The first type of failure that occurs in the operation of the electromagnetic flowmeter is the failure of the instrument itself, that is, the failure caused by damage to the instrument structure or components . The second category is failures caused by external causes, such as improper installation flow distortion, deposition and scaling. 1, , debugging failure This type of fault appears when the electromagnetic flowmeter is initially installed and debugged, but once it is improved and eliminated, it will not appear again under the same conditions. The common debugging problems include three reasons: improper installation, environmental interference, and fluid characteristics. ( 1 ) Pipeline system and installation Typically electromagnetic flow sensor failure caused by incorrect installation location, for example, a common flow sensor installed in the gas tend to be deposited retention of high network; no back pressure after the flow sensor, the liquid discharged into the atmosphere runoff straight, form a measurement Non-full pipes in pipes; empty pipes, etc. may be installed on vertical pipes running from top to bottom. ( 2 ) Environmental aspects Mainly the pipe stray current interference, space electromagnetic interference, large-field magnetic interference. The stray current interference of the pipeline usually adopts a good individual grounding protection to obtain a satisfactory measurement. However, if there is a strong stray current in the pipeline (such as a pipeline in the electrolysis workshop), it may not be overcome, and the flow sensor and the piping must be adopted. Space electromagnetic interference - generally through the signal cable I, usually single or multi-layer shielding to protect, but also encountered shielding protection can not be overcome (see Case 10). ( 3 ) Fluids Liquids containing uniformly distributed fine air bubbles usually do not affect the normal measurement, but the measured volumetric flow is the sum of both the liquid and the gas; the increase of the air bubble causes the output signal to fluctuate, if the air bubble is large enough to flow over the electrode to cover the entire electrode surface, so that the electrode signal The circuit turns off instantaneously and the output signal will produce more fluctuations. Low-frequency (50/16 Hz-50/6 Hz) rectangular-wave excitation magnetic flowmeters produce slurry noise when the liquid contains more than a certain amount of solids, and the output signal will also fluctuate to some extent. When two or more liquids are used as a pipe mixing process, if the conductivity of the two liquids (or their respective potentials with respect to the electrodes) is different, the flow sensor will enter the flow measurement before the mixing is not uniform, and the output signal will also fluctuate. The poor matching of the electrode material and the measured medium produces chemical effects such as passivation or oxidation. The formation of an insulating film on the surface of the electrode, as well as electrochemical and polarization phenomena, will interfere with the normal measurement. 2 , the operating period failure After the initial commissioning and normal operation of a period of time after the operation of the failure occurred, the common causes of the failure are: the inner wall of the flow sensor layer, lightning strikes, changes in environmental conditions. ( 1 ) Adhesion layer on the inner wall Because electromagnetic flowmeters have a much higher chance of measuring suspended solids or contaminants than other flow meters , the probability of failure due to the inner wall adhesion layer is relatively high. If the adhesion layer conductivity is similar to the liquid conductivity, the meter can still output signals normally, but only change the flow area, forming a hidden failure of the measurement error; if the high conductivity adhesion layer, the electromotive force between the electrodes will be short circuit; if the insulating adhesion layer The electrode surface is insulated and the measurement circuit is disconnected. The latter two phenomena will make the instrument unable to work. ( 2 ) Lightning Strike Lightning strikes instantaneous high voltages and inrush currents in the line, which can damage the instrument when it enters the instrument. There are three ways to introduce a lightning damage meter: the power line, the sensor flow signal line and the excitation line between the converter . However, analysis of damaged components from lightning failures causes most of the induced high voltage and surge currents of the fault to be introduced from the control room power lines, and the other two approaches are less. Also from the accident scene lightning learned, not only electromagnetic flow meter fails, the other instruments in the control room electric lightning accidents often occur at the same time. Therefore, the use of the unit to understand the importance of setting up the control room instrument power line lightning protection facilities. ( 3 ) Changes in environmental conditions The main reason is the same as the fault environment in the debugging period in the previous section, except that the source of the interference is not in the commissioning period but is involved in the operation again. For example, an electromagnetic flowmeter with unsatisfactory grounding protection has normal operation due to no interference source during the commissioning period. However, new interference sources (for example, pipelines near the measuring point or where welding is performed at a far distance) interfere with the normal operation of the instrument. , There is a large fluctuation in the output signal.
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How to deal with common electromagnetic flowmeter failures