What is a Pneumatic Actuated Valve?
Pneumatic actuated valves used compressed air flowing into and exhausting from an actuator to open and close the valve. The type of pneumatic actuator required depends on the actuator valve. Valves requiring linear motion (gate, globe or diaphragm valves) typically require a pneumatic diaphragm actuator. Quarter turn valves (ball and butterfly valves) typically require either a Rack & Pinion air actuator or a Scotch Yoke air actuator. We'll discuss the two types on quarter-turn air actuators in greater detail.
Scotch Yoke actuators are composed of a piston directly connected to a slotted yoke. A pin on the rotating shaft rides in the yoke slot, converting the linear motion to rotational motion in much the same way a crankshaft converts the linear motion of a piston connecting rod. Scotch Yoke actuators have a nonlinear torque profile, with the highest torque at the beginning and end of the stroke. Scotch Yoke construction is readily scale-able, making this type of actuator suitable for large valves requiring high torque.
Rack & Pinion actuators use a piston to drive a toothed shaft (the rack) which is geared to the actuator output drive (the pinion). They are typically used in lower torque applications, offer a more precise control and an even torque throughout the stroke.
Double acting actuators use compressed gas driving a piston to both open and close the valve. These actuators have two ports to pass and exhaust the gas. Compressed gas is fed through the first port, causing the actuator to close the valve, and then exhausted out the second port. To return to the original position the supply and exhaust functions are reversed.
Spring return actuators typically have a single port to both pass and exhaust the compressed gas. Unlike double acting actuators, spring return actuators are driven in a single direction and, when the gas is exhausted, use springs to return to the original position. The valve can be installed to return to either the closed or open position; the more common mode is normally closed.
As we've seen, pneumatic actuators require compressed air to be supplied and exhausted at certain ports at certain times. In the case of double acting actuators, both ports serve as supply and exhaust ports depending on actuator position, and for spring return actuators a single port serves as both the supply and exhaust port. This necessitates some sort of external switching.
This is often accomplished via an external bank of pilot valves. An electronic controller (like a PLC) opens and closes pilot valves which in turn actuate the main valves. However, in large plants or outdoor applications the valve bank may be far from the actuator. In this case long runs of small-diameter pneumatic lines are impractical due to the line pressure loss and the potential for leaks at connections. Instead, it is preferable to run electrical wires directly to the valve and control the valve locally. Compressed pilot air may be supplied by a short run from the piping of the main plant.
Direct mount solenoid valves accomplish this by using an electrically actuated solenoid valve to control the compressed gas. The solenoid valve mounts directly to the pneumatic actuator. When energized, the solenoid moves a plunger which directs compressed gas into or out of the valve depending upon configuration.
Double-acting actuated valves are controlled by solenoid valves utilizing a five port, two position (or 5/2) configuration. The five ports are two output, two exhaust and one input. For spring return control solenoid valves use a 3/2 configuration (one output, one exhaust, one input).
Where a source of compressed air is readily available and simple On/Off performance is needed, pneumatic valve actuators offer several advantages over electric actuators. The above discussion is just a brief outline of the many types of actuated valves; the variations are as numerous as the applications. For more information, please visit our technical resources page, or give us a call at 800-511-0100.