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"Automated instruments have functions such as measurement, display, recording, and alarming. However, in actual production processes, instrument faults inevitably occur due to factors such as aging or outdated process equipment, improper operation, or issues related to installation location or instrument selection. These faults can reduce measurement accuracy or cause failures, resulting in sudden changes in process parameters that are detrimental to production safety. Timely detection and resolution of instrument faults should be the primary concern.
Instrument faults typically manifest as abnormal indication values—for example, readings that are too high, too low, constant, or abnormally fluctuating. The causes of these faults fall into two categories: (1) abnormal conditions due to unreasonable process parameter settings, and (2) faults in the measurement stage leading to inaccurate instrument displays.
To ensure instruments operate normally, it is essential to correctly diagnose and analyze the causes of faults. This requires:
A sound understanding of the instrument’s measurement principles, selection criteria, and operational characteristics;
Familiarity with the process flow, properties of materials, and characteristics of equipment and containers;
Knowledge of the measurement system’s workflow.
During operation, automated instruments may experience various faults. To quickly address and restore normal operation—thereby reducing losses—it is necessary to master several basic diagnostic methods:
Visual Inspection:
Check the instrument’s appearance and cables for any damage; also verify that the buttons and knobs on the control cabinet are responsive and functional.
Internal Inspection:
Examine internal connectors, pins, and terminal blocks for signs of oxidation, corrosion, or poor contact; also ensure that built-in fuses, relays, and other components are functioning properly.
Measurement of Voltage and Current:
Use a multimeter to check that the voltage, current, and resistance of components are within normal ranges.
Temperature Instruments
Common temperature instruments include thermocouples and resistance temperature detectors (RTDs). For thermocouples, accurate measurement and signal transmission require dedicated compensating wires that correspond to the specific type of thermocouple thermometer. Mixing compensating wires or reversing polarity can cause measurement errors and result in an unstable process system.
RTDs are typically used in lower-temperature environments and may use 2-wire, 3-wire, or 4-wire configurations. During normal operation, if the temperature measurement is inaccurate or displays maximum or minimum values, first check whether the on-site sensor can detect a resistance or thermoelectric potential signal. Then, inspect the wiring for any short circuits, open circuits, or poor contacts.
Differential Pressure Transmitters with Dual Flange for Level Measurement
Dual-flange differential pressure transmitters are commonly used for level measurement. For a transmitter installed on a tank, if it displays normally during initial calibration and parameter setup but later shows significant measurement errors once the tank is in use—such as segmented fluctuations at the static liquid level—this can lead to process issues like tank overflow or running dry. The main causes include:
The capillary tube may rupture, causing silicone oil leakage.
Ambient temperature changes may cause the silicone oil in the capillary tube to expand or contract.
The medium may crystallize or be overly viscous.
Process factors may lead to significant changes in the density of the medium within the tank.
When analyzing such faults, first rule out blockage at the level flange, then focus on the level measurement itself and the properties of the medium, as well as the influence of environmental factors.
If segmented fluctuations in the level display are observed, it may be due to temperature-induced expansion and contraction of the silicone oil in the capillary tube; insulation measures can be used to maintain the capillary temperature within a specific range.
Sometimes, after prolonged operation, the measured level gradually deviates significantly from the local indication. This may be caused by sensor zero drift, which necessitates recalibrating the zero point.
For measurement errors caused by changes in medium density, recalculating and adjusting the transmitter’s range parameters may be required.
Control Valves
Common issues with control valves include failure to actuate, slow operation, excessive leakage, fluctuations, and significant vibration.
Valve Failure to Actuate:
First, check whether there is an air supply at the condensate drain of the filter-reducer valve. If there is no air, it could be because the air supply valve is closed or the air pressure is insufficient; blockages in the filter-reducer valve, filter, or air supply piping may also be responsible. If the air supply is fine, further investigation should include: checking whether the DCS is outputting commands; inspecting the valve’s diaphragm head for air leakage (which can lead to insufficient air pressure); and examining the valve stem for wear or deformation that might cause it to stick.
Slow Valve Operation:
This could be due to slight deformation or friction in the valve stem or core, which may require replacement; a ruptured diaphragm in the pneumatic actuator can cause air pressure imbalance, necessitating diaphragm replacement; or low air pressure may require increasing the air supply pressure.
Excessive Leakage:
Causes include severe internal leakage from a worn valve core (which may need replacement); an improperly sized valve stem that prevents full closure (requiring adjustment); a rusted or deformed spring in the diaphragm head (which may need to be replaced); damaged sealing elements in the valve body; or a large differential pressure across the valve. Additionally, foreign objects at the valve seat that prevent the valve core from closing fully must be removed.
Significant Valve Fluctuations:
These may result from improperly set PID control parameters. In this case, adjust the PID parameters. Also, if the connection between the positioner and the valve stem is not secure, secure it properly; if there is leakage in the positioner’s output tubing, fix the leak; and if the signal line shows intermittent contact, identify and rectify the issue.
Flow Meters Not Displaying
If a flow meter suddenly shows no flow during operation, first check whether the signal and power connections are secure—if not, rewire them. Also, inspect the display module’s pins for looseness and reinsert them if necessary. Next, check if the transformer or fuse is damaged and replace them if needed. Finally, verify whether the negative terminal of the signal cable is inadvertently connected to ground and correct any insulation issues.
Pressure Instruments
Pressure instruments are typically used to measure media in liquid, gas, or solid states. When pressure measurements become abnormal, diagnose based on the physical state of the measured medium.
If the pressure instrument shows fluctuations, first check whether the process parameters have changed, as such fluctuations are often due to adjustments in process parameters or abnormal PID settings.
If the pressure instrument displays a constant value despite changes in process parameters, it is usually due to a fault in the pressure measurement system. Check whether the impulse line is blocked—if it is, clear the blockage. If not, verify that the pressure instrument’s output module is functioning properly, and replace it if necessary.
If the pressure reading is too high or too low, it may be due to improper installation or nearby electromagnetic interference affecting measurement accuracy. In such cases, resolve the interference through proper isolation or choose a new tapping point away from the interference.
Instrument Signal Cable Faults
A common fault in automated instruments is transmission failure in signal cables, usually manifested as short circuits, open circuits, or poor insulation due to cable damage. Causes include:
Rough handling or dragging of cables during installation, which can damage the cable jacket; prolonged exposure to environmental factors can further degrade the insulation.
Poor cable routing design, where cables are placed too close to high-temperature equipment or piping, resulting in overheating or carbonization that causes short circuits or open circuits.
Ensuring the integrity of signal transmission is crucial. Therefore, during the initial construction phase, avoid rough handling of cables and carefully plan the routing. Where possible, install conduits and cable trays at a safe distance from high-temperature equipment or pipes. If installation near high-temperature equipment is unavoidable, position conduits and cable trays below or to the side of the equipment.
To ensure the normal operation of instruments, a comprehensive instrument maintenance and inspection system should be established and implemented. Daily inspections should be conducted to promptly detect any leaks or faults during operation. The instruments’ exteriors should be kept clean, and mechanical transmission parts should operate smoothly. If any parts become stiff or show signs of rust, they should be promptly cleaned, wiped, and lubricated. In environments with strong corrosive properties or high dust generation, components such as transmitters, valve seals, and connectors must be kept clean—this is fundamental for the proper functioning of the instruments."