Differential Pressure Transmitter: Fault Analysis and Working Principle
With the continuous development of the economy and the improvement of industrial automation, differential pressure transmitters have become widely used in industries such as steel manufacturing, food processing, chemical production, and papermaking. These devices play a crucial role in flow measurement due to their accuracy and reliability. In daily applications, there are several common methods for measuring flow using differential pressure transmitters:
1. Utilizing the pressure difference caused by the weight of the liquid itself to determine the height of the liquid.
2. Combining with other components, such as orifice plates, to measure flow rate based on the pressure difference before and after the throttling element.
3. Directly measuring the pressure difference between different pipes or tanks.
As the application technology continues to evolve, it's essential that maintenance personnel are well-trained to handle any issues that may arise. If problems are not resolved promptly, they can disrupt production schedules and affect the normal operation of the instruments. This highlights the critical role of on-site technicians in ensuring the smooth functioning of these systems.
**Working Principle of Differential Pressure Transmitters**
The basic design principle of a differential pressure transmitter is based on the formula ΔP = pgΔh, where ΔP represents the pressure difference, p is the density of the fluid, g is the gravitational acceleration, and Δh is the height difference. When the tank is cylindrical, the cross-sectional area S remains constant, so the weight G = ΔP × S = pgΔh × S. Since S is constant, G is directly proportional to ΔP. Therefore, accurately detecting the ΔP value allows us to determine the liquid level.
When temperature changes, the volume of the fluid may expand or contract, but the actual liquid level will change accordingly. However, the pressure detected remains stable. To display the true liquid level, temperature compensation can be applied.
In terms of operation, the differential pressure transmitter divides a space into two chambers using sensitive components like bellows. When pressure is applied to both sides, the sensor generates displacement proportional to the pressure difference. This displacement is then converted into a standard output signal reflecting the magnitude of the differential pressure. The structure, chamber configuration, and conversion method vary depending on the model and application.
**Important Considerations During Use**
1. Avoid applying voltage higher than 36V to the transmitter, as this can cause damage.
2. Ensure the measured medium does not freeze, as this could damage the isolation diaphragm.
3. Never touch the diaphragm with hard objects, which may cause permanent damage.
4. For steam or high-temperature media, use a heat pipe to connect the transmitter to the pipe and prevent direct contact.
5. Do not exceed the maximum operating temperature specified for the transmitter.
6. Ensure all connections are leak-free, and open valves slowly to avoid damaging the diaphragm.
**Fault Diagnosis and Troubleshooting**
1. **Direct Inspection Method**: Many faults can be identified quickly through observation and experience. Maintenance personnel should check for physical damage, unusual smells, or signs of overheating.
2. **Gradual Inspection Method**: After identifying a fault, maintenance staff should systematically isolate potential causes. For example, if communication issues occur, checking the power supply and cable interference can help diagnose the problem.
3. **Replacement Method**: If a component is suspected of failure, replacing it with a known good one can help identify the issue.
**Common Faults and Case Studies**
One common issue is leakage in the pressure guiding tubes. For example, in a heating furnace control system, an orifice plate combined with a differential pressure transmitter was used to measure airflow. Over time, the flow reading dropped significantly.
Upon inspection, the secondary instrument and circuits were found to be in good condition, and the transmitter passed calibration. The maintenance team suspected a leak in the positive pressure tube. Upon further examination, a poorly sealed weld was discovered. After repairing the weld, the system returned to normal operation.
**Analysis of Positive Pressure Tube Leakage**
When the positive pressure tube leaks, the output of the differential pressure transmitter may fluctuate. For instance:
- If the static pressure in the pipeline increases and the positive pressure tube leaks, the transmitter output may remain unchanged.
- If the static pressure is greater than the pressure in the positive tube, the output may increase.
- If the pressure in the positive tube is higher, the output may decrease when the flow rate increases.
Similarly, when the flow rate decreases, the transmitter output tends to be lower than the actual flow, leading to inaccurate readings.
These examples highlight the importance of regular maintenance and proper troubleshooting techniques to ensure the reliable operation of differential pressure transmitters.
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