Practical Guide to Control Valve Failures and Solutions

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Unlike manual valves, control valves must remain in a constant state of motion and adjustment during operation. They have numerous moving parts and are subjected to various forces, such as unbalanced forces exerted by the medium. As a result, all kinds of unforeseen malfunctions may occur. These malfunctions can originate from the actuator or the regulating mechanism, or they may stem from connected accessory devices. 　　1. Failures caused by packing
Failures resulting from packing issues manifest as increased external leakage, heightened friction, and chattering of the valve stem. The analysis is as follows: 1. The filler material is not suitable. Failures caused by an inappropriate filler material are mainly an increase in external leakage and increased friction; for example, in high-temperature applications, polytetrafluoroethylene fillers are used. The troubleshooting method is to replace the packing. 　　2. Improper design of the packing structure. The packing and related accessories are not installed in the correct positions within the packing chamber, and the packing height is also inappropriate. The solution to this issue is to install the packing and related accessories according to the instructions provided in the product manual. 　　3. The filler is not installed properly. For example, the spiral installation of graphite fillers results in uneven packing pressure and misalignment at the center. The troubleshooting method is to install layer by layer to ensure uniform clamping force. 　　4. There are impurities in the filler. Debris in the packing causes scuff marks on the valve stem. The troubleshooting method is to clean the filler and remove debris. 　　5. Improper installation of the upper valve cover. Improper installation of the upper valve cover results in uneven stress on the packing. The troubleshooting method is to reinstall the gasket on the upper valve cover and tighten the bolts securing the upper valve cover evenly in a diagonal pattern. 　　II. Failures caused by the airtightness of the actuator Failures resulting from the airtightness of the actuator are manifested as an increased response time and sluggish movement of the valve stem. The analysis is as follows: 1. The diaphragm of the pneumatic diaphragm actuator is not properly compressed. The diaphragm not being properly compressed or experiencing uneven stress leads to leakage of the incoming air signal, which results in a sluggish response of the actuator to signal changes and an increased response time. If a valve positioner is installed, its effect is reduced. The method for troubleshooting is to apply soapy water for inspection and eliminate the leak point. 　　2. The piston seal ring of the pneumatic piston actuator is worn out. This results in the control valve being unable to respond quickly, and the valve stem moving sluggishly. The troubleshooting method is to replace the sealing ring and check whether there is any wear on the inner wall of the cylinder. 　　3. The diaphragm of the pneumatic diaphragm actuator is damaged. It is manifested as sluggish movement of the valve stem, and a sound of gas leakage can be heard. The troubleshooting method is to replace the diaphragm, and it is also necessary to check whether there are burrs or similar issues with the limit mechanism or tray. 　　4. The connection pipeline is leaking air. This results in sluggish valve stem movement and an increased response time. The troubleshooting method is to apply soapy water to the connection pipes, identify the leak points, and replace or weld them. 　　III. Failures caused by unbalanced forces Failures resulting from unbalanced forces are manifested as unstable operation of the control valve and poor sealing. The fault analysis is as follows: 1. Incorrect flow direction. Improper installation of the control valve causes the actual fluid flow direction to differ from the direction indicated on the valve, resulting in changes in unbalanced forces. For example, the flow-on control valve is installed in flow-off mode. The troubleshooting method is to reinstall. 　　2. The actuator does not match. This results in insufficient thrust or torque, causing the control valve to fail to operate properly. The troubleshooting method is to replace the actuator. 　　IV. Faults in electric actuators 　In addition to common faults such as wire short circuits or open circuits, electric actuators can also suffer from problems with the servo amplifier and motor. The common faults are analyzed as follows: 　1. Loose connectors, or open or short circuits in the wiring. It causes poor contact and increases or decreases the impedance of the relevant circuits. The troubleshooting method is to check and wiggle the connecting wires, and replug and insert all connectors. 　　2. Mechanical transmission components of the reducer. Check whether the operation is normal and whether the gear meshing is good; the methods for dealing with faults are to replace or repair damaged gears and add lubricant. 　　