Rack and Pinion Style Pneumatic Valve Actuator

One example of a pneumatic rack and pinion valve actuator
Courtesy Rotork

Three primary kinds of valve actuators are commonly used: pneumatic, hydraulic, and electric.
Pneumatic actuators can be further categorized as scotch yoke design, vane design, and the subject of this post - rack and pinion actuators.

Rack and pinion actuators convert linear movement of a driving mechanism to provide a rotational movement designed to open and close quarter-turn valves such as ball, butterfly, or plug valves and also for operating industrial or commercial dampers. The rotational movement of a rack and pinion actuator is accomplished via linear motion and two gears. A circular gear, known as a “pinion” engages the teeth of one or two linear gears, referred to as the “rack”. Pneumatic actuators use pistons that are attached to the rack. As air or spring power is applied the to pistons, the rack changes position. This linear movement is transferred to the rotary pinion gear (in both directions) providing bi-directional rotation to open and close the connected valve.

Rack and pinion gear
Courtesy Wikipedia

The actuator pistons can be pressurized with air, gas, or oil to provide the linear the movement that drives the pinion gear. To rotate the pinion gear in the opposite direction, the air, gas, or oil must be redirected to the other side of the pistons, or use coil springs as the energy source for rotation. Rack and pinion actuators using springs are referred to as "spring-return actuators". Actuators that rely on opposite side pressurization of the rack are referred to as "direct acting".

Most actuators are designed for 100-degree travel with clockwise and counterclockwise travel adjustment for open and closed positions. World standard ISO mounting pad are commonly available to provide ease and flexibility in direct valve installation. NAMUR mounting dimensions on actuator pneumatic port connections and on actuator accessory holes and drive shaft are also common design features to make adding pilot valves and accessories more convenient.

Pneumatic rack and pinion actuators are compact and effective. They are reliable, durable and provide good service life. There are many brands of rack and pinion actuators on the market, all with subtle differences in piston seals, shaft seals, spring design and body designs. Some variants are specially designed for very specific operational environments or circumstances.

Share your process valve control and automation challenges with application experts, and combine your process experience and knowledge with their product application expertise to develop effective solutions.

Rotary Vane Actuators for Damper Control

One example of a rotary vane pneumatic damper drive
Courtesy Rotork

A rotary vane actuator is simply a part of an automated damper assembly: its role is to change the position of the damper, converting the motive force of fluid pressure into torque and applying it to a mechanism that will position the damper.

Vane actuators are widely used on quarter turn valves in industrial process automation, but their application also extends to dampers on all types of equipment and installations. Vane actuators are well suited for applications with operation requiring fully open or fully closed damper positions, although some do provide modulating service. A rotation of the actuator drive mechanism through a 90 degree arc, in combination with connecting linkage, quickly moves a damper between open and closed positions. A rotary vane actuator is well suited for driving this type of actuation, with its own 90 degree arc of movement.

A rotary vane actuator is specific to quarter turn opeartion. A pressure tight housing contains a movable vane which is sealed to the sides of the pressure chamber by means of a low friction gasket. Inlets and outlets to the chamber on opposing sides of the vane allow a controller to produce a pressure differential across the vane. The vane will move, in response to the pressure differential, in either direction. A shaft is connected to the vane and the vane acts like a lever to rotate the shaft as the vane is moved by fluid pressure. The torque produced by the actuator assembly is dependent upon the applied fluid pressure.

Hydraulic rotary vane actuators have the ability to handle large amounts of fluid and dynamic motions, exhibiting also qualities of durability and compactness. Pneumatic vane actuators use plant air pressure as the motive force. Both types generally provide fast operation, have few moving parts, and require little regular maintenance. A variety of typical automation accessories and options are available to customize a unit for a particular application.

More information is available from product specialists, with whom you should share your application requirements and challenges. Combining your process and facilities knowledge with their product application expertise will produce effective solutions.

Why Do We Debate Air vs. Electric Powered Valve Actuators?

Industrial Process Control Valve
Courtesy Rotork

Which is better, air or electric? We're talking about valve actuators here. Just for perspective on how original this article might turn out to be, I commanded my friendly Google search engine to look for "air vs. electric actuators". Google always finds something, no matter what you ask, giving the humble user an impression that the big "G" has all the answers....and they truly may. Don't misunderstand me. I am a big Google fan. Someday I might even pay for something that they provide me. Anyway, I was humbled by the avalanche of search returns on my the subject, 17,200,000 articles. That's a large number, even for a Google search. Many of the articles related to industrial machinery automation, not valves. Changing my search to "electric vs pneumatic valve actuator" shaved the returns down to 236K, a more manageable volume.
Dutifully reading the highest ranked articles and following threads in forums, I started to wonder why, like figurative gladiators, we pit these two valve actuator motive power sources against one another. There is not a single winner in this case. One is not universally better, more advantageous, than the other. Both methodologies have instances where they can be used to best advantage. A good recommendation is to not be too influenced by the past, by what your own industrial process control experience may have been. The manufacturers of these products are continuously modifying designs and releasing products with newer technologies and better performance that may eliminate some shortcomings of the past.

