API, ASME Standards & Protocols for Pressure Safety Relief Valves

The performance of a control valve is directly dependent on its Actuator, which is responsible for converting the electrical or pneumatic signals into mechanical motion that positions the valve and modulates the flow of the fluid. In this article, we will go into the various types of actuators used in control valves, their working principles, advantages, and applications.

What is an Actuator?

An actuator is a hardware device that converts energy (usually electrical, hydraulic, or pneumatic) into motion (mechanical). In control valves, actuators are responsible for opening, closing, or modulating the valve position to maintain desired process conditions. Actuators are classified based on their source of energy, mechanism of action, and functionality.

Why Are Actuators Important in Control Valves?

The importance of actuators in control valves can be summarized as follows:

• Precision Control: Actuators provide accurate and reliable control over the process fluids, ensuring that systems operate within required parameters.
• Automation: They enable automation by allowing remote or automatic control of valves, reducing the need for manual interventions.
• Safety: Actuators can enhance safety by ensuring that valves respond quickly and accurately to changes in process conditions, protecting against potential hazards.
• Efficiency: By optimizing flow and pressure, actuators help improve the overall efficiency of industrial systems.

Actuator Types

Three main types of actuators are available in the Industrial processes which are Pneumatic, Electric and Hydraulic Actuators. Following are the the main types:

1. Pneumatic Actuators

Working Principle: Pneumatic actuators use compressed air to create mechanical motion. When air pressure is applied to one side of a diaphragm or piston, it moves in the opposite direction, thereby adjusting the valve position.

Advantages:

• Quick response times, making them suitable for rapid control applications.
• High power-to-weight ratio, allowing for the movement of larger valves.
• Simplified integration with existing pneumatic systems.

Disadvantage: However, they require a constant supply of compressed air, which can be expensive, and their response time can be affected by changes in air pressure.

Applications: Pneumatic actuators are widely used in industries like oil and gas, chemical processing, and water treatment, where fast and reliable control is essential.

2. Electric Actuators

Working Principle: Electric actuators are powered by an electric motor (stepper or servo) and use a set of gears to convert the rotary motion of the motor into linear motion to operate the valve.

Advantages:

• They are commonly used in small control valves and are suitable for use in both on/off and modulating control applications.
• Easy integration with digital control systems and sophisticated automation.
• Their precise and accurate control, which makes them suitable for applications where fine control is required.
• They also have a fast response time and do not require a separate power source.
• Typically lower energy consumption compared to pneumatic systems.

Disadvantages: However, their initial cost can be higher than other types of actuators, and they may not be suitable for use in hazardous environments.

Applications: Electric actuators are commonly used in food and beverage, pharmaceuticals, and HVAC applications where precision is critical.

3. Hydraulic Actuators

Working Principle: Hydraulic actuators operate using pressurized hydraulic fluid. The pressure causes a piston inside the actuator to move, resulting in valve movement. Hydraulic actuators use hydraulic pressure instead of compressed air.

Advantages:

• Exceptional force generation, making them ideal for large valve applications.
• High reliability and ability to maintain position without continuous energy input.
• They are commonly used in larger control valves as they have a higher force output compared to pneumatic actuators.
• Smooth operation with minimal wear and tear.

Disadvantages: However, they are more complex and expensive than pneumatic actuators, require a separate power source, and can be prone to leakage issues.

Applications: Hydraulic actuators are often found in heavy industries like mining, marine applications, and power generation, where large forces are required.

From the above types, The following types branch out: 

* Electro-hydraulic Actuators

Electro-hydraulic actuators combine the features of both hydraulic and electric actuators. They use an electrical motor to drive a hydraulic pump, which in turn operates the valve.

The working principle of electro-hydraulic actuators involves the use of a servo valve to control the hydraulic pressure, thereby positioning the valve.

Advantages: Their ability to provide fast and accurate control, which makes them suitable for use in critical applications.

Disadvantages: However, they are more complicated and expensive compared to other types of actuators.

Applications: They are commonly used in applications where high force output and precise control are required.

* Spring-Return Actuators

Working Principle: These actuators feature a built-in spring mechanism that returns the valve to a predefined position (usually the fail-safe position) when not powered. They can be pneumatic or electric in operation.

Advantages:

• Enhanced safety features, as they revert to a safe position in case of power loss.
• Simple design with fewer components, reducing the risk of failure.
• Cost-effective for applications requiring fail-safe operations.

Applications: Common in fire safety systems and critical process facilities where maintaining safety is paramount. 

* Linear Actuators

Working Principle: Linear actuators provide straight-line motion and are typically powered by electric motors, hydraulic systems, or pneumatic systems. They are used to adjust the valve position in a linear manner.

Advantages:

• Directly converts power into linear motion, providing high accuracy and precision.
• Easy to control and integrate with computer systems.
• Versatile for a wide range of valve types and sizes.

Applications: Used in various industries, including automation, robotics, and manufacturing processes requiring precise movement.

* Rotary Actuators

Working Principle: Rotary actuators provide circular motion and are commonly used for valve types such as ball and butterfly valves. They can be powered by electric, hydraulic, or pneumatic energy.

Advantages:

• Excellent torque generation suitable for handling large valve sizes.
• Variable speed options allow for flexible control.
• The compact manufacturing makes the actuators suitable for the installations with space constraints.

Applications: Found in oil and gas, chemical sectors, and water treatment facilities where rotational movement is necessary for effective process control.

Choosing the Right Actuator for Control Valves

Selecting the appropriate actuator for a control valve is necessary for ensuring reliable system performance. The key factors which needs to be considered in the following points:

1. Type of Process: Analyze the process fluid’s properties, such as viscosity, temperature, and pressure, to determine the actuator type that can handle the specific conditions.
2. Response Time: Determine how quickly the actuator needs to respond to changes in process conditions. Pneumatic actuators are often favored for rapid-response applications.
3. Operating Environment: Evaluate the environmental conditions where the actuator will be installed. For example, electric actuators may not be suitable in hazardous locations without proper protection.
4. Control Requirements: Assess the level of control precision required. Electric actuators tend to provide superior control for applications needing tight tolerances.
5. Installation Footprint: Space limitations may dictate the choice of actuator type, with compact rotary actuators being suitable for confined areas.
6. Cost Considerations: Account for both initial investment and long-term operational costs, including maintenance and energy consumption.

Edit This Article