Operating Principle of the Positioner
Identify the Positioner Parts and Their Function
Introduction
In previous unit, you have learned about valves and actuators.
Figure 1 below, shows how a controller is connected to an actuator to
drive a valve to close and open a piping system.
As
in any control situation, instability is one of many problems in process
system. Instability is caused when time
lags, causes the control loops to react too late to any disturbance. Time lags also occurred in control loops
where large actuator is being used. A
large actuator is like a capacitance in the process. Valves do not change position when airflow
starts to enter in the diaphragm chamber of the actuator. Valves will move only when enough air
accumulates to build up pressure inside the diaphragm chamber and cause motion
to actuator stem.
To
overcome time lags due to actuator capacitance a device called positioner is
installed in between controller and control valve connections. The positioner provides a substantial
improvement in valve and control loop performance because it produces large
airflow amplification. The effect of
large airflow amplification reduces the effective actuator capacitance in a
control loop.
In
this unit you will learn to:
S
Identify the parts and
function of a positioner
S
Identify some other
control situations where application of positioners is necessary
S
Install and calibrate a
Fisher 3582 pneumatic valve positioner.
Describe the Operating Principle of the Positioner and Its Application
The
principle of operation
Figure
5 Positioner Response to an Increasing
Instrument Signal
When
the instrument pressure increases, the bellow expands and moves the beam. The beam pivots the flapper and restricts the
nozzle. The nozzle pressure increases
and moves the relay diaphragm assembly to open the supply valve.
Output
pressure to the diaphragm actuator increases moving the actuator stem
downward. Stem movement is fed back to
the beam by means of a cam, which causes the flapper to pivot slightly away
from the nozzle. Nozzle pressure
decreases and the relay supply valve closes slightly to prevent any further
increases in output pressure. The
positioner will now be in equilibrium but in a higher instrument pressure, a
slightly different flapper position and a new actuator stem position.
When
the instrument signal decreases, the bellows contracts (aided by an internal
range spring) to move the beam and to pivot the flapper slightly further from
the nozzle. Nozzle pressure decreases
and through relay operation, the exhaust valve in the relay opens to release
diaphragm actuator pressure to atmosphere, permitting the actuator stem to move
upward. Stem movement is fed back to the
beam by the cam to reposition the beam and flapper. When equilibrium conditions are obtained the
exhaust valve closes to prevent any further decrease in diaphragm case
pressure.
The
principle of operation for reverse-acting positioner is similar except that as
the instrument pressure increases, the diaphragm case pressure is
decreased. Conversely, a decreasing
instrument air signal causes an increase in the pressure to the diaphragm
actuator.
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