Pneumatic Valve and Damper Actuators: A Design Comparison

Industrial valves, dampers and louvers are operated either electrically or pneumatically. This post compares the three major categories of  pneumatic valve actuators, namely:

1. Rotary vane
2. Scotch-yoke
3. Rack & pinion

All three categories provide the same basic function - converting air pressure to rotational movement intended to open, close, or position a quarter-turn valve (ball, plug, butterfly), louver or damper.

All three can be configured in either direct acting or spring return versions. Direct acting actuators use the air supply to move the actuator in both directions (open and close). Spring return actuators, as the name implies, uses springs to move the actuator back to its "resting" state. Converting 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.

Rotary Vane Actuator

Rotary Vane Actuators

Vane actuators generally provide the most space savings when comparing size-to-torque with rack and pinion and scotch yoke. They have an outstanding reputation for long life because then contain only one moving part, as opposed to rack and pinion and scotch yoke actuators that have many. They tend to withstand dirty and corrosive atmospheres better than rack and pinion and scotch yoke actuators. Vane actuators also use externally mounted, helically wound "clock springs" for their spring return mechanism.

Scotch Yoke

Scotch-yoke actuators use a pneumatic piston mechanism to transfer movement to a linear push rod, that in turn engages a pivoting lever arm to provide rotation. They come in a wide variety of sizes, but are very often used on larger valves because they are capable of producing very high torque output. Spring return units have a large return spring module mounted on the opposite end of the piston mechanism working directly against the pressurized cylinder.

Rack and Pinion

A rack & pinion pneumatic actuator uses opposing pistons with integral gears to engage a pinion gear shaft to produce rotation. Rack & pinion actuators (sometimes referred to as a lunch box because of their shape) tend to be more compact than scotch yoke, have standardized mounting patterns, and produce output torques suitable for small to medium sized valves. They almost always include standard bolting and coupling patterns to directly attach a valve, solenoid, limit switch or positioner. Rack and pinion actuators use several smaller coil springs mounted internally and provide the torque to return the valve to its starting position.

The practical difference between these three types of pneumatic actuators comes down to size, power, torque curve and ease of adding peripherals. For the best selection of valve actuator for any quarter turn valve application, you should seek the advice of a qualified valve automation specialist. By doing so your valve actuation package will be optimized for safety, longevity, and performance.

Scotch yoke mechanism image courtesy of Wikipedia.
Rack and pinion mechanism image courtesy of Wikipedia.

Epoxy Stove Enamel Finish: A Kinetrol Advantage

Kinetrol actuators are well-known for their bright yellow, epoxy stove enamel finish. This coating is engineered to protect the exterior of their pneumatic actuators and accessories. But what is "epoxy stove enamel"?

Epoxy stove enamel is a high quality stoving finish that provides excellent adhesion to a substrate. Epoxy stove enamel forms a very hard, abrasion resistant, chemical and solvent resistant coating that stands up to many extreme environmental conditions. Epoxy stove enamel finish provides high level chemical/corrosion resistant qualities, and is used often in applications such as machinery, industrial equipment, and heavy duty transport machinery.

Kinetrol actuators are engineered specifically for high-cycle, dirty, and corrosive applications. The combination of epoxy stove enamel finish and stainless steel external hardware provide Kinetrol actuators outstanding protection from dirt and harmful chemicals.

Kinetrol USA
https://kinetrolusa.com
972-447-9443

Industrial Valve Actuator Types

Lever actuator
(with failsafe  spring return)

Valve actuators are selected based upon a number of factors including torque necessary to operate the valve and the need for automatic actuation. Types of actuators include manual handwheel, manual lever, electrical motor, pneumatic, and solenoid. All actuators except manual handwheel and lever are adaptable to automatic actuation.

Manual

Manual actuators are capable of placing the valve in any position but do not permit automatic operation. The most common type mechanical actuator is the handwheel. This type includes handwheels fixed to the stem and handwheels connected to the stem through gears.

