Table 3-2. Control valve problem causes and countermeasures
Phenomenon Cause Countermeasure
Valve not operating
Supply air pressure is
normal, but signal pressure
does not increase
Leakage from the signal pipe
•
Check the signal pipe (especially the fitting)
Leakage from the diaphragm or
damage to the diaphragm
•
Replace the diaphragm.
Leakage from the signal receiver of the
positioner or damage to it
•
Replace the bellows receiver and/or the
diaphragm
Failure of the controller
•
Check the controller.
Signal pressure is normal,
but supply air pressure
drops or air is not supplied
Clogging of the filter in the air pressure
regulator
•
Clean the filter.
Leakage or clogging of the air pipe
•
Check the air pipe (especially the fitting)
Failure of the air pressure regulator
•
Check the air pressure regulator.
No positioner output Failure of the positioner and pilot valve
•
Check the positioner and pilot valve.
Leakage from the actuator diaphragm
or damage to the diaphragm
•
Replace the diaphragm.
The valve does not operate
even though air is supplied
to the actuator
The valve stem, valve plug guide, etc., is
stuck due to heat or foreign matter.
•
Disassemble and inspect the valve body, and
reprocess or replace the valve stem or guide
bushing with new ones.
Foreign matter is caught in the valve
plug.
•
Disassemble, inspect, and clean the valve
body.
The valve stem is bent.
•
Repair the valve stem.
Actuator failure
•
Check the operation of the actuator.
Valve does not fully open /
leakage from the valve interior in large amounts
The valve stem is at the
valve fully-open position.
The valve plug and/or the seat ring is
corroded, eroded, abraded, or other-
wise damaged.
•
Lap the valve plug and seat
•
Re-machine the valve plug and seat.
•
Replace the valve plug or/and the seat ring
(consider using hardened parts)
Parts on the outer circumference of the
seat ring (thread or gasket) are corroded
or eroded.
•
Replace the seat ring or the gasket.
•
Use a different mounting method for the
seat ring (e.g., welding).
The fluid leaks from the partition of the
valve body.
•
Weld pinholes.
•
Replace the valve body.
The valve stem does not
reach the fully closed
position.
The differential pressure of the fluid is
too great.
•
Reduce the differential pressure.
•
Increase the output from the actuator.
Foreign matter is caught in
•
Disassemble, inspect, and clean the valve
body.
The valve stem, valve plug guide, etc., is
stuck due to heat
•
Separate these parts.
3-3
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