【Daily Question 200090408】Case Studies on On-site Fault Handling of Electromagnetic Flow Meters
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Please share the failures encountered when using electromagnetic flowmeters in the field, as well as the corrective measures. Format: Installation site: Fault symptoms: Cause of the fault: Solution: Original content is encouraged; spamming is not allowed. Original posts earn 10 points, while spam posts result in a deduction of 10 points. This post was last edited by Mobei Yihai on 2009-4-7 21:59.]
To spark further discussion: Installation site: wastewater treatment system of a chemical plant. Problem observed: After installation, the electromagnetic flowmeter operated normally with high measurement accuracy. However, after some time of use, it was found that the meter’s display would sometimes return to zero, and there were also fluctuations in the displayed values. Cause of the failure: Upon on-site inspection, it was found that since the electromagnetic flowmeter is of a split-type design with the sensor installed in a shaft, heavy water accumulation accumulated in that shaft due to rain. The sensor being submerged for an extended period allowed moisture to enter the wiring box, which reduced the insulation resistance between the excitation coil and the ground, resulting in the flowmeter failing to function properly. Solution: Open the junction box at the sensor, use a hair dryer to dry out the moisture inside the junction box, so that the insulation resistance is greater than 20 MΩ. Seal the inlet of the junction box with silicone.
Fault symptom: No flow signal. Cause of fault: (1) Power supply issues such as the power not being connected； (2) System faults in the connection cables (excitation circuit, signal circuit) ； (3) Faults related to liquid flow conditions ； (4) Failures caused by damaged sensor components or deposits on the inner wall surface ； (5) Faults related to damage to the converter components. Troubleshooting methods: 1. Check for faults in the power supply. 2. Check for faults in the connection cable system. 3. Check the direction of liquid flow and whether the pipes are fully filled with liquid. 4. Check the integrity of the sensors and the condition of the inner walls of the measurement tubes. 5. Check for faults in the converter.
Fault symptom: Fluctuations in the output.
Causes of the fault: 1. Fluctuations (or pulsations) in the flow itself. 2. The pipes are not fully filled with liquid or the liquid contains bubbles. 3. External electromagnetic interference. 4. Verify the physical properties of the liquid. 5. Check the compatibility between the liquid and the electrode material.
Troubleshooting method: If there are fluctuations in the flow itself, then the fluctuations in the instrument’s output reflect these fluctuations accurately. The inspection method can be to ask the operators and process technicians on-site, or to check for sources of variation. There are usually three reasons for flow fluctuations (or pulsations) in piping systems: (1) The flow driving source upstream of the electromagnetic flowmeter is a reciprocating pump or diaphragm pump (often used for adding chemicals in industries such as fine chemicals, food, pharmaceuticals, and water purification), and the pulsation frequency of these pumps is typically ranging from a few times per minute to over a hundred times per minute ； (2) The flow characteristics and size of the control valve downstream of the instrument were not selected appropriately, resulting in hunting; this can be observed by checking whether the valve stem moves oscillatorily ； (3) Other perturbation sources that cause flow fluctuations, such as the presence of flow-blocking elements in the pipeline upstream of the electromagnetic flowmeter (such as fully open butterfly valves) that generate vortices (such as the vortex rows produced by the vortex generator in a vortex flowmeter), gaskets at the sensor inlet extending into the flow channel, or strip-shaped fragments of gaskets swinging within the liquid flow, and so on. On pipelines with pulsating flow sources, to reduce their impact on the measurements taken by flow meters, it is common practice to place the flow sensors away from these pulsating sources, utilizing the flow resistance in the pipeline to attenuate the pulsations ； Or, chamber buffers known as passive filters can be installed at appropriate locations along the pipeline to absorb pulsations. The pipe not being filled with liquid or the presence of bubbles in the liquid is mainly caused by poor design of the pipeline system, which results in the sensor’s measurement tube not being filled with liquid, or by improper installation of the sensor. Gases are present in liquids. The formation of bubble-like gases in liquids can occur through two pathways: absorption from the outside environment, and the transformation of dissolved gases (air) in the liquid into free bubbles. The liquid contains a small number of bubbles, and the diameter of these bubbles is much smaller than that of the electrodes; although this reduces the volume of the liquid to some extent, it does not cause fluctuations in the output of the electromagnetic flowmeter ； Larger bubbles, by brushing against the electrode, can cover the entire electrode, causing the flow signal circuit to become open momentarily; as a result, the output signal fluctuates more significantly. Electromagnetic flowmeters are susceptible to external interference due to the weak flow signal; the main sources of interference include stray currents in the pipes, static