Pneumatic Valve Actuators Information
Pneumatic valve actuators adjust valve position by converting air pressure into linear or rotary motion. There are two main forms: the piston actuators and diaphragm actuators.
Piston actuators are used when the stroke of a diaphragm actuator would be too short or the thrust is too small. Compressed air is applied to a solid piston contained within a solid cylinder. Simple designs have the air fed into a central chamber and the air forces the piston upwards. When the air pressure is removed, the shaft moves in the opposite direction due to the reverse force spring. Piston actuators can also being double acting, meaning the air can be fed into either side of the piston since there is not a return spring.
Piston Valve Actuators. Image credit: Spirax Sarco
Diaphragm actuators have a thin flexible membrane that actuates via a compressed air supply. This type of actuator is single acting because the air is only supplied to one side of the diaphragm, and they can either be direct acting (spring-to-retract) or reverse acting (spring-to-extend).
Diaphragm valve actuator. Image credit: Spirax Sarco
The advantages of pneumatic valve actuators are that they are strong, light, simple, and fast. The disadvantage is that precise position control is not possible except at full stops.
Types of Valve Actuators
Each type of actuator is designed for a particular type of valve. Valves can be sorted by their motion and their function.
Rotary hydraulic valve actuators - Rotary actuators are used for rotary motion valves such as ball, plug, and butterfly valves through a quarter-turn or more from open to close. The closing element is generally a disc or ellipse which turns about an angular shaft. Rotary motion valves specifications include:
- Actuator torque- Torque is the measure of force needed to produce rotary motion. It is determined by multiplying the applied force by the distance from the pivot point to the point where the force is applied.
- Range of rotary motion- Common ranges of motion include 90° (quarter-turn), 180°, 270°, and 360° (multi-turn).
Linear hydraulic valve actuators - Linear actuators move linear valves such as gate, globe, diaphragm, and pinch valves by sliding a stem that controls the closure element. Linear motion valve specifications include:
- Valve stem stroke length- Stroke is a term used to define the travel required by the valve from fully open to fully closed. The stroke of an actuated valve is determined by the actuator if the actuator selected has a stroke that is less than the stroke of the valve. Using an actuator with fewer strokes than the valve will "short stroke" the valve and the full CV rating of the valve will not be realized.
- Actuator force or sealing thrust- The actuator must supply enough force to overcome the pressure in the system to close the closing element and keep it shut.
Throttling valves can be moved to any position, including fully open or fully closed, within the stroke of the valve. Typically, valve actuators are added to throttling valves as part of a control loop that includes a sensing device and circuitry.
Start/stop valves, also known as on/off or isolating devices, limit actuator motion to preset open and closed positions.
Pneumatic valve actuators and hydraulic valve actuators use several different types of actuators.
|Diaphragm actuators||Diaphragm actuators are used mainly with linear motion valves, but are suitable for rotary motion valves with a linear-to-rotary motion linkage.||
Image Credit: Wisc-Online
|Rack-and-pinion||Rack-and-pinion actuators transfer the linear motion of a piston cylinder actuator to rotary motion. They are ideal for automating manually-operated valves.||
Rack and Pinion Actuator. Video credit: BTC Instrumentation / CC BY-SA 4.0
|Scotch yoke actuators||Scotch yoke actuators also transfer linear motion to rotary motion. The mechanism creates lower torque in the middle than at the ends of travel. This characteristic is best for valves that exhibit high end of travel torque requirements. Scotch yoke mechanism animation.||
Scotch yoke valve actuator. Video cedit: M Mehmoodul Hassan / CC BY-SA 4.0
|Lever and link actuators||Lever and link actuators have a splinted or slotted lever which attaches to the valve shaft in order to transfer the linear motion of a diaphragm or piston cylinder to rotary motion. This type of actuator can be found in construction equipment because it allows a linear actuator to actuate a link through a very large angular range of motion. This mechanism has a more even torque capability.||
Image Credit: Chao-Hsu Yao
Vane actuators are used only with rotary motion valves.
Rotary vane actuator. Video Credit: Micrometallic Channel
Important specifications for pneumatic valve actuators include:
Pneumatic actuator for valves and dampers. Video Credit: Industrial Controls / CC BY-SA 4.0
Actuation time is the time it takes to fully close the linear motion valve.
Acting type is an additional specification.
With single-acting devices, fluid pressure actuates the valve in one direction while a compressed spring actuates the valve in the other.
With double-acting devices, fluid pressure actuates the valve in both directions.
Number of turns applies to multi-turn actuators. It defines the number of turns preformed as the rotating valve stem moves from the fully closed to fully open position.
Stem diameter is the diameter of the stem which lifts and lowers the valve disc.
Control signal inputs include milliampere, voltage, and pressure signals are common control signal inputs. Throttling valves regulate the flow, pressure or temperature of the service. They can move to any position within the stroke of the valve and hold that position, including full on or full off. The addition of an actuator to a throttling valve, which has the ability to adjust to a signal, is called a control valve. An actuator used in throttling or modulating service is part of the control loop consisting of a sensing device, controller and an actuator mounted on a valve. The actuator receives a control input signal from the controller or positioner and adjusts closure element position accordingly.
Failsafe position determines whether pneumatic valve actuators open or close the valve in the event of a power failure or the loss of the control signal. Failsafe position options include close at no power, close at no control, open at no power, and open at no control.
Air supply pressure range is the input pressure needed to achieve the desired torque or thrust output. A good supply of compressed air is essential to provide clean and dry air at the right quantity and pressure.
Operating temperature is the range of ambient temperature that the valve actuator can properly function within.
Features for pneumatic valve actuators include NEMA enclosures and actuator action.
The National Electrical Manufacturers Association (NEMA), a non-profit trade organization, rates enclosures for electrical equipment. Devices with NEMA 4 and 4X ratings are suitable for indoor or outdoor use and provide protection against dirt, rain, sleet, and snow.
Constructed for indoor or outdoor use. Provides a degree of protection against contact with enclosed equipment, falling dirt, rain, sleet, snow and windblown dust, splashing water and hose-directed water. Undamaged by external formation of ice on enclosure.
Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water, and corrosion; and that will be undamaged by the external formation of ice on the enclosure.
Constructed for indoor use in hazardous locations classified as Class I, Division 1, Groups A, B, C or D as defined by NFPA70.
Enclosures constructed for indoor use in hazardous locations classified as Class II, Division 1, Groups E, F, or G as defined in NFPA (National Fire Protection Agency) 70.
Valve motion - Manual valve actuators will have direct (clockwise) actuator action or reverse (counterclockwise).
Overtorque protection - The actuator uses a torque sensor to "switch off" electric motor when safe torque level is exceeded.
Local position indication - Local position indicators provide a visual display of valve position based on operation via integral pushbuttons and controls. An electric position sensor will compare the valve signal to the control signal and transmit an appropriate signal to the electric motor. Some positioners vary the motor speed in proportion to the error between the two signals.
Integral pushbuttons and controls - The actuator has controller options that enable the actuator to be operated locally.
Pneumatic valve actuator. Image Credit: Nuclear Power Training
Travel stops or limit stops - Travel stops restrict the actuator's linear or rotary motion. Limit switches are electromechanical contacts or non-contact proximity sensors that allow the position/status of the actuator to be monitored from a remote location. Usually the signal (or lack of signal) generates some action in the process.
Manual overrides - Handwheels, levers, and hydraulic hand pumps which can be used to manually override electric value actuators in the event of an emergency. This safety feature is highly recommended for use in emergency situations which may require the valve to be actuated manually.
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