In this article, you will learn how to do maintenance of control valve which is stuck in the field at a fixed position, and its detailed procedure.
- Hand tools such as spanner & Screw Diver
- Digital Multimeter
- Handheld configurable HART communicator if positioner is smart.
- Loop calibrator or multipurpose calibrator to feed 4 to 20 mA signal
- Cloth for cleaning
- Standard fittings with tubing to check actuator movement
Maintenance of Control Valve
If Valve not operating properly follow the below points
- Inspect the control valve along with its associated accessories.
- Inspect for physical damage or air leakages. Fix them if any problems are detected.
- Apply constant air supply to the control valve.
- Inspect instrument air supply to valve positioner coming from Air Filter Regulator.
- Ensure the set pressure of the Air Filter Regulator is matching with the value specified in the Control Valve datasheet.
- If air pressure is less than required then the valve moves very slowly.
- If air pressure goes high or more than required then there is a chance of actuator damage.
- To avoid this damage & slow working kindly adjust the air pressure to the required value.
- Evaluate the condition of the isolation valve located in the air supply header in absence of an air supply.
- Keep the isolation Valve in closed condition.
- Send the confirmation for control valve operation from the control room based on field condition.
- Check the current signal on the electro-pneumatic positioner, if the valve remains stationary.
- If there is no current signal available, check cable connections by referring loop drawing on both the field side and the marshaling cabinet side.
- Inspect the fuse condition in the marshaling cabinet. Replace the fuse with an equal rating if it is blown.
- Check the condition and resistance of the cable once again if there are any issues.
- If there is any issue with the cable pair, replace a new good cable with an equal rating as per design.
- Provide 0 to 100% instruction from the control system to perform valve stroke checking tests and ensure accurate control valve feedback in SCADA or HMI.
- Change the configuration or feedback setting if feedback is not available even after a healthy valve stroke is confirmed by the system command.
- Two valve feedback concepts such as open feedback closed feedback, and another one is analog feedback from 0% to 100% which depends upon valve design.
- The problem with analog feedback if occurred can be resolved by control valve calibration.
- Check system interlocks for analog output signals within the program, & control system permissive conditions.
- The control system doesn’t transmit any instruction for incorrect logic, and no stem movement occurs in the control valve.
- Calibration of a valve addresses if any problems occurred within the control valve stroke.
- After calibration is complete check the valve stroke again, & check mA to the valve positioner’s input for stroke problem if exists.
- Send some commands to the control system and check the field current in mA in an electro-pneumatic positioner or I-P converter.
- If any problem is found in the analog output channel within the control system, then replace the faulty analog I/O module with the new module of the same specification.
- Check the working condition of the air filter regulator.
- Check the operating condition of the volume booster, airlock relay, trip valve, vent port, and solenoid valve for subsequent addition.
- Check for any kind of blockages identified and ensure for no blockages by arresting.
- Replace respective components if any significant problems were identified.
- Ensure that the whole tube line is clear of choking.
- Ensure the absence of obstruction, and replace it if major issues are identified.
- Check and make sure no issues are present, if found any issues replace them.
- Check the condition of the valve positioner, & its output.
- Calibrate and repair the valve positioner if there is no output signal.
- Refurbish a control valve every time and calibrate a positioner accordingly with reference to calibration instructions, since the calibration procedure of each model varies whit is not the same.
- If there is any functional error in the valve positioner then remove and calibrate once again by ensuring the control valve’s stroke checking after calibration is done.
- If the above possibilities are not satisfying true then the problem is located in the actuator or valve body.
- If any problems occurred in the control valve function, remove all tubing and test control valve movement by supplying external air to the actuator directly.
- Perform manual valve operation to relieve the stuck state.
- Remove the actuator stem, & stem connection to check the actuator function to determine the point of the issue.
- Verify whether the actuator is passing or not since most control valve comes with air-passing ports to check the deterioration in the diaphragm.
- If the actuator is experiencing passing problems conduct an overhaul on the actuator.
- If the actuator is not operating well remove the cap and check the plate, spring, and other components within the control valve actuator.
- Inspect all internal actuator components.
Control Valve Inspection
During calibration inspect the following
- Moving components such as cam, lever, spring, etc.
- Diaphragm pressure Gauges
- Nozzle & its Flapper.
- Supply air leakage.
- Ensure that Electric coils and electronic cards in I-P positioners are kept dry, & moisture free.
Why does a control valve get stuck, if the control valve is having stiction?
- Another problem is that the control valve loop is referred to as stiction.
- Basically, stiction means the combination of the words “stick” and “friction.”
- This condition allows the valve to retain in a constant position, without allowing it to travel freely like a valve Dead band.
- When a Stiction problem in a valve occurs the valve movement needs greater effort.
- This increased force makes the valve overshoot its position and dead band set-point, which makes the valve stacked in a new or incorrect position.
- The Valve Stiction problem exists mostly in all preset mode control loops, revealing a continuous cycle pattern in controller output and a square wave pattern in process variables, resulting in process disruption and valve tears.
Critical Components of Control Valve
Critical Components of the Control Valve Subjected to Inspection
- Actuator Diaphragm
- Actuator Shaft
- Trim elements such as Plug, Seat, Cage, and Stem.
- Bonnet Gland Packing Box
- Gasket Seating Surface
- Body Wall NDT Test
- Valve Positioner Condition
- Valve body Stud Bolts and Nuts
- Actuator Spring
- Hand-Wheel (if installed)
- Flange Gasket Surfaces
How to Repair a Control Valve having a Stiction issue?
Follow the below points to repair a control valve.
- The valve actuator and positioner must be sized correctly to change the necessary force for the valve travel to address Stiction issues within the valve.
- Check the air pressure is within the specified range.
- Check the torque of the valve packing gland & inspect it.
- Inspect the valve’s internal portion for signs of scale, scarring, wear & tear, and replace it if required.
- Investigate the gasket seating surface on the valve bonnet
- Check for any fouling on the valve stem plug and seat assembly.
- If you are confused about fouling, replace it with a new one.
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