The influence of medium mixture and attachment on the measurement of pipeline electromagnetic flowmeter

In the measurement of pipeline electromagnetic flowmeter, it is not affected by the changes of fluid density, viscosity, temperature, pressure and conductivity; there are no obstructive flow parts in the measuring tube, no pressure loss, and the straight pipe section requirement is low. It has unique adaptability to slurry measurement. However, the unevenness caused by other mixtures in the medium will have a certain impact on the measurement, such as bubbles and magnetic particles, which are factors that cause fluctuations in measurement results. Below we will give a specific explanation of these factors affecting the measurement:

1. The influence of the contained substances in the medium on the measurement:

1. Bubbles: tiny bubbles mixed into the bubbling flow can still work normally, but the measured volume flow rate is the mixed volume flow rate containing the bubble volume; if the gas content increases to form a bullet (block) flow, the electrode may be covered by the gas, causing the circuit to be disconnected instantly, resulting in output shaking or even failure to work normally.

2. Magnetic substances: solid-liquid two-phase fluids containing non-ferromagnetic particles or fibers can also measure the volume flow rate of the two phases. Fluids with high solid content, such as drilling mud, drilling cementing slurry, pulp, etc., are actually non-Newtonian fluids. Since the solid flows together in the carrier liquid, there is sliding between the two, and there is a difference in speed, so the single-phase liquid calibration instrument will produce additional errors when used for solid-liquid two-phase fluid.

3. Solid matter: Although there is no systematic experimental report on the effect of EMF on solids in solid-liquid two-phase fluid, there are reports from abroad that the error is within 3% when the solid content is 14%; the experimental report of the Water Conservancy Science Research Institute of the Yellow River Conservancy Commission of China stated that when measuring the flow of high-sand water, the volume ratio of sand content is 17% to 40% (the median particle size of sand is 0.35mm), the instrument measurement error is less than 3%.

4. Larger particles: When larger particles in the slurry rub against the electrode surface of the pipeline electromagnetic flowmeter, spike-shaped slurry noise will be generated in the EMF with a lower frequency rectangular excitation, making the flow signal unstable. It is necessary to select a higher frequency instrument or an instrument with a stronger ability to suppress slurry noise. You can also use an AC excitation instrument or a dual-frequency excitation instrument.
Fluids containing ferromagnetic substances will produce measurement errors for the usual EMF because the magnetic permeability in the measuring tube changes due to the different contents of ferromagnetic substances. However, the EMF compensated by the magnetic flux detection coil in the magnetic circuit can reduce the influence of the mixed ferromagnetic substances. Jiangsu Zhongneng Instrument Factory stated in the experimental report of the AC excitation instrument that the slurry containing iron ore with a liquid-solid weight ratio of about 4:1 and a particle size of ≤0.15mm was tested for the flow rate of clean water and slurry with an 80mm diameter instrument. The usual instrument indicated a change of 7% to 10%. The instrument equipped with a magnetic flux detection coil has an indication error within ±2%FS.

5. For the application of slurry containing ore particles, attention should be paid to the degree of wear of the sensor lining of the pipeline electromagnetic flowmeter . The expansion of the inner diameter of the measuring tube will produce additional errors. In this case, ceramic linings or polyurethane rubber linings with good wear resistance should be selected. At the same time, it is recommended that the sensor be installed on a vertical pipeline to make the pipeline wear evenly and eliminate the disadvantage of severe local wear in the lower half of the horizontal installation. A nozzle-shaped sheath can also be installed at the inlet end of the sensor to relatively extend the service life.

2. The influence of adhesion and precipitation in the pipeline on the measurement results:

When measuring fluids that are easy to adhere to and precipitate on the pipe wall, if the attached material is a conductive material with a higher conductivity than the liquid, the signal potential will be short-circuited and cannot work. If it is a non-conductive layer, the pollution of the pipeline electromagnetic flowmeter electrode should be paid attention to first, such as the use of pointed or hemispherical protruding electrodes that are not easy to adhere, replaceable electrodes, scraper-type scale-removing electrodes, etc. The scraper-type electrode can manually scrape out the deposits outside the sensor regularly.

Foreign products once had ultrasonic transducers installed on the electrodes to remove the surface scale layer, but now it is rare. There is also a temporary disconnection of the measurement circuit, and a low-voltage high current flows during the electrode interruption time to burn and remove the attached grease layer. The flow rate can be increased in places where adhesion is easy to occur to achieve the purpose of self-cleaning. It can also be connected to a more convenient and easy-to-clean pipeline, and the pipeline electromagnetic flowmeter sensor can be cleaned without disassembly.