ACTUATORS


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.



(Copyright Valtorc International,
Kennesaw, GA)

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.



(Animation based on a schematic
Copyright ITT Engineered Valves, Lancaster, PA)

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.



(Copyright Valtorc International,
Kennesaw, GA)

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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.




(Animation based on picture
Copyright ITT Engineered Valves, Lancaster, PA)
(Copyright Flowserve Corporation,
Irving, TX)

When the fluid pressure in the pipe becomes great (dark) enough, it displaces the diaphragm, resulting in flow. This relieves the pressure build up. Flow continues until the pressure in the pipe is no longer sufficient to keep the spring compressed.


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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.



(Copyright Leslie Controls Inc.,
Tampa, FL)

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 a desired valve position.


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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.



(Copyright Emerson Process Management)

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.




(Animation based on picture
Copyright McCanna, Carpentersville, IL)
(Copyright Emerson Process Management)

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.


(Copyright Valtorc International,
Kennesaw, GA)
(Copyright Swagelok,
2011 Swagelok Company.)

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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

DISADVANTAGES

  • Compressed air system is unnecessary
  • Much more expensive than diaphragm actuators
  • Require a constant current

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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 a 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.



(Copyright Columbus McKinnon Corporation, Amherst, NY)

ADVANTAGES

DISADVANTAGES

  • More efficient than electro-hydraulic units
  • Expensive

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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.


(Copyright Columbus McKinnon Corporation, Amherst, NY)

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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.



(Copyright Hydro-Lek Ltd., Berkshire, UK)

USAGE EXAMPLES

A common application for hydraulic actuators is a hydraulic car jack, as shown below.



(Copyright Hydro-Lek Ltd., Berkshire, UK)

ADVANTAGES

DISADVANTAGES

  • High power-to-weight ratio
  • Very precise control of movement and position
  • High force may cause damage

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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.


(Copyright Valtorc International, Kennesaw, GA)

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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


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