Common faults in the use of electromagnetic flowmeters

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This post was last edited by hamtyb on 2012-4-26 at 14:38. Common faults in electromagnetic flowmeters can be caused by damage to the components of the instrument itself, or they may result from improper selection, incorrect installation, environmental conditions, or characteristics of the fluid – issues such as fluctuating readings, reduced accuracy, or even damage to the instrument. It can generally be divided into two types: faults that occur during installation and debugging (debugging-phase faults) and faults that occur during normal operation (operational-phase faults). 1. Faults during the commissioning period: Faults that occur during commissioning generally appear during the installation and setup of the instrument. Once resolved, the electromagnetic flowmeter will not experience such issues under the same conditions in the future. Common faults during the commissioning period are usually caused by improper installation, environmental interference, and the effects of fluid properties. 1) Regarding installation, faults are usually caused by the incorrect placement of the electromagnetic flow meter; a common issue is installing the sensor at the highest point of the pipeline where gases tend to accumulate ； Or installed on a vertical pipe from top to bottom, emptying may occur ； Or there is no back pressure behind the sensor, and the fluid flows directly into the atmosphere, resulting in a non-full condition within the measurement tube. 2) In terms of the environment, the issues are usually mainly pipeline stray current interference, strong electromagnetic wave interference in the surrounding area, and magnetic field interference from large motors. For interference caused by stray currents in pipelines, satisfactory results can usually be achieved by employing proper separate grounding protection. However, in the presence of strong stray currents (such as those in electrolysis plants, where the peak value of the alternating voltage induced across the two electrodes can sometimes reach 1 V), additional measures are necessary, including insulating the flow sensors from the pipelines. Space electromagnetic wave interference generally enters through signal cables, and is usually protected using single-layer or multi-layer shielding. 3) Regarding fluids, the presence of uniformly distributed tiny bubbles in the liquid being measured generally does not affect the normal operation of electromagnetic flowmeters. However, as the bubbles grow larger, fluctuations occur in the instrument’s output signal. When the bubbles are large enough to cover the entire electrode surface, their passage over the electrodes can cause an instantaneous interruption in the electrode circuit, resulting in even greater fluctuations in the output signal. When an electromagnetic flowmeter driven by a low-frequency square wave is used to measure slurries with too high a solid content, slurry noise is also generated, causing fluctuations in the output signal. When measuring a mixed medium, if flow sensing is performed before the mixture has been homogenized, it will also cause fluctuations in the output signal. An inappropriate selection of electrode materials and the medium to be measured can also affect accurate measurements due to chemical reactions or polarization effects. The electrode material should be selected appropriately based on the instrument or relevant manuals. 2. Operational faults: Operational faults are those that occur in electromagnetic flowmeters after they have been tuned and are operating normally for some time. Common operational faults are usually caused by deposits on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions. 1) Adhesive layer on the inner wall of the sensor: Since electromagnetic flowmeters are often used to measure dirty fluids, an adhesive layer tends to accumulate on the inner wall of the sensor over time, leading to malfunctions. These failures are often caused by the conductivity of the adhesion layer being too high or too low. If the attachment is an insulating layer, the electrode circuit will be open, preventing the instrument from functioning properly ； If the conductivity of the adhesive layer is significantly higher than that of the fluid, a short circuit will occur in the electrode circuit, and the instrument will not function properly. Therefore, the accumulated scale layer inside the measuring tube of the electromagnetic flowmeter should be removed in a timely manner. 2) Lightning strikes: Lightning can easily induce high voltages and surge currents in the instrument circuits, causing damage to the instruments. It is introduced mainly through the power cable, the excitation coil, or the flow signal line between the sensor and the converter, with the majority coming from the power cable in the control room. 3) Changes in environmental conditions: During commissioning, when the environmental conditions are favorable (for example, there are no interference sources) and the flow meter operates properly, it is easy to overlook the installation conditions (such as inadequate grounding). Under such circumstances, once the environmental conditions change and new sources of interference arise during operation (such as welding on pipes near the flow meter or the installation of large transformers in the vicinity), it will disrupt the normal functioning of the instrument, causing fluctuations in the output signal of the flow meter. Article sourced from: http://www.hamtyb.com