Electric Valve Actuators Information
Electric valve actuators mount on valves which, in response to a signal, automatically move to a desired position using an outside power source. Single-phase or three-phase AC or DC motors drive a combination of gears to generate the desired torque level. Electric valve actuators are the most popular actuator used due to their high level of precision and their fast response time.
Electric valve actuation system in action. Video Credit: videoproektant / CC BY-SA 4.0
Types of Electric Valve Actuators
There are two basic types of electric valve actuators: rotary and linear. Each type of actuator uses special valves.
Rotary electric valve actuators are used with ball, plug, and butterfly valves that rotate a quarter-turn or more from open to close. Rotary electric valve actuators are used in the electric power industry, high-power switching gears, and packaging applications.
Linear electric valve actuators are used with gate, globe, diaphragm, pinch, and angle valves that feature a sliding stem that opens or closes the valve. Linear electric valve actuators are well-suited for operating in tight tolerances.
More information on rotary and linear motion valves can be found on Engineering360's Valve Types page.
Image Credit: Wikipedia
Rotary Electric Valve Actuators
Electric rotary valve actuators drive components rotationally via electromagnetic power from a motor. They often provide control and indexing capabilities to allow multiple position stops along strokes. Rotary electric valve actuators include an electrical enclosure, electric motor, reduction gearing, drive coupling between the final drive gear and the valve stem, and travel limiting devices. The rotational element can be either a circular shaft or a table. Circular shafts often include keyways while tables provide a bolt pattern for mounting other components.
Specifications for rotary electric valve actuators include:
Actuator torque- Actuator torque, the force that rotates the axis, is determined by multiplying the applied force by the distance from the pivot point to the point where the force is applied.
Range of motion- The full range of motion can be 90° (quarter-turn), 180° (nominal), 270° (nominal), or 360° (multi-turn).
Linear Electric Valve Actuators
Linear electric valve actuators provide linear motion via a motor-driven ball screw or ACME screw assembly. ACME screws typically hold loads without power but are usually less efficient than ball screws. Ball screws are power screws with a train of ball bearings riding between the screw and the nut in a re-circulating track. They exhibit lower friction and higher efficiency than lead screws. With linear electric valve actuators, the load is attached to the end of a screw or rod and is unsupported. Typically, the screw is belt or gear driven. The linear electric actuator will include the electrical enclosure, electrical motor, reduction gearing, valve stem drive nut/bushing, and travel limiting devices.
Specifications for linear electric valve actuators include:
- Valve stem stroke length- Stroke length is measured in inches (in). The term stroke is used to define the travel required to have the valve fully opened to fully closed. 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.
Actuating force or seating thrust- Actuator force is measured in pounds (lbs). The actuator must supply enough force to overcome the pressure in the system to close the closing element and keep it closed.
Number of turns- Number of turns performed by a multi-turn actuator, as the rotating valve stem moves from the fully closed to fully open valve position.
General specifications for both rotary and linear electric valve actuators include:
Control signal input- Electric valve actuators that use throttling valves receive control signal inputs from positions that adjust the valve’s closure position. Input control signals are measured in either milliamperes or volts.
Stem diameter- The valve stem diameter can be combined with the lead and pitch of the valve stem thread in order to size the automation required for the valve. It can also be used with the valve size and the pressure drop across the valve to calculate torque demand.
Actuation time- Actuation time is the time required to fully close the liner motion valve.
Failsafe method- Failsafe methods involve the actuators turning off or on during the event of a power outage or loss of control signal. The actuator will either opens or closes depending on the requirement of the system.
AC or DC voltage- AC or DC voltage requirements will depend on the voltage available in the system. The actuator should be able to provide its rated torque within tolerances. The tolerance
Duty cycle- Duty cycle is a measurement of how long an actuator can actually work before it needs to rest. Electric motors typically generate a lot of heat, which requires occasional idling time. If the valve is going to be used in isolating applications (occasional use) then an intermittent rating, 20% duty cycle with 60 starts per hour, should be adequate. If the valve is going to be used for control applications, the actuator should be continuously rated.
More information on the specifications for rotary and linear motion valves can be found on Engineering360's How to Select Valve Actuators page.
Features for electric valve actuators include:
- NEMA enclosures- Electric valve actuators are often housed in enclosures that are rated by the National Electrical Manufacturers Association (NEMA), a trade organization which defines safety standards for electrical equipment. There are four standard choices.
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.
Overtorque protection-Electric valve actuators that feature overtorque protection use a sensor to switch off the motor when a specified torque level is exceeded.
Travel stops- Travel stops restrict the actuator’s linear or rotary motion.
Limit switches- The actuator has an electromechanical limit switch (contacts) or a non-contact proximity sensor that allows the position / status of the actuator to be monitored from a remote location using proper connections. Usually the signal (or lack of a signal) generates some action in the process.
Local position indicators- 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.
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.
Thermostats and thermal overload protection- Thermal overload protection is used to protect an electric motor if the motor is doing more work than it is designed to handle. The overload protection will adjust the motor speed if it gets too hot from use.
Nesbitt, Brian. Valves Manual International: Handbook of Valves and Actuators. Oxford: Butterworth-Heinemann, 2007. Print.
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