Fault Inspection and Analysis of Electromagnetic Flowmeters [Lecture 1] — Fault Inspection and Analysis of Electromagnetic Flowmeters (1)

　Section 4: Inspection of Output Fluctuations and Countermeasures 　　　　　I. Causes of Faults 　　　　　Output fluctuations can generally be attributed to 5 types of fault causes, which are: 　　　　　(1) The flow itself is fluctuating or pulsating; this is not actually a fault of the electromagnetic flowmeter, but rather a reflection of the actual flow conditions； 　　　(2) The pipe is not fully filled with liquid or the liquid contains bubbles ； 　　　(3) Electrical and magnetic interference from external stray currents, etc ； 　　　(4) Reasons related to the properties of the liquid (such as uneven liquid conductivity or slurries containing a large number of variable particles/fibers, etc.) ； 　　　 (5) The electrode material is not properly matched to the liquid. 　　II. Inspection Procedure The diagram in Figure 4 shows the process for checking the fluctuations in the output of electromagnetic flowmeters. First, conduct a preliminary investigation and assessment following the flowchart, and then carry out detailed checks item by item to troubleshoot the issues. The principle governing the sequence of checks listed in the process is as follows: ① Those that can be determined through observation or questioning without requiring extensive procedures come first; in other words, the easier tasks are handled first ； ②Based on past on-site maintenance experience, those that occur frequently and are likely to happen again in the future are listed first ； ③Check the sequential requirements of the process itself. If the causes of several faults are confirmed through a preliminary investigation, a detailed inspection can also be carried out in advance. 　　 　　 　　 　　 Figure 4: Inspection procedure for fluctuations in the output of electromagnetic flowmeters 　　　III. Fault diagnosis and corrective actions 　　　This section discusses, respectively, the methods for diagnosing the causes of faults in the above 5 aspects as well as the corrective actions to take. 　　 　　1. Fluctuations (or pulsations) in the flow itself 　　　 Check item 1 in the process diagram. If the flow itself fluctuates, the fluctuation in the instrument’s output accurately reflects that variation. 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 in industries such as fine chemicals, food, pharmaceuticals, and water purification for adding chemicals), 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 disturbance 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 readings of flow meters, it is common practice to place the flow sensor at a distance from the pulsating source, using the flow resistance in the pipeline to attenuate the pulsations ； Or, gas chamber buffers known as passive filters can be installed at appropriate locations in the pipeline to absorb pulsations. 　　 　　2. The pipeline is not filled with liquid or the liquid contains bubbles. Refer to item 2 of the inspection flowchart; such faults are mainly caused by poor design of the pipeline network, which results in the sensor’s measurement tube not being filled with liquid, or by improper installation of the sensor. Measures should be taken to avoid installation at positions a and e as shown in Figure 3, as well as at position b when using the pipe indicated by the dashed line, and to instead modify it to positions c and d. 　　 　　There is no backpressure or insufficient backpressure downstream of the sensor; if it is installed at position e, the liquid flows through a short section of pipe before being discharged into the atmosphere. If valve 2 is fully open, it is possible that the sensor’s measurement tube is not filled with liquid. Sometimes, when the flow rate in the process is high, the instrument can be filled to capacity and function properly; however, when the flow rate decreases, there may not be enough liquid, which can cause the instrument to malfunction. 　　 　　Gases are present in liquids. The formation of gas bubbles in liquids occurs 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. 　　 　　●In a fluid stream, tiny bubbles gradually accumulate at high points or dead corners during flow. If an electromagnetic flowmeter is installed at such high points in the piping system, the trapped gas reduces the area available for liquid flow within the sensor, thereby affecting measurement accuracy. When a large amount of gas is trapped, it can also generate interference signals (see Case 3) ； If the sensor is installed at a high point, gas accumulates there beyond its capacity, or due to pressure fluctuations, the gas flows with the liquid in bubble or sheet form, covering the electrodes and causing fluctuations in the output. 　　 　　●The common ways air enters from the outside in water supply systems are mainly bubbles present in river water as the raw material, or an excessively low water level at the intake point – it is generally required that there be a distance of 2 to 5 times the diameter of the intake opening, depending on the intake flow rate – which creates suction vortices that draw in air. In the field of process industries, air gets mixed in during the stirring in mixing vessels, as well as at points where there is poor sealing at the pump inlet or in other sections of the piping system. Such faults are also commonly encountered in practice. 　　 　　