**1. Foreword**
In highly automated chemical control systems, the control valve serves as the final actuating device, adjusting process parameters based on control signals to maintain system stability. Its sensitivity directly impacts the performance of the entire control loop. According to industry data, approximately 70% of system failures originate from control valves. Therefore, it is crucial to conduct regular maintenance and analyze factors that affect the safe and reliable operation of these valves.
**2. Stuck Valve**
A common issue in control valves is sticking, often occurring during the initial commissioning of a new system or after maintenance. This can be caused by debris such as welding slag or rust trapped in the valve seat, leading to restricted flow and poor media circulation. Additionally, improper maintenance may increase internal friction, causing the valve to respond only to large signals while remaining inactive under small ones.
To resolve this, try quickly opening and closing the valve or using a bypass line to flush out contaminants. If the stem is jammed, use a pipe clamp to rotate it manually while applying external pressure. If this fails, increasing the air supply pressure and cycling the valve several times may help. If all else fails, disassembly and inspection are necessary.
**3. Leakage**
**3.1 Stem Length Mismatch**
If the stem is too long or too short, it may not allow the valve plug to fully close against the seat, resulting in internal leakage. Adjusting the stem length to ensure proper seating is the solution.
**3.2 Packing Leakage**
Packing leakage often occurs due to uneven contact between the packing and the stem. Over time, wear and aging can reduce the sealing effectiveness. To improve this, chamfer the top of the stuffing box, install a metal guard ring at the bottom, and use high-quality flexible graphite packing. Ensuring smooth surfaces and proper tightening of the gland bolts also helps.
**3.3 Deformation of Plug and Seat**
Corrosion or erosion from the process medium can cause deformation of the plug and seat, creating gaps that lead to leakage. Selecting corrosion-resistant materials and replacing damaged components when necessary ensures long-term performance.
**4. Oscillation**
Oscillation in control valves is often caused by insufficient spring stiffness, mismatched frequencies with the system, or operation at small openings. This leads to unstable output and vibration. To address this, increase the spring stiffness, add support to vibrating pipes, replace the valve if frequency conflicts exist, or reselect the valve’s flow capacity to avoid operating at small openings.
**5. Valve Positioner Faults**
Traditional positioners rely on mechanical balance and nozzle-baffle technology, making them prone to issues like temperature sensitivity, clogging, and spring degradation. Intelligent positioners, on the other hand, use microprocessors and digital signal processing, eliminating force balance. However, they require regular testing, especially in emergency situations, to prevent failure due to small signal inactivity or sensor drift.
**6. Conclusion**
By understanding the causes of control valve failures and implementing appropriate maintenance strategies, the reliability and efficiency of the system can be significantly improved. This not only reduces downtime but also enhances production efficiency, economic benefits, and energy savings. Proper care of control valves plays a vital role in ensuring the long-term stability and performance of industrial processes.
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