In order to achieve the automation of industrial production, it is essential to monitor various parameters such as temperature, pressure, flow, and level throughout the production process. These measurements are carried out using specialized detection instruments. Any failure in these instruments can lead to significant disruptions and damage to the normal operation of industrial processes.
Therefore, industrial control personnel must have a thorough understanding of the physical structure, measurement principles, and performance specifications of these four key instruments. They should be capable of accurately diagnosing and addressing any instrument failures to ensure smooth and uninterrupted industrial operations.
First, let's take a look at the four major instruments used in industrial automation:
1. **Temperature Meter**
In petrochemical production, chemical reactions and changes occur under specific temperature and pressure conditions. To monitor temperature changes in real time and maintain precise control, a number of temperature meters are installed. The most common method for contact temperature measurement involves thermocouples and resistance temperature detectors (RTDs). Field temperature technology is used to achieve automated temperature control.
**Thermocouple and Thermal Resistance Identification**
The appearance of industrial thermocouples and RTD protective sleeves is often similar, especially for compact types like armored sensors. To identify them:
- If there is a nameplate, check the model. Thermocouples use standardized indexing numbers like S, B, E, K, R, J, and T. RTDs typically use designations such as Pt10, Pt100, Pt1000, Cu50, and Cu100.
- If no nameplate is present, examine the number of lead wires. Thermocouples usually have two wires, while RTDs may have three or four. For four-wire systems, measuring resistance between pairs can help determine whether it’s a dual thermocouple or a four-wire RTD.
2. **Pressure Gauge**
Pressure gauges come in various forms, including pressure sensors, transmitters, and specialized gauges. They are suitable for extreme environments such as high temperatures, corrosive conditions, and media that tend to crystallize or become granular.
3. **Level Meter**
Level meters are categorized based on their measurement methods: buoyancy type, direct reading, differential pressure, radar, and others. In petrochemical applications, radar level meters are preferred due to their high accuracy and adaptability to various materials.
4. **Flow Meter**
Flow meters operate on either volume or mass flow measurement principles. Common types include orifice plate differential pressure meters, electromagnetic meters, ultrasonic meters, and vortex meters. Most flow meters measure volume, but with modern intelligence, they can also detect mass flow by adjusting internal parameters.
**Instrument Failure Analysis**
A typical automatic control system consists of three parts: the sensor, transmitter, and display. Sensors detect analog signals, transmitters convert them into standard 4–20 mA signals, and displays provide visual readings.
Common instrument failures manifest as abnormal readings—such as high, low, stagnant, or fluctuating values. These issues can stem from either process abnormalities or faults within the measurement system.
To diagnose effectively, one must understand the instrument’s working principle, its structure, and the overall measurement workflow. A deep knowledge of the industrial process and equipment properties is also crucial.
**Common Failures of the Four Major Instruments**
1. **Flow Meter Issues**
- If the flow meter reads maximum, manually adjust the valve. If the flow decreases, it may be a process issue; if not, check the signal transmission or pressure system.
- Abnormal fluctuations may indicate PID parameter problems or a faulty meter.
- If the flow is minimal, first check the field instrument. If it reads low, inspect the valve opening. If the opening is normal, consider blockage, crystallization, or low pressure.
2. **Level Meter Problems**
- When the level reaches a maximum or minimum, check the field instrument. If it’s normal, switch to manual control and observe stability.
- For differential pressure level meters, mismatched readings may indicate leakage in the pressure guiding tube or incorrect negative migration settings.
- Fluctuations in small-volume equipment are more likely caused by the instrument, while in large-volume systems, process changes are more probable.
3. **Pressure Gauge Malfunctions**
- Abnormal pressure readings depend on the state of the measured medium (solid, liquid, or gas).
- Fluctuating readings often result from process changes or PID parameter issues.
- Stagnant readings suggest a problem in the pressure measurement system, such as clogged piping or faulty transmission devices.
4. **Temperature Instrument Failures**
- Sudden high or low readings usually point to an instrument fault, as such changes are rare in process systems.
- High-frequency fluctuations are often due to improper PID settings.
- Slow fluctuations may indicate process changes, or if those are ruled out, a faulty control system.
**Conclusion**
Industrial automation has significantly improved efficiency and safety in the manufacturing sector. It enhances the interaction between operators and control systems, enabling more accurate and reliable operations.
When dealing with instrument failures, a combination of theoretical knowledge and practical experience is essential. Quick diagnosis and timely troubleshooting are critical to maintaining continuous and safe industrial production.
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