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technical article
Failure and cause of electromagnetic flowmeter
2011-6-4 16:47:34
Jinhu Dongxiang Instrument Co., Ltd. summed up the electromagnetic flowmeter production and sales experience for many years- electromagnetic flowmeter failure types

The first type of failure that occurs during the operation of the electromagnetic flowmeter is the failure of the instrument itself, that is, the failure caused by the damage of the instrument structure or components; the second type is the failure caused by external causes, such as improper installation, flow distortion, deposition and scaling, etc. . This chapter focuses on the application aspects and the above-mentioned second types of external causes of failure. According to the classification of faults, they can be divided into: ① faults during commissioning; ② faults during operation. Faults during commissioning occur in the early stage of commissioning after new installation. The main reasons are improper selection or setting of the instrument and improper installation. Faults during operation occur after a period of operation. The main reasons are impurities in the fluid adhering to the electrode lining, and new interference sources due to changes in environmental conditions. According to the external source of the failure, it comes from three aspects: ① caused by the pipeline system and installation; ② caused by the environment; ③ caused by the fluid. The source ① is mainly displayed during the commissioning period; the sources ② and ③ will appear during the commissioning and operation periods.

I. Failure during commissioning

This type of fault occurs when the electromagnetic flowmeter is initially installed and debugged, but once it is improved to eliminate the fault, it will generally not reappear under the same conditions in the future. Common failures during commissioning are mainly caused by improper installation, environmental interference, and fluid characteristics.

1. Piping system and installation

It is usually caused by incorrect installation position of electromagnetic flow sensor. Commonly, for example, the flow sensor is installed at the high point of the pipe network where it is easy to accumulate retained gas; there is no back pressure behind the flow sensor, and the liquid plutonium is directly discharged into the atmosphere, forming a non-linearity in the measuring tube. Full pipe; installed on vertical pipe from top to bottom, emptying may occur.

2. Environmental aspects

Mainly the pipeline stray current interference, space electromagnetic wave interference, large motor magnetic field interference, etc. Pipeline stray current interference can usually be satisfactorily measured with good separate grounding protection. However, if the pipeline has strong stray current (such as the pipeline in the electrolytic workshop), it may not be overcome, and measures must be taken to isolate the flow sensor from the pipeline (see below Case 12). Space electromagnetic interference-usually through the signal cable bow, usually using single or multiple layers of shielding to protect, but also encountered shielding protection can not be overcome (see case 10).

3. Fluid aspect

The liquid contains uniformly distributed fine air bubbles, which usually does not affect the normal measurement, but the measured volume flow is the sum of both liquid and gas; the increase of air bubbles will cause fluctuations in the output signal. The loop is disconnected instantaneously, and the output signal will produce greater fluctuations. Low-frequency (50/16 Hz-50 / 6 Hz) rectangular wave excitation electromagnetic flowmeter will produce slurry noise when the solid content in the liquid exceeds a certain content, and the output signal will fluctuate to a certain extent. When two or more liquids are used in the pipeline mixing process, if the conductivity of the two liquids (or the potential between each liquid and the electrode) is different, it will enter the flow sensor for flow measurement before the mixing is not uniform, and the output signal will fluctuate. Improper selection of the electrode material and the measured medium will cause chemical effects such as passivation or oxidation, the formation of insulating films on the electrode surface, and electrochemical and polarization phenomena, which will prevent normal measurement.
Failure during operation

Faults that occur during operation after initial commissioning and normal operation for a period of time. Common causes of failure include: adhesion layer on the inner wall of the flow sensor, lightning strike, and changes in environmental conditions.

1.Inner wall adhesion layer

Because the electromagnetic flowmeter has a greater chance of measuring suspended solids or dirt than other flow meters, the probability of failure due to the adhesion of the inner wall 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 change the flow area to form a hidden fault of the measurement error. If it is a high conductivity adhesion layer, the electromotive force between the electrodes will be short-circuited; The electrode surface is insulated to disconnect the measuring circuit. The latter two phenomena make the meter inoperable.

2.Lightning strike

Lightning strikes induce transient high voltages and surge currents in the line, and entering the meter will damage the meter. There are three ways to introduce the lightning damage instrument: the power line, the flow signal line and the excitation line between the transducer and the transducer. However, from the analysis of the damaged parts in the lightning fault, most of the induced high voltage and surge current that caused the failure were introduced from the power line in the control room, and the other two paths were less. It was also learned from the scene of the lightning strike that not only the electromagnetic flowmeter failed, but also other instrumentation and electricity in the control room often had lightning strikes at the same time. Therefore, users should recognize the importance of setting lightning protection facilities for control room instrument power lines.

3. Changes in environmental conditions

The main reason is the same as the fault environment in the debugging period, except that the interference source does not appear during the debugging period and then intervenes during the operation. For example, an electromagnetic flowmeter with unsatisfactory grounding protection. During the commissioning period, the meter operates normally because there are no interference sources. However, during the operation period, new interference sources (such as pipelines near the measuring point or remotely implemented pipe welding) interfere with the normal operation of the meter The output signal fluctuates greatly.

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