Failure analysis of electromagnetic flowmeter

Causes for electromagnetic flowmeter failures: 1. Failure during commissioning. Failure during commissioning generally occurs during the installation and commissioning phase of the instrument. Once eliminated, it will not reoccur under the same conditions in the future. Common faults during commissioning are usually caused by improper installation, environmental interference, and fluid characteristics. 1) In terms of installation, the fault is usually caused by the incorrect installation position of the electromagnetic flow sensor. The liquid in the pipe is not full due to insufficient back pressure or the poor installation position of the flow sensor, resulting in the liquid in the measuring pipe not being filled. The fault phenomenon is due to the degree of underfill and the flow condition. There are different performances. If a small amount of gas appears as stratified flow or wavy flow in the liquid pipe, the fault phenomenon will be an increase in error, that is, the flow measurement value does not match the actual value; if the flow is bubble flow or plug flow, the fault phenomenon will be except that the measured value does not match the actual value. In addition to the discrepancy, output oscillation will also occur due to the gas phase momentarily covering the electrode surface; if the gas phase part of the flow cross-sectional area increases in the stratified flow of the horizontal pipeline, that is, the liquid is not full of the tube, output oscillation will also occur.

If the liquid is not full, If the tube condition is serious enough that the liquid level is below the electrode, the output will be over full. 2) Environmental aspects are usually mainly caused by stray current interference in pipelines, strong electromagnetic wave interference in space, magnetic field interference from large motors, etc. Pipeline stray current interference usually requires good independent grounding protection to obtain satisfactory results. However, if a strong stray current is encountered (such as electrolytic workshop pipelines, sometimes the AC potential peak value Vpp induced on the two electrodes can be as high as 1V), it is still possible to obtain satisfactory results. Additional measures need to be taken to insulate the flow sensor from the pipeline. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single or multi-layer shielding. 3) In terms of fluids, the measured liquid contains evenly distributed tiny bubbles which usually do not affect the normal operation of the electromagnetic flowmeter. However, as the bubbles increase, the instrument output signal will fluctuate. If the bubbles are large enough to cover the entire electrode surface, As the bubbles flow through the electrode, the electrode circuit will be instantly disconnected, causing greater fluctuations in the output signal. When an electromagnetic flowmeter with low-frequency square wave excitation measures slurry with excessive solid content, it will also produce slurry noise, causing the output signal to fluctuate. When measuring mixed media, if the flow sensor is entered for measurement before the mixture is uniform, the output signal will also fluctuate. Improper selection of electrode material and measured medium will also affect normal measurement due to chemical effects or polarization phenomena.

The electrode material should be correctly selected according to the instrument selection or relevant manual. 2. Operational faults: Operational faults are faults that occur after the electromagnetic flowmeter has been debugged and operated normally for a period of time. Common running faults are generally caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions. (1) Adhesion layer on the inner wall of the sensor. Since electromagnetic flowmeters are often used to measure dirty fluids, after running for a period of time, an adhesion layer will often accumulate on the inner wall of the sensor and cause failure. These failures are often caused by the conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be open-circuited and the instrument will not work properly; if the conductivity of the attachment layer is significantly higher than the conductivity of the fluid, the electrode circuit will be short-circuited and the instrument will not work properly. so, the attached scaling layer in the electromagnetic flowmeter measuring tube should be removed in time. (2) Lightning strike Lightning strikes can easily induce high voltage and surge current in the instrument circuit, causing damage to the instrument. It is mainly introduced through power lines or excitation coils or flow signal lines between sensors and converters, especially the majority of them introduced from the power lines in the control room. (3) Changes in environmental conditions. During debugging, because the environmental conditions are still good (for example, there is no interference source), the flow meter works normally. At this time, it is often easy to ignore the installation conditions (for example, the grounding is not very good). In this case, once the environmental conditions change and new sources of interference appear during operation (such as electric welding on the pipeline near the flow meter, large transformers installed nearby, etc.), it will interfere with the normal operation of the meter and the output of the flow meter. The signal will fluctuate. Therefore, during the installation and actual use of the electromagnetic flowmeter, we must carefully check to avoid possible failures, and further strengthen its daily maintenance and upkeep, so that it can operate reliably and greatly improve the measurement performance of the electromagnetic flowmeter. and service life.