Chemical Instrumentation Maintenance Tips: Pneumatic Control Valve Failure Response Strategy Explained

In the field of industrial automation, pneumatic control valves are critical, but often due to various types of failure to affect production. The following is a detailed analysis of its common failures and response strategy.

When the regulating valve is jammed, the medium flow is seriously blocked. Small signal input no response, the arrival of a large signal is excessive action, deviating from the normal adjustment range.

• Pipeline construction or long-term use, internal welding slag, iron filings and other impurities are easy to accumulate in the throttle port and guide parts, impeding the passage of media.
• If the packing is installed too tightly during maintenance, the friction of the valve stem movement increases greatly, resulting in difficult action of the control valve, leading to jamming.

• Try to quickly switch the regulating valve, with the help of fluid impact to flush away the blockage, restore the medium flow.
• Clamp the valve stem with a pipe wrench, and rotate it positively and negatively under the pressure of external signals to help the spool jump over the jammed place.
• If the above methods are invalid, you can properly raise the pressure of the external source, enhance the driving power, and repeatedly promote the valve stem; still can not be solved, the need for professional personnel to disassemble and clean up.

• Internal leakage: when the valve is closed, there is media leakage between the valve spool and the valve seat, and the flow control is not allowed.
• Packing leakage: packing box at the media leakage, resulting in material loss and environmental pollution.
• Spool and seat deformation leakage: the shape of the spool and seat changes, gap, triggering leakage.

• Internal leakage: the valve stem of the air-open valve is too long or the valve stem of the air-closed valve is too short, so that the valve spool and the valve seat can not be tightly adhered to.
• Packing leakage: high temperature, high pressure and high permeability fluid environment, packing plastic deformation, uneven contact with the valve stem, sealing performance is reduced.
• Valve spool and seat deformation leakage: erosion by corrosive media, high-speed fluid impact and long time wear and tear, spool and seat material damage and deformation.

• Internal leakage:
  • Accurately measure and adjust the length of the valve stem to ensure that the spool and the valve seat are in close contact.
• Packing leakage:
  • Chamfer the top of the stuffing box, add corrosion-resistant metal protection ring at the bottom, finish the contact surface, and choose flexible graphite packing.
• Spool and seat deformation leakage
  • Select materials with high corrosion resistance, eliminate defective products, and replace new parts in case of serious wear.

Irregular and unstable vibration occurs when the regulating valve is working, affecting the valve life and system stability.

• Insufficient spring stiffness, unable to provide stable response force for the valve spool.
• The output signal is unstable, and the control valve receives wrong commands.
• The inherent frequency of the valve is the same as the system frequency, resulting in resonance.
• Piping and base vibration is transferred to the regulating valve.
• Improper selection, large flow resistance when the valve works at small opening.

• Replace the regulating valve with large stiffness spring in case of slight oscillation.
• When the pipeline and base vibrate, increase the support structure to reduce the vibration transmission.
• If the oscillation is caused by resonance, replace the regulating valve with appropriate frequency.
• Improper selection, then re-selection, or the use of split-range control, sub-mother valve and other strategies.

Regulating valve face normal signal and gas source does not respond, can not open or close normally, seriously affecting the system automation control.

• The gas source is not open or faulty, the air duct is blocked, the pressure reducing valve is blocked or failed.
• Positioner or converter failure, can not accurately convert the control signal.
• Valve spool falling off, valve stem bending or breaking.

• Check the gas source equipment and adjust the pressure reducing valve to the specified pressure.
• Check the positioner connector, pipeline, maintenance of amplification holes, throttle holes.
• Spool off or stem damage, disassemble the valve to repair or replace parts.

The action of the control valve fluctuates significantly, unable to adjust accurately according to the set signal, and the system control accuracy decreases.

• The pressure of gas source is unstable, and the driving force is sometimes big and sometimes small.
• The output signal of regulator is unstable.
• The friction resistance of the valve stem movement is large, such as the packing is too tight and the parts are rusty.
• Wear and tear of amplifier ball valve in positioner, signal leakage.

• Check the signal pipeline, etc., to eliminate leakage points and stabilize the pressure of the gas source.
• Clamp the loose parts and add lubricant to the parts with high friction.

The valve stem moves slowly and responds to the input signal with a lag, affecting the real-time control performance of the system.

• The pneumatic membrane is broken and the power is insufficient.
• Weak response performance of the positioner.
• There is obstruction inside the regulating valve.
• Poor quality of packing or too tight filling.

• Replace the broken pneumatic membrane.
• Adjust or replace the positioner.
• Clean the blockage inside the regulating valve.
• Adjust the packing gland or replace the packing.

By analyzing and solving these common failures, it can effectively protect the stable operation of industrial production, improve production efficiency and reduce costs. I hope the above content on the relevant staff to help.