How to troubleshoot a control valve and How to do the preventive maintenance of the control valve

How to do the control valve inspection and maintenance

The service life of a control valve could be increased by doing proper maintenance. The internal parts of the control valves must be checked, before doing any inspection the valves must be dismantled. The major parts of the control valve are the disc and seat. The leakage chances of the valves would be high if there is any damage to these parts. These parts of the valve could get damaged because of corrosion and erosion. The valves must be inspected to know if the components are functioning properly in their operating conditions. In order to keep the valve in a better operating condition sometimes certain adjustment to the packing or the operator is needed. While doing the inspection of control valves the stuffing box and flanges must be checked so that we could know if there is any leak in the packing or in the gasket. Periodic maintenance could make the valve perform better if we use lubricants for the body and seat rings.

Why do we need to do the maintenance of the control valve

  • The efficiency of the control valve could be improved
  • The repair of the valve can be done if we find out any damage during the inspection
  • The cost can be reduced
  • Periodic maintenance is done by without disassembling the valve, only if there is a loss of operating efficiency valves are taken out of service
  • Before doing any maintenance we must check the manufacturer’s instructions
  • In certain cases, the parts of the control valves must be replaced instead of refurbishing it, and it could be known by the inspection of the control valve.
  • By doing the maintenance the parts of the valves could be free from dirt.

How to do the inspection before starting the operation of the control valve

  • Check whether the flow direction of the control valve is equal to the process fluid flow direction
  • Diaphragms bolts and nuts must be checked
  • Check the bonnet clamp bolts and yoke clamp bolts
  • Check for any air leakage in the valve
  • Actuator spring and the gland packing must be checked

What are the testing methods of the control valve

  • Shell test
  • Backseat test
  • Low-pressure closure test
  • High-pressure closure test
  • Visual examination of casting

How to do the periodical inspection and maintenance of the control valve

  • Air leak from the diaphragm must be checked
  • Check for any leak from flanges and bonnet
  • Yoke clamp must be tight and there should not be any air leak from the air piping of the actuator
  • The stem connectors should be checked for a loose connection
  • The actuator stem and valve stem must be checked
  • Check for any vibrations from the control valve
  • Gland packing must be checked, and confirm there is no leak

How to do the actuator maintenance of the control valve

Before doing the maintenance of the actuator, it must be disassembled and the disassembly must be done according to the manual. The disassembly and the reassembly of the actuator must be done by an experienced person other-wise it would create many problems. After the disassembly of the actuator, the parts must be cleaned so that we can remove or prevent rust and corrosion. Grease must be provided to the cylinder for its smooth operation. The diaphragm must be kept clean and we should check for any defects in the diaphragm. The tightening of the diaphragm must be done according to the info provided by the manufacturer. Check whether the stem seal is installed properly and proper lubricant must be provided to it. The actuator stem must be checked so that we could know if there is any damage. If there is a need for spring replacement in the actuator, then all the moving parts must be lubricated before replacing the spring.

How to do the positioner maintenance of the control valve

While using an electronic positioner in the control valve we must check that if the control valve makes a lot of vibration if so we must not use an electronic positioner for this type of control valve. Electronic positioners should only be used in control valves that don’t make a lot of vibrations. The important step is to mount the positioner into the control valve and this must be done very carefully. The calibration and the operation are depended on how the positioner is mounted if it is not mounted properly then it would cause a lot of problems. Check for any air leaks from the positioner. Integral relays are the important part of the positioner if there is no relay or didn’t get any replacement for the relay then the whole positioner must be changed. While linking the positioner to the control valve it must be done properly without any wear or tear. There must not be any wear in the linkage parts and the linkage binding must be properly done.

How to select a control valve

  • It must be selected according to the operating pressure and temperature
  • Rate of flow and type of fluid
  • It must be selected according to the cost of installation and maintenance
  • It must be selected according to the viscosity and temperature of the fluid
  • According to the inlet pressure, flow capacity, and outlet pressure
  • Noise level
  • Pressure drop during normal conditions and pressure difference during shutoff

How to troubleshoot a control valve and what are the preventive maintenance of the control valve

Check the day to day operation of the control valve and if there are any sudden changes in the operating conditions then the chances of faults are high. By doing the visual inspection we can detect the faults in control valves like if the fault is caused by dirt or something. In certain cases, control valves are not able to achieve full closure and it could be because the valve disc and seat could have some foreign materials or other products. So in order to prevent or remove these faults, we must polish the seat and take preventive measures for corrosion. If there is cuts or scratches in certain parts of the control valve like the disc, then it must be replaced

  • Valve stem leak – It can be caused by worn stem seals or because of the damaged stem. So the valve stem must be removed or if it is a metal valve then stem packing nut can be tightened.
  • Valve leakage past ball – it could be because of damaged stem or bore, so it can be removed by installing repair kit or adjust the limit switches of the actuator
  • Body leak of the valve – It may be caused because of not tightening the body bolts or due to the operating pressure. The proper solution for this fault could be checking the bolt and the valve should be used for the applications that match its pressure ratings another solution is replacing the valve or installing a repair kit
  • Excessive operating torque of the valve – it can be caused because of the over-tightening of the valve bolts or stem nut. The solution for this fault could be the loosening of the stem nut or valve bolts. Another solution will be cleaning or replacing the valve.

If the Valve fails to open or close, it could be because of diaphragm damage or stem binding.

Diaphragm damage

  • This problem can be removed by loosening the tubing connection in the bonnet which would remove the residual pressure.
  •  All diaphragm plates must be tightened
  • Remove the diaphragm
  • Spring replacement
  • Control tubing reinstallation

Stem binding

  • Tubing connection must be loosened so the residual pressure can be removed
  • Removal of the bonnet nuts
  • Check the stem for cuts and scratches and polish it if necessary
  • Reinstall the spool assembly

Preventive maintenance

Visual inspection is an important step for this, before installing the valve check the valve for any damages in the piping, fittings. The valve must be cleaned so the foreign materials in the valve can be removed and it would affect the valve operation. The valve must be installed in the line based on the inlet flange arrow. Valve lubrication must be done. Motor operated valve must be operated electrically.

How to prevent cavitation in control valves

In some of the control valves, the cavitation is reduced by increasing the distance between the trim parts and implosion of vapor bubbles, and this is done by providing a flow aiding to close direction. In certain anti-cavitation valves, the chances of the cavitation are reduced by dropping the pressure by steps.

To get more info about the control valve check the following link

News Reporter
post-graduate in Electronics & communication