3. Power supply. Check whether the fuse is blown, and observe whether there is any smoking or unusual odors from the position feedback of the servo amplifier, such as a burnt smell coming from the insulation layer of the transformer casing or its resistors. The troubleshooting method is to replace the damaged components. 　　V. Malfunctions caused by mismatched flow characteristics The flow characteristics of control valves are used to compensate for the different characteristics of the controlled object. If the selected flow characteristic is not appropriate, it will degrade the control quality of the control system. For example, the sensitivity of the control system differs at low and high flow rates. The fault analysis is as follows: 1. The controlled object exhibits saturated nonlinear behavior (for example, in temperature control systems); at low flow rates, the control system operates normally, but it becomes sluggish at high flow rates. At low flow rates, the control system is extremely sensitive, and oscillations and instability may occur; however, it functions properly at high flow rates. The cause of the failure was the use of control valves with linear or quick-opening flow characteristics. The troubleshooting methods involve replacing the trim or the control valve itself, or installing a valve positioner, so that the control valve meets the requirements for equal percentage or parabolic flow characteristics. 　　2. The controlled object has linear characteristics (e.g., a flow follow-up control system). The control system operates normally at low flow rates, but exhibits oscillation or instability at high flow rates; or it is sluggish at low flow rates while functioning properly at high flow rates. The cause of the malfunction is the selection of control valves with equal percentage or parabolic flow characteristics. The fault resolution method is to replace the internal components of the control valve or the control valve itself, or to install a valve positioner, so that the control valve meets the requirements for linear flow characteristics. 　　3. The rated flow coefficient of the control valve was selected improperly. The selected rated flow coefficient is either too large or too small, resulting in an increase or decrease in the minimum or maximum flow rate that can be regulated by the control valve, thus failing to meet the operational requirements of the process. When the control valve operates at a small or large opening, the control quality deteriorates. The troubleshooting method is to recalculate the flow coefficient of the control valve and install a control valve that meets the requirements. For example, selecting control valves based solely on the diameter of the process piping can result in an excessively high rated flow coefficient, while an increase in production scale can lead to an excessively low rated flow coefficient. 　　VI. Failures caused by improper flow path design and installation 　Failures resulting from improper design or installation of the control valve’s flow path are manifested as increased noise, easy accumulation of debris inside the valve body, inadequate sealing of the control valve, increased leakage, or jamming. The fault analysis is as follows: 1. The leakage rate of the two-seat valve has increased. The two-seat valve does not feature an integrated design, which results in different expansion coefficients of the valve components when temperatures change, thereby increasing the leakage rate. The fault handling method is to use an integrated two-seat valve, or a sleeve valve with a balancing function. 　　2. When a three-way valve is used for merging streams, the temperature difference between the two fluids being combined leads to an increased leakage rate. The troubleshooting method involves changing the fluid convergence to divergent control by installing a tee valve in front of the heat exchanger, thereby ensuring consistent fluid temperatures. 　　3. Improper flow direction causes increased noise. For example, flow-open control valves are used in flow-close applications, which causes increased noise at low flow rates. The troubleshooting method is to check the flow direction and reinstall it. 　　4. The upstream and downstream shut-off valves and bypass valves are not installed properly. This prevents the discharge of contaminants, condensate, or non-condensable gases. The fault handling method is to install the drain valve at the lowest point of the control valve assembly, and the vent valve at the highest point of the same assembly. 　　5. The guide bushing is not installed properly. This causes misalignment of the center, increasing friction and leading to the valve stem getting stuck. The troubleshooting method is to reinstall the guide bushing. 　　VII. Malfunctions caused by leakage
