1.1 measurement principle of electromagnetic flowmeter
Electromagnetic flowmeter works based on Faraday’s law of electromagnetic induction and consists of sensor and converter. The sensor is mainly composed of measuring tube lined with insulating material, a pair of electrodes installed through the wall of measuring tube, a pair of coils and iron core to generate working magnetic field. When the conductive medium flows through the measuring tube of the sensor, there is a stable magnetic field with magnetic induction intensity generated by the transmitter in an interval perpendicular to the flow direction of the medium. When the conductive fluid with average flow velocity flows through the magnetic field area, it moves to cut the magnetic line of force. Therefore, the induced potential is generated on the two detection electrodes parallel to the cross-section of the pipeline and perpendicular to the direction of the magnetic line of force. According to the law of electromagnetic induction, the instrument constant is a constant when the pipeline diameter D has been determined and the magnetic induction intensity remains unchanged. At this time, the induced potential has a linear relationship with the volume flow, that is, the induced potential E and the volume flow Q of the medium are measured
1.2 main technical parameters
Maximum flow rate: 15m / S; Accuracy: DN15 ～ DN600, ± 0.3% of the indicated value, DN700 ～ dn24000, ± 0.5% of the indicated value; Fluid conductivity: not less than 5p S/cm； Maximum fluid temperature: ordinary type, 70oC; Parting type: PTFE lining, 100oC, neoprene lining, 80 ℃, polyurethane lining, 80 ℃, polypropylene lining, 100 ℃.
1.3 advantages of electromagnetic flowmeter
① There are no throttling and movable parts in the measuring conduit, so the pressure loss is very small, energy is saved, and the flow of fluid containing particles, suspended solids and so on can be measured. ② As long as the lining and electrode are in contact with the medium, the purpose of anti-corrosion can be achieved by selecting the data of both; As long as the electrode is not polluted, the accuracy of the instrument will not be reduced after long-time use. ③ There is a linear relationship between the output current and flow of electromagnetic flowmeter. And not affected by the physical properties of the liquid; Therefore, after water calibration, it can be used to measure the flow of other conductive liquids or solid-liquid two-phase media without correction. ④ Smart converter adopts novel excitation mode, with low power consumption, stable zero point, high precision, flow range up to 100:1, convenient parameter setting, reliable programming, and self inspection and self diagnosis functions. ⑤ No mechanical inertia, flexible response, able to measure pulsating flow.
2 Application of electromagnetic flowmeter in chlor alkali metering
2.1 suitability of electromagnetic flowmeter in chlor alkali measurement
The chlor alkali industry mainly measures brine and caustic soda, which have common characteristics:
① The composition of brine is complex, sometimes acidic, sometimes alkaline, and may contain free chlorine due to unclean dechlorination of ionic membrane. Both of them have strong corrosivity.
② Both are conductive media.
③ In the chlor alkali plants in the north, due to the low temperature in winter, the salt deposition phenomenon occurs in the brine pipeline, and the salt will be precipitated with the decrease of temperature during the escort process; Ionic membrane caustic soda will crystallize at about 3 ~ C; When the pipeline is long and intermittently escorted, our factory also showed 32% caustic soda crystallization in winter, resulting in pipeline infarction, so both measured media will contain solid particles.
④ The temperature of both media is below 100oC.
According to the characteristics of the measured medium and the advantages of electromagnetic flowmeter, it can be seen that electromagnetic flowmeter is very suitable for chlor alkali measurement. Although the price of electromagnetic flowmeter is expensive, it has high precision, small maintenance, long service life and high cost performance.
Differential pressure flowmeter, turbine flowmeter and elliptical gear flowmeter are affected by fluid working state, fluid density, viscosity and other parameters, which increases the measurement error. Due to the wear of solid particles, the service life of the measuring element is reduced, and even the blockage can not be used, and the corrosivity can not be solved. Our factory used elliptical gear flowmeter to measure diaphragm electrolyte, but it was stopped due to the precipitation and blockage of salt in electrolyte, and finally replaced with electromagnetic flowmeter.
2.2 selection of electromagnetic flowmeter
2.2.1 selection principle of electromagnetic flowmeter:
① Considering the caliber and range, although the range of electromagnetic flowmeter is set arbitrarily, its setting range is limited by the caliber. The setting of measuring range should consider that the normal flow exceeds half of the full range, so that the measurement accuracy is high. The flow rate is generally 2 ~ 4rn / s. if the medium wears the electrode or the medium flows by itself according to the level difference of the high-level groove, a slightly lower flow rate can be selected; If the medium is easy to adhere, the appropriate high flow rate can be selected. After comprehensive consideration, the diameter of the instrument shall be selected according to the flowmeter.
② The measured fluid must be conductive liquid or slurry, and its conductivity shall not be less than 5 ~ s / cm. The measured fluid shall not contain more ferromagnetic substances or bubbles; According to the type, working pressure and corrosivity of the instrument, the type and material of the lining shall be selected.
Post time: 14-02-22