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Can electromagnetic flowmeter measure mixture?

Date: 2019-03-19
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1, electromagnetic flowmeter measuring liquid containing bubbles


The tiny bubbles mixed into bubble-like flow can still work normally, but the mixed volume flow with bubble volume is measured.  If the gas content increases to form a slug flow, the circuit may be instantaneously disconnected due to the electrode being covered by the gas, resulting in output shaking and even failure to work normally.




2, electromagnetic flowmeter measurement of solid-liquid two-phase flow of liquid


Solid-liquid two-phase fluid containing non-ferromagnetic particles or fibers can also measure the volume flow of the two phases.  Fluids with high solid content, such as drilling mud, drilling cement slurry and pulp, are actually non-Newtonian fluids.  As solids flow together in the carrier liquid, there is slippage between the two and there is a difference in speed, the single-phase liquid calibration instrument will produce additional errors when used in solid-liquid two-phase fluid.  There are reports abroad that the error is within 3% when the solid content is 14%.  According to the test report of the water conservancy science research institute of the yellow river water conservancy Commission, the volume ratio of sediment concentration is 17% to 40% (the median particle size of sand is 0. 35 mm), and the measurement error of the instrument is less than 3%.


Larger particles in the slurry rub over the electrode surface, and spike slurry noise will be generated in rectangular excited electromagnetic flowmeter with lower frequency, which will make the flow signal unstable. Therefore, higher frequency instrument or instrument with stronger capability of inhibiting slurry noise, commercial AC excited instrument or dual-frequency excited instrument shall be selected.




3, electromagnetic flowmeter measuring liquid containing ferromagnetic substances


Fluids containing ferromagnetic substances will produce measurement errors for common electromagnetic flowmeters because the permeability in the measurement tube varies with the content of ferromagnetic substances.  However, EMF of electromagnetic flowmeter compensated by magnetic flux detection coil in magnetic circuit can reduce the influence of mixed ferromagnetic.  The test report shows that the water contains iron ore slurry with a liquid-solid weight ratio of about 4: 1 and a particle size of less than or equal to 0.15 mm. A 80mm gauge is used for the comparison flow test of clear water and slurry. The normal gauge shows a change of 7% to 10%, and the gauge equipped with magnetic flux detection coil shows an indication error within+/-±2% Fs.




4, electromagnetic flowmeter measuring pulp


For the application of ore pulp containing ore particles, attention should be paid to the abrasion degree of the sensor lining, and additional errors will be generated when the inner diameter of the measuring tube is enlarged.  In this case, ceramic lining or polyurethane rubber lining with good wear resistance should be selected. At the same time, it is suggested that the sensor should be installed on the vertical pipeline to make the pipeline wear evenly and eliminate the defect of severe local wear in the lower half of horizontal installation.  A nozzle-shaped sheath can also be additionally arranged at the inlet end of the sensor, so that the service life is relatively prolonged.


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