Pneumatic actuators use air pressure pushing against either a flexible diaphragm or a piston to move a valve mechanism. The following photograph shows a cut-away control valve, with a pneumatic diaphragm actuator mounted above the valve body. You can see the large coil spring providing default positioning of the valve (air pressure acting against the diaphragm moves the valve against the spring) and the rubber diaphragm at the very top. Air pressure applied to the bottom side of the diaphragm lifts the sliding stem of the valve in the upward direction, against the spring’s force which tries to push the stem down:
The air pressure required to motivate a pneumatic actuator may come directly from the output of a pneumatic process controller, or from a signal transducer (or converter ) translating an electrical signal into an air pressure signal. Such transducers are commonly known as I/P or “I to P” converters, since they typically translate an electric current signal (I) of 4 to 20 mA DC into an air pressure signal (P) of 3 to 15 PSI.
The following photographs show I/P transducers of different make and model. A Fisher model 846 appears in the upper-left photograph, while an older Fisher model 546 appears in the upper-right (with cover removed). A Foxboro model E69F I/P appears in the lower-left photograph, while a Moore Industries model IPT appears in the lower-right:
Despite their differing designs and appearances, they all function the same: accepting an analog DC current signal input and a clean supply air pressure of about 20 PSI, outputting a variable air pressure signal proportional to the electric current input. An interesting feature to compare between these four I/P transducers is their relative ruggedness. Every transducer shown except the Moore Industries model (lower-right) is built to withstand direct exposure to a process atmosphere, hence the heavy cast-metal housings and electrical conduit fittings. The Moore Industries unit is intended for a sheltered location, and may be plugged in to a “manifold” with several other I/P transducers to form a compact bank of transducers capable of driving air pressure signals to several valve actuators.
Some pneumatic valve actuators are equipped with hand jacks which are used to manually position the valve in the event of air pressure failure. The next photograph shows a sliding-stem control valve with pneumatic diaphragm actuator and a “handwheel” on the top:
Note the three manual valves located around the control valve: two to block flow through thecontrol valve and one to bypass flow around the control valve in the event of control valve failure or maintenance. These manual valves happen to be of the gate design, with rising-stem actuators to clearly show their status (stem protruding = open valve ; stem hidden = closed valve). Such blockand- bypass manual valve arrangements are quite common in the process industries where control valves fulfill critical roles and some form of manual control is needed as an emergency alternative.
Note also the air pressure tubing between the valve actuator and the air supply pipe, bent into a loop. This is called a vibration loop, and it exists to minimize strain on the metal tubing from vibration that may occur.
Pneumatic actuators may take the form of pistons rather than diaphragms. Illustrations of each type are shown here for comparison:
Piston actuators generally have longer stroke lengths than diaphragm actuators, and are able to operate on much greater air pressures. Since actuator force is a function of fluid pressure and actuator area (F = PA), this means piston actuators are able to generate more force than diaphragm actuators. The combination of greater force and greater displacement yields more work potential for piston actuators than diaphragm actuators of equivalent size, since mechanical work is the product of force and displacement (W = Fx).
Perhaps the greatest disadvantage of piston actuators as applied to control valves is friction between the piston’s pressure-sealing ring and the cylinder wall. This is not a problem for on/off control valves, but it may be a significant problem for throttling valves where precise positioning is desired. Diaphragm actuators do not exhibit the same degree of friction as piston actuators because the elastic diaphragm rolls and flexes rather than rubs against a stationary surface as is the case with piston sealing rings.
A double-piston pneumatic actuator appears in the next photograph, providing the mechanical force needed to turn an on/off butterfly valve:
In this particular actuator design, a pair of pneumatically-actuated pistons move a rack-andpinion mechanism to convert linear piston motion into rotary shaft motion to move the butterfly trim. Note the rotary indicator (yellow in color) at the end of the rotary valve stem, showing what position the butterfly valve is in. Note also the travel switch box (black in color) housing multiple limit switches providing remote indication of valve position to the control room.
A rack-and-pinion mechanism looks like this, as viewed looking into the axis of the rotary shaft:
Compressed air applied to the bottom tube (with the top tube vented) pushes both pistons toward the center, spinning the pinion gear counter-clockwise. Applying compressed air to the top tube (with the bottom tube vented) pushes both pistons outward, rotating the pinion gear clockwise.
A smaller version of this same actuator design, cut away to reveal its inner workings, appears here:
Another pneumatic piston actuator design uses a simple crank lever instead of a rack-and-pinion gear set to convert linear piston motion into rotary motion. This next photograph shows such a piston actuator connected to a ball valve: