Seven-step method for troubleshooting instruments

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When an instrument malfunctions, to quickly identify the cause of the problem, it is necessary to understand the production process flow, as well as the structure, characteristics, performance, and parameters of the instrument control system. On this basis, the following principles should be followed when troubleshooting instrument failures. 1. Ask about the process first before checking the instruments. Since it is the operators who use the instruments directly, it is necessary to ask them in detail about the situation before and after the instrument malfunctioned, to find out whether production in the preceding and subsequent stages was normal, and whether there were any adjustments or changes in the process parameters, so as to determine whether the issue lies with the process or the instruments. While asking the operators questions, it is necessary to observe the changes in the instrument readings. If the recording curve of the instrument was normal before a failure occurred, but then became highly erratic, making it difficult to control the system and even difficult to perform manual operations, this situation may be due to process issues or problems with the equipment. If the flow and level control systems experience large fluctuations and no issues are found upon checking the transmitters, manual operation can be attempted to see if the flow or level can stabilize. If the fluctuations remain severe, it may be due to process-related factors. If the pressure fluctuations are significant, the causes can be sought in the process itself; factors such as changes in load, addition or removal of materials, opening and closing of reflux valves, and improper operation can all lead to changes in the pressure inside the equipment. 2. First go to the control room and then to the site. When an instrument malfunctions, based on inquiries with the operators, one can observe the display of the instrument to roughly determine where the problem lies. A digital multimeter can be used to measure the wiring terminals specifically; by performing these measurements, it is possible to determine the condition of the wiring from the measuring components to the terminals in the control room, such as whether there are any open circuits, short circuits, or grounding faults. Check whether the thermocouple generates a thermoelectric potential, or use needle-nose pliers to short-circuit the input terminal of the display instrument to see if the instrument can indicate the room temperature. In the case of a thermal resistor, check whether there is any change in its resistance value ； Use needle-nose pliers to short-circuit the input terminal of the display gauge to see if it reads zero, or disconnect the terminals to see if it reads maximum or shows an overflow. For such transmitters, check whether there is current output; using a HART communicator makes it easier to diagnose faults. For the control system, it is possible to check whether the hand and automatic switch are in the correct positions; if necessary, the operator can be switched to manual mode to observe whether the actuator operates properly and whether there are any valve position feedback signals. When interference is suspected, measure whether there is an interference voltage and its magnitude. 3. Start with the simpler cases before dealing with the more complex ones. First, determine whether it is just one instrument that is not functioning properly or multiple instruments. Then check whether the power supply for the instruments and the power supply unit are working correctly, as well as whether any fuses have blown ； Check the relevant wiring to see if there are any poor contacts or short circuits, and verify that the switch is in the correct position. Check the pressure guide tube and valves for any signs of leakage. For instruments used to measure micro-pressure, it is possible to check whether the rubber or plastic tube connected to the pressure guide tube has come loose or is leaking. If pressure and flow meters show no fluctuations or changes, it is often due to a clogged pressure guide tube. 4. First check the primary instruments, then the secondary ones. Based on observations in the control room, if there is suspicion that the primary instruments on-site are faulty, those instruments can be inspected – checking whether the terminals of thermocouples and thermal resistors are loose, whether there is water intrusion, or if they are damaged. It is also necessary to check whether the actuators are stuck or lack lubrication, so that any issues can be addressed accordingly. When pressure and differential pressure transmitters are not functioning properly, it is necessary to first drain the fluid and clean the pressure guiding tubes; at the same time, check whether there is any blockage or leakage in the three-valve assembly and other valves. For the flow transmitter, close the positive and negative pressure valves of the three-valve assembly, and open the balance valve to check whether the transmitter’s zero point is normal ； Then, with the meter in operation, quickly turn on and off the positive tube drain valve to see if the output current increases; similarly, quickly turn on and off the negative tube drain valve to check whether the output current decreases. If the current changes as expected, then there is no major issue with the transmitter. For the level transmitter, close the positive and negative pressure valves of your three-valve manifold, and open the balance valve to check whether the output current of the transmitter is normal. This current is related to drift; in the case of negative drift, the output current should be 20 mA when the differential pressure is zero, while in the case of positive drift, the output current should be less than 4 mA when the differential pressure is zero. For transmitters with negative migration, with the meter in the on state, quickly open and close the drain valve for the positive line to observe whether the output current decreases; then quickly open and close the drain valve for the negative line to check if the output current increases. If the current changes as expected, then there is no major issue with the transmitter. During the above checks, the discharge valve should be closed as soon as a change in the output current is observed; this way, too much condensate will not be discharged. There are patterns to follow when diagnosing faults in secondary instruments. For example, if there is a large delay in temperature readings, it is impossible for the instrument’s display value to change suddenly; if the display does change abruptly to the maximum or minimum value, then, after ruling out problems with the primary components, it is usually an issue with the display unit itself. If the pressure indication shows no fluctuations or changes only slowly, then, apart from the possibility of blockages in the pressure conduit and valves, it is likely a problem with the display instrument. If the flow meter does not show fluctuations and the readings remain approximately linear, this may indicate a malfunction in the instrument, as fluctuations in flow parameters should be fairly significant; any changes in these parameters should be reflected in the meter’s readings. When there are doubts about the display parameters of the DCS or recorder, one can check the conventional backup instruments, or other instruments on site such as the indicator gauges of transmitters, to see how large the difference in their displays is, thereby identifying the fault. To check whether the controller is functioning properly, one can manually change the setpoint to create a new deviation, and then observe the changes in the current output by the controller in order to determine if it is working correctly. 5. First check the outside before examining the interior of the instrument. Following a step-by-step approach starting with the easier tasks, first check the exterior of the instrument, such as whether the power supply is functioning properly ； Check for any leakage points in the pressure guide tube, and observe whether the waste discharge is smooth, in order to determine whether there is any blockage in the pressure guide tube or valves ； Check the wire connections and terminals for issues such as looseness, rust, or poor contact. For thermocouples, shorting can be used, while for thermal resistors, opening the terminals can be employed to determine the location of the fault. Measure the voltage at the instrument panel terminal block or the instrument wiring terminals to diagnose the fault. On this basis, it is then determined whether the instrument needs to be removed for processing. 6. Check the visible areas first before looking at the hidden ones. When dealing with instrument faults at the site, first inspect the terminals and wiring inside the instrument panel; if no problems are found but there is still suspicion that the wires are faulty, then check the wires inside the cable trays and in the trenches. Those with sewage pipes leading into the gutter are also checked last. When suspecting that the protective sleeve of a thermocouple or thermal resistor is damaged, it should also be removed for inspection only after confirming that there are no issues in other areas. 7. First check the software parameters, and then examine the hardware. With the widespread use of smart meters, when dealing with faults in field instruments, the approach used cannot be limited to the methods originally employed for analog instruments. The functionality of smart meter hardware depends on software support; without it, such meters cannot operate. Therefore, when checking and servicing smart meters, it is first necessary to verify whether the meter’s settings are correct. The author has encountered situations where the meter stopped working after a power outage caused by lightning at night; once power was restored, the meter would not function properly, only to work again after some of its setting parameters were adjusted. I once encountered a situation where, after replacing the network card in an industrial computer, it became unable to connect to the internet. I tried installing drivers and replacing the network card, spending several hours with no success. Eventually, I discovered that a setting in the computer’s CMOS had changed; resetting it resolved the issue.