With the continuous advancement of automation in industries such as petrochemicals, steel, paper, food, and pharmaceuticals, the demand for skilled field instrument maintenance personnel has significantly increased. To reduce downtime, ensure safe operations, and enhance overall profitability, this article shares practical on-site maintenance experiences that can serve as a reference for technicians working with industrial instruments.
First, understanding the basic steps to analyze field instrument system failures is crucial. Field instrument measurements typically involve four main parameters: temperature, pressure, flow, and level. Each parameter presents unique challenges when troubleshooting.
Before diving into fault analysis, it's essential to have a solid understanding of the production process, the conditions under which the instruments operate, and the design intent behind the system. Familiarity with the structure, performance, and operational requirements of the instruments helps in identifying potential issues more effectively.
It's also important to review the load and raw material parameters from the production operators and examine the historical data recorded by the faulty instruments. This allows for a comprehensive assessment of whether the issue lies within the instrument itself or in the broader process.
If an instrument records a straight line (no change) or a previously fluctuating curve suddenly becomes static, it's likely that there’s an issue with the instrumentation system. Modern DCS systems are highly sensitive, so any lack of response to manual adjustments strongly suggests a problem in the system.
Similarly, if the recorded curve shows sudden jumps to maximum or minimum values, it may point to a fault in the instrument system. However, if the curve behaves erratically after a process change, the issue might be related to the process rather than the instrument.
Checking the on-site indicator against the DCS display can also help identify discrepancies. If the values differ significantly, it's a strong indication of an instrument failure.
In summary, when diagnosing field instrument failures, special attention should be given to changes in the controlled object and control valve behavior, as these can often be the root cause. A thorough evaluation of both the instrument system and the process operation is necessary to determine the true source of the problem.
Next, let’s explore the specific steps for analyzing failures in the four key measurement parameters:
1. **Temperature Control System Failure Analysis**
When dealing with temperature control issues, two factors are critical: the use of electrical instruments for measurement and the inherent lag in temperature readings. Sudden extreme values usually indicate a fault, such as a broken thermocouple, damaged transmitter, or faulty amplifier. Rapid oscillations often point to improper PID settings, while slow fluctuations may suggest either process changes or internal instrument faults.
2. **Pressure Control System Failure Analysis**
Sudden pressure fluctuations may stem from process changes or incorrect regulator settings. If the pressure reading remains constant despite process variations, the issue is likely in the pressure measurement system. Checking for blockages in the pressure lines or leaks in the transmitter can help pinpoint the problem.
3. **Flow Control System Failure Analysis**
When flow readings drop to a minimum, the first step is to check the on-site detection equipment. If the sensor is functioning normally, the issue could be with the valve or process conditions such as clogged pipelines or pump failure. Conversely, if the flow reading is at its maximum, checking the valve operation and signal transmission system can help identify the cause.
4. **Level Control System Failure Analysis**
Sudden changes in level readings may indicate either a process issue or a malfunction in the measuring system. Comparing the differential pressure level gauge with a direct-reading instrument can help determine if the problem lies in the calibration or the system integrity.
In real-world scenarios, many control loops are complex, involving cascade, split, program, and interlock controls. These require more in-depth analysis and a systematic approach to diagnose and resolve the underlying issues effectively.
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