Pneumatic Actuators

Valves cannot control a process by themselves. 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, electric solenoids or motors. In this article we look at specifically pneumatic actuators.

The figure below shows an illustration of a pneumatic actuator:

This pneumatic actuator 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. The bottom chamber contains a spring that forces the diaphragm against mechanical stops in the upper chamber. Lastly, a local indicator is connected to the stem to indicate the position of the valve.

Operation of the Pneumatic Actuator

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 the supply air pressure is increased from zero, its force on the 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 be begin to force the diaphragm upward and open the control valve. Furthermore, if supply pressure is held constant at some value between zero and maximum, the valve will position at an intermediate position. Hence, the valve can be positioned anywhere between fully open and fully closed in response to changes in supply air pressure.

Positioner

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 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.

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 changes in supply air pressure and holds the valve at the new position. This in turn corrects the controlled variable’s deviation from setpoint.

For instance, as the control signal increases, a valve inside the positioner admits more supply air to the actuator. As a result, the control valve moves downwards. The linkage transmits the valve position information back to the positioner. This forms a small internal feedback loop for the actuator. When the valve reaches the position that correlates to the control signal, the linkage stops supply air flow to the actuator. This causes the actuator to stop.  Conversely, if the control signal decreases, another valve inside the positioner opens and allows the supply air pressure to decrease by venting the supply air. This causes the valve to move upward and opens. When the valve has opened to the proper position, the positioner stops venting air from the actuator and stops movement of the control valve.

Fail-safe Valve

This is an actuated valve or automatic valve that reverts to a pre-determined position after the actuating force is removed.

An actuator or automatic valve can be designed to position a control valve in a safe position if a loss of supply air occurs. We have two arrangements:

Fail-open also called spring to-open or air-to-close

Fail-closed also called air-to-open, spring-to-close