Common faults and repairs of control valves

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The control valve directly controls the medium and serves as the terminal element of automatic control; therefore, apart from failures in the valve itself, all faults or misoperations in the control instruments are manifested in the control valve. Common faulty components of control valves are listed in the table below: Serial Number, Fault, Cause, Inspection Method. 1. Valve does not operate: 1. Air supply issue, severe leakage in the air supply pipeline. 2. The input signal disappears. 3. Positioner, converter failure. 4. The film ruptures, and the spring breaks. 5. Stuck valve stem, valve core, etc. First, check the upstream instruments, starting from the signal converter to the locator. Check the gas source pressure and supply volume. Is there air leakage in the actuator? Finally, check the valve. 2 Slow valve operation 1. Low air supply pressure. 2. Diaphragm and piston ring leakage. 3. The packing is too tight, causing the valve stem to deform. 4. The locator has poor response performance. Check the gas source and the performance of the locator. Check whether the diaphragm and piston rings are leaking air. Check the packing gland. Finally, check the valve core and valve stem. 3 The valve cannot be fully closed. 1. There is a problem with the input signal. 2. The operating air pressure is insufficient or the spring force is insufficient. 3. The locator did not reach its full travel distance during debugging. 4. The operating pressure difference is greater than the design pressure difference. 5. There are debris on the valve seat and valve core. Check the signal and air source in the same way as above. Check the medium pressure difference under operating conditions. Check the spring pressure range. Verify the locator. 4 Large leakage when the valve is closed. 1. Low thrust of the actuator, or insufficient spring force. 2. The valve core and valve seat are damaged. 3. The valve seat is loose, and the gasket is damaged. 4. There is debris between the valve seat and the valve core. Adjust the actuator and disassemble it for inspection. 5 Valve vibration 1. The positioner is not adjusted properly. 2. The valve opening is too small, and the flow direction is incorrect. 3. The filler is too tight. 4. The gap between the valve core and the guide sleeve is too large. 5. There are vibration sources in the vicinity. First, check whether the locator is oscillating. Check flow direction, opening degree ； Adjust the filler. If the opening is too small and the gap is too large, replace the valve model or diameter during the major repair. Check the support and shock absorption structures. 6 Packing leakage 1. The packing is not properly compressed. 2. The filler material does not match the medium. 3. The valve stem is deformed and rough. 4. Deformation of the filler gland. Compress the filler. Check the filler material. Disassemble the valve to inspect the valve stem and gland. 7 Leakage of valve body gasket 1. Insufficient tightening torque. 2. The gasket is damaged. 3. The sealing surface is damaged, causing leakage from the screw holes. Increase the pre-tensioning force. Disassemble to inspect the gasket, the sealing surface, and the screw holes of the double-headed studs. The maintenance of control valves is generally divided into two categories: 1. Preventive maintenance: includes preventive measures taken during installation, preventive measures taken during routine inspections, and temporary repairs. Also known as planned maintenance, it refers to preventive measures taken before a failure occurs; it can also be understood as prevention prior to installation and routine maintenance. It mainly consists of two parts: 1) Issues to note before installation. Cleaning: The control valve should be cleaned before installation, and the pipes should also be flushed to remove any remaining debris, weld slag, and other impurities. This prevents such debris from getting stuck in the valve core or causing damage to the valve or other equipment due to high-speed movement of those particles. Avoid installation stress: When installing control valves, it is common to encounter a situation where the two pipe flanges are not aligned with each other, resulting in severe misalignment ； Sometimes the distance between the two flanges differs too much from the distance to the valve’s end face ； The pipe was pulled and bent forcefully using a crowbar to force the valve into place; as a result, the valve will be under stress for an extended period, leading to deformation to varying degrees. Secure the supports: Large-diameter valves have a significant weight; for example, a 41000 sleeve valve weighs 510 kg, while an 8″ Class 600 valve weighs 895 kg. Such valves need to be supported beneath them so that they do not hang from the pipeline. For valves installed horizontally, a support frame is generally provided at the connection between the valve body and the actuator to prevent the valve from experiencing torque and bending moments. Avoid vibration: If the valve is placed too close to power machinery such as compressors and pumps, it will be subjected to forced vibration, which may lead to resonance; therefore, vibration isolation devices should be used between them. For valves with a large pressure difference, the impact of high-speed fluid on the valve core can also cause vibration; therefore, when selecting such valves it is necessary to pay attention to limiting the flow velocity at the inlet, as well as using an appropriate design. The inlet section of the valve should have a sufficiently long straight pipe section, generally 10 times the valve diameter, to prevent the turbulent flow from elbows from impacting the valve internals. 