Answer / bunny

Valve positioner is an instrument used with automatic
control valves,not with a manual valve.An automatic control
valve open/close basicaly according to the control signal
from a controler(e.g a PLC),say,4-20MA.For every control
signal comming from the controler,there would be a
corrsponding position for the valve plug,which determines
the flow rate.But in practical applications,the valve
position might not get maintained by the controller action
alone.So if there is any deviation in that position,we need
to rectify that and for that we are using a valve
positioner.

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