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Classification according to the time of failure, can be divided into: 1 operating period failure; 2 debugging period failure. Debugging faults appear in the initial stage of debugging after new equipment installation. The main reason is that the instrument is selected or improperly set and the installation is not proper. Run-time faults occur after a certain period of operation, mainly due to impurities in the fluid adhering to the electrode lining, and new interference sources such as changes in environmental conditions.
According to the external sources of failure analysis from three aspects: 1 caused by the piping system and installation; 2 caused by the environment; 3 caused by the fluid. Source 1 is mainly demonstrated during the commissioning period; Sources 2 and 3 are both present during commissioning and operation.
First, the operation of the fault after the initial commissioning and normal operation after a period of time during the operation of the failure, the common causes of the failure are: flow sensor attached to the inner wall, 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 contaminated bodies 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 thunder and lightning shock induction transient high voltage and inrush current in the line, enter the instrument will damage 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. It also learned from the scene of a lightning accident that not only did the electromagnetic flowmeter malfunction, but other instrument power in the control room often had a lightning strike 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. The incumbent has already met with the design team and explored solving this problem.
3, the main cause of changes in environmental conditions in the same period as the fault environment during the debugging period, but the interference source is not in the debugging period and then involved in the operation period. For example, an electromagnetic flowmeter with unsatisfactory grounding protection will operate normally during the commissioning period due to no disturbance to the plant. However, new interference sources (for example, pipelines near the measuring point or pipelines at a far distance) interfere with the normal operation of the instrument. Operation, there is a large fluctuation in the output signal.
Second, the debugging period of failure This type of failure in the initial installation of electromagnetic flowmeter debugging appears, but once improved troubleshooting, after the same conditions in general will not reappear. The common debugging faults include three reasons: improper installation, environmental interference, and fluid characteristics.
1. The piping system and installation are usually faults caused by incorrect installation position of the electromagnetic flow sensor. Commonly, for example, the flow sensor is installed at the high point of the pipe network where accumulation of entrapped gas is easy; there is no back pressure after the flow sensor, and the liquid is directly discharged. The human atmosphere forms a non-full pipe in its measuring pipe; it may be empty on a vertical pipe running from top to bottom. The installation of an open channel flow meter is rather special. Since it is underwater and underwater, it must be ensured that there is no floating.
2, the environment is mainly the pipe stray current interference, space electromagnetic interference, large motor magnetic field interference. The stray current interference of the pipeline usually adopts a good individual grounding protection to obtain a satisfactory measurement, but if there is a strong stray current in the pipeline (such as the electrolysis workshop pipeline), it may not be overcome, and the measures of the flow sensor and the piping must be adopted. The introduction of space electromagnetic wave interference-general signal cables usually uses single-layer or multi-layer shielding for protection, but it has also encountered shielding protection that cannot be overcome.
Third, fluids Liquids containing uniformly distributed fine air bubbles usually do not affect the normal measurement, but the measured volume flow is the sum of both liquid and gas; the increase in air bubbles will cause the output signal to fluctuate if the air bubbles are large enough to flow over the electrodes to cover the entire electrode surface. , Make the electrode signal circuit break instantaneously, the output signal will produce greater fluctuation.
Low-frequency (50/16Hz-50/6Hz) rectangular-wave excitation electromagnetic flowmeters will produce slurry noise when the liquid contains solids exceeding a certain level, and the output signal will also fluctuate to a certain 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 actions 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 normal measurement.
Analyze two common types of faults in the use of electromagnetic flowmeters
Overview of the electromagnetic flowmeter often encountered in the use of the fault, the first category is generally the fault of the instrument itself, that is, instrument structure or component damage caused by failure; the second type is caused by external causes. Such as improper installation flow distortion, deposition and scaling and so on. The focus here is on the application and failures of the second type of external reasons mentioned above.