Your best course of action is to consider the following:

• What is the expected useful life of the process? Short term, long term, permanent?
• Will existing air supply and piping system accommodate the anticipated additional pneumatic valve load, if that type actuator seems otherwise advantageous?
• Are there sufficient maintenance and technical resources in place to keep either system in top operating condition and successfully deal with operational and repair issues that will arise? Does the current maintenance staff have sufficient knowledge and training to perform needed tasks?
• Are there rated hazardous zones where valves will be located?
• What needs to happen to valve position if motive power (air pressure or electricity) fails?
• What valve positioning requirements are associated with proper control of the process?
• What interfaces with any existing control systems, if any, need to be accomplished?

Carefully consider these points, add several of you own. Consult with knowledgeable sales engineers that specialize in valve automation. Combine experience and knowledge from a number of sources and a good solution will materialize. 

Heavy Duty Scotch Yoke Pneumatic Valve Actuator

Scotch yoke refers to a sliding linear mechanism that provides a reciprocating motion. One of the most common uses for scotch yokes are pneumatic valve actuators.

Scotch yoke mechanism
(courtesy of Wikipedia)

Scotch yoke valve actuators are designed to provide higher torque output for larger industrial valves. They typically have large pistons and air cylinders that provide the linear force required to open and close large quarter turn valves (ball, butterfly, plug).

Scotch yoke pneumatic actuators installed
in sea terminal
(courtesy of Rotork)

Scotch yoke actuators can be energized by either compressed gases, or hydraulic fluids. Here are some images of scotch yoke actuators in use:

Large spring return scotch yoke actuator
(courtesy of Rotork)

Class 1500 Ball Valve and Actuator Retrofit

Class 1500, metal seated ball valve
with Rotork pneumatic spring return
actuator and hydraulic override.

CTi Controltech's broad engineering experience was recently challenged with the retrofit of a worn out globe valve and hydraulic actuator, to a new high pressure (Class 1500), metal seated ball valve and actuator package used on 10,000 lb. per hour steam system. 

The customer is a large US Navy contractor located in Northern California. The application is the main steam stop valve on the contractor's steam boilers that provide steam for their turbine testing facility. The contractor's facility simulates conditions on US Navy ships so that turbines and related equipment can be properly tested. 

One of the biggest challenges was to replace the valve and actuator in a very tight compartment. Space was extremely limited.

CTi, along with actuator manufacturer Rotork, came up with the right package to meet the very demanding operating conditions, as well as the working with the small space available.

The job involved the removal of the old globe valve and hydraulic actuator, and then having the new ball valve and pneumatic actuator package lowered through a hole cut in the facilities' roof. A very large crane, with a 200 foot boom, was used for the placement.

The new metal seated, Class 1500 ball valve was outfitted with a Rotork GP200 pneumatic, spring return actuator that included an hydraulic manual override.

The new valve and actuator package works exceptionally well, and the customer is very pleased CTi engineers came up with a solution that fit in to the existing space.

Rack and Pinion Valve Actuators - A Look Inside

Here is an animation of the assembly components of a Flowserve Automax Supernova rack and pinion actuator with a transparent look inside to see its operation.

The critical components to take note of during the video are:

• body 
• pinion gear
• o-ring seals
• piston "racks"
• travel adjustments
• end caps and seals
• housing bolts

At the end, the video allows you to see the operation (in both directions) of the rack and pinion actuator as it is pressurized from either port. The pinion gear rotates 90 degrees, which in most applications, is used to open or close a valve or damper.

Rack and Pinion Pneumatic Actuator Basics

A rack and pinion gear.
(Courtesy of  Wikipedia)

Rack & Pinion actuators are designed for operating quarter-turn valves such as butterfly, plug, and ball valves or for actuating industrial or commercial dampers.

The rotational movement of a rack and pinion actuator is accomplished via linear motion and two gears. A circular gear, referred to a “pinion” engages the teeth of a linear gear “bar” referred to as the “rack”.

In a pneumatic actuator, pistons are attached to the rack. As air or spring power is applied the to piston, the rack is “pushed” inward or “pulled” outward. This dual direction linear movement is transferred to the rotary pinion gear providing bi-directional rotation.