Ultimate Guide to Electric Valve Actuator: Definition, Working Principles, Types and Components

Electric valve actuators are devices powered by electricity to move or control a valve.

Unlike pneumatic actuators, which depend on compressed air (and thus, are susceptible to pressure fluctuations), electric actuators rely on electricity, offering more consistent and precise control.

This makes them ideal for applications requiring accurate positioning or where small flow changes matter (like dosing in chemical processes).

1. How Electric Valve Actuators Work

If you’ve never heard of actuators and are wondering, how do actuators work, they simply are devices that convert energy into mechanical motion.

Electric valve actuators, specifically, convert electrical energy to initiate movement and position the valve. 

The process starts with an input signal, which can either be digital (on/off) or analog (e.g. a range like 4-20mA). This signal tells the actuator how much to open or close the valve

Once the signal is received, an electric motor inside the actuator powers up, generating motion.

Now, this motion is typically fast but not powerful enough to directly move large, heavy industrial valves. The gear mechanism absorbs and distributes the mechanical dress generated, slowing down the motor speed and increasing the torque.

Finally, the motion is applied to the valve stem, which either rotates or moves linearly to adjust the valve’s position:

1. Rotary motion: Involves turning the valve stem, usually by  90° or 180°, and is commonly used with ball, butterfly, and plug valves.
2. Linear motion: Involves moving the valve stem in a straight line, which is the case for gate or globe valves that require up-and-down motion to regulate flow.

Note that advanced electric actuators can sometimes combine these motions or convert rotary motion to linear motion using special gear systems.

2. Types of Electric Valve Actuators

Linear Electric Actuators

These actuators produce a straight-line motion (up-and-down or in-and-out). But, how does a linear actuator work?

This motion is typically produced by a lead screw or ball screw mechanism, where the motor turns the screw, moving the stem up or down. Of course, the thrust force (i.e. the amount of linear force) needs to be high enough to move the valve’s stem effectively without overloading the actuator.

As discussed, a linear valve actuator is ideal for valves that require linear movement, such as globe valves and those heavy gate valves.

They work best in the following areas:

1. Precise Flow Control: Because of their ability to move in small, incremental steps, linear actuators offer fine control over the valve’s position.
2. Heavy-Duty Applications: They’re perfect for larger valves that require significant force to open and close (like globe valves for throttling and regulating flow).
3. High-Accuracy Industrial Applications: They’re prevalent in settings such as water treatment plants or chemical processing.

However, they may not be suitable where you’re concerned of the following:

1. Speed: Linear actuators tend to operate slower compared to rotary actuators and may not be ideal if your system requires quick valve cycling.
2. Size and Space Requirements: For very large valves, the actuator needs to be significantly robust, which can take up more space and can be heavier.

Rotary Valve Actuators

As its name suggests, a rotary valve actuator produces rotational motion that turns the valve stem (typically by a specific angle), either opening or closing the valve.

Instead of the thrust force linear actuators rely on, rotary actuators require torque output. This simply refers to the rotational force the actuator can exert to turn the valve stem.

So where are they best suited?

1. High-Speed Systems: Rotary actuators are generally faster than linear actuators in terms of opening/closing the valves, which is perfect in oil and gas pipelines.
2. Compact Design: These actuators are usually more compact and lightweight, which is advantageous in tight spaces where larger actuators would be impractical.
3. General Purpose Applications: They are widely used where only a limited range of motion (usually 90°) is required, typically applications involving ball valves or butterfly valves.

Circling back to “turning by specific angles”, rotary actuators are further divided into two categories: multi-turn actuators and quarter-turn actuators.

Multi-Turn Actuators

A multi-turn electric actuator is designed to operate valves that require more than one full rotation to move from fully closed to fully open. That said, it’s commonly used with gate valves and globe valves.

Key advantages of multi-turn actuators include the following:

1. High-Torque Capacity: They can generate a huge amount of force to turn larger, heavier valve stems.
2. Suited for Thick Fluids and High-Pressure Pipelines: When the valve stem becomes difficult to rotate due to high pressure or viscous media, these actuators excel in maintaining reliable operation.
3. Well-Suited for Modulating Duty Operations: They allow precise incremental adjustments to valve positions, ensuring accurate flow rate or pressure control without the risk of overshooting.

However, they also have their share of drawbacks, including:

1. Multi-turn actuators are typically larger and heavier than quarter-turn actuators, which can be disadvantageous in space-constrained environments.
2. Due to their design, multi-turns aren’t the fastest. They generally operate at a slower pace, making them less suitable for time-sensitive operations or emergency systems.

Quarter-Turn Actuators

Unlike multi-turn actuators that allow continuous rotation, quarter-turn actuators are mechanically limited to 90°, which allows for fast valve actuation suited for applications that involve ball valves, butterfly valves, and plug valves.

Apart from quick operation, other advantages of quarter-turn actuators include:

1. Compact Design: Their smaller size compared to multi-turn or linear actuators makes them easier to integrate into confined areas.
2. Cost-Effectiveness: They’re generally more affordable due to their straightforward design and limited motion range.
3. Maintenance-free: With fewer moving parts and self-lubricating mechanisms in place, they can perform at peak over long periods without frequent servicing.

However, you shouldn’t use them for throttling applications (they’re poor with fine adjustments), or where you’re dealing with large-sized valves that accommodate high pressures or large flow volumes.

3. Components of Electric Valve Actuators

Electric Motor

Electric motors are the core components of the actuator as they generate the mechanical force (either rotational or linear motion) needed to move the valves. They can be one of the following types:

1. AC motor: Suited for heavy-duty applications requiring steady performance.
2. DC motor: Prevalent in compact or low-power applications
3. Servo motor: Used in advanced actuators

The squirrel cage motor, a type of AC motor, is commonly used in industrial settings because of its durability and minimal maintenance needs. It consists of a rotor shaped like a squirrel cage, which makes it simple and effective for continuous operation.

For hazardous environments where there’s a risk of explosion, explosion proof motors are used.

The motor’s power rating (typically expressed in HP or kW) determines the actuator’s ability to handle different valve sizes and heavy load.

Gearbox

The gearbox transfers the motion from the motor to the valve, converting high-speed, low-torque motor output into low-speed, high-torque motion required to operate the valve.

The most common of the types is the worm gearbox, which has a screw-like structure, with a spiral thread that meshes with a toothed wheel (the gear). It is self-locking, which means that when the actuator motor is turned off, the valve remains in place without the need for additional locking mechanisms.

Another type is the bevel gearbox, which is conical in shape and with teeth that are cut at an angle to allow the gears to mesh at a 90-degree angle. These gearboxes are generally bulkier than worm gearboxes but offer a more efficient power transfer for certain applications.

Lastly, planetary gearboxes consist of a central “sun” gear, which meshes with multiple “planet” gears that orbit around it, with a surrounding “ring” gear. The best thing about them? They’re compact but can still produce a high torque output (i.e. high torque-to-size ratio).

Feedback Mechanism

The feedback mechanism, often utilizing sensors, keeps track of the valve’s position as it moves. It compares the actuator’s position with the control signal sent to it and provides real-time updates back to the control unit to ensure the valve is in the correct position.

Limit switches play a crucial role here by providing a physical signal when the actuator reaches its fully open or closed position.

The feedback can come in different forms – either analog (e.g. using potentiometers) or digital (e.g. using encoders), depending on the complexity and precision required.

In some advanced systems, the feedback mechanism may include self-calibration features, where the system can automatically adjust and align itself, minimizing manual intervention.

Torque and Thrust Sensors

These sensors track the rotational force (torque) or linear force (thrust) applied to the valve, adjusting the actuator’s movement to prevent overloading or damaging the valve or actuator components.

Additionally, the duty cycle is an important factor when monitoring the continuous operation of these sensors. It refers to how long an actuator can run at full load before requiring a cooling-down period.

Control Unit

The actuator control system or unit processes the input signal and directs the actuator motor accordingly to move the valve to the desired position. Simply said, it acts as the brain of the actuator.

For simpler systems, the control unit may be designed to work with basic switches for simple on/off commands to the actuator.

In more sophisticated systems, the control unit can connect with Programmable Logic Controllers (PLCs). PLCs allow you to control several actuators at once, handle more complex tasks, and connect the actuator system to other parts of the plant for smooth overall control.

With a PLC, you can also access smart features like performance monitoring, remote troubleshooting, and scheduled maintenance, as well as integration into IoT ecosystems. 

However, local control remains valuable as it allows operators to manually adjust actuator settings directly on-site.

Manual Override Mechanism

This is a backup control method that allows you to manually handle the actuator in case of a malfunction (i.e. when bad actuator symptoms are present) or when automated control is unavailable. As a result, operations can continue even if the automated system fails.

Fail Safe Mechanism

This is a safety feature designed to automatically bring the system to a safe state in case of a failure, such as a power loss or component malfunction. 

When the power supply fails, battery backup systems provide power to the actuator, while spring return mechanisms automatically move the valve to a default or safe position, such as fully open or fully closed.

Conclusion

By now, you can see how electric valve actuators are non-negotiables for accurate valve, monitoring, and most importantly, ensuring safety for both operations and everyone in the work environment.
If you want to learn more about electric actuators and how they can fit into your current project, feel free to call us and speak with one of our experts.

Reference

Electric Actuator

Electric Valve Actuator – How They Work