The causes of common failures of gas flowmeters can be analyzed from these two aspects

The causes of common failures of gas flowmeters can be analyzed from these two aspects. The failures in the operation of gas flowmeters are the failures of the instrument itself, and the liquid flowmeters are failures caused by damage to instrument structural parts or components; the second category is caused by external factors. Faults, such as improper installation, flow distortion, deposition and fouling, etc. This chapter focuses on failures in application and external causes of the second category above. According to the classification of the fault occurrence period, it can be divided into: ① faults during debugging; ② faults during operation. The faults in the debugging period occur in the initial stage of debugging after the new installation is used. The main reason is that the instrument is not selected or set properly, and the installation is not proper. The faults during the operation period appear after a period of operation. The main reasons are that impurities in the fluid adhere to the electrode lining, and new interference sources appear due to changes in environmental conditions. According to the analysis of the external source of the fault, it comes from three aspects: ①caused by the piping system and installation; ②caused by the environment; ③caused by the fluid. Source ① is mainly manifested in the debugging period; sources ② and ③ will appear in both the debugging period and the running period. 1. Faults in the commissioning period This type of faults occurred during the initial installation and commissioning of the gas flowmeter, but once the compressed air flowmeter is improved and the faults are eliminated, it will generally not reappear under the same conditions in the future. The common faults during the commissioning period mainly include improper installation, environmental interference, and fluid property influence. 1. The pipeline system and installation are usually caused by the incorrect installation position of the electromagnetic flow sensor. Commonly, the flow sensor is installed at a high point of the pipe network where trapped gas is easy to accumulate; there is no back pressure behind the flow sensor, and the liquid is discharged directly If it is installed in a vertical pipe flowing from top to bottom, it may be empty and so on. 2. Environmental aspects Mainly interference from pipeline stray current, space electromagnetic wave interference, large motor magnetic field interference, etc. Pipeline stray current interference can usually be satisfactorily measured with a good separate grounding protection, but if there is a strong stray current in the pipeline (such as the electrolysis workshop pipeline), it may not be able to overcome it. Measures must be taken to isolate the flow sensor from the pipeline (see below Case 12). Space electromagnetic wave interference is generally introduced through signal cables, and is usually protected by single-layer or multi-layer shielding, but shielding protection has also been encountered and cannot be overcome (see case 10). 3. In terms of fluid, the liquid contains evenly distributed fine air bubbles, which usually does not affect the normal measurement. The volume flow measured by the sewage flowmeter is the sum of the liquid and the gas; the increase of the air bubbles will cause the output signal to fluctuate. If the air bubbles are large enough to flow through the electrodes to cover The entire electrode surface, so that the electrode signal circuit is disconnected instantaneously, the output signal will have greater fluctuations. When the low-frequency (50/16 Hz-50/6 Hz) rectangular wave excitation gas flowmeter measures the liquid containing solids exceeding a certain content, slurry noise will be generated, and the output signal will also fluctuate to a certain extent. When two or more liquids are used for 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 mixing is not uniform, and the output signal will also fluctuate. Poor selection of the electrode material and the measured medium will cause chemical effects such as passivation or oxidation, the formation of an insulating film on the electrode surface, and electrochemical and polarization phenomena, etc., will hinder normal measurement. 2. Faults during operation After initial debugging and normal operation for a period of time, faults occur during operation. The common causes of faults are gas flowmeter: adhesion layer on the inner wall of the flow sensor, lightning strike, and changes in environmental conditions. 1. Adhesion layer on the inner wall Since the gas flowmeter has more opportunities to measure suspended solids or dirt than other flow meters, the probability of failure caused by the inner wall adhesion layer of the air flowmeter is relatively high. If the conductivity of the adhesion layer is similar to the conductivity of the liquid, the instrument can still output signals normally, but the flow area is changed, forming a hidden fault of 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 electrode surface is insulated and the measurement circuit is disconnected. The latter two phenomena will make the meter unable to work. 2. Lightning strike The lightning strike induces instantaneous high voltage and surge current in the line, and if it enters the instrument, it will damage the instrument. There are 3 ways to introduce the lightning strike damage instrument: the power line, the flow signal line between the sensor and the converter, and the 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 power line of the control room, and the other two paths are less. It is also learned from the scene of the lightning accident that not only the gas flowmeter failed, but also other instrumentation in the control room often had lightning accidents at the same time. Therefore, the user must understand the importance of setting up lightning protection facilities for the instrument power lines in the control room.

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