Automated instrumentation systems play a crucial role in modern chemical industry production. They act as the eyes and ears for field operators, guiding them on when to add or remove materials. Therefore, the stability of automated instrumentation systems is paramount. This also demands high levels of troubleshooting skills from instrument maintenance personnel. This document provides a brief explanation of how to maintain and troubleshoot primary instruments in the field.
Due to the characteristics of petrochemical production operations, such as pipeline-based, process-oriented, and fully enclosed processes, especially the high level of automation in modern chemical enterprises, process operations are closely related to detection instruments. Process personnel use various process parameters displayed by detection instruments, such as temperature, material flow rate, container pressure and liquid level, and raw material composition, to determine whether the production process is normal and whether the product quality is qualified. Based on the instrument indications, they can increase or decrease production, or even shut down the plant.
Abnormal instrument readings (indicating readings too high, too low, unchanging, unstable, etc.) inherently involve two factors: first, process factors, where the instrument accurately reflects abnormal process conditions; and second, instrument factors, where a malfunction in a component of the instrument (measuring environment) causes the indicated process parameters to deviate from reality. These two factors are often intertwined, making it difficult to immediately pinpoint the exact location of the fault. To improve their fault diagnosis capabilities, instrument maintenance personnel need not only familiarity with the instrument's principles, structure, and performance characteristics, but also with every component of the measurement system. Furthermore, an understanding of the process flow, the characteristics of the process media, and the properties of chemical equipment is essential. This knowledge broadens their perspectives and facilitates the analysis and diagnosis of fault phenomena.
Temperature detection fault diagnosis: Symptoms include abnormal temperature indication (too high or too low, slow change, or no change at all). This is explained using a resistance temperature detector (RTD) as the measuring element. First, understand the process conditions. Inquire with process personnel about the measured medium and the instrument's installation location (gas phase, liquid phase, or other process conditions). Since this is a fault occurring during normal production and not a newly installed RTD, reversed RTD polarity can be ruled out. After eliminating the above factors, proceed with the following troubleshooting steps as shown in the diagram.
Troubleshooting common faults in temperature instrumentation systems:
(1) Sudden increase in temperature: This fault is mostly caused by open circuit of the resistance temperature detector (thermocouple), loose wiring terminals, broken (compensation) wires, temperature failure, etc. At this time, it is necessary to know the location of the temperature and the wiring layout. By using the resistance (millivolt) range of a multimeter to measure several sets of data at different locations, the cause can be found quickly.
(2) Sudden temperature drop: This fault is mostly caused by short circuit of thermocouple or RTD, short circuit of wires, and temperature malfunction. We should start by checking weak points that are prone to failure, such as wiring ports and wire bends, and check them one by one. If the on-site temperature rises but the main control indication remains unchanged, it is mostly caused by the presence of a liquid with a low boiling point (water) at the measuring element.
(3) When the temperature fluctuates greatly or oscillates rapidly: At this time, the main thing to check is the process operation (check the control system involved in the adjustment).
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