Thread Content
The common faults encountered in the use of electromagnetic flowmeters are usually defects in the instrument itself, that is, faults caused by damage to the instrument’s structural components or parts； The second category is caused by external factors. Proper installation is necessary to avoid issues such as flow distortion, deposition, and scaling. What is focused on here are the application aspects and failures caused by the second type of external factors mentioned above. Classified by the time when the failure occurs, they can be divided into: ① Failures during operation ； ②Fault during the debugging period. Failures during the commissioning period occur in the early stages of testing after new installation, and the main causes are improper selection or setting of instruments, as well as incorrect installation. Operational failures occur after the system has been in use for a period of time; the main reasons include impurities in the fluid adhering to the electrode lining, as well as new sources of interference arising from changes in environmental conditions. The analysis of external fault sources is based on three aspects: ① Those caused by the pipeline system and installation, etc ； ②Caused by environmental factors ； ③Caused by fluid-related factors. Source ① is mainly evident during the debugging phase ； Sources ② and ③ appear during both the debugging phase and the operation phase. I. Operational failures: Failures that occur during operation after initial debugging and some time of normal operation. Common causes of such failures include deposits on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions. 1. Inner wall deposit layer: Since electromagnetic flowmeters are more likely to measure fluids containing suspended solids or contaminants compared to other flowmeters, the probability of failures caused by inner wall deposits is relatively higher. If the conductivity of the adhesive layer is similar to that of the liquid, the instrument can still output signals normally; it is only the change in the flow area that causes a latent fault resulting in measurement errors ； If it is a high-conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited ； If it is an insulating adhesive layer, the electrode surface becomes insulated, breaking the measurement circuit. Both of the latter phenomena will prevent the instrument from functioning. 2. Lightning strike: Lightning induces instantaneous high voltages and surge currents in the circuit, which can damage instruments when they come into contact with such currents. There are 3 pathways through which lightning can damage instruments: the power supply cable, the flow signal line between the sensor and the converter, and the excitation wire. However, analysis of the components damaged in lightning faults shows that the induced high voltages and surge currents causing these faults mostly originate from the power supply lines in the control room, with the other two pathways being less common. It was also learned from the site of the lightning strike incident that not only did the electromagnetic flowmeter fail, but other instruments in the control room also frequently experienced problems due to lightning strikes. Therefore, the user units must recognize the importance of installing lightning protection facilities for the power cables of the control room instruments. Several design firms are currently working to understand and find solutions to this problem. 3. Changes in environmental conditions: The main reasons are the same as those related to environmental issues during the debugging phase mentioned in the previous section; the only difference is that the sources of interference appear not during debugging but rather during operation. For example, in the case of an electromagnetic flowmeter whose grounding protection is not optimal, it operates normally during the calibration phase since there are no external disturbances from the factory. However, during operation new sources of interference arise – such as pipe welding near or at a distance from the measurement point – which disrupt the normal functioning of the instrument, resulting in significant fluctuations in the output signal. II. Faults during the commissioning period: These types of faults occur during the initial installation and commissioning of the electromagnetic flowmeter. However, once the issues are resolved through corrective actions, they generally do not reoccur under the same conditions thereafter. Common faults during the debugging period are mainly caused by improper installation, environmental interference, and the effects of fluid properties. 1. Issues related to pipeline systems and installation are usually caused by incorrect placement of electromagnetic flow meters; a common example is installing the flow meter at high points in the pipeline network where gases tend to accumulate ； There is no back pressure behind the flow sensor, so the liquid flows directly into the atmosphere, resulting in a non-full condition within its measurement tube ； Installed in vertical pipes that allow flow from top to bottom, issues such as emptying may occur. The installation of open-channel flow meters is rather special; since they are submerged underwater, it is necessary to ensure that there is no floating. 2. In terms of the environment, the main issues include interference from stray currents in pipelines, spatial electromagnetic wave interference, and magnetic field interference from large motors. Interference caused by stray currents in pipelines can usually be mitigated through proper separate grounding, which yields satisfactory measurement results; however, in cases where there are strong stray currents in the pipelines (such as those in electrolysis plants), this approach may not be sufficient, and measures to isolate the flow sensors from the pipelines are necessary. Space electromagnetic wave interference generally enters through signal cables, and is usually protected by single-layer or multi-layer shielding; however, there have been cases where even such shielding protection was insufficient to overcome the issue. 3. Regarding fluids, liquids containing uniformly distributed tiny bubbles generally do not affect normal measurements; however, the volume flow rate measured is the sum of that of the liquid and the gas ； An increase in the size of the bubbles causes fluctuations in the output signal. If the bubbles become large enough to cover the entire surface of the electrode as they pass over it, the electrode signal circuit is momentarily interrupted, resulting in even greater fluctuations in the output signal. In low-frequency (50/16 Hz–50/6 Hz) rectangular-wave excitation electromagnetic flowmeters, the presence of a certain amount or more of solids in the liquid generates slurry noise, resulting in fluctuations in the output signal as well. When two or more liquids are mixed through pipelines, if there are differences in their conductivities (or in the potential between each liquid and the electrodes), and flow measurement is taken using a flow sensor before the mixing is complete, the output signal will also exhibit fluctuations. Improper selection of the electrode material and the medium to be measured can lead to chemical reactions such as passivation or oxidation, the formation of an insulating film on the electrode surface, as well as electrochemical and polarization effects – all of which can hinder proper measurement. For more information, please visit the company’s official website at http://www.jshdyb.com/. Please retain this link when reproducing the content! http://www.yb1518.com/UploadFiles/2012717174126829.jpg