Nov.08.2024
Pneumatic control valves are different from manual valves. They must be in a state of constant movement and adjustment during use and have many moving parts. They must also withstand the impact of various forces such as unbalanced forces in the medium, and various unexpected failures are inevitable. These faults can come from the actuator, adjustment mechanism, or connected accessory devices. So, what faults are prone to occur in pneumatic control valves? How to solve it?
1. The regulating valve does not operate
Reason 1: No air source or the air source pressure is too small
measure:
You should first check whether the air source (instrument air) is unobstructed and whether the air source pressure meets the usage requirements of the valve.
Reason 2: There is a gas source, but there is no output signal gas pressure.
measure:
For mechanical controllers or positioners, the mechanical pressure controller or positioner should be replaced.
For the regulating valve of the mechanical controller, check whether the signal collection pipeline valve of the medium on the regulating valve installation pipeline is fully open or leaks seriously. If there is any abnormality, it should be dealt with in time.
Reason 3: The output signal air pressure is normal but still does not operate.
measure:
Check whether the diaphragm of the pneumatic diaphragm actuator has serious air leakage. If it leaks, the diaphragm and related seals should be replaced in time.
If it is checked that the diaphragm is intact and there is no air leakage, the main valve core, bushing, and valve seat should be stuck. The main valve should be disassembled and inspected, and any debris should be cleaned.
If the valve stem is severely bent and deformed, the main valve should be disassembled and inspected, and replaced according to the inspection situation.
There is a leak in the signal gas source pipeline. Check and deal with the leak point.
The signal amplifier is faulty or improperly adjusted. The main air source does not enter the diaphragm cavity through the amplifier. The amplifier should be adjusted or replaced in time.
2. The regulating valve produces regulating oscillation and unstable control during operation.
Reason 1: The air source pressure changes greatly or the filter and pressure reducing valve are not working properly.
measure:
Check the operating status of the compressed air system.
Check and replace filter or pressure reducing valve.
Reason 2: The air source pressure is stable but the signal pressure is unstable.
measure:
If it is still unstable after replacing the intelligent positioner, the PID parameters should be adjusted.
For the mechanical controller or positioner, the mechanical controller or positioner should be replaced and the controller should be adjusted.
If the mechanical controller or positioner is still unstable after replacing it, you should check and replace the signal air source amplifier and adjust it.
Reason 3: The air source and signal pressure are stable, but the regulating valve is still unstable.
measure:
Check the air tightness of the pneumatic diaphragm actuator. Carefully check whether there is slight air leakage in the drive push rod seal. If there is air leakage, the seal should be disassembled and replaced. Disassemble the diaphragm and check whether there is slight air leakage caused by scratches or punctures. If there is air leakage, the diaphragm should be replaced.
Carefully check whether there is any gap at the connection between the positioner and the main valve, and replace or tighten the relevant connections between the positioner and the main valve.
Use soapy water to carefully check whether there is any slight air leakage in the signal gas pipeline. Once found, it should be dealt with in time.
The setting of the signal air source amplifier is unreasonable. Adjust the balance screw of the signal air source amplifier.
If the resistance of the regulating valve movement mechanism (including the main valve and pneumatic diaphragm actuator) is too high, the main valve should be disassembled, and abnormal components should be checked and replaced, including the valve core, valve seat, valve stem, valve stem seal and driving push rod.
Check whether the pressure balance spring in the pneumatic diaphragm actuator is damaged and whether the spring has fatigue deformation. If there is any abnormality, replace the pressure balance spring.
3. The regulating valve moves slowly
Reason 1:
There is sticky matter blocking the valve body.
measure:
Disassemble the main valve body and clean the sticky matter inside the valve body.
Reason 2: The valve stem packing has deteriorated and hardened or the graphite and asbestos packing lubricating oil has dried.
measure:
Disassemble the main valve body and replace the valve stem packing.
Reason 3: The filler is added too tightly and the friction resistance increases.
measure:
After loosening the valve stem packing compression nut, reciprocate the main valve several times, and then adjust the valve stem packing compression nut to the appropriate torque.
If Measure 1 has no effect, replace the valve stem packing and adjust the valve stem packing compression nut to the appropriate torque.
Reason 4: The valve stem is not straight, resulting in large friction resistance.
measure:
Disassemble the main valve and replace the valve stem.
Reason 5: There is slight air leakage in the pneumatic diaphragm actuator diaphragm or signal gas pipeline.
measure:
Replace the pneumatic diaphragm actuator diaphragm and deal with air leakage in the signal gas pipeline.
4. The regulating valve operates normally, but the process control parameters are abnormal.
Reason 1: The regulating valve core falls off.
measure:
Disassemble and check the main valve body of the regulating valve, and restore the valve core to fall off according to the inspection situation.
Reason 2: The connection between the regulating valve core and the valve stem is relatively displaced but not disengaged.
measure:
Disassemble and inspect the main valve body of the regulating valve, and restore the valve core position according to the inspection conditions.
Reason 3: The regulating valve stem is broken.
measure:
Disassemble and inspect the main valve body of the regulating valve and replace the valve stem.
Reason 4: There is dirt clogged in the regulating valve core.
measure:
Disassemble and inspect the main valve body of the regulating valve and clean the dirt in the valve core.
Reason 5: Other control parameters of the controlled medium system are inappropriate, such as medium flow and pressure not within the design requirements.
measure:
Adjust system control parameters within the design requirements.
