Fault inspection and analysis of electromagnetic flowmeter in application

1 Introduction

Common faults in the use of electromagnetic flowmeters include faults caused by damage to the instrument's own components, and faults caused by improper selection, improper installation, environmental conditions, fluid characteristics and other factors, such as display fluctuations, reduced accuracy and even instrument damage. It can generally be divided into two types: faults that occur during installation and commissioning (commissioning period faults) and faults that occur during normal operation (operation period faults).

2 Working Principle

The electromagnetic flowmeter consists of a transmitter and a converter, which are connected to each other by connecting wires (including excitation wires and signal wires). 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 lines of force, so an induced electromotive force ex is generated in the direction perpendicular to the flow of the medium and the magnetic lines of force.

Figure 1: Structural principle of electromagnetic flowmeter

ex=bdvⅹ10-4v

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 during the debugging period

Faults during the debugging period usually occur during the initial installation and debugging of the instrument, but once the fault is improved and eliminated, it will generally not reappear under the same conditions in the future. Common debugging period faults are usually caused by improper instrument selection or setting, improper installation, environmental interference, and fluid characteristics.

(1) Installation

is usually caused by incorrect installation position of the electromagnetic flow sensor. Common examples include installing the sensor at the highest point of the pipeline network where gas is easily accumulated; or installing it on a vertical pipe from top to bottom, which may cause emptying; or there is no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere, forming a non-full pipe in the measuring pipe. [page]

(2) Environmental aspects

are usually mainly caused by pipeline stray current interference, strong electromagnetic wave interference in space, and magnetic field interference from large motors. Pipeline stray current interference usually takes good single grounding protection to obtain satisfactory results. For example, on April 12, 2007, the electromagnetic flowmeter FT-31235 of the first set of melamine equipment of Henan Coal Chemical Group Zhongyuan Dahua Company encountered strong stray current and could not indicate normally (the electrolysis workshop pipeline, sometimes the AC potential peak VPP induced on the two electrodes can be as high as 1V). The staff immediately took single grounding protection, but its indication still could not return to normal. Therefore, measures such as insulating the flow sensor from the pipeline were taken before FT-31235 could work normally. Spatial electromagnetic wave interference is generally introduced through the signal cable, and is usually protected by single or multi-layer shielding.


(3) In terms of fluid,

the measured liquid contains uniformly distributed tiny bubbles, which usually do not affect the normal operation of the electromagnetic flowmeter. However, as the bubbles grow, 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 instantly disconnected as the bubbles flow through the electrode, causing greater fluctuations in the output signal. When the electromagnetic flowmeter with low-frequency square wave excitation measures slurry with excessive solid content, it will also generate slurry noise, causing the output signal to fluctuate. When measuring mixed media, if the conductivity of the two liquids (or the potential between each and the electrode) is different, the output signal will also fluctuate if they enter the flow sensor for measurement before the mixture is evenly mixed. Improper matching of electrode materials and the measured medium will produce chemical reactions such as passivation or oxidation, the formation of an insulating film on the electrode surface, and electrochemical and polarization phenomena, which will hinder normal measurement. The electrode material should be correctly selected according to the instrument selection or relevant manual.

3.2 Operational failures

Operational failures are failures that occur after the electromagnetic flowmeter has been debugged and has been running normally for a period of time. Common operational failures are generally caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes, and changes in environmental conditions.

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

Since the electromagnetic flowmeter has a much greater chance of measuring suspended solids or dirty bodies than other flow meters, the probability of failure caused by the adhesion layer on the inner wall is relatively high. If the conductivity of the adhesion layer is similar to that of the liquid, the instrument can still output signals normally, but the flow area will change, 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 will be insulated and the measurement circuit will be disconnected. Therefore, the adhesion scaling layer in the measuring tube of the electromagnetic flowmeter should be removed in time.

(2) Lightning strike

Lightning strikes can easily induce high voltage and surge current in the instrument circuit, causing damage to the instrument. It is mainly introduced through power lines or excitation coils or flow signal lines between sensors and converters, especially from the power lines in the control room, which account for the vast majority.

(3) Changes in environmental conditions

During the commissioning period, the flowmeter works normally because the environmental conditions are still good (for example, there is no interference source). At this time, it is often easy to neglect the installation conditions (for example, the grounding is not very good). For example, the electromagnetic flowmeter FT-81235 of the third set of melamine plant of Henan Coal Chemical Group Zhongyuan Dahua Company operated normally during the commissioning period, but on the morning of March 12, 2006, the output signal suddenly fluctuated greatly. After checking, the sensor and converter of the instrument itself and the cable connecting the two were found to be normal. It turned out that the pipeline welding nearby interfered with the normal operation of the instrument. The staff immediately implemented good grounding protection before the instrument could indicate normally.

4 Fault phenomenon and inspection process

Common fault phenomena of electromagnetic flowmeters are:

(1) No flow signal;
(2) Output shaking;
(3) Zero point instability;
(4) Flow measurement value does not match the actual value;
(5) The output signal exceeds the full value.

The procedure for checking the entire measurement system and judging the fault is usually shown in Figure 1. The inspection links include the sensor and converter of the electromagnetic flowmeter itself and the cable connecting the two, the process pipeline above the electromagnetic flowmeter, and the cable connecting the lower (back) display instrument. 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 components of the circuit board in the converter to determine the location of the fault by substitution method.
- Signal trace method: Use an analog signal device to replace the sensor, and provide a flow signal under the condition that the liquid is not flowing, so as to test the electromagnetic flow converter.

The inspection starts with whether the display instrument is working normally, and proceeds in the opposite direction of the flow signal transmission. Use an analog signal device to test the converter to determine whether the fault occurs in the converter and its subsequent instruments or in the upper sensor of the converter. If the converter fails, if conditions permit, it can be conveniently borrowed from the converter or the circuit board in the converter for substitution method debugging; if the sensor fails and needs to be replaced, it must be stopped and the pipeline system must be shut down, which involves a wide range and is often difficult to do. Especially for large-caliber flow sensors, the trial replacement project is large, and usually only after completing other inspections, it is finally determined to remove the pipeline to check the internal condition of the sensor measuring tube or replace it. The relevant maintenance items are shown in Table 1.

5 Conclusion

During the use of electromagnetic flowmeter, various faults may occur, but generally speaking, all faults can be attributed to two categories, namely, commissioning failures and operation failures. As long as we always pay attention to these two types of failures in daily work and eliminate and solve them well, I believe that the electromagnetic flowmeter will be able to play its due role.