●When air is dissolved in a liquid, it separates into free bubbles; as the pressure in the pipeline decreases, the previously dissolved air (or gas) separates into free bubbles. For example, in a pipe system filled with liquid where the valves at both ends are closed, the system gradually cools down after operation stops. Due to the different coefficients of thermal expansion, the liquid contracts far more than the pipe system does, resulting in a contraction space within the pipe system that creates a local vacuum condition. Air dissolved in the liquid separates out to form bubbles, which accumulate at the highest points of the piping system. Upon restart, the flow of liquid containing bubbles past the electrode surface can cause fluctuations in the output of the electromagnetic flowmeter. This may be one of the reasons for the fluctuation in the output of the electromagnetic flowmeter at the beginning of the pipeline system’s operation, before it stabilizes. For example, at 1 atmosphere and 0°C, water can dissolve up to about 0.3% of VN air; as the water temperature rises during a process, the air separates into free bubbles (at 30°C, only about 0.15% can be dissolved). Faults may also occur when they accumulate. 　　 　　3. External electromagnetic interference 　　　 Refer to item 3 in the inspection flowchart. 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. 　　 　　●Pipeline stray current is mainly controlled through proper grounding of the electromagnetic flowmeter; the grounding resistance should typically be less than 100 ohms. It should not share the same grounding connection as other motors and electrical devices. Sometimes, under favorable environmental conditions, an electromagnetic flowmeter can function properly even without being grounded, but we still consider it advisable to ground it anyway. Because once the favorable environmental conditions are no longer present and the instruments malfunction, it will affect their use, and carrying out various inspections at that point will cause a lot of trouble. 　　 　　Sometimes, even though an electromagnetic flowmeter is properly grounded, strong stray currents in the pipeline (such as those in pipelines used in electrolytic processes or in systems with cathodic protection) can interfere with its proper functioning. In such cases, it is necessary to provide electrical insulation between the electromagnetic flowmeter sensor and the pipeline. For specific examples and their inspection and troubleshooting processes, refer to Case 12. 　　 　　●Electrostatic and electromagnetic interference can be introduced through the signal lines between the sensor and converter of the electromagnetic flowmeter; it can usually be prevented by proper shielding, such as using shielded cables for the signal lines and placing the cables inside protective metal tubes. However, there have been cases where strong electromagnetic waves could not be mitigated; in such situations, the converter was moved closer to the sensor, the signal cable connecting them was shortened, or an integrated instrument without an external cable was used. For specific details of the example, please refer to Case 10. 　　 　　●Magnetic field interference can usually be addressed only by placing the electromagnetic flowmeter away from strong magnetic field sources. The ability of an electromagnetic flowmeter to resist magnetic fields depends on the structural design of the sensor; for example, if the protective casing surrounding the sensor’s excitation coil is made of non-magnetic materials such as aluminum or plastic, its resistance to magnetic field effects is weaker, whereas a casing made of steel provides greater resistance. 　　 　　4. Verification of liquid properties 　　　 Check item 4 in the process flow diagram. There are 3 factors in liquid properties that can cause fluctuations in the output. They are: (1) the liquid contains solid particles or bubbles, (2) in a two-component liquid the conductivities of the two components differ and they have not been mixed evenly, or the chemical reaction in the pipeline has not yet been completed, (3) the conductivity of the liquid is close to its lower limit value. 　　 　　●If the liquid being tested contains a high amount of solid particles, it will cause spikes and pulse-like noise in the flow rate signal, just as bubbles do, leading to fluctuations in the output. If the solid phase is in powder form, it generally does not cause output fluctuation. 　　 　　●In the fine chemical industry, food industry, pharmaceutical industry, and water treatment projects, chemicals are often added to the main fluid, and these chemicals are typically injected in proportion to the flow rate of the main fluid by reciprocating pumps or diaphragm pumps. After the liquid medicine is injected, the flow shows segments separated by areas without the medicine; if two liquids with different conductivities are not mixed evenly, the output of the electromagnetic flowmeter used to measure the flow rate downstream will exhibit fluctuations. In such a case, the liquid addition point should be moved downstream, or the electromagnetic flowmeter should be placed entirely upstream of the liquid addition point ； If constrained by site conditions or due to the large scale of modification work, a mixer can also be installed downstream of the liquid addition point as a remedy. However, the installation of a static mixer will generate a small rotational flow in the liquid stream, which may cause an additional error of 1% or more. However, compared to the inability to measure output fluctuation, it is a measure taken to choose the lesser of two evils. 　　 　　