Failures of Control Valve Positioners and Their Solutions

 
Control valve positioners play an essential role in modern industrial automation control systems. As critical accessories for control valves, they are typically used with pneumatic control valves to achieve precise control over fluid flow and pressure. Valve positioners receive electrical signals from the control system to adjust the opening of the pneumatic valve, thereby regulating fluid flow. Due to their critical nature within the overall control system, valve positioners have a relatively high failure rate. When a failure occurs, it can severely impact the stability and reliability of the entire system. Therefore, understanding common failures of valve positioners and their solutions is crucial for maintaining efficient system operation.

Low or No Output with Input Signal Present

 
In a control valve system, the valve positioner receives input signals from the control system and outputs corresponding control signals to adjust the opening of the pneumatic valve. However, there may be instances where the input signal is normal, but the output signal is very low or absent. Such failures can directly affect fluid control and may result in system malfunctions.

1. Possible Causes

 
Improper Adjustment of Stroke Fine-Tuning Screw: If the fine-tuning screw is over-adjusted, it may lead to the torque motor coil becoming detached or poorly connected, affecting the output signal.
Coil Failure: The torque motor coil may have broken wires or be burnt due to overcurrent, significantly deviating from normal resistance values.
Signal Line Issues: Poor contact or reversed polarity in the signal lines can weaken or eliminate the output signal.
Incorrect Nozzle Position: If the nozzle's baffle is not adjusted correctly, it can impede airflow.
Loose Fixing Screws: If the nozzle's fixing screws are loose, it can result in unstable signal output.
Amplifier Failure: There may be an internal fault in the amplifier, preventing it from processing the input signal correctly.
Blocked Air Restriction or Vent: If the air restriction or vent is blocked, it can disrupt the normal operation of the valve positioner, causing abnormal output signals.

2. Relevant Solutions

 
Check the Fine-Tuning Screw: Reassess and adjust the fine-tuning screw to ensure it is within a reasonable range, avoiding excessive adjustments that could cause damage.
Measure Coil Resistance: Use a multimeter to check the resistance of the torque motor coil, which should normally be around 250Ω. Replace the coil if it deviates significantly from this value.
Inspect Signal Lines: Check the connections of the signal lines to ensure good contact and correct polarity.
Adjust Nozzle Position: Readjust the nozzle position to ensure it is parallel with the valve to allow proper airflow.
Tighten Fixing Screws: Ensure the fixing screws for the nozzle are secure to prevent loosening that could lead to unstable signal output.
Examine the Amplifier: Verify the proper functioning of the amplifier; replace it promptly if a fault is found.
Clear Air Restriction and Vent: Regularly check and clean the air restriction and vent to ensure they are unobstructed, preventing faults due to blockages.

Output Pressure Oscillation

 
Output pressure oscillation is a common failure phenomenon in control valve systems, which can lead to unstable flow control and frequent system fluctuations. This situation not only affects process stability but can also potentially damage equipment, necessitating timely identification and resolution.

1. Major Causes

 
Dirt Accumulation in Amplifier: Accumulated dirt in the amplifier may cause unstable signal transmission.
Leakage: Air leaks in the output line or diaphragm can result in unstable output pressure.
Aging Diaphragm: An aged or damaged diaphragm can compromise output stability.
Incorrect Magnet Position: If the magnet is not adjusted correctly, it can lead to an unstable magnetic circuit.
Loose Fixing Screws: Loose screws on the feedback lever can cause valve vibrations.
Excessive AC Components in Input Signal: Interference from AC components can affect output stability.

2. Corresponding Solutions

 
Regularly Clean Amplifier: Keep the amplifier clean to maintain normal operation.
Inspect Air Lines for Leaks: Check air lines for any leaks to ensure smooth valve operation.
Replace Aging Diaphragm: Change any old diaphragms to ensure the diaphragm head operates correctly.
Readjust Magnet Position: Ensure the magnet is parallel with other components.
Tighten Feedback Lever Screws: Secure the screws for the feedback lever to eliminate valve vibrations.
Use Capacitors: Add capacitors in parallel to the input to filter out AC interference.

Output Present with No Input

 
When there is output even without an input signal in a control valve system, it typically indicates a malfunction. This can lead to operational instability and fluid control failure, impacting production processes. Thus, it is crucial to identify and resolve this issue promptly.

1. Potential Causes

 
Blockage in Back Pressure Section: If the back pressure section is clogged with debris, the system may fail to respond to input signals, resulting in output even without an input signal.
Improper Positioning of Auto/Manual Switch: If the auto/manual switch is not correctly set, the valve may operate even without an input signal.
Feedback Loop Failure: There may be a fault or poor wiring in the feedback loop, preventing the system from recognizing changes in input signals accurately.
Electrical Noise Interference: External electrical noise may disrupt signal transmission, causing output in the absence of an input signal.

2. Recommended Solutions

 
Clear Blockage in Back Pressure Section: Regularly inspect and clear the back pressure section to ensure unobstructed flow and avoid signal failure due to blockage.
Check Switch Settings: Confirm the status of the auto/manual switch to ensure it is correctly positioned to prevent operational errors.
Inspect Feedback Loop: Check the connections and operation of the feedback loop to ensure it functions normally and accurately recognizes input signals.
Eliminate Electrical Noise Interference: Implement measures such as shielding signal lines or using filters to reduce the impact of electrical noise on signal transmission.

Poor Accuracy

 
Poor accuracy during control valve operation directly affects fluid control, leading to instability and reduced efficiency in production processes. Thus, understanding the causes of poor accuracy and implementing effective solutions is essential.

1. Major Causes

 
Improper Adjustment of Nozzle and Baffle: The relative position of the nozzle and baffle may not be set correctly, affecting measurement accuracy.
Air Leaks in Gas Line: Leakage in the back pressure gas line can lead to unstable output signals.
Excessive Radial Displacement of Control Valve: Mechanical faults in the control valve may reduce accuracy.
Incorrect Adjustment of Zeroing Screw: If the zeroing screw is not adjusted according to requirements, it can affect accuracy.
Mismatched Position of Feedback Lever and Fixed Pin: If the feedback lever is not adjusted according to travel requirements, it can lead to inaccurate control.

2. Common Solutions

 
Readjust Nozzle and Baffle: Ensure their parallelism and the positioning of fixing screws meet accuracy requirements.
Inspect and Eliminate Gas Line Leaks: Check for and resolve any leaks in the back pressure gas line.
Repair Control Valve: Ensure the mechanical components of the control valve are functioning correctly.
Re-adjust Zeroing Screw: Adjust the zeroing screw to meet accuracy requirements.
Readjust Feedback Lever and Fixed Pin: Position the feedback lever and fixed pin according to travel specifications.

Conclusion

 
Valve positioners for control valves are critical components in industrial automation control systems, and their failures directly impact the performance and reliability of the entire system. By identifying common failures and applying effective solutions, instrumentation personnel can maintain and manage valve positioners effectively, ensuring efficient system operation. Regular maintenance and inspections will significantly extend the service life of valve positioners and help prevent potential failures.