2) Daily maintenance considerations after the valve is put into use: Ensure that the air supply is clean and the power supply is reliable; the standards for control valve products specify clear requirements regarding the air supply and power supply. Electric valves must be powered in strict accordance with requirements regarding voltage, number of phases, AC/DC, grounding, etc., with reliable wiring. The air supply must be dry and clean, free of oil, water, dust, and other corrosive substances, to prevent accelerated aging of the rubber diaphragms in the actuators and positioners, and to avoid blockage of the constant orifice in the positioners. The air filter-regulator used to supply air to pneumatic valves or valve positioners should be used properly; oil, water, and dirt accumulated in the filter should be removed promptly, and the filtering elements should be cleaned regularly. Regular inspection and refueling: After using the control valve, it should be inspected regularly, with a focus on whether its operation is smooth. Check whether the connection between the push rod and the valve stem is loose, and whether there are any leaks at the packing and the valve body gaskets. A common issue is leakage at the packing area; tighten the packing nuts promptly, and if an oil injector is used, apply oil regularly. In areas with acid mist or corrosive gases, the exposed valve stems are protected by plastic tubes or rubber bellows; if any damage to these protective covers is detected, they should be replaced promptly. Implement appropriate protective measures in particularly harsh environments: Although control valves are made of steel, they can still be damaged quickly in extreme conditions such as intense sunlight, flying sand and debris, or heavy wind and snow. Such situations are often not anticipated in advance; by adding simple protective devices after installation to shield against sunlight and rain and to block wind and sand, the service life of the valves can be extended. 2. Fault repair: Repairs carried out when the valve malfunctions or when the performance of the control valve fails to meet the requirements of automatic control. Once there are problems with the safety performance of a control valve, or if it cannot operate properly and fails to meet the requirements of an automatic control system, it indicates that there is a fault that must be repaired. Periodic major repairs of chemical plants and power stations are also classified under this type of maintenance. Complete unit cleaning: After depressurizing and cooling the pipeline, the valves must be removed for cleaning. Ordinary media are purged and soaked with water or steam. Special treatment processes are used for acids, alkalis, radioactive substances, or other toxic, harmful, and corrosive media, in order to avoid impacts on human health and prevent environmental pollution. Removing the valve: First, separate the actuator from the valve; attach labels before doing so, and make marks at the connection points. Then remove the upper valve cover from the valve body, and make marks at the connection point of the middle flange as well. Remove the spool-stem assembly, sleeve, valve body gasket, and double-headed studs from the valve body ； Unscrew the valve seat and valve seat gasket (if any). Remove the packing flange and gland from the upper valve cover, as well as the packing and gaskets ； After removal, clean it thoroughly, and pay special attention to inspecting the following components: the valve body, as well as any damage to its inner walls ； The degree of damage to the valve core, valve seat, and throttle surface determines whether repair is possible ； Valve stem: Is the thread connecting it to the valve core loose? Is the valve stem bent, deformed, or worn? ； Fillers, whether deformed or aged, are generally not reused ； Valve body gaskets and valve seat gaskets are generally not reused. Removing the actuator: Remove the bolts surrounding the pneumatic diaphragm head, take out the diaphragm and spring-rod assembly, as well as the O-rings in the bracket. Clean it thoroughly and check carefully. (Diaphragm: check for aging, cracks, and delamination of the mesh fabric) ； Spring: Check the degree of surface rust, for any cracks, and for any permanent deformation ； O-ring: whether it is aged, worn, or broken ； Check whether the push rod is deformed or corroded ； For piston-type actuators, check whether there is wear or scuffing on the inner wall of the cylinder, whether the piston rings and guide elements are aged or worn, and whether the piston and piston rod are deformed or worn. Reassembly: After the above checks, the parts can be reassembled after being repaired or replaced. Then connect the two components to form the complete device. During assembly, care should be taken to add grease. The associated valve positioners, air filters, pressure reducers, and other auxiliary instruments were repaired and tested to ensure they met the required standards before being installed in their original positions. Performance testing: The assembled control valves are tested and inspected according to the factory-specific requirements, including airtightness, sealing performance, pressure resistance, leakage rate, basic error, hysteresis, dead zone, rated stroke, and deviation from the set point. Tests for rated flow rate and flow characteristic are conducted in special required situations. For commonly used spare parts, before carrying out repairs in case of accidents, the following vulnerable components are usually prepared: valve body parts, packing, valve body gaskets, seat gaskets, double-headed studs, safety pins, etc ； Pneumatic actuator components: O-rings, piston rings, bolts for diaphragm covers ； Prepare diaphragms and springs at a certain percentage of the quantity of actuators of the same specification. Other vulnerable components such as valve cores, valve seats, valve stems, sleeves, packing flanges, etc., are generally not prepared in advance due to their high cost; only after disassembly and inspection to determine if replacement is necessary are they purchased from the original manufacturer.