If the mixture enters the electromagnetic flowmeter measurement before the chemical reaction within the pipeline is complete, output fluctuations may also occur. In this case, the position of the measurement point can only be changed, ensuring that it is located upstream of the mixing point or downstream, away from the mixing section. However, the distance required between the sections far from the mixing zone must be very large; for example, with a reaction time of 60 seconds and a liquid flow rate of 3 m/s, 180 meters is required if no safety factor is taken into account. 　　 　　●Oscillations may also occur if the liquid conductivity is close to the lower limit value. This is because the lower limit specified in the manufacturer’s instrument specifications is the lowest value that can be measured under optimal operating conditions, whereas actual conditions can never be ideal. We have encountered on many occasions the situation where low-conductivity distilled water or deionized water, with a conductivity close to the lower limit specified by the electromagnetic flowmeter standards, namely 5×10-6 S/cm, causes fluctuations in the output when used. It is generally believed that the lower limit for conductivity that can be measured stably is 1–2 orders of magnitude. 　　 　　Liquid conductivity can be found in appendices or relevant manuals; if no available data is present, samples can be taken and measured using a conductivity meter. But sometimes, samples are taken from the pipeline and sent to the laboratory where they are found to be suitable, yet in reality the electromagnetic flowmeter does not function. This is because the liquid used for measuring conductivity differs from the liquid in the pipeline; for example, the liquid may have absorbed CO2 or NOx from the atmosphere, forming carbonic acid or nitric acid, which alters its conductivity. 　　For noise-containing slurries generated by liquids with particles or fibers, increasing the excitation frequency can effectively improve output fluctuation. As shown in Table 7-1, the IFM 3080F model DN 300 electromagnetic flowmeter with adjustable frequency was used to measure a pulp slurry with a concentration of 3.5% corrugated low-density pulp, and the fluctuation in the instantaneous flow rate was measured on-site at different excitation frequencies. When the frequency is low at 50/32 Hz, the shaking reaches up to 10.7% ； When the frequency is increased to 50/2 Hz, the shaking is reduced to 1.9%, showing a very significant effect. 　　 　　Table 1: Instantaneous flow fluctuation magnitude at different excitation frequencies 　　　　　5. Investigate the compatibility between the liquid and electrode materials 　　　Check item 5 in the process flowchart. When selecting electrode materials, the corrosion resistance of the electrodes against the liquid to be measured is considered 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. There is not as much available information on the matching of dielectric and core materials as there is regarding corrosion resistance; only limited experience exists, and further data needs to be accumulated through practical use. 　　 　　With non-acidic solutions such as tantalum monohydrate and alkalis, an insulating layer is formed on the tantalum electrode when measuring water flow, which can cause the instrument to malfunction or result in significant noise after it has been in operation for a short period of time. In the manufacturing process, even brief contact of the tantalum electrodes with water or \"non-acidic\" liquids, such as when the tubes are rinsed with clean water, can affect the proper functioning of the instruments. Tantalum electrodes cannot be used either for alkali solutions such as sodium hydroxide. 　　 　　Hastelloy B – High-concentration hydrochloric acid: There are several successful examples of Hastelloy B being used in hydrochloric acid with moderate temperature and concentration levels. However, noise is generated when the concentration exceeds a certain value; practical experience shows that using tantalum electrodes or acids such as nitric acid and sulfuric acid can produce similar effects. 　　 　　Platinum-hydrogen peroxide platinum electrodes are used for measuring low-pressure hydrogen peroxide (pressure below 0.3 MPa); due to the catalytic action, an aerosol is generated on the electrode surface, which interrupts the electrical circuit and affects its performance. 　　 　　Hydrochloric acid with a thallium concentration of over 10% generates noise in the presence of a platinum electrode; using a tantalum electrode instead resolves this issue. 　　 　　Hastelloy B – aluminum sulfate solution: Aluminum sulfate used in water treatment plants to coagulate suspended particles by mixing it with raw water. We once encountered fluctuating outputs when using a Hastelloy B electrode to measure 15% aluminum sulfate solutions; switching to an acid-resistant steel electrode then yielded satisfactory results. 　　 　　 　　Section 5: Inspection for Zero-Point Instability and Measures to Take 　　　　　I. Causes of Zero-Point Instability There are 5 main categories of causes for zero-point instability in human bodies, namely: 　　(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. 　　 　　