Application of plug-in electromagnetic flowmeter in mine water treatment
In order to reduce pollution and save water and reduce emissions, it is necessary to strictly measure and control domestic water, industrial water, circulating water, and discharged sewage. The measurement of flow occupies an important position in the implementation of total water resources control, quota management, and petrochemical production and process control.
1 Technical analysis
1.1 How does it work?
The working principle of plug-in electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When measuring flow, the liquid flows through the magnetic field perpendicular to the direction of flow and induces a voltage proportional to the average flow rate (volume flow). The induced voltage signal is detected by two electrodes that are in direct contact with the liquid and is transmitted to the smart transmitter through a dedicated cable. The smart transmitter performs calculations based on the magnitude of the potential and mathematical model of the electromagnetic flow and converts it into a flow signal (including instantaneous The amount and cumulative amount are displayed in the field, and the standard 4-20MA and 1-5VDC signals are output to the flow secondary instrument or DCS.
1.2 Composition
Plug-in electromagnetic flowmeters mainly consist of sensors, smart transmitters and special cables.
(1) The sensor is composed of an electrode and an excitation coil, and its function is to generate a corresponding induced potential (5 to 60 mV) on the electrode according to the speed of the conductive medium (fluid) flowing through the excitation coil.
(2) The role of the smart transmitter is to amplify the sensor's induced potential signal, calculate it according to the mathematical model and formula of potential flow, and obtain the flow results and standard flow signals. Various flow parameter adjustments and calibration parameters are set on the smart transmitter.
(3) The role of the dedicated communication cable is that the sensor transmits a millivolt potential signal to the smart transmitter, and the smart transmitter provides an oscillating voltage to the excitation coil. Since the potential signal is a weak signal of 5 to 60 mV, and the excitation signal is an oscillation signal of high frequency and low amplitude, the cable must fully consider the shielding and anti-interference. The length of the cable is proportional to the cross-sectional area, which is proportional to the conductance of the medium. The stronger the conductive characteristics, the longer the transmission distance. The general transmission distance is 20 to 500 meters.
1.3 Installation Requirements
1.3.1 Ensure sufficient straight sections
When the direction of the fluid, the diameter of the valve, and the opening of the valve are changed, eddy current and flow field distortion will occur, which will affect the measurement of the sensor electrode, thus affecting the accuracy of the instrument. Therefore, as far as possible from the elbow, hand valve, valve and other components a certain distance. In the table, D indicates the pipe diameter, and L indicates the distance from the sensor to the valve or elbow.
1.3.4 Ensure good grounding
The ground point of the sensor output signal should be electrically connected to the measured medium. When a fluid cuts a magnetic force line to generate a flow signal, the fluid itself acts as a zero potential, a positive potential is generated on one electrode, and a negative potential is generated on the other electrode, alternating alternately. Therefore, the midpoint of the converter input (signal cable shield) must be at 0 potential with the fluid and turn on, so as to form a symmetrical input loop. The midpoint of the input of the converter is in electrical communication with the fluid under test through the ground point of the sensor output signal. Because the output signal of the sensor is very small, only a few millivolts, in order to improve the anti-interference ability of the instrument, the potential of 0 in the input loop must be grounded. Since general metal pipes are connected to the ground and the flowing medium is electrically connected to the ground through the metal pipe, it is not required to provide a separate grounding device for the electromagnetic flowmeter, especially a small-diameter electromagnetic flow sensor. If it is a non-metallic pipe, it must be connected to a separate ground wire.
1.3.5 Ensure the correct insertion depth of the sensor
Due to the laminar flow phenomenon of the fluid in the pipeline, the velocity of the fluid near the wall is slower, and the fluid velocity near the center of the pipeline is faster, according to calculating the average velocity of the closest fluid at D/8 of the pipeline. The wall thickness of the pipe and the length of the flange nozzle are to be calculated during installation to ensure that the electrode depth of the sensor is at the D/8 position. However, not all plug-in flowmeter sensors are installed in the D/8 position of the pipe, and a few are installed in the D/2 position of the pipe. Be sure to carefully study the installation instructions before installation.
2 Conclusion
Plug-in electromagnetic flowmeter is more suitable for large-diameter pipe installation. The application of plug-in electromagnetic flowmeter on the pipeline above Φ250mm is more cost-effective than the flange-type electromagnetic flowmeter, and the larger the diameter, the more obvious the cost-effectiveness. Its most notable feature is that it can be installed with pressure. The thickness of the pipe wall has no effect on it, but its installation technology is more complicated. If it is improperly installed, it will cause a large error. According to the experience, it is not allowed to stop work and the pipe with larger diameter is not allowed. Installing plug-in electromagnetic flowmeters should be an economically viable option.
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