Actuators are used to open and close valves. Pneumatic actuators use pressurized air for control. Electric actuators use electrical power (e.g. an electrical motor).
Pneumatic
Diaphragm
General Information
The picture below shows a diaphragm actuator attached to the top of a weir-type valve. If the pressure below the diaphragm becomes greater than the pressure in the chamber above it, the diaphragm moves upward and the valve opens.
Equipment Design
This animation shows a diaphragm actuator attached to the top of a weir-type valve. Air can be pumped into or out of the space below the actuator diaphragm.
When the pressure above the diaphragm is greater (darker) than the pressure below it, the valve is in the down, or “off,” position. When the pressure below the diaphragm is greater, the valve is in the up, or “on,” position.
Usage Examples
The picture below shows a pneumatic diaphragm actuator that has applications in the poultry industry.
Kennesaw, GA)
Spring-Diaphragm
General Information/Equipment Design
Spring-diaphragm actuators are comprised of a large spring and a diaphragm, as shown in the animation. The spring is set to the pressure required to keep the diaphragms in place.
When the fluid pressure in the pipe becomes great (dark) enough, it displaces the diaphragm, resulting in inflow. This relieves the pressure build-up. Flow continues until the pressure in the pipe is no longer sufficient to keep the spring compressed.
Piston
General Information/Equipment Design
This animation shows a piston actuator, also called a cylinder actuator. This type of actuator is very powerful because the air-supply pressure that may be used is twice the maximum of a spring-diaphragm actuator.
The pressure on the piston determines the position of the valve. Air can be loaded to or unloaded from the top of the piston to result in the desired valve position.
Rack And Pinion
General Information
Rack and pinion actuators consist of a linear rack of gears and a rotating pinion. The rack is attached to a piston. When the actuator is activated, the piston moves the gear rack, rotating the pinion. The pinion, in turn, rotates a valve.
Equipment Design
Vents inside the actuator divide it into two chambers, marked in the animation below on the left in yellow and red. As the “red” chamber is pressurized, and the “yellow” chamber exhausted, the pistons move apart. This turns the pinion counterclockwise, opening the valve. A schematic close-up of the rack and pinion assembly is shown on the right. When the “yellow” chamber is pressurized and the “red” chamber vented, the pistons are forced together. This turns the pinion clockwise, closing the valve.
Usage Examples
Pneumatic rack and pinion actuators are available for virtually any range of air pressure, torque, valve size, and type. These pictures show rack and pinion actuators attached to the top of ball valves.
Electric
Electro-Hydraulic
General Information/Equipment Design
This animation shows how an electro-hydraulic actuator works. Electro-hydraulic actuators consist of a pump connecting the two sides of a cylinder surrounding a piston. The pump, driven by a variable current motor, controls the amount of fluid on either side of the piston.
If the input current is varied by a sensor in the pipe, not shown here, some fluid is transferred from one side of the piston to the other. This fluid transfer moves the piston, rotating the valve.
Advantages
- Compressed air system is unnecessary
Disadvantages
- Much more expensive than diaphragm actuators
- Require a constant current
Electro-Mechanical
General Information
The picture below shows a typical electro-mechanical actuator, consisting of a motorized gear driving the valve stem, and an electronic feedback system. A varying input signal corresponds to the desired valve position. This signal activates the motor, which moves the valve stem, creating a voltage that is monitored by the feedback system. When the feedback voltage and the input voltage are equal the motor stops, leaving the valve stem in the desired position.
Advantages
- More efficient than electro-hydraulic units
Disadvantages
- Expensive
Mechanical
Mechanical actuators operate by translating mechanical movement, often rotary, into linear movement.
Screw
General Information
Two common types of mechanical actuators are machine screw and ball screw actuators.
Other
Hydraulic
General Information
Hydraulic systems are very similar to pneumatic systems except that they use liquids instead of gases. This allows hydraulic actuators to work at 100 to 200 bar pressure, whereas pneumatic actuators are typically used up to 7 bar. Also, hydraulic systems have one power pack to control many actuators, while pneumatic systems have their own power source. Hydraulic actuators can also be referred to as hydraulic cylinders.
Usage Examples
A common application for hydraulic actuators is a hydraulic car jack, as shown below.
Advantages
- High power-to-weight ratio
- Very precise control of movement and position
Disadvantages
- High force may cause damage
Manual
General Information
Most valves are operated manually by turning a handle or lever to open or close the valve. Manually operated valves are designed for instances that require low torque and infrequent number of operations.
Acknowledgements
- Columbus McKinnon Corporation, Amherst, NY
- Emerson Process Management
- Flowserve Corporation, Irving, TX
- Hydro-Lek Ltd., Berkshire, UK
- ITT Engineered Valves, Lancaster, PA
- Leslie Controls Inc., Tampa, FL
- Swagelok Company
- Valtorc International, Kennesaw, GA
References
- Merrick, Ronald C. Valve Selection and Specification Guide. New York: Van Nostrand Reinhold, 1991: 217, 300, 310 – 311, 345. Print.
- Perry, Robert H. and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New York: McGraw-Hill, 1997: 8-66 – 8-67. Print.
- Skousen, Philip L. Valve Handbook . New York: McGraw-Hill, 1988: 336-380. Print.
- Ulanski, Wayne. Valve and Actuator Technology. McGraw-Hill, Inc., 1991: 163-166, 234-236. Print.
Developers
- Jeff Scramlin
- Steve Wesorick
- Rob Kendrick
- Kelsey Kaplan
- Steve Cotton