How to Select Optimal Accessories for Your Control Valves

 
When selecting common accessories for control valves, it's crucial to understand the function, application scenarios, and impact of each accessory on the overall control system. Proper accessory configuration not only enhances system precision and stability but also ensures the efficient operation of control valves under complex conditions. Below is a detailed analysis of common control valve accessories and recommendations for their selection.

Valve Positioner

 
The valve positioner is a core accessory for pneumatic actuators, used to improve valve positioning accuracy. Its primary functions include overcoming stem friction and unbalanced forces from the medium, ensuring the valve accurately positions itself according to the regulator's commands.
 
High Pressure Differential Scenarios: In conditions where the medium pressure is high or there is a significant pressure differential, the valve positioner ensures precise valve positioning, preventing deviations.
Large-Diameter Control Valves (DN>100): As the diameter of the control valve increases, stem friction and unbalanced forces also increase. The positioner provides sufficient thrust to address these challenges.
Extreme Temperature Conditions: In high or low-temperature environments, the valve's response may be affected by temperature changes. The positioner ensures accurate valve movement by stabilizing the control signal.
Fast Response Requirements: In scenarios requiring rapid response, the positioner effectively reduces action delay, improving system response speed and ensuring timely process control.
Non-Standard Actuators: When using actuators with non-standard spring ranges (e.g., outside 20–100 kPa), the valve positioner provides necessary signal matching, ensuring normal system operation.
Special Function Requirements: The positioner is also required for specific functions, such as split-range control, reverse action (switching between air-to-open and air-to-close), and changing valve flow characteristics by adjusting the positioner cam.

Solenoid Valve

 
The solenoid valve is a key component for achieving program control and two-position control, widely used in various automation control systems.
 
Power Requirements: Choose a solenoid valve that matches the system's power type (AC or DC), voltage, and frequency.
Function Type Matching: The solenoid valve's function type (e.g., "normally open" or "normally closed") should match the control requirements of the control valve to ensure stable system operation.
Capacity Enhancement Needs: To shorten action time or enhance capacity, two solenoid valves can be used in parallel, or the solenoid valve can act as a pilot valve in combination with a high-capacity pneumatic relay, achieving faster response speed and greater control capability.

Limit Switch

 
A limit switch detects and indicates the two extreme open and closed positions of the control valve.
 
Switch State Monitoring: The limit switch sends a signal to the control room when the valve is fully open or closed, helping operators confirm the valve’s actual state.
Safety Measures Execution: Based on the feedback from the limit switch, the control room can take necessary actions to ensure the system operates safely and prevent accidents due to abnormal valve positioning.

Pneumatic Relay

 
A pneumatic relay is a power amplification device that transmits pneumatic signals over longer distances, eliminating delay effects caused by extended signal lines.
 
Long-Distance Signal Transmission: Between field transmitters and the central control room, or between regulators and field control valves, the pneumatic relay ensures the accuracy and timeliness of signal transmission.
Signal Amplification and Attenuation: When it's necessary to adjust the strength of pneumatic signals, the pneumatic relay can effectively amplify or reduce the signal to meet the needs of different operating conditions.

Converter

 
Converters, including I/P (current to pneumatic) and P/I (pneumatic to current) converters, are used to convert between pneumatic and electrical signals.
 
Electrical Signal Control of Pneumatic Actuators: When the system needs to control pneumatic actuators via electrical signals, converters can translate 0–10 mA or 4–20 mA electrical signals into 0–100 kPa pneumatic signals, or vice versa, ensuring correct transmission of control signals.
Adaptation to Different Signal Formats: Converters can accommodate different signal formats, ensuring signal compatibility between various devices in complex control systems.

Air Filter Regulator

 
The air filter regulator plays an important role in industrial automation systems, primarily filtering and purifying compressed air from air compressors and stabilizing the pressure at a preset value. Its application scenarios include.
 
Pneumatic Instrument Air Supply: Provides a stable and clean air source for pneumatic instruments, solenoid valves, cylinders, and other equipment, ensuring their normal operation.
Small Pneumatic Tools: Supplies air and maintains pressure for small pneumatic tools, extending the equipment’s service life and improving work efficiency.

Lock-Up Valve

 
The lock-up valve is a device used to maintain the valve position, particularly critical in case of an air supply failure.
 
Air Supply Failure Protection: If the air supply to a pneumatic control valve fails, the lock-up valve can cut off the air signal and maintain the pressure within the diaphragm or cylinder at the pre-failure state, preventing unintended valve movement.
System Stability Maintenance: The lock-up valve ensures that the control valve remains in its preset position during an air supply interruption, preventing system instability.

Valve Position Transmitter

 
A valve position transmitter is used for remote monitoring of the control valve’s open or closed position, especially crucial when the control valve is located far from the control room.
 
Real-Time Valve Position Monitoring: The valve position transmitter converts the mechanical displacement of the valve opening into an electrical signal and transmits it to the control room, allowing operators to monitor the valve’s operating status in real-time.
Reverse Action Signal: Some transmitters also provide reverse action signals from the valve positioner, offering more comprehensive system feedback.

Other Accessories

 
In specific scenarios, other accessories like position feedback devices, handwheel operating mechanisms, and limiters can be selected to enhance the system's operational flexibility and safety.
 
Choosing the right accessories for control valves requires careful consideration of system application needs, operating conditions, and requirements for control precision and response speed. Properly configuring these accessories can not only improve the overall performance of the control valve but also ensure stable operation of the system under various complex conditions. For any industrial control system, selecting the appropriate control valve accessories is key to ensuring efficient, safe, and reliable system operation.