Common Faults Of Pneumatic Actuators And Their Causes

The regulating valve does not operate. The fault symptoms and causes are as follows

No signal, no air source

  • The air source is not turned on;
  • Because the water in the air source freezes in winter, the air duct is blocked or the filter and pressure-reducing valve are clogged and malfunctioned;
  • The air compressor fails;
  • The air source main pipe leaks.

There is an air source but no signal.

  • Regulator failure;
  • Signal tube leakage;
  • The bellows of the positioner is leaking;

The positioner has no air source.

  • The filter is clogged;
  • Pressure-reducing valve failure;
  • Pipe leakage or blockage.

The positioner has an air source but no output.

  • The positioner’s orifice is clogged.

The positioner output is normal and the regulating valve does not operate.

  • The valve core falls off;
  • The valve core and valve seat are stuck;
  • Pipe leakage or blockage;
  • The valve seat and valve core are frozen or have coke and dirt;
  • The actuator spring is rusted due to long-term use.

The action of the regulating valve is unstable. The fault symptoms and causes are as follows

The air source pressure is unstable

  • The compressor capacity is too small;
  • The pressure-reducing valve is faulty.

Signal pressure is unstable

  • The time constant (T=RC) of the control system is inappropriate;
  • The regulator output is unstable.

The air source pressure is stable and the signal pressure is also stable, but the action of the regulating valve is still unstable.

  • When the amplifier in the positioner is worn or dirty, and the air consumption increases particularly, output oscillation will occur;
  • The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle;
  • The output pipe and line leak;
  • The diaphragm box leaks;
  • The friction resistance of the valve stem is large.

The regulating valve oscillates. The fault site and cause analysis are as follows:

The regulating valve oscillates at any opening.

  • The support is unstable;
  • There is a vibration source nearby;
  • The valve core and bushing are severely worn;
  • The diaphragm box leaks;
  • The positioner leaks;
  • The pipeline between the positioner output and the diaphragm box leaks;
  • The packing is pressed too tightly;
  • The packing is pressed eccentrically;
  • The friction of the valve stem is large.

The regulating valve oscillates when it is close to the fully closed position.

  • The regulating valve is larger and is often used at a small opening;
  • The medium flow direction of a single valve seat is opposite to the closing direction.

The regulating valve is sluggish in action. The symptoms and causes of sluggishness are as follows:

The valve stem is only slow when moving in one direction.

  • The diaphragm in the pneumatic diaphragm actuator is damaged and leaking;
  • The “O” type seal in the actuator is leaking.

The valve stem is slow when reciprocating.

  • There is sticky matter blocking the valve body;
  • The polytetrafluoroethylene packing becomes hard or the graphite-asbestos packing lubricating oil dries;
  • The packing is pressed too tightly and the friction resistance increases;
  • Friction occurs because the valve stem is not straight The resistance is large;
  • Pneumatic control valves without positioners can also cause slow movement.

The leakage of the regulating valve increases. The reasons for the leak are as follows:

The leakage increases when the valve is fully closed

  • The valve core is worn and internal leakage is serious;
  • The valve is not adjusted properly and cannot be closed tightly;

The valve cannot reach the fully closed position.

  • The medium pressure difference is too large, the rigidity of the actuator is small, and the valve is not closed tightly;
  • There are foreign objects in the valve;
  • The bushing is sintered.

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