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Summary - Positioners
1. A positioner ensures that there is a linear relationship
between the signal input pressure from the control system
and the position of the control valve. This means that for
a given input signal, the valve will always attempt to
maintain the same position regardless of changes in valve
differential pressure, stem friction, diaphragm hysteresis
and so on.
2. A positioner may be used as a signal amplifier or
booster. It accepts a low pressure air control signal and,
by using its own higher pressure input, multiplies this to
provide a higher pressure output air signal to the actuator
diaphragm, if required, to ensure that the valve reaches
the desired position.
3. Some positioners incorporate an electropneumatic
converter so that an electrical input (typically 4 - 20 mA)
can be used to control a pneumatic valve.
4. Some positioners can also act as basic controllers,
accepting input from sensors.
A frequently asked question is, ‘When should a positioner
be fitted?’
A positioner should be considered in the following
circumstances:
1. When accurate valve positioning is required.
2. To speed up the valve response. The positioner uses
higher pressure and greater air flow to adjust the valve
position.
3. To increase the pressure that a particular actuator and
valve can close against. (To act as an amplifier).
4. Where friction in the valve (especially the packing)
would cause unacceptable hysteresis.
5. To linearise a non-linear actuator.
6. Where varying differential pressures within the fluid
would cause the plug position to vary.
To ensure that the full valve differential pressure can be
accepted, it is important to adjust the positioner zero
setting so that no air pressure opposes the spring force
when the valve is seating.
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