Common faults and main causes of pneumatic control valves

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Common Faults and Their Main Causes
The valve does not operate: There is air flow in the actuator, but no output.
① The constant-throttle orifice in the actuator’s amplifier is blocked.
② Moisture in the compressed air accumulates at the ball valve of the amplifier.

There is a signal present but no operation:
① The valve core is stuck to the sleeve or seat.
② The valve core has fallen off.
③ The valve stem is bent or broken.
④ There is a fault with the actuator.

Unstable valve operation: The air supply pressure remains constant, yet the control valve still operates unstably.
① There is a problem with the actuator.
② There is air leakage in the output pipeline.
③ The actuator’s stiffness is too low, resulting in insufficient thrust due to changes in liquid pressure.
④ The friction within the valve is high.

Vibration and noise from the valve: The valve vibrates when it approaches the fully closed position.
① The control valve is too large for the application, so it is used at a small opening degree.
② The flow direction of the medium is the same as the direction in which the valve should close.

Vibration at any opening degree:
① The support is unstable.
② There is a vibration source nearby.
③ The valve core and sleeve are worn out.

Slow valve operation: The back-and-forth movement of the valve stem is slow.
① There is mud or a viscous medium inside the valve, causing blockages or coking.
② The PTFE packing has hardened and deteriorated.

Slow operation in one direction only:
① The diaphragm in the air chamber is damaged.
② There is air leakage in the air chamber.

Valve leakage: Leakage is severe when the valve is fully open.
① The valve core or seat is corroded or worn out.
② The threads on the outer surface of the seat are corroded.

The valve cannot reach the fully closed position:
① The pressure difference of the medium is too high, resulting in insufficient output force from the actuator.
② There are foreign objects inside the valve.

Leakage at the packing and connections:
Sealing packing leakage:
① The packing gland is not tightened properly.
② The PTFE packing has aged and deteriorated.
③ The valve stem is damaged.

Leakage at the connections between the valve body and the upper/lower valve caps:
① The hex nuts used for tightening are loose.
② The sealing gaskets are damaged