Electrohydraulic Valve Actuators and Hydraulic Valve Actuators Information
Electrohydraulic valve actuators and hydraulic valve actuators convert fluid pressure into motion in response to a signal. They use an outside power source and receive signals that are measured in amperes, volts, or pressure. Hydraulic actuators can be used when a large amount of force is required to operate a valve.
The piston-type hydraulic actuator is the most common and consists of a cylinder, piston, spring hydraulic supply and return line, and stem. The cylinder is separated into two chambers. The top chamber contains the spring and the lower chamber contains the hydraulic oil. The piston moves vertically along the stem in-between the top and bottom chambers. When there is no hydraulic fluid pressure, the spring force holds the valve in the closed position. As fluid enters the lower chamber, pressure in the chamber increases and moves the bottom of the piston up the stem and against the force caused by the spring. As the piston moves upwards and compresses the spring, the valve opens. The valve is closed again when the hydraulic fluid is drained from the lower chamber. Hydraulic valve actuators can be used for throttling by adjusting the amount of fluid in the lower chamber.
Hydraulic valve actuator. Image Credit: Engineers Edge
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.
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.
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.
Electrohydraulic 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.||
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.||
Video Credit: 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: Micrometicllc / CC BY-SA 4.0
Important specifications for electrohydraulic valve actuators and hydraulic valve actuators include:
Actuation time -- The time it takes to fully close the linear motion valve.
Hydraulic fluid supply pressure range -- The input pressure necessary to achieve desired torque or thrust output.
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).
Valve stem stroke length -- Stroke is a term used to define the travel required by the valve from fully open to fully close. 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.
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- The 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.
Features for electrohydraulic valve actuators and hydraulic 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.
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 allows 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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