The first type of fault that occurs in the operation of electromagnetic flow juice is the failure of the instrument itself, that is, the failure caused by the damage of the instrument structural components or components; the second type is the failure caused by external causes, such as improper installation of flow distortion, deposition and scaling, etc. . This chapter focuses on the application aspects and the failure of the second type of external causes mentioned above. Electromagnetic flowmeter fault inspection and analysis Section 1 Fault type Electromagnetic flow The first type of fault occurred in the operation of the juice is the fault of the instrument itself, that is, the fault caused by the damage of the instrument structural component or component; the second type is the fault caused by the external cause, Such as improper installation of flow distortion, deposition and scaling. This chapter focuses on the application aspects and the failure of the second type of external causes mentioned above. According to the classification of the fault occurrence period, it can be divided into: 1 debugging period failure; 2 running period failure. The failure of the commissioning period occurs in the initial stage of commissioning after the new installation. The main reason is that the instrument is improperly selected or improperly installed, and the installation is not appropriate. The operating period failure occurs after a period of operation. The main reason is that impurities in the fluid are attached to the electrode lining, and new interference sources appear in the environmental conditions. According to the fault source, the source analysis comes from three aspects: 1 caused by piping system and installation; 2 caused by environmental aspects; 3 caused by fluid. Source 1 is mainly displayed during the commissioning period; sources 2 and 3 are present during both the commissioning phase and the runtime phase.
I. Trouble during commissioning This type of fault occurs when the electromagnetic flowmeter is initially installed and debugged, but once it is improved, it will not appear again under the same conditions. Common failures during the commissioning period mainly 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. For example, the flow sensor is installed at the high point of the pipe network which is easy to accumulate the retained gas; after the flow sensor, there is no back pressure, and the liquid is straight. The human atmosphere forms a non-full tube in the measuring tube; it is installed on a vertical pipe that flows from the top to the bottom, and may be evacuated.
2, the environmental aspects are mainly pipeline stray current interference, space electromagnetic wave interference, large motor magnetic field interference. Pipe stray current interference usually takes good separate grounding protection to obtain satisfactory measurement, but if there is strong stray current in the pipeline (such as electrolysis workshop piping), it may not be overcome, and the flow sensor and pipeline must be taken (see below) Case 12). Space electromagnetic interference - usually transmitted through a signal cable, usually protected by a single layer or multiple layers of shielding, but it has also been overcome by shielding protection (see Case 10).
3, the fluid aspect of the liquid contains evenly distributed fine bubbles usually does not affect the normal measurement, only the measured volume flow is the sum of the liquid and the gas; the increase of the bubble will cause the output signal to fluctuate, if the bubble is large enough to flow through the electrode to cover the entire electrode surface To make the electrode signal loop momentarily open, the output signal will produce more fluctuations. Low-frequency (50/16 Hz-50/6 Hz) rectangular wave excitation electromagnetic flowmeter will 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 in the pipeline mixing process, if the conductivity of the two liquids (or the potential between the electrodes) is different, the flow sensor is measured before the mixing is not uniform, and the output signal also fluctuates. The electrode material and the measured medium are poorly matched, and chemical effects such as passivation or oxidation, formation of an insulating film on the surface of the electrode, and electrochemical and polarization phenomena may hinder normal measurement.
Second, the operating period failure After the initial commissioning and normal operation for a period of time after the failure occurred during the operation, the common causes of failure are: the inner layer of the flow sensor, lightning strikes, environmental conditions change.
1. Inner wall adhesion layer Since the electromagnetic flowmeter measures the possibility of containing suspended solid phase or dirty body much more than other flow meters, the probability of failure of the inner wall adhesion layer is relatively high. If the conductivity of the adhesion layer is close to the liquid conductivity, the meter can output the signal 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; if it is an insulating adhesion layer The surface of the electrode is insulated to disconnect the measuring circuit. Both of the latter phenomena will make the meter inoperable (see Case 7).
2. Lightning lightning strikes induce instantaneous high voltage and surge current in the line, and entering the meter will damage the meter. There are three ways to introduce the lightning damage meter: the power line, the flow signal line between the sensor spoon converter and the excitation line. However, from the analysis of damaged components in lightning faults, the induced high voltage and surge currents that cause faults are mostly introduced from the control room power line, and the other two approaches are less. It was also learned from the scene of the lightning strike that not only the electromagnetic flowmeter was faulty, but also other instrumentation in the control room often had lightning strikes at the same time. Therefore, the user must know the importance of setting up the lightning protection facilities of the control room instrument power cord. The current team has been able to identify and explore this problem in the design unit, such as the Qilu Petrochemical Design Institute [1].
3. The main reason for the change of environmental conditions is the same as the fault environment in the commissioning period of the previous section, except that the interference source does not appear during the commissioning period and then intervenes during the operation. For example, an electromagnetic flowmeter whose grounding protection is not ideal is not normal due to factory disturbance, and the instrument is operating normally. However, new interference sources appear during operation (such as pipelines near the measuring point or pipeline welding at a remote location). When running, there is a large fluctuation in the output signal.
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