Application of Control Valves in Industrial Automation

Control valves are essential components in industrial automation process control systems. They precisely adjust parameters like flow, pressure, temperature, and liquid levels based on signals from the control unit. As final control elements, control valves ensure the stability, reliability, and safety of production processes. Their applications span various fields, including chemicals, oil, pharmaceuticals, power, and food industries.

Basic Components of Control Valves

Control valves mainly consist of two parts: the actuator and the valve itself. The types of actuators include pneumatic, electric, and hydraulic, each with its own working principles and suitable applications.

Pneumatic Actuators: Use compressed air as the power source, offering quick response times, ideal for processes that require frequent starts and stops.
Electric Actuators: Driven by motors, suitable for precise control and complex automation systems, allowing for remote operation.
Hydraulic Actuators: Use hydraulic oil as power, suitable for high-load and high-pressure environments, providing significant output force.
The design and material selection for the valve directly impact its performance and durability, with different media and working conditions requiring various valve structures.

Considerations for Selecting Control Valves

When choosing the right control valve, multiple factors must be considered to ensure it meets specific process requirements.

1. Medium Characteristics

Flow Characteristics and Unbalanced Force: The shape and structure of the valve core should be designed based on flow characteristics (e.g., linear, equal percentage, or quick opening) to match the process requirements.
Corrosion Resistance: For corrosive media, it is advisable to use corrosion-resistant materials (e.g., stainless steel, PTFE) to minimize wear and extend valve life.

2. Temperature and Pressure

High-Temperature and High-Pressure Conditions: Materials for the valve core and seat must possess good resistance to heat and pressure, such as alloys or high-temperature plastics, to ensure stable operation under extreme conditions.

3. Wear Resistance

Fluid Medium Properties: For media containing high concentrations of abrasive particles, the valve's flow path should be smooth, and materials with high hardness (e.g., tungsten carbide, ceramics) should be used to

4. Preventing Flashing and Cavitation

Flashing and cavitation can cause flow fluctuations, vibrations, and noise, which may reduce the valve's lifespan. When selecting a valve, measures should be taken to prevent these phenomena by choosing appropriate valve types and flow adjustment methods.

Selection and Coordination of Actuators

The normal operation of control valves relies on sufficient output force from the actuator to overcome various resistances during valve opening, ensuring a tight seal. Different types of actuators have unique characteristics.

Double-Acting Actuators: Commonly found in pneumatic, hydraulic, and electric actuators, these do not use return springs, and the output force is independent of the direction of operation.
Single-Acting Actuators: In pneumatic actuators, output force is closely related to valve opening. It’s essential to maintain force balance throughout the entire opening range for stable operation.

Modes of Operation for Control Valves

The mode of operation is crucial when selecting pneumatic actuators, combining actuator and valve actions. Common combinations include:

Direct Action (Normally Closed): The actuator closes the valve, suitable for safety applications.
Reverse Action (Normally Open): The actuator opens the valve, ideal for startup processes.
Automatic Opening: The valve opens automatically when the actuator releases, suitable for specific process needs.
Automatic Closing: The valve closes automatically when the actuator releases, ensuring system safety.
When choosing the mode of operation, consider three key factors:
Safety in Production: Ensure reliability under extreme conditions to minimize accident risks.
Medium Characteristics: Choose suitable valves based on the physical and chemical properties of the media to avoid reactions or damage.
Product Quality and Economic Loss: Optimize the balance between cost and quality to reduce economic losses.

Flow Characteristics and Size Selection

The flow characteristic of a control valve refers to the relationship between the relative flow rate of the media and the valve opening. Ideal flow characteristics include:

Linear Characteristics: Suitable for applications requiring precise flow control.
Equal Percentage Characteristics: Ideal for processes with large flow variation.
Quick Opening Characteristics: Suitable for fast start-stop applications, such as two-position regulation and program control.
Choosing the right flow characteristics directly affects the system's control accuracy and response speed, which must be thoroughly considered in the design.
The size of the control valve is based on its flow capacity (Cv). During engineering design and selection, Cv calculations should be performed, providing a detailed design specification to ensure the valve’s effectiveness and reliability under specific conditions.

Maintenance and Management of Control Valves

Regular maintenance and management are crucial for the reliable long-term operation of control valves. Common maintenance measures include:
Regular Inspections: Check the sealing performance, operational flexibility, and wear of the valve.
Cleaning and Care: Regularly clean the interior and exterior of the valve to prevent blockages or damage.
Timely Replacement: Replace damaged or severely worn components promptly to ensure normal valve operation.

Conclusion

Control valves play an indispensable role in industrial automation. By comprehensively considering medium characteristics, temperature and pressure, wear resistance, and actuators, it is possible to select suitable control valves that meet precise control needs for parameters like flow and pressure. This enhances process stability and brings greater economic benefits to enterprises. A robust maintenance management system ensures the long-term reliability and performance stability of control valves, supporting sustainable development for businesses.