Accurate measurement of flowing objects has always been a problem, especially in the steel industry. Due to a lot of dust, severe vibration, high temperature, a

Accurate measurement of flowing objects has always been a problem, especially in the steel industry. Due to a lot of dust, severe vibration, high temperature, and humidity in the steel plant, the working environment of the instrument is poor, and it is more difficult to ensure the long-term accuracy and reliability of the measurement data. But now, enterprise management assessment has higher and higher requirements for data accuracy. The situation forces us to improve the quality of our work and produce data products that meet the assessment requirements. For the measurement of fluids, there are already a wide variety of flow meters with various measurement principles. After years of installation, use and maintenance practices, I think that the measurement effect of electromagnetic flowmeters for industrial water in steel mills is ideal. Now summarize it for your reference.
Keywords: industrial water metering, electromagnetic flowmeter, operation and maintenance

1. Working principle and structure of electromagnetic flowmeter
 
The electromagnetic flowmeter that uses Faraday's law of electromagnetic induction as the measurement principle is a commonly used instrument for measuring the flow of conductive liquids in the industry. It has a wide range of applications and can measure various corrosive media and slurries with suspended particles. Its working principle is shown in Figure (1):

Figure (1) Schematic diagram of electromagnetic flowmeter
In a uniform magnetic field, there is a pipe with a diameter D perpendicular to the direction of the magnetic field. The pipe is made of non-magnetic material and the inner surface of the pipe is lined with an insulating lining. When the conductive liquid flows in the conduit, the conductive liquid cuts the magnetic lines of force, so an induced potential is generated in a direction perpendicular to the magnetic field and the flow direction. If a pair of electrodes is installed, a potential difference proportional to the flow rate is generated between the electrodes.
    E=BDυ*10-8 volts
    Where: E - induced potential (volts);
    B——magnetic induction intensity (Gauss);
    D - inner diameter of the pipe (cm);
    υ—the average velocity of the liquid in the pipe (cm/s)
    The electromagnetic flowmeters in actual use all work with alternating magnetic fields.
    B=Bmax*simωt
    E＝Bmax*simωt*D＊υ＊10-8
    =(4Q/Dπ)*10-8*Bmax*simωt=KQ
    Where K=4*10-8*Bmax*simωt/πD
    The flow Q=πDE*108/4Bmax*simωt, that is, the induced potential E is proportional to the flow Q.
    For a meter, the magnetic induction is known and basically constant. Therefore, as long as E is tried to be measured, the flow Q will be known.
    Two installation of electromagnetic flowmeter
    1. The installation position of the electromagnetic flowmeter can be selected according to the needs. And its sensor can be installed obliquely or vertically. However, no matter what form of installation is adopted, it should be noted that the axes of the two electrodes must be roughly in the horizontal direction. As shown in (Figure 2).

Figure II)
 2. It should also be noted that the sensor of the electromagnetic flowmeter should be installed in a place that can ensure that the measuring tube is filled with the measured medium, and there should be no phenomenon that the tube is not full or bubbles gather in the measuring tube. So don't install the sensor on the highest part of the pipe, but on the lower, upward-sloping pipe. When installing on vertical pipes, do not install the liquid flow from top to bottom, but install it on bottom-up pipes. The sensor cannot be installed at the inlet of the water pump, because it is easy to form a vacuum here, which will affect the metering effect.
3. When installing the sensor, pay attention to the direction of the arrow on the sensor, which refers to the direction of liquid flow.
4. Although the sensor of the electromagnetic flowmeter is not very sensitive to vibration, if it is installed in a place with strong vibration, it needs to be supported on both sides of the pipeline where the sensor is installed.
5. If the measured medium is a seriously dirty liquid or a liquid that is easy to scale in the pipe, a bypass pipe can be added to the measuring pipe where the sensor is installed to facilitate disassembly and cleaning. As shown in (Figure 3).

Figure (3)
For pipelines with DN ≥ 350, if the cost of adding a bypass pipe is high, a sensor with a scraper electrode can also be used. When it is necessary to clean the dirt on the sensor electrode, you can open the shell cover and turn the knob a few times. A detachable electrode sensor can also be used. When the electrode is detached, it can be carried out under pressure without affecting the operation of the process equipment. However, the above two methods only clean the electrodes of the sensor. If the measuring pipe of the sensor is seriously polluted and scaled, the sensor still needs to be removed for cleaning.
6. In terms of the installation requirements of the electromagnetic flowmeter sensor, the requirement for the length of the straight pipe section is generally not less than 10 D on the upstream side of the sensor (D is the diameter of the pipe), and not less than 2D on the downstream side. But in the actual installation, because our company is located in a mountainous area, restricted by the geographical environment, especially for large-diameter pipelines, it is difficult to meet the 10 D measurement on the upstream side. According to our practice, the upstream side can generally guarantee 5 D measurement The effect is not bad.
7. The installation of the converter is generally installed indoors, and it is sufficient not to expose it directly to sunlight.
8. For the installation of the grounding ring of the electromagnetic flowmeter, three different installation methods should be adopted for three different pipelines with no insulating coating, insulating coating and cathodic protection on the inner surface of the process pipeline.
    Three operation and maintenance of electromagnetic flowmeter
    The failure of the electromagnetic flowmeter during operation refers to the failure of the flowmeter after it has been debugged and operated normally for a period of time. Common failures during operation are generally caused by factors such as the adhesion layer on the inner wall of the flow sensor, lightning strikes, changes in operating environment conditions, and changes in the zero position of the system.
    1. Sensor inner wall adhesion layer
    If the measured medium is a dirty fluid, after a period of operation, an adhesion layer will often accumulate on the inner wall of the sensor and cause failure. These failures are often caused by the conductivity of the adhesion layer being too large or too small. If the attachment is an insulating layer, the electrode circuit will be disconnected and the meter will not work normally; if the conductivity of the adhesion layer is significantly higher than that of the fluid, the electrode circuit will be short-circuited and the meter will not work normally. Therefore, the attached 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 line, which can damage the instrument. It is mainly introduced through power lines, excitation coils or flow signal lines between sensors and converters, especially from the control room power lines.
    3. Changes in environmental conditions
    During commissioning, because the environmental conditions are good (for example, there is no interference source), the flowmeter works normally. At this time, it is often easy to ignore the installation conditions (for example, the grounding is not very good). In this case, once the environmental conditions change, new interference sources appear during operation (such as electric welding on nearby pipelines, large transformers installed nearby, etc.), which will interfere with the normal operation of the instrument, and the output signal will fluctuate.
    4. Zero point change of the system
    Under normal operating conditions, the system zero point of the electromagnetic flowmeter will cause the system zero point to be caused by factors such as the aging of components, the reduction of the insulation strength of the excitation coil, the polarization and pollution of the measurement electrode, and the increase in the system grounding resistance (potential) with the long-term operation of the system. changes and drift. Therefore, the system zero point of the flowmeter should be checked regularly, adjusted and maintained. The system zero point of a magnetic flowmeter occurs when the sensor is filled with medium and there is no flow. At this point the system can be zero adjusted.
    Four conclusions
    In industrial water metering, we have used standard orifice plates, Annubar flowmeters and ultrasonic flowmeters before. By comparison, electromagnetic flowmeters are more suitable for industrial water metering in steel plants, because it has many characteristics: 1. Higher accuracy 2. There is no need to consider the problem of antifreeze and heat preservation in winter; 3. Energy saving, almost no pressure loss 4. The performance is relatively stable and the maintenance is small. In actual use, the effect is ideal.