The valve positioner is a key accessory of the regulating valve, which is used to improve the accuracy and response speed of valve control. It receives the control signal (such as 4-20mA or pneumatic signal) and ensures that the valve opening is consistent with the set value through the feedback system.
Which valves need a positioner?
The positioner is usually used for the following valves or working conditions:
1. High-precision regulation system Such as key control loops in chemical, petroleum, power and other industries, requiring the valve opening to accurately match the control signal.
2. High pressure difference (ΔP >1MPa) valves High pressure difference will cause the valve core to be affected by unbalanced force, and the positioner can enhance the actuator thrust and reduce errors.
3. High temperature, high pressure, low temperature or hazardous medium valves Such as LNG ultra-low temperature valves and high temperature steam valves, the positioner can overcome the packing friction and medium resistance.
4. Large diameter valves (DN>100mm) Require greater thrust, and the positioner can enhance the actuator output.
5. Long-distance signal transmission (>60m) Reduce signal lag and improve valve response speed.
6. Split-Range Control One controller controls two valves, responding to high and low signal ranges respectively.
7. Viscous or solid particle-containing media Prevent media from blocking the valve stem and ensure stable adjustment.
Classification of positioners
1. Classification by input signal
|
|
Input signal | Output signal |
|
| Pneumatic positioner | 20-100kPa gas signal | Gas signal |
Mechanical structure, explosion-proof, but requires additional electrical converter |
| Electro-pneumatic positioner |
|
Gas signal | Integrated electrical conversion, the most widely used |
| Smart locator |
|
Gas signal | With CPU, supports self-diagnosis, PID adjustment, and remote monitoring |
2. Classification by action direction
Unidirectional positioner: only one-way drive (such as spring return actuator).Bidirectional positioner: double-acting cylinder, bidirectional drive.
3. Classification by function
Ordinary positioner: only basic positioning function.Intelligent positioner (such as FISHER FIELDVUE, Siemens SIPART PS2): Support HART/FF communication Self-tuning, fault diagnosis Programmable flow characteristics (linear/equal percentage).
4. According to feedback detection method
Mechanical linkage type (traditional, easy to wear) Hall effect type (non-contact, high precision) Electromagnetic induction type (anti-vibration, suitable for harsh environments).
The role of the positioner
1. Improve control accuracy Ordinary valve error may reach 20%, and the intelligent positioner can be controlled within 0.5%.
2. Enhance thrust Overcome the influence of high pressure difference and high friction.
3. Accelerate response speed Reduce signal lag and improve regulation efficiency.
4. Improve flow characteristics Programmable curve (such as linear → equal percentage).
5. Self-diagnosis and remote monitoring Intelligent positioners can detect valve stem wear, air source failure, etc.
Working principle of positioner
1. Signal input: Receive 4-20mA or digital signal from DCS/PLC.
2. Deviation detection: Compare the set signal with the valve stem feedback position.
3. Pneumatic output: Output air pressure to drive the actuator through I/P converter (intelligent type) or nozzle flapper (traditional type).
4. Feedback adjustment: The valve stem displacement is fed back through mechanical/non-contact sensors to form a closed-loop control. Additional functions of intelligent positioner: Automatic adjustment (such as ABB TZID-C can complete calibration within 1 minute). Fault self-protection (three-break protection: lock the valve position when gas/power/signal are cut off).
In summary
Valve positioners are key components of industrial automation control, suitable for complex working conditions such as high precision, high pressure difference, and hazardous media. Intelligent positioners are becoming a trend, with advantages such as self-diagnosis and remote control, which can greatly improve system reliability and maintenance efficiency.
