Control valve positioner calibration and working principle

Control valve positioner calibration:

The positioner is mounted on the control valve and calibrated in factory. Hence calibration is not required. However if positioner has been disconnected for some reasons and if it is to be reconnected then calibration is required.

  • Apply regulated air pressure to actuator and adjust pressure until stem is positioned at mid stroke.
  • Adjust motion connector assembly until positioner lever and motion connector are parallel and horizontal at mid stroke.
  • Disconnect regulated air supply to diaphragm and connect tubing from output port of positioner to diaphragm.
  • Check numbers stamped on the positioner lever and move stroke-adjusting pin to desired stroke and tighten.
  • To set positioner start point, apply desired starting pressure (usually 3 PSI) and adjust the spring adjuster until valve stem starts moving.
  • Set desired maximum pressure of signal range (usually 15 PSI) and check for the stem stroke. If stroke is less, move stroke-adjusting pin to the higher value i.e. right. If stroke is achieved at pressure less than desired pressure move to lower value i.e. left.
  • Recheck the start point.
  • Repeat step 5 & 6 until travel is found within acceptable limits.

Download valve cv sizing calculator for liquids and gases flow systems.

 

How a valve positioner works

control valve positioner

Importance of bypassing the control valve positioner:

Bypass system directs the pressure signal to the actuator and nullifies the valve positioner action. It is important to design the bypass system to the CV positioner because it helps to perform the maintenance work during the control valve is online in process stream.
To bypass the positioner

  • Loosen the bypass knob fixing screw
  • Rotate selector knob clockwise and set to “bypass” position
  • Tighten the bypass knob fixing screw. The positioner is ready for “Bypass”.

Maintenance:

Replacing pilot relay assembly:
Pilot relay assembly is factory calibrated and sealed, field disassembly is not recommended. If required replace factory calibrated spare pilot relay assembly in following steps:

  • Before removing pilot relay assembly, note whether action of positioner is direct or reverse to avoid misalignment when reassembly.
  • Remove three screws fastening assembly to body
  • Inspect existing gasket and replace if necessary
  • Check the action by reversing the positioner relay assembly and mount in the desired position
  • Fix the 3 screws to securely fasten assembly to the body

Gasket replacement:

  • When manifold or pilot assembly removed, if the gasket found to be worn out, change the same

Changing positioner action:

To change the action, follow the steps given below:

  • Release feedback spring and remove travel pin.
  • Remove the 3 Nos screws, which fasten the pilot assembly to the housing.
  • Rotate the pilot by 180 degrees  and remount it to the housing. Make sure that DA or RA marking is properly lined with arrow mark.
  • Remove the cam and fasten it in the opposite direction.
  • Readjust the travel pin and feedback spring.
  • Recalibrate as illustrated in “Calibration”.

Note: Above are an idea about positioner operation, they may vary depending on the control valve design.