The first type of fault type sewage water meter operation in the first type of failure occurs is the instrument itself failure, that is, instrument structural parts or components caused by damage to the failure; the second type is external causes of failure, such as improper installation flow distortion, Deposition and scaling. This chapter focuses on the application and the above-mentioned second type of external causes of failure.
According to the classification of failures, sewage flow meters can be divided into faults during commissioning and faults during operation. Debugging fault appears in the initial stage of debugging after new installation. The main reason is that the instrument is improperly selected or set, and the installation is not proper. Run-time faults occur after a period of operation, mainly due to impurities in the fluids adhering to the electrodes and linings, and new interference sources such as changes in environmental conditions.
According to the external source of failure analysis from three aspects: 1 caused by the piping system and installation; 2 caused by the environment; 3 caused by the fluid. The first aspect is mainly manifested during the commissioning period; the latter two aspects will appear during the debugging period and the running period.
First, the debugging period failure This type of fault occurs in the initial installation of sewage water meter debugging, but once improved to eliminate the fault, after the same conditions will not generally appear again. The common debugging faults include three reasons: improper installation, environmental interference, and fluid characteristics.
1. Pipeline systems and installations are usually faults caused by incorrect installation positions of electromagnetic flow sensors. Commonly, flow sensors are installed at the high point of the pipe network where accumulation of entrapped gas is common; there is no back pressure behind the flow sensor, and the liquid is discharged directly into the atmosphere. The formation of its non-full pipe inside the measuring pipe; installed in the vertical pipe from top to bottom, there may be emptying and other reasons.
2. The main aspects of the environment are spurious interference in the pipeline, electromagnetic interference in the space, and magnetic interference in large electrical machines. Stray current disturbances in pipelines are usually measured with good individual grounding to obtain satisfactory measurements, but if there is strong stray current in the pipeline (such as the electrolytic workshop pipe), it may not be overcome, and measures must be taken to insulate the flow sensor from the pipeline (see Case 12). ). Space electromagnetic interference is usually introduced via signal cables and is usually protected by single or multi-layer shielding. However, it has been found that shielding protection cannot be overcome (see Case 10).
3. In the case of fluids, fluids containing uniformly distributed fine air bubbles do not normally affect normal measurements, but when the measured volumetric flow is the sum of both liquid and gas, the output signal fluctuates due to the increase in air bubbles, if the air bubbles flow to the electrode to cover the entire surface of the electrode, The momentary disconnection of the electrode signal loop causes greater fluctuations in the output signal.
Low-frequency (50/16~50/6Hz) square-wave excitation electromagnetic flowmeters produce liquid noise when the liquid contains solids exceeding a certain level, and the output signal may 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.
Second, the operation of the fault After the initial commissioning and normal operation of a period of time, the common causes of the fault occurred during the operation are: flow sensor inner wall adhesion layer; lightning strike; environmental conditions change.
1. Internal Wall Adhesion Layers Sludge flowmeters have a much higher chance of measuring suspended solids or contaminants than other flow meters, and 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. Both of the latter phenomena make the instrument inoperable (see Case 7).
2 Lightning strikes on lightning strikes instantaneous high voltage and inrush currents in the line, which can damage the instrument when it enters the instrument. There are three introduction methods for lightning damage to the instrument: power line, flow signal line and excitation line between sensor and converter. However, from the analysis of damaged components in a lightning fault, most of the induced high voltage and inrush currents caused by the fault are 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 instruments in the control room also often experienced lightning strikes at the same time. Therefore, the use of units to understand the importance of setting up the control room instrument power line lightning protection facilities. Several design units have now recognized and explored solutions to this problem, such as the Qilu Petrochemical Design Institute [1].
3. The main reason for the change of environmental conditions in the sewage flow meter is similar to that in the fault environment during the commissioning period, except that the source of the interference is not in the commissioning period but is introduced during the operation period. For example, an electromagnetic flowmeter with unsatisfactory grounding protection has no interference source and the instrument is operating normally during the commissioning period. However, during the operation period, new interference sources appear and the instrument operates normally. However, during the operation period, new interference sources occur (such as near the measuring point). Pipes or pipes welded farther away from the implementation of interference with the normal operation of the instrument, the output signal fluctuations.
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