should not be less than 5 points, 0%, 25%, 50%, 75%, 100%. Slowly input each point signal to the positioner and observe whether the stroke pointer corresponds to the scale scale. Otherwise, it should be adjusted repeatedly until it reaches the standard. 4. Test and acceptance. (1) After the valve is adjusted to the standard, it shall be installed on the pipeline, and after confirming that there are no problems, it will notify the workshop and go through the formalities for delivery. (2) with the process personnel from the on-site manual operation to the total control operation. (3) Check whether the records after inspection and repair are complete and accurate. (4) When the valve is introduced into the medium, check the packing and the flanges on both sides for leaks. (5) After 72 hours of normal operation of the regulating valve, the relevant technical supervisor shall sign for acceptance. 5. Safety precautions. (1) Maintenance safety precautions: Maintenance must be performed by two or more people; when entering the site, maintenance must be done by wearing the specified protective articles; if an abnormal valve is found that needs to be dealt with, it must be approved by the technologist and safety measures must be followed. (2) Inspection safety precautions: The online use of the valve must be completed when the inspection and repair of the valve; before the disassembly of the online valve must discharge the process media clean, and confirmed that no leakage, can be carried out operations; shut down the gas source during maintenance; In this case, special tools must be used and parts must not be damaged. The disassembled parts and the exposed valve body should be covered with soft materials. (3) Safety precautions for operation: Prior to commissioning, the technologist should be notified to handle the handover procedures; the two parties involved in the process and maintenance should work closely together. Fourth, the pneumatic valve disassembly process (1) Cut off the power supply and gas source; remove the power line and signal line and then remove the gas pipes; (2) Connect the air pipe (via the pressure reducing valve and positioner to the membrane head), and set the pressure of the pressure reducing valve to the minimum working pressure value of the spring on the nameplate; (3) Then remove the split nut, close and remove the gas source after removal; (4) Loosen the indicator block and lock nut; (5) Loosen the bracket round nut and remove the bracket; (6) Loosen and remove the bonnet nuts at the opposite corners, remove the upper bonnet, valve plug, guide sleeve and toothed gasket in sequence; after disassembly, inspect the cause of the malfunction and reconnect the valve plug to the valve stem. Then assemble the valve body in the reverse order of removal
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