Electric Motor Actuators

Gear operator
(attached to pneumatic actuator)

Electric motors permit manual, semi-automatic, and automatic operation of the valve. Motors are used mostly for open-close functions, although they are adaptable to positioning the valve to any point opening. The motor is usually a, reversible, high speed type connected through a gear train to reduce the motor speed and thereby increase the torque at the stem. Direction of motor rotation determines direction of disk motion. The electrical actuation can be semi-automatic, as when the motor is started by a control system. A handwheel, which can be engaged to the gear train, provides for manual operating of the valve. Limit switches are normally provided to stop the motor automatically at full open and full closed valve positions. Limit switches are operated either physically by position of the valve or torsionally by torque of the motor.

Pneumatic Actuators

Pneumatic actuator

Pneumatic actuators provide for automatic or semi-automatic valve operation. These actuators translate an air signal into valve stem motion by air pressure acting on a vane, diaphragm, or piston connected to the stem. Pneumatic actuators are used in throttle valves for open-close positioning where fast action is required. When air pressure closes the valve and spring action opens the valve, the actuator is termed direct-acting. When air pressure opens the valve and spring action closes the valve, the actuator is termed reverse-acting. Double acting actuators have air supplied to both sides of the vane, diaphragm, or piston. The differential pressure across the diaphragm positions the valve stem. Automatic operation is provided when the air signals are automatically  controlled by circuitry. Semi-automatic operation is provided by manual switches in the circuitry to the air control valves.

Hydraulic Actuators

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve, similar to the pneumatic actuators. These actuators use a piston to convert a signal pressure into valve stem motion. Hydraulic fluid is fed to either side of the piston while the other side is drained or bled. Water or oil is used as the hydraulic fluid. Solenoid valves are typically used for automatic control of the hydraulic fluid to direct either opening or closing of the valve. Manual valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic operation.

Solenoid Actuated Valves

Solenoid valve
(attached to pneumatic actuator)

Solenoid actuated valves provide for automatic open-close valve positioning. Most solenoid actuated valves also have a manual override that permits manual positioning of the valve for as long as the override is manually positioned. Solenoids position the valve by attracting a magnetic slug attached to the valve stem. In single solenoid valves, spring pressure acts against the motion of the slug when power is applied to the solenoid. These valves can be arranged such that power to the solenoid either opens or closes the valve. When power to the solenoid is removed, the spring returns the valve to the opposite position. Two solenoids can be used to provide for both opening and closing by applying power to the appropriate solenoid.

Single solenoid valves are termed fail open or fail closed depending on the position of the valve with the solenoid de-energized. Fail open solenoid valves are opened by spring pressure and closed by energizing the solenoid. Fail closed solenoid valves are closed by spring pressure and opened by energizing the solenoid. Double solenoid valves typically fail "as is." That is, the valve position does not change when both solenoids are de-energized.

One application of solenoid valves is in air systems such as those used to supply air to pneumatic valve actuators. The solenoid valves are used to control the air supply to the pneumatic actuator and thus the position of the pneumatic actuated valve.

Contact Kinetrol USA for any valve actuation challenge. They can be reached at https://kinetrolusa.com or by calling 972-447-9443.

Industrial Dampers and Drives

Round dampers with
pneumatic vane type drives.

By definition, a damper is a device used to control pressure, flow, or flow direction in an air or gas system. Different types of dampers can be used, depending on specific functional requirements. Table 5.7 below lists the types of dampers and their functions, and Table 5.8 lists the damper configurations. Selection of the proper damper type and blade configuration is important to achieve the required damper performance. The type and configuration of damper can significantly impact pressure drop, leakage rates, and controllability.

Click table for larger view.

Click table for larger view.

Pneumatic vane damper drive.

A very important part of damper design is determination of damper torque and sizing and selection of damper actuator for the maximum torque. Actuator torque should be selected for a minimum of 1.5 times the damper maximum torque to provide margin and allow for degradation over the life of the damper. Actuators should be evaluated for damper blade movement in both directions, at the beginning of blade movement, and while stroking blades through the full cycle of movement.

Damper operators can be one of three types: pneumatic, electric, or electro-hydraulic, as described below.

