Valve positioners are an important part of the control valve system and can be divided into three types according to their structure: pneumatic, electrical and intelligent. As the main accessories of the control valve, they are used in conjunction with the pneumatic control valve and are responsible for receiving the output signal of the regulator and converting it into instructions for controlling the pneumatic control valve. When the control valve is actuated, the displacement of the valve stem is fed back to the valve positioner through a mechanical device to achieve closed-loop control. The valve position status is transmitted to the upper system through an electrical signal to ensure the efficient operation of the system.
Function and importance of valve positioners
The key role of valve positioners is to improve the accuracy and response speed of the control valve and ensure the stability of the fluid control system. Especially in industrial applications involving liquid or gas flow, the accuracy of the valve positioner directly affects the performance of the entire system. For example, in the chemical, petroleum and water treatment industries, the reliability of valve positioners is crucial to ensuring product quality and operational safety.
Common faults and their analysis
The input signal is normal but the output is insufficient
This type of fault is usually caused by the following factors:
Over-adjustment of the fine-tuning screw: If the fine-tuning screw is adjusted too tight, it may cause the torque motor coil to be unsoldered and the lead wire needs to be re-welded.
Internal wire breakage of the coil: Check the coil resistance, which should be about 250Ω under normal circumstances. If the resistance deviates too much, the coil needs to be replaced.
Poor contact of the signal line: Check the terminal regularly to ensure that there is no looseness.
The nozzle baffle is not in the correct position: The parallelism needs to be readjusted to see if the output changes.
Loose nozzle fixing screw: Tighten the screw to meet the stroke requirements.
Unstable output pressure
Common causes include:
Internal dirt in the amplifier: Clean the amplifier to ensure normal operation.
Output pipeline leakage: Eliminate air leakage to ensure smooth valve movement.
Aging of the diaphragm: Check and replace the aged diaphragm regularly.
Loose feedback lever: Re-tighten the feedback lever screw to eliminate vibration.
Inaccurate zero point and range
This problem is mostly caused by improper debugging, changes in the on-site environment, or changes in the feedback lever position. Make sure that the control valve operates normally, and then recalibrate it through the standard signal to make the stroke of the control valve consistent with the control signal.
Blocked throttle hole
Due to the blockage of dirt, the positioner has no output signal, so that the control valve does not operate. Check and clean the throttle hole regularly to ensure normal operation.
Accumulation of dirt between the nozzle and the baffle
The on-site environment may cause dirt to adhere, affecting the back pressure of the nozzle, thereby affecting the output of the positioner, resulting in unstable state of the control valve and oscillation.
The output speed of the positioner is slow
This failure may be caused by a variety of reasons, including:
Input air chamber leakage: Check and replace the damaged diaphragm.
Constant throttle hole blockage: Clean the throttle hole in time.
Leakage in the air source pipeline: Detect and deal with the leak.
Poor position of the baffle: Re-adjust the position of the baffle to ensure precise control.
Zero position misalignment
It may be caused by loose zero adjustment screws, leakage of input air chamber diaphragm, etc. The relevant screws should be checked and tightened regularly to maintain system stability.
Output misalignment
Common causes include leakage of input air chamber, loose cam, etc. Damaged parts should be replaced and related parts should be tightened.
Maintenance and preventive measures
In order to reduce the failure rate of valve positioners, the following preventive measures are recommended:
Regular inspection: Develop a regular maintenance plan to ensure the normal working condition of each component.
Cleaning and maintenance: Regularly clean the nozzle, throttle orifice and feedback device to prevent dirt accumulation.
Train personnel: Regularly train on-site operators to improve their fault identification and handling capabilities.
Record data: Establish equipment operation records, analyze fault causes, and optimize maintenance processes.
Conclusion
The valve positioner is a critical component in the fluid control system. It is crucial for on-site staff to understand its common faults and solutions. Through regular maintenance and timely handling of faults, the overall operating efficiency of the system can be effectively improved to ensure safety and stability.