electricity, electromagnetic waves, and magnetic fields. When selecting electrode materials, the corrosion resistance of the electrodes against the liquid to be measured is taken into consideration first; however, an inappropriate selection that leads to surface effects on the electrodes can cause issues such as fluctuations in the output. Electrode surface effects include the formation of insulating layers such as passivation films or oxide films on the electrode surface, as well as polarization phenomena and electrochemistry. Fault symptom: Unstable zero point. Cause analysis: (1) The pipeline is not filled with liquid or the liquid contains bubbles ； (2) Subjectively believing that there is no flow in the pipeline fluid when in fact there is slight flow ； In fact, it’s not a fault of the electromagnetic flowmeter; rather, it’s a misunderstanding that it fails to accurately reflect the flow conditions ； (3) Imperfect grounding of the sensors, leading to interference from external factors such as stray currents; (4) Issues related to the liquid itself (such as uniformity of the liquid’s conductivity, electrode contamination, etc.) ； (5) Decrease in insulation of the signal circuit. Solution: There is no flow inside the flow sensor, yet a slight amount of flow actually exists. The main cause of this type of fault is the poor sealing performance of the shut-off valves in the pipeline; the slight leaks detected by the electromagnetic flowmeter are misinterpreted as variations or instability in the zero point. It is quite common to encounter situations where valves fail to seal properly due to long-term use or contamination by liquids, especially in the case of large valves. Another common reason is that the flow meter has several branch pipes in addition to the main pipe, and the valves for these branch pipes are forgotten or overlooked and left open. External disturbances such as stray currents in pipelines are mainly mitigated by the proper grounding of electromagnetic flowmeters; it is generally required that the grounding resistance be less than 1000, and the same grounding should not be shared with other motors and electrical devices. Sometimes, when the environmental conditions are favorable, an electromagnetic flowmeter can function properly even without being grounded. However, once those favorable conditions are no longer present, the instrument will malfunction, and attempting to diagnose the issue at that point will cause a lot of trouble. Changes or unevenness in liquid conductivity cause the zero point to shift when at rest, and cause fluctuations in the output when the liquid is in motion. Therefore, the flow meter should be placed far away from the point where the liquid is injected or downstream of the section of the pipeline where chemical reactions occur; it is best to install the flow sensor upstream of these locations. Inspect the insulation of signal lines; check the electrode contact resistance and electrode insulation resistance
Fault symptom: Fluctuations in the output.
Causes of the fault: 1. Fluctuations (or pulsations) in the flow itself. 2. The pipes are not fully filled with liquid or the liquid contains bubbles. 3. External electromagnetic interference. 4. Verify the physical properties of the liquid. 5. Check the compatibility between the liquid and the electrode material.
Troubleshooting method: If there are fluctuations in the flow itself, then the fluctuations in the instrument’s output reflect these fluctuations accurately. The inspection method can be to ask the operators and process technicians on-site, or to check for sources of variation. There are usually three reasons for flow fluctuations (or pulsations) in piping systems: (1) The flow driving source upstream of the electromagnetic flowmeter is a reciprocating pump or diaphragm pump (often used for adding chemicals in industries such as fine chemicals, food, pharmaceuticals, and water purification), and the pulsation frequency of these pumps is typically ranging from a few times per minute to over a hundred times per minute ； (2) The flow characteristics and size of the control valve downstream of the instrument were not selected appropriately, resulting in hunting; this can be observed by checking whether the valve stem moves oscillatorily ； (3) Other perturbation sources that cause flow fluctuations, such as the presence of flow-blocking elements in the pipeline upstream of the electromagnetic flowmeter (such as fully open butterfly valves) that generate vortices (such as the vortex rows produced by the vortex generator in a vortex flowmeter), gaskets at the sensor inlet extending into the flow channel, or strip-shaped fragments of gaskets swinging within the liquid flow, and so on. On pipelines with pulsating flow sources, to reduce their impact on the measurements taken by flow meters, it is common practice to place the flow sensors away from these pulsating sources, utilizing the flow resistance in the pipeline to attenuate the pulsations ； Or, chamber buffers known as passive filters can be installed at appropriate locations along the pipeline to absorb pulsations. The pipe not being filled with liquid or the presence of bubbles in the liquid is mainly caused by poor design of the pipeline system, which results in the sensor’s measurement tube not being filled with liquid, or by improper installation of the sensor. Gases are present in liquids. The formation of bubble-like gases in liquids can occur through two pathways: absorption from the outside environment, and the transformation of dissolved gases (air) in the liquid into free