Faults that are likely to occur during the operation of electromagnetic flowmeters and how to avoid them.
Faults that are likely to occur during the operation of electromagnetic flowmeters and how to avoid them:
The first type of faults that occur during the operation of electromagnetic flowmeters are faults of the instrument itself, that is, faults caused by damage to the structural parts or components of the instrument; The second category is faults caused by external reasons, such as improper installation, flow distortion, deposition and scaling, etc. The focus here is on the application aspects and the above-mentioned second type of external cause failures.
According to the classification of fault occurrence period, it can be divided into: ①Faults during debugging; ②Faults during operation. Debugging faults occur in the early stages of commissioning after new installation. The main reasons are improper selection or setting of instruments, improper installation, etc. Operational faults occur after a period of operation. The main reasons include impurities in the fluid adhering to the electrical lining, and new interference sources arising from changes in environmental conditions.
According to the external source analysis of the fault, it comes from three aspects: ① Caused by piping system and installation; ② Caused by environment; ③ Caused by fluid. Source ① mainly appears during the debugging period; sources ② and ③ will appear during both the debugging and running periods.
1. Faults during commissioning
This type of fault occurs during the initial installation and commissioning of the electromagnetic flowmeter. However, once the fault is improved and eliminated, it will generally not reoccur under the same conditions in the future. . Common faults during the commissioning period mainly include improper installation, environmental interference, and fluid effects.
1. Pipeline system and installation aspects
Usually the fault is caused by incorrect installation position of the electromagnetic flow sensor. A common example is installing the flow sensor in a pipe network that is prone to accumulation of trapped gas. High point; there is no back pressure behind the flow sensor, and the liquid is discharged directly into the atmosphere, causing the measuring tube to be less than full; when installed on a vertical pipe that flows from top to bottom, emptying may occur.
2. Environmental aspects
Mainly include stray current interference from pipelines, space electromagnetic wave interference, large motor magnetic field interference, etc. Stray current interference in pipelines can usually be measured satisfactorily with good single grounding protection. However, if there is a strong stray current in a pipeline (such as an electrolytic workshop pipeline), it may not be able to overcome it. Measures must be taken to isolate the flow sensor from the pipeline edge. Space electromagnetic wave interference is generally introduced through signal cables and is usually protected by single or multi-layer shielding. However, there have been cases where shielding protection cannot be overcome.
3. In terms of fluids
Liquids containing uniformly distributed fine bubbles usually do not affect normal measurement, but the measured volume flow rate is the sum of liquid and gas; the increase in bubbles will cause the output The signal fluctuates. If the bubbles are so large that they flow through the electric circuit and cover the entire electric surface, causing the electric signal loop to be instantly disconnected, the output signal will have 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 measured liquid exceeds a certain level, and the output signal will also fluctuate to a certain extent.
When two or more liquids are mixed in a pipeline, if there is a difference in the conductivity of the two liquids (or the potential between each liquid and the electric potential), they will enter the flow sensor for flow measurement before the mixture is uniform. The output signal will also produceproduce fluctuations.
Poor selection of the electrical material and the medium to be measured will produce chemical effects such as passivation or oxidation, the formation of a thin film on the electrical surface, and electrochemical neutralization phenomena, etc., which will hinder normal measurement.
2. Faults during operation
Faults that occur during operation after initial debugging and normal operation for a period of time. Common fault causes include: attachment layer on the inner wall of the flow sensor, lightning strike, environment Conditions change.
1. Inner wall adhesion layer
Since the electromagnetic flowmeter has far more opportunities to measure suspended solids or dirt than other flow meters, the probability of failure caused by the inner wall adhesion layer is also high. It's 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, causing a hidden fault in 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 electrical surface is blocked and the measuring circuit is disconnected. The latter two phenomena will cause the instrument to fail to work.
2. Lightning strike
Lightning shock induces instantaneous high voltage and surge current in the line, and if it enters the instrument, it will damage the instrument. There are three ways to introduce lightning-damaged instruments: power lines, flow signal lines and excitation lines between sensors and converters. However, from the analysis of damaged parts in lightning faults, most of the induced high voltage and surge current that caused the fault were introduced from the power supply line in the control room, and the other two paths were less. We also learned from the scene of the lightning strike accident that not only the electromagnetic flowmeter failed, but also other instruments in the control room often experienced lightning strike accidents at the same time. Therefore, users must understand the importance of setting up lightning protection facilities for instrument power lines in the control room. The current design team has identified and explored solutions to this problem.
3. Changes in environmental conditions
The main reason is the same as the failure environment during the commissioning period in the previous section, except that the interference source does not appear during the commissioning period but intervenes again during operation. For example, an electromagnetic flowmeter with unsatisfactory grounding protection. During the commissioning period, the instrument operates normally because there are no factory interference sources. However, during the operation period, new interference sources (such as pipelines near the measurement point or pipeline welding at a distance) appear to interfere with the normal operation of the instrument. During operation, the output signal fluctuates greatly.
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