Process Controls
VALVE ACTUATORS
The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the
upper chamber. Finally, a local indicator is connected to the stem to indicate the position of the
valve.
The position of the valve is controlled by varying supply air pressure in the upper chamber. This
results in a varying force on the top of the diaphragm. Initially, with no supply air, the spring
forces the diaphragm upward against the mechanical stops and holds the valve fully open. As
supply air pressure is increased from zero, its force on top of the diaphragm begins to overcome
the opposing force of the spring. This causes the diaphragm to move downward and the control
valve to close. With increasing supply air pressure, the diaphragm will continue to move
downward and compress the spring until the control valve is fully closed. Conversely, if supply
air pressure is decreased, the spring will begin to force the diaphragm upward and open the
control valve. Additionally, if supply pressure is held constant at some value between zero and
maximum, the valve will position at an intermediate position. Therefore, the valve can be
positioned anywhere between fully open and fully closed in response to changes in supply air
pressure.
A positioner is a device that regulates the supply air pressure to a pneumatic actuator. It does
this by comparing the actuator’s demanded position with the control valve’s actual position. The
demanded position is transmitted by a pneumatic or electrical control signal from a controller to
the positioner. The pneumatic actuator in Figure 35 is shown in Figure 36 with a controller and
positioner added.
The controller generates an output signal that represents the demanded position. This signal is
sent to the positioner. Externally, the positioner consists of an input connection for the control
signal, a supply air input connection, a supply air output connection, a supply air vent connection,
and a feedback linkage. Internally, it contains an intricate network of electrical transducers, air
lines, valves, linkages, and necessary adjustments. Other positioners may also provide controls
for local valve positioning and gauges to indicate supply air pressure and control air pressure (for
pneumatic controllers). From an operator’s viewpoint, a description of complex internal workings
of a positioner is not needed. Therefore, this discussion will be limited to inputs to and outputs
from the positioner.
In Figure 36, the controller responds to a deviation of a controlled variable from setpoint and
varies the control output signal accordingly to correct the deviation. The control output signal
is sent to the positioner, which responds by increasing or decreasing the supply air to the
actuator. Positioning of the actuator and control valve is fed back to the positioner through the
feedback linkage. When the valve has reached the position demanded by the controller, the
positioner stops the change in supply air pressure and holds the valve at the new position. This,
in turn, corrects the controlled variable’s deviation from setpoint.
Rev. 0
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