Valve Positioners are devices used to put the valve in the correct position by increasing or decreasing the air load pressure on the actuator. Valve positioners are used for controlling valve where accurate and rapid control is required without error or hysteresis.
Before calibrating the pneumatic positioner:
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Before calibrating the positioner we have to familiar with all of the instrument components and safety precautions.
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Check whether the positioner is properly positioned at valve
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Before connecting the air supply to the positioner make sure that the air supply is maintained at the proper pressure. In most cases, it is 20 Psi.
Components of Valve positioner:
Calibration:
There is D shaped beam you can see inside the positioner, through which the flapper assembly moves on. The summing beam is labelled direct and reverse in either side. The cam attached to the feedback system is down behind the summing beam.
Procedure:
The Zero adjustment is done with the Nozzle pin and Span adjustment is done by moving the flapper adjustment.
Zero adjustments:
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The supply gauge shows the maximum pressure to line.
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Move the flapper assembly to the mid of range of the Direct side in the Force beam.
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Set the input value to the LRV, that is 3Psi in most cases → Adjust the Nozzle pin to set the output zero.
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Slowly increase the input pressure to 3 Psi and check the output is zero. Zero adjustment is completed.
Span adjustment:
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Sent the maximum pressure to the positioner, by slowly increasing the pressure.
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The positioner input signal is set to 15 Psi which is the URV. Check the output is maximum output 20psi.
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To adjust the Span, move the flapper through the force beam. Which is already set to the middle of the direct reading.
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Move the flapper to a smaller value if the output saturated for a smaller input value. Move the flapper to a larger value if the output saturated for a larger value.
Thus the Zero and Span adjustment is done
To know more about [Control valve pressure test] (https://automationforum.co/2018/01/control-valve-pressure-test.html)