A Maintenance Guide to Faulty Control Valves

In industrial production, control valves play an indispensable role. They act as precise "regulators," ensuring that the flow rate, pressure, and other parameters of fluid media are always maintained at an ideal operating state. However, even the most sophisticated equipment is not immune to malfunctions. When a control valve fails to operate normally or cannot meet operational requirements, maintenance for fault repair becomes the key to restoring its normal function. Today, let's delve into the intricacies of maintaining faulty control valves.

When a control valve is found to be faulty and is removed from the pipeline, the first task is to thoroughly clean it. During operation, control valves are highly susceptible to contamination by process fluids. If the process medium is corrosive, explosive, or radioactive, these contaminants pose significant health hazards to humans. Therefore, it is essential to use special methods to clean the contaminated parts. The choice of cleaning method depends on the nature of the process medium. In most cases, simple water rinsing or steam blowing can achieve the desired cleaning effect. However, for specific contamination scenarios, such as alkaline medium contamination, acid washing is required; and for heavy metal contamination, special treatment under high temperature and extended duration is necessary. During the cleaning process, it is crucial to avoid recontamination, ensure that operations are conducted in a safe location, and have all necessary tools, containers, heating equipment, steam hoses, and washing devices prepared in advance to ensure the smooth progress of the cleaning work.

Disassembling the control valve is a critical step in the maintenance process. Before disassembly, it is imperative to mark the connection positions of the actuator corresponding to the valve body. This ensures that all components can be accurately repositioned during subsequent maintenance and reassembly. During disassembly, first separate the actuator from the upper valve cover, then detach the upper valve cover from the valve body, and finally disassemble the upper valve cover from the packing box components. This allows for the smooth removal of key parts such as the valve core and valve stem from the valve body.

After disassembly, the next step is to carefully inspect all the removed parts and components. This is a crucial phase in determining which parts need repair and which need replacement. During the inspection, several areas require special attention:

The pressure resistance and corrosion resistance of the inner wall of the valve body are key inspection points. Since control valves are subjected to various media pressures and corrosive effects during operation, the condition of the inner wall directly affects the safety and reliability of the control valve. Corrosion or damage to the inner wall may lead to media leakage and even safety accidents.

The valve seat is one of the critical components of a control valve. The ingress of media can corrode the threaded surfaces that secure the valve seat, causing it to loosen. In high-pressure applications, the corrosion or damage to the sealing surface of the valve seat requires careful inspection. Some modern control valve designs use high-temperature sealing or other methods to fix the valve seat instead of threaded connections. Regardless of the structure, the sealing areas of the valve seat must be thoroughly inspected to ensure good sealing performance.

The valve core is the core component of a control valve, as it directly contacts the media and controls its flow. Therefore, it is essential to carefully inspect the valve core for corrosion or wear, especially under high differential pressure conditions, where wear can be more severe. The valve stem should also be inspected for similar issues, as well as for any looseness in its connection with the valve core. These details can significantly impact the normal operation of the control valve.

The condition of the packing and gaskets directly affects the sealing performance of the control valve. Carefully inspect them for aging, cracking, and the integrity of their mating surfaces. Any issues with the packing or gaskets can lead to media leakage and affect the normal operation of the control valve.

After disassembling the actuator, it is also important to carefully inspect components such as the bracket, diaphragm, spring, push rod, and threaded connections. The bracket, as a load-bearing component, must be free of cracks; the diaphragm, a critical element, should not be damaged or aged; the spring should be free of cracks. For multi-spring actuators, damage to even one spring can severely impact the performance of the entire actuator. If any parts are found to be corroded or rusted, it is best to replace them with new ones to ensure the normal operation of the actuator.

After careful inspection and necessary repairs or replacements, the next step is to reassemble the control valve. During reassembly, several key points need special attention:

During assembly, it is crucial to ensure accurate alignment between components to guarantee the normal operation of the control valve. The tightening force of bolts or double-ended bolts should be even, with the torque load related to the bolt size. Over-tightening should be avoided to prevent damage to components or other issues.

The type, quantity, and installation method of packing directly affect sealing performance, so care must be taken during assembly. Select the appropriate packing according to requirements and install it correctly to ensure it provides a seal while minimizing friction on the valve stem.

After reassembling the actuator, apply suitable lubricants to all friction points. These include the valve stem and guide components, actuator push rod and guide components, bearings, transmission parts, and adjusting nuts. Lubricant application reduces friction between components, extends their service life, and helps ensure the normal operation of the control valve.

After reassembly, the control valve must undergo a series of tests, which is a critical step in ensuring its performance. The tests include basic performance testing, air-tightness testing, and leakage rate testing. Only by passing these rigorous tests can we ensure that the control valve will function properly when installed on-site and meet the various requirements of the production process.

In addition to fault repair maintenance, routine maintenance of control valves is equally important. It can effectively prevent faults and extend the service life of the control valve. Routine maintenance mainly includes the following aspects.

Improper installation or assembly may generate various stresses on the control valve. For example, high-temperature media can produce thermal stresses, and uneven tightening forces during installation can also cause stress. These unbalanced stresses can deform the valve stem and guide components, leading to leakage and increased variability. Therefore, stress relief should be carried out during routine maintenance to ensure that all components of the control valve are in good working condition.

Regularly check for rust, welding slag, and other contaminants in the pipelines connected to the control valve and remove them promptly. These contaminants can cause wear on the valve core and seat, affecting normal operation. Typically, a filter screen or other filtering devices can be installed upstream of the control valve and cleaned regularly to ensure pipeline cleanliness.

The support of the control valve keeps its components in a position unaffected by gravity. Improper support can misalign the valve stem and seat, increasing variability and reducing sealing performance. Therefore, it is necessary to check whether the support of the control valve is appropriate and ensure that it is stable and rational.

Air and hydraulic sources are the energy sources for the operation of control valves. Impurities in instrument compressed air and hydraulic oil can clog throttle holes and pipelines, causing malfunctions. Therefore, it is crucial to regularly inspect the air and hydraulic oil and regularly drain the filtration devices to ensure the normal operation of the control valve.

Regularly inspect the gear transmission devices of handwheel mechanisms, electric actuators, and hydraulic actuators, and apply lubricants to prevent binding. Additionally, check whether the braking and limiting devices are flexible and functional to ensure the normal operation of the transmission devices.

Regularly inspect the wear condition and clamping force of the packing, and replace the packing box in a timely manner to ensure that the packing provides a seal while minimizing friction. For oil-free lubricated packing boxes, lubricating oil should not be added.

In summary, the maintenance of faulty control valves is a complex and meticulous task, with every step being crucial. From disassembly, cleaning, and inspection to reassembly, testing, and routine maintenance, each procedure must be strictly followed according to requirements. Only through meticulous inspection, precise repair, and rigorous testing can we ensure that the control valve functions properly after repair and meets the various demands of the production process. Moreover, regular routine maintenance can effectively prevent faults and extend the service life of the control valve, providing a strong guarantee for the stable operation of industrial production.