Choosing Electric or Pneumatic Valve Actuators

Introduction

Valve actuators can be categorized into manual and automatic types. Manual operation can use handwheels, handles, or gear drives, while automatic operation includes pneumatic valves, electric valves, hydraulic, and their combinations. With the development of industrial production and increased automation, valves requiring centralized control, remote control, and program control, those in hazardous areas inaccessible to humans, those needing rapid action, and those needing synchronized operation with other devices all necessitate electric, hydraulic, or pneumatic actuators. Statistics show that a refinery processing 5 million tons annually uses up to 1500 electric actuators.

Classification of Valve Actuators

1. By Power Source Type:
   • Manual: Operated by human force.
   • Pneumatic: Uses compressed air, including pneumatic ball valves, pneumatic gate valves, pneumatic globe valves, and pneumatic butterfly valves.
   • Hydraulic: Driven by oil or water pressure.
   • Electric: Powered by electromagnets or motors, including electric ball valves, electric gate valves, electric globe valves, and electric butterfly valves.
   • Recent advancements include gas-liquid linkage, electro-hydraulic linkage, and self-actuating emergency shutoff devices.
2. By Stem Movement Form:
   • Multi-turn Actuators: The output shaft rotates more than one full turn, suitable for gate, globe, throttle, and diaphragm valves.
   • Part-turn Actuators: The output shaft rotates less than one full turn, used for ball, butterfly, and plug valves.
   • The primary connection standards are ISO 5210 and ISO 5211, suitable for multi-turn and part-turn valve actuator attachments, respectively.
   
   Note:
   1. ISO 5210 standard
   2. ISO 5211 standard

Comparison of Valve Actuator Features

1. Manual Actuators:
   • Basic and limited by human strength.
   • Small valves can be operated directly; larger valves may require a gearbox for easier operation.
   • Handwheels are typically installed on globe and gate valves, while handles are used for butterfly valve ball valves, and plug valves.

2. Electric Actuators:
   • Advantages: High stability, constant thrust, and high control precision. The largest actuators can generate up to 225,000 kgf of thrust.
   • Can switch between direct and reverse action easily with a servo amplifier, and maintain position in case of a fault.
   • Types include part-turn and multi-turn electric actuators, used for valves like electric ball valves, electric butterfly valves, and electric gate valves.

3. Pneumatic Actuators:
   • Advantages: Fast action, reliable, and easy control. Typically operate with a 0.4-0.7 MPa air source, with special units up to 5-10 MPa.
   • Operate in temperatures from -15 to 80°C, some up to -60 to 120°C. Use compressed air or nitrogen as the power source.
   • Common in remote and automated valve controls.

4. Hydraulic Actuators:
   • Components: Power source, control section, and actuator.
   • Advantages: Simple, compact, large output force, stable, and reliable. Allows for easy speed adjustment and remote control.
   • Disadvantages: Viscosity changes with temperature, potential for leaks, complex piping, and maintenance. Not suitable for signal processing, and not recommended for flammable or explosive environments.
5. Gas-Liquid Linkage Actuators:
   • Often used in gas pipelines and areas without power sources, utilizing the gas in the pipeline as the power source.
   • Advantages: Compact, stable transmission, large output torque, adjustable torque, and suitable for various automatic control methods.
   • Disadvantages: Expensive and complex structure.

Conclusion

Valve actuators play a critical role in modern industrial operations, providing necessary control and automation. They are classified by power source and stem movement, each with unique advantages and drawbacks. Understanding these distinctions is vital for selecting the right actuator for specific industrial applications.

FAQs

1. What is the main advantage of electric actuators over pneumatic ones?
   • Electric actuators offer higher control precision and maintain their position in case of a fault, unlike pneumatic actuators that require a backup system.
2. Why are hydraulic actuators not recommended for flammable environments?
   • Hydraulic actuators use oil, which can pose a risk in flammable environments due to potential leaks and oil's flammable nature.
3. How does a gas-liquid linkage actuator work without an external power source?
   • It uses the gas present in the pipeline as its power source, making it ideal for remote locations without an external power supply.
4. What is the typical operating pressure range for pneumatic actuators?
   • Pneumatic actuators usually operate within 0.4-0.7 MPa, with special versions handling pressures up to 5-10 MPa.
5. What standards are used for valve actuator connections?
   • The primary standards are ISO 5210 for multi-turn actuators and ISO 5211 for part-turn actuators, ensuring proper connection dimensions for different valve types.

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