Types of Pneumatic Valve Actuators

Scotch-yoke actuators (Morin)

Pneumatic valve actuators all provide the same function:  They convert air pressure to rotational

movement and are designed to open, close, or position a quarter-turn valve.  These include ball valves, plug valves, butterfly valves, or other types of 90 degree rotational valves.

The basic design variations of pneumatic valve actuators are as follows:

• Rack and pinion
• Scotch-yoke
• Rotary vane

Let's review each of these in detail:

Rack and Pinion Actuators

Rack and pinion actuator (Unitorq)

These actuators are sometimes referred to as, “lunch box,” because they, well, look like a lunch box. This actuator uses opposing pistons with integral gears to engage a pinion gear shaft to produce rotation. They are usually more compressed than a scotch yoke, have standardized mounting patterns, and produce output torques suitable for small-to-medium sized valves.  Rack and pinion nearly always include standard bolting and coupling patterns to directly attach a valve, solenoid, limit switch or positioner.  One of their features include several smaller coil springs mounted internally, which provide the torque to return the valve to its starting position.

Scotch-yoke Actuators 

Scotch-yoke actuators internal view.

These actuators come in a multitude of sizes, but are usually used on larger valves because they can produce a very high torque output.  They employ a pneumatic piston mechanism to transfer movement to a linear push rod.  That rod, in turn, engages a pivoting lever arm to provide rotation. Spring return units have a large return spring module mounted on the opposite end of the piston mechanism working directly against the pressurized cylinder.

Rotary Vane Actuators 

Rotary vane actuator animation.

These actuators are usually used when the application requires a significant space savings.  They take up less space when comparing size-to-torque with rack and pinion and scotch yoke. Rotary van actuators also benefit from a reputation of longevity.  They contain fewer moving parts than other types of pneumatic valve actuators.  Rotary vane actuators use externally mounted, helically wound "clock springs" for their spring return mechanism.

These style of valve actuators can all be secured with direct acting or spring return versions. Direct acting actuators use the air supply to move the actuator in both directs (open and close). Spring return actuators, as the name describes, uses springs to move the actuator back to its "resting" state. Converting a version from direct acting to spring return is done through simple modifications, typically just adding an external spring module, or removing the end caps from rack and pinion actuators and installing several coil springs.

When considering the choice of pneumatic valve actuators, your decision comes down to size, power, torque curve and the ease of adding peripherals. To ensure that your valve actuation package will be optimized for safety, longevity, and performance, the advice of a qualified valve automation expert should be sought out. That expert will be able to help you with the best selection of the appropriate valve actuator for any quarter turn valve application.

For more information on valve actuation, contact Piping Specialties, Inc.
https://psi-team.com
800-223-1468

Industrial Valve Basics

Industrial multi-port ball valves (Flo-Tite)

Valves are mechanical devices, essential control and regulating components of a piping system. They are the controlling element within any fluid handling systems; they control the flow and/or pressure of fluids such as liquids, gases, vapors, slurries, and more.

Because of the variety of fluids valves can accommodate, care and consideration are needed when selecting a valve that provides the right service level at the right price point.

For this reason, the types, models, and classifications of valves vary, however, they all offer the same basic function:

• Stopping and starting flow
• Increasing or reducing flow
• Controlling the direction of flow
• Regulating a flow or process pressure

To begin, the first classification of valves are the valves themselves; there are seven common types: gate, globe, plug, ball, butterfly, check, and diaphragm. Each of these valves has models, the second classification. Depending on the valve of choice, the valves can be self-operated, manually operated, or controlled with an actuator that is pneumatic, electric, or hydraulic.

The third classification is based on mechanical motion of the valve closure.

Internal view of
linear industrial valve
(Conval)

Linear Valve: the valve closure moves in a straight line between open and closed positions,
providing fully closed, a range of partially open, and fully open positions. Partially open positions provide throttling of the fluid flow at levels between no flow and full flow. Gate, globe, and diaphragm valves are characterized by linear motion. These valves are also referred to as multi-turn valves, because of the mechanical drive arrangement that some utilize to move the valve closure.

Internal view of rotary (ball) valve.
Courtesy of Flo-Tite.

Rotary Valve: the valve closure travels along a circular or angular path; e.g. butterfly, plug, and ball valves. Rotary valves generally require an approximate quarter turn to complete the motion between fully open and fully closed positions.

There are many product and performance attributes to consider in the valve selection process, low maintenance burden and cost usually being highly ranked. It is also important to match the valve construction to the fluid the valve will be handling, e.g. is it corrosive or erosive? The level of physical stress, including frequency of use, temperature, pressure, and the speed at which flow is to be interrupted may be of concern.

Ultimately, each industrial process application will benefit from a carefully selected valve that closely matches the process performance requirements. Share your fluid control requirements and challenges with an industrial valve expert, combining your own process knowledge and experience with their product application expertise to develop effective solutions.