
Positioners
Positioners in Valve Control Systems
What is a Positioner?
A positioner is a device used to control the position of a valve or actuator based on a control signal. It ensures that the valve or actuator achieves and maintains the desired position, improving process control accuracy and efficiency in automated systems, such as those found in process industries.
How does a Positioner work?
A positioner receives a control signal (usually 4-20 mA or a digital signal) from a process controller or controller system. It compares this signal to the current position of the valve and adjusts the actuator accordingly, ensuring the valve moves to the correct position to control the flow, pressure, or level.
What is the purpose of a Positioner?
The primary purpose of a positioner is to precisely position the valve or actuator in order to control the flow, pressure, temperature, or other process variables within a specific range. It provides accurate and responsive control for automatic valves in industries such as oil and gas, chemicals, water treatment, and power generation.
Types of Positioners
Different Types of Positioners
Positioners come in two main types:
- Pneumatic Positioners: These use pneumatic signals (air pressure) to control the actuator. They are commonly used for traditional pneumatic valve control systems.
- Electro-pneumatic Positioners: These positioners receive electrical input signals (e.g., 4-20 mA) and convert them into pneumatic output signals to drive the actuator.
- Digital (Smart) Positioners: These are advanced positioners that communicate digitally with control systems and offer features like diagnostics, self-calibration, and remote adjustment via fieldbus protocols (e.g., HART, Profibus).
Difference between Pneumatic and Electro-pneumatic Positioners
- Pneumatic Positioner: Operates entirely on pneumatic pressure and is used in systems where the control signal is pneumatic.
- Electro-pneumatic Positioner: Receives an electrical control signal (4-20 mA) and converts it into a pneumatic output to position the actuator. These positioners are used in modern digital control systems and provide better precision and flexibility.
What is a Smart Positioner?
A smart positioner is a digital device that integrates with modern control systems to provide advanced features, including self-diagnostics, error detection, automatic calibration, and communication with process control systems using digital protocols such as HART or Profibus.
Installation and Setup
How to Install a Positioner
To install a positioner:
- Mounting: Attach the positioner to the actuator or valve as per the manufacturer’s instructions, ensuring proper alignment.
- Wiring: Connect the control signal (electrical or pneumatic) to the input of the positioner. Ensure proper wiring for electrical positioners (e.g., 4-20 mA).
- Air Supply (for Pneumatic Positioners): Connect the appropriate air supply to the positioner, ensuring the air is clean and dry.
- Calibration: After installation, calibrate the positioner to ensure it responds correctly to control signals and accurately positions the actuator.
- Testing: Test the positioner by sending control signals and verifying that the actuator moves smoothly to the correct position.
How to Calibrate a Positioner
To calibrate a positioner:
- Set the zero and span: Adjust the positioner’s zero and span settings to match the required range of valve positions.
- Input control signal: Apply the control signal (4-20 mA or other) and observe the movement of the actuator to ensure it responds correctly.
- Fine-tuning: Adjust the calibration for fine-tuning, ensuring that the valve reaches the required position at each signal level.
- Test the system: Verify that the valve consistently moves to the desired positions across the entire signal range.
Maintenance and Troubleshooting
How to Maintain a Positioner
To maintain a positioner:
- Regular Inspection: Check the positioner for any signs of wear, leaks, or damage.
- Cleanliness: Ensure that the positioner and actuator are kept clean and free from contaminants, especially if it is a pneumatic positioner.
- Calibration: Perform periodic recalibration to ensure the positioner is providing accurate control.
- Air Supply Maintenance (for Pneumatic Positioners): Maintain the air supply by using filters and regulators to prevent contaminants and moisture from affecting the positioner’s operation.
- Replace Worn Parts: Replace seals, gaskets, or any other worn parts as necessary to keep the positioner functioning efficiently.
What to Do if the Positioner is Not Responding Correctly
If the positioner is not responding correctly:
- Check the control signal: Ensure the control signal (e.g., 4-20 mA) is within the correct range and properly connected.
- Inspect for leaks: Check for air leaks in pneumatic positioners, or wiring issues in electro-pneumatic positioners.
- Verify Calibration: Ensure that the positioner is correctly calibrated and adjust it if necessary.
- Test the actuator: Ensure the actuator itself is functioning properly, as issues with the actuator could affect the positioner’s performance.
Performance and Operation
How Accurate is a Positioner?
Positioners are generally highly accurate, providing precise control of valve positions within a small margin of error. Smart positioners, in particular, offer exceptional accuracy and are often used in high-precision applications.
Typical Pressure Range for Pneumatic Positioners
Pneumatic positioners typically operate with an air supply pressure range of 20-100 psi (1.4-6.9 bar). Ensure that the pressure falls within the manufacturer’s specifications for proper operation.
How to Know if the Positioner is Working Correctly
To verify the positioner’s performance, you should:
- Monitor the response to control signals.
- Check the actuator movement to ensure it aligns with the expected position.
- Use diagnostics (available in smart positioners) to detect faults, deviations, or errors.
- Perform a test by sending signals and observing if the valve reaches the desired positions without delays or issues.
Safety and Operating Tips
Are Positioners Safe to Use?
Yes, positioners are designed with safety in mind, especially in automated systems. They ensure that valves and actuators move precisely to the correct positions, preventing over-travel, under-travel, or system malfunctions. Always follow safety protocols during installation and maintenance.
Can Positioners be Used in Hazardous Environments?
Yes, positioners can be used in hazardous environments, but it is essential to select a positioner rated for such conditions. For environments with explosive or dangerous gases, you should use explosion-proof positioners that meet relevant safety certifications (e.g., ATEX, IECEx).
How to Ensure Proper Sealing in a Positioner
To ensure proper sealing in a positioner:
- Inspect seals and gaskets regularly to check for wear or damage.
- Replace any worn or damaged seals immediately.
- Ensure the positioner is mounted securely to avoid any movement that could cause leaks.
- For pneumatic positioners, ensure the air supply is clean and dry to avoid clogging or wear on seals.
Selection and Purchasing
How to Choose the Right Positioner for Your System
When selecting a positioner, consider the following factors:
- Control type: Choose between pneumatic, electro-pneumatic, or smart positioners based on your control system (analog vs. digital).
- Application: Match the positioner to the type of actuator and valve being used (e.g., rotary or linear actuators).
- Environmental conditions: Select a positioner designed for the operating environment, including temperature, humidity, and potential exposure to hazardous conditions.
- Features: Consider features like diagnostic capabilities, ease of calibration, and compatibility with existing control systems.
Can I Buy Replacement Parts for Positioners?
Yes, replacement parts such as actuators, seals, sensors, and internal components are available from the manufacturer or authorized distributors. Always ensure that replacement parts are compatible with your specific model and make of positioner.