By themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators.
Actuators may be pneumatic, hydraulic, or electric solenoids or motors.
Pneumatic Actuators :
A simplified diagram of a pneumatic actuator is shown in Figure 1. It operates by a combination of force created by air and spring force. The actuator positions a control valve by transmitting its motion through the stem.
A rubber diaphragm separates the actuator housing into two air chambers. The upper chamber receives supply air through an opening in the top of the housing.
Figure 1 : Pneumatic Actuator: Air-to-Close/Spring-to-Open
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 1 is shown in Figure 2 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 (4-20mA), a instrument 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 – old controlling methods).
Figure 2 : Pneumatic Actuator with Controller and Positioner
In Figure 2, 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.