Solution to Bubble Noise of Electromagnetic Flowmeter

The measurement principle of the electromagnetic flowmeter is Faraday's law of electromagnetic induction to measure the volume flow of conductive liquids and slurries in closed pipelines, including highly corrosive liquids such as acids, alkalis, and salts. Most of the false alarms of electromagnetic flowmeters in the measurement of cooling water in the steel industry are caused by air bubbles passing over the electrodes, forming a short-term induction signal that is zero. This is a kind of cavitation phenomenon, and we call this kind of fault bubble noise. The following introduces the avoidance and solution of the bubble noise problem. First of all, the installation should meet the length requirements of the upstream straight pipe section of the electromagnetic flowmeter, standardize the installation of the instrument, choose the installation place far away from the heat source, use the pipeline flow rate reasonably, and choose the smooth PFA fluoroplastic lining and high-purity oxidized industrial ceramic conduit. These measures will help prevent or reduce the occurrence of vortices and gas separation. That is to say, measures such as improving the sensor manufacturing process, improving the environmental conditions and installation conditions of the instrument, and installing an exhaust valve upstream of the instrument may avoid the occurrence of the problem. Secondly, setting the damping time and function of the electromagnetic flowmeter reasonably can also solve the false alarm of bubble noise measurement. The damping time is selected according to the pulse width of the bubble noise in the flow signal. Generally, the damping time should be 3~5 times of the pulse width of the bubble noise. If the pulse width of bubble noise is 10s, the damping time should be 30~50s. The specific selection should be based on the required control accuracy. The control error of 3 times pulse width is 5%, and the control accuracy of 5 times pulse width is higher than 1%. Increasing the damping time of the instrument can effectively solve the impact of this pulse-shaped bubble noise, but it also brings the disadvantage of slow response, that is, when the real flow fluctuates, the instrument responds very slowly. This is undoubtedly a difficult problem for cooling water systems that require sensitive control. In order to solve this problem, the intelligent electromagnetic flowmeter can use software logic judgment, that is, the method of coarse error processing. When this kind of failure occurs, adjust the insensitivity time of the flow rate and the limit of the change range to judge whether it is the change of the flow rate or the air bubbles rubbing against the electrode. If it is not the noise of air bubbles rubbing against the electrode, the CPU performs normal sampling, calculation and digital filtering; if it is determined that the noise is air bubbles, the measured value is cut off and the previous flow measured value is maintained. In this way, the damping time is still 3~6s during normal flow measurement. Only when there is bubble noise, the insensitivity time will be lengthened according to the pulse width setting, and the system control time will also be lengthened. When we reasonably select the change rate limit value and insensitive time value of the electromagnetic flow converter with coarse error suppression function, the converter can not only suppress false alarms caused by bubble noise, but also maintain the response speed of the instrument during normal operation. Set the damping time value. The research on the bubble noise of electromagnetic flowmeter should be to simulate the electrode of electromagnetic flow sensor with bubbles, but there is no such condition at present. Therefore, we only use the switching of the electromagnetic flow signal generator signal to simulate the bubble noise. Proper selection of damping time and intelligent electromagnetic flowmeter's method of dealing with bubble noise faults will have an obvious effect on observing flowmeter display and output signal changes and judging the effect of dealing with bubble noise. Switch the switch of the standard signal source of the intelligent electromagnetic flowmeter, quickly set the flow rate and zero point, keep the signal at zero time as required, and simulate the occurrence and existence of bubble noise. Change the damping time of the meter and set different rate-of-change limit values ​​and insensitive time values ​​to test the change of the meter output. The results show that increasing the damping time and intelligent bubble noise processing can achieve no large changes in the output, and the latter is more conducive to the improvement of the measurement response speed during normal measurement.

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