bubbles. The liquid contains a small number of bubbles, and the diameter of these bubbles is much smaller than that of the electrodes; although this reduces the volume of the liquid to some extent, it does not cause fluctuations in the output of the electromagnetic flowmeter ； Larger bubbles, by brushing against the electrode, can cover the entire electrode, causing the flow signal circuit to become open momentarily; as a result, the output signal fluctuates more significantly. Electromagnetic flowmeters are susceptible to external interference due to the weak flow signal; the main sources of interference include stray currents in the pipes, static electricity, electromagnetic waves, and magnetic fields. When selecting electrode materials, the corrosion resistance of the electrodes against the liquid to be measured is taken into consideration first; however, an inappropriate selection that leads to surface effects on the electrodes can cause issues such as fluctuations in the output. Electrode surface effects include the formation of insulating layers such as passivation films or oxide films on the electrode surface, as well as polarization phenomena and electrochemistry. Fault symptom: Unstable zero point. Cause analysis: (1) The pipeline is not filled with liquid or the liquid contains bubbles ； (2) Subjectively believing that there is no flow in the pipeline fluid when in fact there is slight flow ； In fact, it’s not a fault of the electromagnetic flowmeter; rather, it’s a misunderstanding that it fails to accurately reflect the flow conditions ； (3) Imperfect grounding of the sensors, leading to interference from external factors such as stray currents; (4) Issues related to the liquid itself (such as uniformity of the liquid’s conductivity, electrode contamination, etc.) ； (5) Decrease in insulation of the signal circuit. Solution: There is no flow inside the flow sensor, yet a slight amount of flow actually exists. The main cause of this type of fault is the poor sealing performance of the shut-off valves in the pipeline; the slight leaks detected by the electromagnetic flowmeter are misinterpreted as variations or instability in the zero point. It is quite common to encounter situations where valves fail to seal properly due to long-term use or contamination by liquids, especially in the case of large valves. Another common reason is that the flow meter has several branch pipes in addition to the main pipe, and the valves for these branch pipes are forgotten or overlooked and left open. External disturbances such as stray currents in pipelines are mainly mitigated by the proper grounding of electromagnetic flowmeters; it is generally required that the grounding resistance be less than 1000, and the same grounding should not be shared with other motors and electrical devices. Sometimes, when the environmental conditions are favorable, an electromagnetic flowmeter can function properly even without being grounded. However, once those favorable conditions are no longer present, the instrument will malfunction, and attempting to diagnose the issue at that point will cause a lot of trouble. Changes or unevenness in liquid conductivity cause the zero point to shift when at rest, and cause fluctuations in the output when the liquid is in motion. Therefore, the flow meter should be placed far away from the point where the liquid is injected or downstream of the section of the pipeline where chemical reactions occur; it is best to install the flow sensor upstream of these locations. Inspect the insulation of signal lines; check the electrode contact resistance and electrode insulation resistance
Installation site: Flow measurement of the finished acid. Fault symptom: Unstable flow measurement. Cause of the fault: The flow meter was installed behind the control valve; frequent opening and closing of the valve caused bubbles to form in the flowing medium, thereby affecting the measurement. Solution: After shutting down the system, move the flow meter to position in front of the valve.
I encountered it in the 1990s; I was quite inexperienced at that time. Installation site: desalination unit. Fault symptoms: flow rate could not be measured; sometimes there was a flow rate, but it was unstable and very low. Cause of the fault: The secondary deionized water is too pure and does not conduct electricity, so the electromagnetic flowmeter is unable to make a measurement. Solution: Replace it with an electromagnetic flowmeter or a time-of-flight ultrasonic flowmeter that is suitable for this medium.
Faults encountered when electromagnetic flowmeters are used in the field and corresponding solutions. Format: Installation site: Oil gathering and transportation station. Fault phenomenon: A large discrepancy exists between the measurement values within the gathering and transportation station and those at the receiving combined station. Cause of the fault: The oil coming from individual wells contains many impurities, which lead to the formation of deposits on the inner walls, thereby causing measurement errors. Solution: Install additional treatment devices
Faults encountered when electromagnetic flowmeters are used in the field and corresponding solutions. (E+H split-type electromagnetic flowmeter) Format: Installation location: Circulation water pipeline. Fault symptom: The displayed value is much higher than the actual flow rate. Cause of the fault: The serial numbers of the split-type meter head and the sensor do not match (the meter head was installed incorrectly). Solution: Locate and replace with a meter head having the same serial number