How to deal with common electromagnetic flowmeter faults

The type of faults that occur during operation is the fault of the instrument itself, that is, the fault caused by the damage of the structural parts or components of the instrument. The second category includes failures caused by external factors, such as improper installation, flow distortion, deposit and scaling.

1. Faults during debugging

This type of faults appeared during the initial installation and commissioning, but once the faults were improved and eliminated, they did not will not reappear under the same conditions in the future. Common faults during the commissioning period are mainly caused by improper installation, environmental interference and the influence of fluid characteristics.

(1) Piping system and installation, etc.

Usually the failure is caused by the incorrect installation position of the electronic flow sensor.electromagnetic. The highlight of the net; there is no back pressure behind the flow sensor and the liquid is vented directly to atmosphere here, forming a non-full pipe in the meter tube; it is installed on a vertical pipe running from top to bottom, and draining may occur.

(2) Environmental aspects

Mainly stray current interference in pipelines, electromagnetic wave interference in space, and magnetic field interference of large motors . Stray current interference from the pipeline can usually be measured satisfactorily with good separate grounding protection, but if there is a strong stray current in the pipeline (such as the pipeline in the electrolysis plant) , he may not be able to overcome it. Steps should be taken to isolate the flow sensor from the pipeline. electromagnetic wave interferenceSpatial interferences are usually introduced by signal cables and are usually protected by single-layer or multi-layer shielding, but shielding protection has also been encountered and cannot be overcome (see 10).

(3) Fluid Appearance

The liquid contains evenly distributed fine bubbles, which usually does not affect the normal measurement, but the measured volume flow rate is the sum of the liquid and some gas ; increasing bubbles will cause the output signal to fluctuate. If the bubbles are so large that they pass through the electrode and cover the entire surface of the electrode, the electrode signal circuit is disconnected instantly and the output signal will have larger fluctuations. The low frequency (50/16Hz-50/6Hz) rectangular wave excitation electromagnetic flowmeter will generate sludge noise when the solid content in the liquid exceeds a certain level, and the output signal will also fluctuate insome extent. When two or more liquids are used for the pipeline mixing process, if the conductivity of the two liquids (or the potential between each and the electrodes) is different, the flow sensor will enter the flow sensor for flow measurement before the mixture is not uniform, and the output signal will also fluctuate. Improper selection of electrode material and measuring red medium will cause chemical effects such as passivation or oxidation, the formation of an insulating film on the surface of the electrode, and electrochemical and polarization phenomena, etc., will interfere with normal measurement.

2. Faults during operation

Faults that occur during operation after initial debugging and normal operation for a period of time, common causes of faults include: layer d adhesion of the internal wall of the flow sensor, lightning strike, change of coenvironmental conditions.

(1) Adhesion layer to the inner wall

Since the electromagnetic flowmeter has more possibilities to measure suspended solids or dirt than other flowmeters, the probability of failure caused by the adhesion layer to the inner wall It is relatively high. If the conductivity of the adhesion layer is similar to the conductivity of the liquid, the meter can still output signals normally, but only changes the flow rate. If it is a high conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited; if it is an insulating adhesion layer, the electrode surface will be insulated and the measurement circuit will be disconnected. These last two phenomena will make the meter unable to function.

(2) Lightning shock

Lightning shock induces instantaneous high voltage and surge current in the line, and if it penetratesinto the instrument, it will damage the instrument. There are 3 ways to introduce the lightning damage instrument: power line, flow signal line between sensor and converter, and excitation line. However, from the analysis of damaged components in lightning faults, most of the induced high voltage and surge current that cause the fault are introduced from the control room power line, and both other paths are lower. It is also learned from the scene of the lightning accident that not only does a fault occur, but also lightning accidents often occur at the same time in other instrumentation circuits in the control room. Therefore, the user should understand the importance of implementing lightning protection facilities for instrument power lines in the control room.

(3) Changes inns the environmental conditions

The main reason is the same as in the previous section on the failure environment during the debug period, except that the interference source does not appear during the debug period . debug period but occurs during the runtime period. For example, if the grounding protection is not ideal, the instrument works normally during the debugging period because there is no interference source, but a new interference source appears during the period of operation (such as the pipeline near the measurement point or the welding of the pipe at a distance) interferes with the normal operation of the instrument, and the output occurs. The signal fluctuates wildly.

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