Fault inspection and analysis of the hottest elect

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Fault inspection and analysis of electromagnetic flowmeter in application

1 introduction

common faults in the use of electromagnetic flowmeter, some are caused by the damage of components of the instrument itself, some are caused by improper selection, improper installation, environmental conditions, fluid characteristics and other factors, such as display fluctuation, accuracy decline and even instrument damage. It can generally be divided into two types: failure during installation and commissioning (failure during commissioning) and failure during normal operation (failure during operation)

2 working principle

electromagnetic flowmeter is composed of transmitter and converter, which are connected with each other by connecting wires (including excitation wire and signal wire). The transmitter works based on the law of electromagnetic induction. As shown in Figure 1, the measured medium flows perpendicular to the direction of the magnetic line of force, so an induced electromotive force ex is generated in the direction that both the medium flow and the magnetic line of force are perpendicular

Figure 1 structure principle of electromagnetic flowmeter

ex=bdv x v

where: B - magnetic induction intensity, t (Tesla); D - distance between two electrodes (pipe diameter), cm; V - average flow rate of the measured medium, cm/s

3 common faults

3.1 faults in the commissioning period

faults in the commissioning period generally occur in the initial installation and commissioning stage of the instrument, but once the faults are eliminated after improvement, they generally will not occur again under the same conditions in the future. Common faults during commissioning are usually caused by improper instrument selection or setting, improper installation, environmental interference and fluid characteristics

(1) in terms of installation

it is usually the fault caused by the incorrect installation position of the electromagnetic flow sensor, such as installing the sensor at the highest point of the pipe easy to accumulate gas if necessary; Or installed on the vertical pipe from top to bottom, which may be emptied; Or there is no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere to form a non full tube in the measuring tube

(2) in terms of environment

generally, it is mainly pipeline stray current interference, space strong electromagnetic wave interference, large motor magnetic field interference, etc. Generally, satisfactory results can be obtained by taking good separate grounding protection for pipeline stray current interference. For example, the electromagnetic flowmeter ft-31235 of the first melamine device of Zhongyuan Dahua Company of Henan coal chemical group encountered a strong stray current on April 12, 2007 and could not indicate normally (the peak value VPP of AC potential induced on the two electrodes of the pipeline in the electrolysis workshop can be as high as 1V), and the staff immediately took single grounding protection, However, its indication still could not return to normal, so the ft-31235 could work normally after taking measures such as isolating the flow sensor from the pipeline. Space electromagnetic wave interference is generally introduced through signal cables, and is usually protected by single-layer or multi-layer shielding

(3) in terms of fluid

the measured liquid contains uniformly distributed tiny bubbles, which usually does not affect the normal operation of electromagnetic current TBEA, which is Xinjiang Zhonghe's first largest shareholder. However, with the increase of bubbles, the output signal of the instrument will fluctuate. If the bubbles are large enough to cover the entire electrode surface, the electrode circuit will be momentarily opened as the bubbles flow through the electrode, resulting in greater fluctuations in the output signal. When the electromagnetic flowmeter excited by low-frequency square wave measures the slurry with excessive solid content, it will also produce slurry noise, causing the output signal to fluctuate. When measuring the mixed medium, if the conductivity of the two liquids (breaking through the bottleneck of the new material or the potential between each and the electrode) is different, it will enter the flow sensor for measurement before the mixing is uneven, which will also cause the output signal to fluctuate. Improper selection of electrode material and the measured medium, resulting in chemical effects such as passivation or oxidation, the formation of insulating film on the electrode surface, and electrochemical and polarization phenomena will hinder normal measurement. Electrode materials should be selected and matched correctly according to instrument selection or relevant manuals

3.2 operation period fault

operation period fault is the fault that occurs after the electromagnetic flowmeter has been debugged and operated normally for a period of time. Common operation period faults are generally caused by the adhesion layer on the inner wall of the flow sensor, lightning strike, changes in environmental conditions and other factors

(1) adhesion layer on the inner wall of the sensor

because the electromagnetic flowmeter has far more opportunities to measure suspended solid or dirty bodies than other flow meters, the failure probability of the adhesion layer on the inner wall is relatively high. If the conductivity of the attachment layer is similar to that of the liquid, the instrument can output signals normally, but change the flow area, forming a hidden fault of measurement error; If it is a high conductivity adhesion layer, the electromotive force between electrodes will be short circuited; If so, how should the tester be installed? The surface of the electrode is insulated and the measuring circuit is disconnected. Therefore, the attached scaling layer in the measuring tube of the electromagnetic flowmeter should be removed in time

(2) lightning strike

lightning strike is easy to induce high voltage and surge current in the instrument circuit, causing damage to the instrument. It is mainly introduced through the power line or excitation coil or the flow signal line between the sensor and the converter, especially from the power line in the control room

(3) changes in environmental conditions

during commissioning, due to the good environmental conditions (e.g. no interference source), the flowmeter works normally. At this time, it is often easy to neglect the installation conditions (e.g. grounding is not very good). For example, the electromagnetic flowmeter ft-81235 of the third melamine plant of Zhongyuan Dahua Company of Henan coal chemical group operated normally during the commissioning period, but the output signal suddenly fluctuated significantly in the morning of March 12, 2006. After checking that the sensor and converter of the instrument itself and the cable connecting them were free of any fault, it turned out that the normal operation of the instrument was disturbed by the electric welding of pipelines near it, The instrument can be normally indicated only after the staff immediately implement good grounding protection

4 fault phenomena and inspection process

common fault phenomena of electromagnetic flowmeter are:

(1) no flow signal

(2) output shaking

(3) zero point instability

(4) the measured value of flow is inconsistent with the actual value

(5) over full value of transmission signal

generally, the procedure of checking the whole measurement system and judging the fault is shown in Figure 1. The inspection link includes the sensor and converter of the electromagnetic flowmeter itself, as well as the cables connecting them, the upper process pipeline of the electromagnetic flowmeter, and the lower (rear) display instrument connecting cables. The frequently used inspection means or methods and their inspection contents are listed as follows:

- general routine instrument inspection

- substitution method: use the interchangeability between the converter and the sensor and between the circuit board components in the converter to identify the location of the fault by substitution method

- signal trace method: replace the sensor with an analog annunciator to provide a flow signal under the condition that the liquid is not flowing, so as to test the electromagnetic flow converter

the inspection starts from whether the display instrument works normally and reverses the direction of flow signal transmission. Test the converter with an analog annunciator to determine whether the fault occurred in the converter and its rear instrument or in the upper sensor of the converter. If the converter fails, the converter or the circuit board in the converter can be easily used for alternative debugging if conditions permit; If the sensor fails and needs to be replaced, it is often difficult to stop the operation and close the pipeline system, which involves a wide range. In particular, the trial replacement of large-diameter flow sensors requires a large amount of work. Usually, it is only after other inspections are completed that the decision is made to remove the pipe to check the internal condition of the sensor measuring pipe or replace it. Relevant maintenance items are shown in Table 1

5 conclusion

various faults may occur in the process of using the electromagnetic flowmeter, but generally speaking, all faults can be divided into two types, namely, faults during commissioning and faults during operation. As long as we pay attention to these two types of faults in our daily work and eliminate them well, we believe that the electromagnetic flowmeter will play its due role

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