Stroke Adjustment of Pneumatic Control Valves

Pneumatic control valves are vital components in modern industrial automation systems. They accurately regulate the flow and pressure of fluid media based on control signals. However, as they age, these control valves may experience issues with stroke precision, leading to significant errors when attempting to open to a specific position. Therefore, adjusting the stroke of pneumatic control valves is essential for maintaining system stability and efficiency.

Calibration Process

The calibration of pneumatic control valves involves several key steps that must be performed carefully to ensure optimal valve performance.

1. Zero Point Adjustment

Setting the zero point is the first step in calibrating the control valve. The process is as follows:
Signal Input: A control room or signal generator should provide a 4mA signal, ensuring the control valve is completely closed at this point.
Valve Inspection: If the valve is not fully closed, zero point adjustment is necessary. The goal is to ensure that the valve closes completely when it receives the lowest signal.
Calibration Process: Adjust the zero point knob on the positioner until the valve stem is fully closed. This step may need to be repeated to achieve an accurate closed state.

2. Full Stroke Adjustment

Full stroke adjustment ensures that the pneumatic control valve performs stably when fully open:
Adjustment Goal: This process is similar to the zero point adjustment, but the objective is to ensure the valve stem is fully open.
Signal Input: Gradually increase the control signal to its maximum while observing whether the valve fully opens and making necessary adjustments based on feedback.

3. Intermediate Point Adjustment

After completing the zero point and full stroke adjustments, it's important to adjust the intermediate position to ensure accurate flow control under various working conditions:
Selection of Points: Choose intermediate points, such as 50% opening, for adjustment.
Span Adjustment: Record the valve stem's response at these points, comparing it with the target opening. Adjust the positioner’s span setting as needed to ensure the feedback rod remains vertical with the valve stem.

Sensitivity and Accuracy Adjustment

To enhance the control precision of pneumatic control valves, adjusting sensitivity and dead time is equally important.

1. Sensitivity Adjustment

Sensitivity adjustment aims to improve the valve's response to control signals:
Signal Input: Input a small control signal to the positioner and observe the valve stem's movement.
Adjust Sensitivity: Use the sensitivity knob on the positioner to ensure the valve stem responds sensitively to small signals, thus improving system response speed.

2. Hysteresis Adjustment

Hysteresis adjustment reduces the difference in the valve’s position during the forward and reverse strokes under the same input signal, improving control accuracy:
Observe Differences: Input a control signal and record the positional difference between the forward and reverse strokes.
Adjust Hysteresis: Use the hysteresis knob on the positioner to gradually reduce the difference, ensuring the valve returns accurately to the specified position under the same conditions.

3. Dead Time Adjustment

The purpose of dead time adjustment is to minimize the lag between the output signal of the pneumatic control valve and the input signal, enhancing control stability:
Signal Input: Input a step signal to the positioner and observe the valve stem’s movement.
Record Output Curve: Calculate the dead time, then adjust the dead time knob on the positioner to reduce it, improving the system's real-time response capability.

Adjustment Methods

Stroke adjustment methods for pneumatic valves primarily include manual and automatic adjustment methods.

1. Manual Adjustment

Manual adjustment is the most common method for adjusting the stroke of pneumatic valves:
Operation Method: Adjust the valve's opening screws or handwheels manually. This straightforward approach is suitable for initial testing and temporary adjustments in emergencies.
Limitations: While easy to operate, this method relies on human intervention and generally offers lower precision.

2. Automatic Adjustment

Automatic adjustment connects pneumatic actuators with control systems to achieve precise stroke adjustments:
Control Signals: Utilize control signal size and frequency to accurately control the valve opening, facilitating precise fluid media control.
Intelligent Adjustment: This method allows for real-time feedback and adjustments, significantly enhancing accuracy and efficiency.

Safety and Stability

Ensuring the system's safety during pneumatic valve stroke adjustments is crucial.

Safety Precautions: Before adjusting, check the system's status to prevent safety hazards caused by incorrect operations, especially in high-pressure or high-temperature environments.
Precision Equipment: Use accurate measuring instruments and tools during the adjustment process, following technical requirements strictly to ensure stroke precision.
Additionally, the stability of the valve stroke directly impacts the control accuracy of the system:
Attention to Detail: During adjustments, monitor the valve's stress, sealing performance, and component wear to ensure stable operation under various conditions.

Conclusion

Adjusting the stroke of pneumatic control valves is key to ensuring the stability and efficiency of industrial automation control systems. Through systematic calibration, sensitivity and accuracy adjustments, and adherence to strict safety measures, precise control over fluid media can be achieved, enhancing production efficiency and safety. Mastering the adjustment methods for pneumatic control valves will help optimize industrial processes, reduce operating costs, and create greater value for businesses.