II. Inspection procedure: The flowchart shown in Figure 5 illustrates the process for checking instability in zero-point electromagnetic flowmeters. First, conduct a comprehensive preliminary investigation and assessment following the established process, and then carry out detailed checks item by item to troubleshoot the issues. The principle governing the order of the inspection items listed in the process is as follows: (1) Those that can be determined through observation or questioning without requiring extensive procedures come first; in other words, the easier tasks are handled first ； 　　(2) Based on past on-site maintenance experience, those with a high occurrence frequency and a high probability of occurring in the future are listed first ； 　　(3) The sequential requirements necessary for the inspection itself. If the causes of several faults are confirmed through preliminary investigation, detailed inspections can also be carried out in advance. 　　 　　III. Fault Inspection and Remedial Measures This section discusses separately the inspection methods for the aforementioned 5 types of fault causes and the corresponding remedial measures. 　　 　　1. The pipeline is not filled with liquid or the liquid contains bubbles – Check item 1 of the flowchart. These types of failures are mainly caused by defects in the design of the pipeline network or by imperfections in the related equipment; refer to point 2 under section 4 on page 9: “2. The pipes are not filled with liquid, or the liquid contains bubbles.” ” 　　 　　2. There is slight flow in the pipeline; check item 2 of the process diagram. Subjectively, it is believed that there is no flow inside the flow sensor, when in fact a slight flow does exist. 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. Sometimes, it is quite difficult to confirm on-site that there is no flow in the piping system. At this time, as shown in Figure 4, a small-diameter leakage monitoring valve 3 can be installed between the shut-off valves 1 and 4 located before and after the flow sensor 2, in order to check for any leakage. 　　 　　 　　?? Figure 6: Dual valve closure and leakage monitoring – stop valves 1, 4, 6 ； 2 Flow sensor 3, 5 Leak monitoring valve 　　　3. Inadequate grounding, affected by external interference and changes in ground potential – see item 3 of the inspection flowchart. 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 problem at that point will cause a lot of trouble. Changes in the condition of electrical equipment near the flow sensor (such as an increase in leakage current) result in changes in the ground potential, which in turn causes variations in the zero point of the electromagnetic flowmeter. For the inspection method, please refer to Section 9. 　　 　　4. Item 4 of the flowchart for investigating liquid property verification. 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. Zero drift can occur if the liquid contains solid phases, if impurities deposit on the inner wall of the measuring tube, if scaling forms on the inner wall of the measuring tube, or if the electrodes are contaminated by grease or other substances. Because the scaling on the inner wall surface and the degree of electrode contamination cannot be exactly symmetrical, this disrupts the balanced state set at the beginning of operation. Proactive measures are sufficient to remove dirt and accumulated scale ； If the zero offset doesn’t change much, you can also try resetting it. 　　 　　5. Item 5 of the flowchart for inspecting the insulation of signal lines. A decrease in the insulation of the signal circuit can cause instability at the zero point. The main cause of the decreased insulation in the signal circuit is the degradation of insulation at the terminal areas; however, it is also possible that the insulation of the signal cables and their connection terminals has degraded or been damaged. This is because the conditions on site can be extremely harsh; even a slight carelessness, such as inadequate sealing of the instrument covers or wire connections, can allow moisture, acidic fumes, or dust particles to penetrate into the instrument junction boxes or cable insulation, thereby reducing the insulation level. The inspection of the insulation resistance of the signal circuit is carried out separately on the cable side and the flow sensor side, using a megohmmeter. Since the signal cable can be prepared first, it’s easier. The flow sensor measurement is carried out in two steps: first, the surface contact resistance of the electrode is measured after filling it with liquid, and then the insulation resistance of the U pole is measured. 　　 　　6. Check the electrode contact resistance and electrode insulation resistance as per item 6 of the test diagram, in 2 steps. (1) Measure the liquid contact resistance on the surface of the measuring electrode filled with liquid. Disconnect the signal cable of the flow sensor, and use a multimeter to measure the resistance between each electrode and the ground point. The difference in the resistances between the two electrodes relative to ground should be within 10%–20%. Further explanation will be provided in Section 9, \"I. Measurement of electrode contact resistance\". 　　(2) Insulation check of control electrode: Empty the measurement tube, wipe its inner surface with a dry cloth; once it is completely dry, use an H500VDC megohmmeter to measure the resistance between each electrode and ground. The resistance value must be above 100MΩ. 　　 　　 　　 　　 Figure 5 Inspection procedure for unstable zero point of electromagnetic flowmeter