Common Operational Issues of Different Control Valves
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Control valves are key components in fluid control systems, and their stability, sealing performance, and corrosion resistance directly impact the overall system performance. Despite continuous improvements in the design and manufacturing of control valves, a range of common issues still arise during operation, affecting their efficiency and lifespan. This article explores the common problems encountered in control valve operation, focusing on oscillation, sealing performance, clogging, hysteresis, corrosion, and actuator issues, and offers corresponding solutions and optimization strategies.
Control Valve Oscillation
Control valves tend to oscillate when operating at small openings, primarily due to the structure of the valve trim and the direction of fluid flow. For single-seated control valves, the valve is more stable when the medium flows in the open direction, while flow in the closed direction leads to reduced stability. Double-seated control valves, having two valve plugs, are more prone to oscillation at small openings. The lower plug operates in a flow-closed direction, while the upper plug works in a flow-open direction. This design, particularly the lower plug's flow-closed structure, creates unstable fluid forces at small openings, resulting in severe vibrations.
Solution: Double-seated control valves are not recommended for small-opening applications to avoid oscillation. Instead, single-seated control valves or other valves designed for stable operation at small openings should be chosen. Proper valve selection and control system setup during installation and commissioning should also align with actual operating conditions.
Control Valve Sealing Performance
The sealing performance of control valves directly affects system safety and reliability. In practice, double-seated control valves struggle to achieve good simultaneous contact between both sealing surfaces, leading to significant leakage, especially in applications requiring strict shut-off. Even improved designs, such as double-seal sleeve valves, cannot fully resolve this issue. Soft-seal valves offer better shut-off performance, but their sealing materials are prone to wear, affecting long-term reliability.
Solution: For applications requiring high sealing performance, hard-sealed control valves are recommended. Hard-seal valves protect sealing surfaces with wear-resistant alloys, improving durability and reducing leakage. Full-function ultra-light control valves are also ideal choices, providing low leakage and high reliability.
Control Valve Clogging
Clogging is another common problem during control valve operation, especially when the medium contains solid particles or sediments. Linear-stroke control valves, with vertical throttling, create complex flow paths in the valve body, allowing dead zones to form, which can lead to sediment build-up and clogging over time. Rotary-stroke control valves, with horizontal throttling, provide a simpler flow path and reduce sediment accumulation, minimizing clogging risk.
Solution: For systems with particulate-laden or unclean media, rotary-stroke control valves are preferred. Their horizontal flow design reduces sediment formation, improving anti-clogging performance. Regular cleaning and maintenance also help reduce clogging occurrences.
Control Valve Hysteresis
Hysteresis refers to the lag in valve response due to friction or other factors, causing a discrepancy between the actual valve position and the setpoint. This issue is particularly common in linear-stroke control valves, where the vertical movement of the valve stem generates significant sliding friction. Tight packing further increases friction, reducing control accuracy.
Solution: Rotary-stroke control valves, which rely on rotational motion, reduce sliding friction. Their thicker, more rigid valve stems, combined with longer-lasting asbestos packing, effectively minimize hysteresis, improving control precision and valve longevity.
Control Valve Corrosion
In certain corrosive media, such as desalinated water containing low concentrations of acids or bases, control valve sealing materials (e.g., rubber) can be corroded. Rubber seals may swell, age, or lose strength, reducing sealing performance and shortening valve lifespan. For instance, rubber-lined butterfly valves and diaphragm valves do not perform well in corrosive environments.
Solution: Fluorine-lined diaphragm valves offer better corrosion resistance for such media, although the diaphragm may degrade from frequent folding. For longer service life, specialized ball valves for water treatment, with superior corrosion resistance, are recommended. These valves can last 5 to 8 years in desalinated water applications.
Control Valve Actuator Issues
Pneumatic actuators are critical components of control valves, as their performance directly affects valve response speed and control accuracy. Traditional diaphragm actuators, with limited thrust and large size, are being replaced by piston actuators. Piston actuators better utilize air pressure, offering a more compact size, greater thrust, and improved sealing with O-rings, resulting in longer lifespans than diaphragm designs.
Solution: To enhance control valve performance and response speed, piston pneumatic actuators are recommended. Their greater thrust and higher reliability make them the preferred choice over diaphragm actuators, positioning them as the future mainstream solution.
Common operational issues with control valves, such as oscillation, leakage, clogging, hysteresis, and corrosion, directly impact their reliability and stability in fluid control systems. Proper valve selection and design optimization can effectively address these issues. For example, avoiding double-seated valves at small openings, choosing rotary-stroke valves to prevent clogging, opting for hard-seal valves for improved sealing, and using piston actuators for better control performance are key strategies. These improvements will significantly enhance the stability and lifespan of control valves in operation.