1. Pneumatic. These damper operators are used whenever controls rely primarily on compressed air (pneumatic) for moving operators or transmitting control signals.
2. Electric. These damper operators are used whenever controls rely primarily on low voltage electric circuits to transmit control signals.
3. Electrohydraulic. These damper operators are the same as the electric type described above, except they have the ability to modulate. They use an electric control signal to position a hydraulic system that, in turn, positions the damper.

Electrically operated damper drives have historically been favored, but the shift to retro-fit electric drives with pneumatic damper drives has been significant in the last two decades. When pneumatic vane actuators were first introduced for damper drive service, their virtues were quickly discovered. Their inherent design and operating advantages apply perfectly for precise damper control. These design and operating advantages are:

• Damper drives on round dampers.
• Precise, smooth signal to movement response.
• 100 percent duty cycle.
• Continuous modulating service.
• No overheating.
• High speed/high-torque.
• Fast full stroke open/close.
• Very easily serviced.
• Excels in harsh, high-temperature operating environments.
• Effectively zero air consumption in resting state.

For more information on any damper drive application, contact Kinetrol USA at 972-447-9443 or visit http://www.kinetrolusa.com.

Actuators for Ergonomic Moving and Lifting Equipment

Ergonomic manipulators (also know as ergo arms, articulating arms, articulating jib arms, and lift assist devices) are lifting assistance devices used to help workers lift, maneuver, and place items that are impossible to safely lift themselves.

Ergonomic lift assist devices increase productivity while offering an economic solution to material handling needs. They create a better way to work providing a safe and healthy workplace for employees, while increasing efficiency and productivity at the same time.

The Kinetrol vane actuator is an ideal pneumatic drive for industrial ergonomic manipulators due to:

• Compact size
• Powerful
• Wide range of torque outputs
• Exceptionally smooth control

For more information, visit http://www.kinetrolusa.com or call 972-447-9443.

A Basic Description of Valve Actuators

Automated plug valve with
actuator, limit switch, and indicator.

Valves are used to control the flow of fluids in process systems. In order to meet the needs of a process system some method of changing the positions of the valve is needed. 

Some valves can be operated with hand wheels or levers, but some valves have to be open and closed or throttled frequently. Manually positioning valves in these types of situations is not always practical. So instead of hand wheels or levers, actuators are used to position the valves. 

An actuator is a mechanism that moves or controls a device such as a valve. Actuators reduce the need for operators to go to every valve that needs repositioning and operate the valve by hand. When actuators are used, valves can be repositioned from a central remote location, such as a control room. This is very important in processes where a number of valves have to be repositioned accurately and quickly.

There are three basic types of actuators pneumatic actuators, hydraulic actuators, and electric actuators. Pneumatic actuators use air pressure to produce motion to position the valve. They're probably the most common type of actuator used in process systems. Actuators that are powered by a pressurized liquid, such as hydraulic fluid, are called hydraulic actuators. Compared to pneumatic actuators, hydraulic actuators of the same size are usually more powerful. Electric actuators use electricity to produce motion. They usually fall into one of two general classifications: solenoid actuators or motor-driven actuators. 

Regardless of the type of actuator used with valves, actuators are normally part of process control systems. From a control room, valves with actuators can be quickly and accurately positioned to deal with frequent flow changes. 

Valve actuators operate to position valves in response to signals from controllers. Controllers are part of the instrumentation systems that monitor plant processes and response to variations in the processes. Basically a controller is a device that receives an input from another part of the instrumentation system, compares that input to a set point, and sends out a corrective signal to regulate the value of a process variable such as temperature, pressure, level, or flow. 

Generally, actuators are used with valves that are designed for throttling or regulating flow. Valves that are used to automatically throttle flow are generally referred to as control valves. However some on-off or isolation valves may also require actuators. Although these isolation valves may require actuators, they're not commonly referred to as control valves. Control valves can be either linear that is the stem moves the valve disc up and down as in globe and gate valves, or they can be rotary which means that they're positioned by rotation. Butterfly valves and ball valves, which open or closed with a 90-degree turn, are examples of rotary control valves.