Accurate measurement of flowing objects has always been a problem, especially in the steel industry. Due to the large amount of dust, severe vibration, high temperature, and humidity generated during production in steel plants, the working environment of instruments is poor, making it more difficult to ensure long-term accuracy and reliability of measurement data. Nowadays, enterprise management assessment has increasingly 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 now a wide variety of flow meters with various measurement principles. After many years of installation, use and maintenance practice, I believe that the electromagnetic flow meter is ideal for measuring industrial water in steel plants. It is summarized now for your reference.
Keywords: Industrial water measurement, electromagnetic flow meter, operation and maintenance
1. Working principle and structure of electromagnetic flow meter
 
    Electromagnetic flow meter that apply Faraday's law of electromagnetic induction as the measurement principle are currently commonly used instruments in industry to measure the flow of conductive liquids. 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 flow meter
    Suppose there is a pipe with diameter D perpendicular to the direction of the magnetic field in a uniform 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 field lines, thus generating an induced electric potential in the direction perpendicular to the magnetic field and the flow direction. If a pair of electrodes is installed, an electric potential difference proportional to the flow rate will be generated between the electrodes.
    E = BDv x 10-8 V
    Among them: E--induced electric potential (volts);
    B--Magnetic induction intensity (Gauss);
    D--Pipe inner diameter (cm);
    Υ--Average velocity of liquid in the pipe (cm/s)
    Electromagnetic flow meter in actual use all work with alternating magnetic fields. At this time
    B = Bmax x sin(ωt)
    E = Bmax x sin(ωt) x D x v x 10-8
    = (4Q/Dπ) x 10-8 x Bmax x sin(ωt) = KQ
    where K = 4 x 10-8 x Bmax x sin(ωt) / πD
    Flow Q=πDE*108/4Bmax*simωt, that is, the induced potential E is proportional to the flow Q.
    For an instrument, the magnetic induction intensity is known and basically unchanged. Therefore, as long as we try to measure E, the flow rate Q will be known.
    2. Installation of electromagnetic flow meter
    1. The installation location of the electromagnetic flow meter can be selected according to your needs. And its sensor can be installed tilted or vertically. However, no matter what form of installation is used, it must be noted that the axes of the two electrodes must be roughly in the horizontal direction. As shown in (Figure 2).

Picture (2)
    2. It should also be noted that the sensor of the electromagnetic flow meter should be installed in a position that ensures that the measuring tube is filled with the medium to be measured, and there should be no incomplete tube or bubbles gathering in the measuring tube. So do not install the sensor at the highest point of the pipe, but install it on a lower upward-sloping pipe. When installing on a vertical pipe, do not install it on a pipe where the liquid flows from top to bottom, but install it on a bottom-up pipe. 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 flow meter is not very sensitive to vibration, if it is installed in a place with strong vibration, it is necessary to add supports on both sides of the pipe where the sensor is installed.
    5. If the measured medium is a severely dirty liquid or a liquid that is easy to scale in the pipeline, 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, you can also use a scraper electrode sensor. When you need to clean the dirt on the sensor electrode, you can open the housing cover and turn the knob a few times. Detachable electrode type sensors can also be used. When the electrodes are disassembled, they 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 contaminated and scaled, the sensor still needs to be removed and cleaned.
    6. Regarding the installation requirements of the electromagnetic flow meter sensor, the length of the straight pipe section is generally not less than 10 D (D is the diameter of the pipe) on the upstream side of the sensor and not less than 2D on the downstream side. However, in actual installation, because our company is located in a mountainous area and is restricted by the geographical environment, especially for large-diameter pipelines, it is difficult to meet 10 D on the upstream side. According to our practice, the upstream side can generally guarantee 5 D measurement effect, which is also good.
    7. When installing the converter, it is generally installed indoors and not directly exposed to sunlight.
    8. When installing the grounding ring of the electromagnetic flow meter, three different installation methods should be adopted for three different pipelines: those with no insulating coating, those with insulating coating, and those with cathodic protection on the inner surface of the process pipeline.
    3. Operation and maintenance of electromagnetic flow meter
    Faults that occur during operation of the electromagnetic flow meter refer to faults that occur after the flow meter has been debugged and operated normally for a period of time. Common faults 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 environmental conditions, and changes in system zero position.
    1. Adhesion layer on the inner wall of the sensor
    If the measured medium is dirty fluid, after running for a period of time, an adhesion layer will often accumulate on the inner wall of the sensor, causing 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 open and the instrument will not work properly; if the conductivity of the attachment layer is significantly higher than the conductivity of the fluid, the electrode circuit will be short-circuited and the instrument will not work properly. Therefore, the attached scaling layer in the electromagnetic flow meter measuring tube should be removed in time.
    2. Lightning strike
    Lightning strikes can easily induce high voltage and surge current in instrument circuits, causing damage to the instrument. It is mainly introduced through power lines, excitation coils or flow signal lines between sensors and converters, especially the majority of them introduced from the power lines in the control room.
    3. Changes in environmental conditions
    During debugging, because the environmental conditions are good (for example, there is no interference source), the flow meter 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 and new interference sources appear during operation (such as electric welding on nearby pipelines, large transformers installed nearby, etc.), it 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 flow meter will change and drift due to factors such as aging of components, reduction in the insulation strength of the excitation coil, polarization and contamination of the measuring electrode, and increase in system grounding resistance (potential) over the long-term operation of the system. Therefore, it is necessary to regularly check the system zero point of the flow meter and perform adjustments and maintenance. The system zero point of the electromagnetic flow meter occurs when the sensor is filled with medium and there is no flow. At this time, the system can be zero-adjusted.
    4. Conclusion
    In industrial water measurement, we have previously used standard orifice plate, Annubar flow meter and ultrasonic flow meter. By comparison, electromagnetic flow meter are more suitable for industrial water measurement in steel plants because they have many characteristics: 1. High precision, which can fully meet the requirements of the production process; 2. There is no need to consider antifreeze and warmth in winter; 3. Energy saving, almost no pressure loss; 4. Stable performance and minimal maintenance. The effect is ideal in actual use.