Internal leakage reduces the adjustable ratio; in severe cases, it prevents the control system from meeting the requirements of process operations and control. External leaks cause environmental pollution and increase costs. The fault analysis is as follows: 1. The leakage increases due to cavitation and erosion. Damage to the valve core and seat caused by cavitation, flashing, and erosion leads to an increase in leakage from the control valve, which is manifested as an increase in gas or fluid dynamic noise. Troubleshooting methods include inspecting valve trim, replacing or grinding the valve stem and seat, hardfacing the valve stem with cemented carbide, reducing the pressure drop across the control valve, eliminating noise sources, and using low-noise control valves. 　　2. The increased leakage was caused by impurities present in the fluid in question. During the operation phase, improper practices such as failing to remove the control valve during pipeline purging can lead to debris entering the valve; or during operation, debris carried by the fluid being controlled can accumulate inside the valve body. These debris cause damage to the sealing surfaces between the valve core and the valve seat, resulting in increased leakage. The troubleshooting method involves grinding the valve core and seat; during pipeline purging, the control valve should be removed. For controlled fluids containing particles, a filtering device can be installed upstream of the control valve. The control valve assembly should be installed at a higher position, and regular drainage should be carried out. 　　3. The actuator is not properly connected to the control mechanism. The troubleshooting method is to reinstall and conduct a leakage test. 　　4. Improper installation of the filler. Improper installation of the packing can lead to increased friction or deformation of the valve stem. The troubleshooting method is to reinstall the packing and reshape the deformed valve stem. 　　5. Improper flange installation. Uneven stress leads to external leakage. The troubleshooting method is to reinstall the connecting flange and gasket, and then tightly press the connecting flange with even force. 　　6. Wear of the valve core and seat due to fluid flow. The method for troubleshooting is to grind the valve core and valve seat. 　　7. Improper packing installation leads to increased friction, while poor sealing of the control valve results in increased external leakage. The troubleshooting method is to reinstall the packing to reduce friction. 　　8. Improper flow direction leads to an increased leakage volume. Improper flow direction selection increases the unbalanced force, thereby increasing the leakage rate. The troubleshooting method is to check the design drawings and reinstall it. 　　VIII. Failures caused by the detachment of the valve core Before the valve core detaches, the control valve will produce significant mechanical noise. After a fault occurs, the control system is unable to regulate properly, resulting in a sudden increase or decrease in the controlled variable. The fault analysis is as follows. 　　1. The flow path design of the control valve is unreasonable, resulting in oscillation of the valve core and exposure to shear forces. Over time, this leads to the breakage of the pin that connects the valve core to the valve stem, causing the valve core to fall off. The troubleshooting method is to check the flow path of the control valve and replace the pin. 　　2. The connecting pin of the valve core is not securely installed, resulting in the valve core falling off. The troubleshooting method is to reinstall the pin and tighten it. 　　IX. Failures of valve positioners Failures in valve positioners degrade the performance of the cascade secondary loop. Since the valve positioner is part of the secondary loop of the cascade control system, it possesses a certain degree of adaptability. Faults in valve positioners manifest as instability in the control system, sticking, etc. 　　The fault symptoms caused by an inappropriate valve positioner cam are similar to those caused by an inappropriate flow characteristic of the control valve; they result in instability or sluggishness in the control system at different operating points. The fault handling method involves selecting an appropriate valve positioner cam based on the characteristics of the controlled object and the flow characteristics of the control valve; after installing the cam, tuning is required. 　　Faults in the valve positioner amplifier include clogged throttle holes and excessive amplifier gain. The former causes a slow change in the output, while the latter leads to resonance in the control system. Therefore, the troubleshooting method involves checking and clearing the amplifier’s throttle orifice; when the amplifier’s gain is too high, it is possible to reduce the elasticity of the compressed reed or replace the amplifier. 　　A mismatch in the valve positioner’s sensing rod results in an increased dead zone, preventing the feedback signal of the valve position from being reflected accurately and in a timely manner. Therefore, the control quality of the control system deteriorates. The troubleshooting method is to check and reinstall the feedback rod.