The measurement principle of electromagnetic flowmeter is Faraday’s electromagnetic induction law to measure the volume flow of conductive liquid and slurry in closed pipeline, including acid, alkali, salt and other highly corrosive liquids. The false alarm of electromagnetic flowmeter in the measurement of cooling water in iron and steel industry is mostly caused by bubbles rubbing the electrode, resulting in a short-term induction signal of zero, which is a kind of cavitation phenomenon. We call this fault bubble noise. The following describes the avoidance and solution of bubble noise.
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, select the installation site away from the heat source, reasonably use the pipeline flow rate, and select PFA fluoroplastic lining with high finish and high-purity alumina industrial ceramic conduit. These measures will help to prevent or reduce the occurrence of vortex and gas separation. In other words, it is possible to avoid problems by improving the sensor manufacturing process, improving the environmental conditions and installation conditions of the instrument, and installing an exhaust valve upstream of the instrument.
Secondly, setting the damping time and function of electromagnetic flowmeter reasonably can also solve the false alarm of bubble noise measurement. The selection of damping time is based on the pulse width of bubble noise in the flow signal. Generally, the damping time should be 3 ~ 5 times of the pulse width of bubble noise. If the pulse width of bubble noise is 10s, the damping time shall be 30 ~ 50s. The specific selection shall be based on the required control accuracy. The control error of three times of pulse width is 5%, and the control accuracy of five times of pulse width is higher than 1%.
Increasing the damping time of the instrument can effectively solve the influence of this pulse bubble noise, but also bring the disadvantage of slow response, that is, when the real flow fluctuates, the response of the instrument is very slow. This is undoubtedly a difficult problem for the cooling water system requiring sensitive control. In order to solve this problem, the intelligent electromagnetic flowmeter can use software logic judgment, that is, the method of gross error processing. In case of such failure, it is determined whether the flow changes or the bubbles rub the electrode by adjusting the insensitive time of the flow and the limit of the change range. If it is not the noise of bubbles rubbing the electrode, the CPU will conduct normal sampling, operation and digital filtering; If it is determined that bubble noise is generated, cut off the measured value and maintain the previous flow measurement value. In this way, the damping time during normal flow measurement is still 3 ~ 6S. Only when there is bubble noise, the insensitive time and the system control time will be prolonged according to the length of pulse width setting.
When we reasonably select the change rate limit value and insensitive time value of the electromagnetic flow converter with gross error suppression function, the converter can not only suppress the false alarm caused by bubble noise, but also maintain the set damping time value when the instrument works normally.
The research on bubble noise of electromagnetic flowmeter should be the simulation test of bubble on the electrode of electromagnetic flow sensor, but there is no such condition at present. Therefore, we only use the switching of electromagnetic flow signal generator signal to simulate bubble noise. Properly selecting the damping time and the method of intelligent electromagnetic flowmeter to deal with the bubble noise fault is obvious for observing the change of flowmeter display and output signal and judging the effect of dealing with bubble noise. Switch the switch of standard signal source of intelligent electromagnetic flowmeter, quickly set the flow rate and zero point, and keep the signal at zero as needed to simulate the occurrence and existence of bubble noise. Change the instrument damping time, set different change rate limit values and insensitive time values, and test the change of instrument output. The results show that increasing the damping time and intelligent bubble noise treatment can achieve that the output does not change greatly, and the latter is more conducive to the improvement of measurement reaction speed during normal measurement.
Post time: 02-03-22