Electromagnetic flowmeters can measure highly corrosive liquids including acids, alkalis, and salts. Most of the false alarms in the measurement of cooling water in the iron and steel industry by electromagnetic flowmeters are due to bubbles passing through the electrodes, forming a short-term induction signal of zero. This is a kind of cavitation phenomenon. We call this kind of failure a bubble noise. Here's how to avoid and solve the bubble noise problem.
First of all, it should meet the requirements of the length of the straight section of the upstream of the electromagnetic flowmeter, regulate the installation of the instrument, select the installation site away from the heat source, use the flow rate of the pipeline reasonably, and use the PFA fluorine plastic lining with high finish and the high purity alumina industrial ceramic conduit. These measures will help prevent or reduce the occurrence of vortex and gas separation. In other words, improving the sensor manufacturing process, improving the environmental conditions and installation conditions of the instrument, and adopting measures such as installing an exhaust valve upstream of the instrument may avoid problems.
Secondly, setting the damping time and function of the electromagnetic flowmeter reasonably can also solve the false alarm that the bubble noise measurement occurs. The damping time is selected based on the pulse width of the bubble noise in the flow signal. Generally, the damping time should be 3 to 5 times the bubble noise pulse width . If the bubble noise pulse width is 10s , the damping time should take 30~50s . Specific selection should be based on the required control accuracy, 3 times the pulse width control error in 5% , 5 times the pulse width control accuracy is higher than 1% .
Increasing the instrument damping time can effectively solve the impact of this pulse-type bubble noise, and at the same time it also brings the disadvantage of unresponsiveness, that is, when the true flow fluctuations, the instrument response is very slow. This is undoubtedly a problem for cooling water systems that require sensitive control. In order to solve this problem, intelligent electromagnetic flowmeters can use software logic to determine the method of coarse error processing. In the event of such a failure, it is determined whether the flow changes by adjusting the insensitivity of the flow rate and the limit of the change in the amplitude. If it is not the noise of the air bubbles rubbing the electrodes, the CPU performs normal sampling, operation, and digital filtering; if it is determined that bubble noise is generated, the measured values ​​are cut off and the previous flow rate measurement is maintained. In this way, the damping time during normal flow measurement is still 3~6s . Only when there is bubble noise, the insensitivity time will be lengthened according to the length of the pulse width setting, and the system control time will also be lengthened.
When we reasonably select the change rate limit value and insensitivity time value of the electromagnetic flow converter with coarse error suppression function, the converter can not only prevent the false alarm caused by bubble noise, but also the response speed of the instrument can still be maintained during normal operation. Set the damping time value.
The study of the electromagnetic noise from the electromagnetic flowmeter should be conducted by using a bubble to simulate the electromagnetic flow sensor electrode. However, such conditions have not yet existed. Therefore, we only use the electromagnetic flow signal generator signal switching to simulate the bubble noise. Appropriate selection of damping time and smart electromagnetic flowmeter to deal with the bubble noise failure method, observe the flowmeter display and output signal changes, determine the effect of processing bubble noise is obvious. Switch the intelligent magnetic flowmeter standard signal source switch, set the flow rate and zero point quickly, keep the signal zero time as needed, simulate the occurrence and existence of bubble noise. Change the instrument damping time and set different limit values ​​of change rate and insensitivity time values ​​to test the change of the meter output. The results show that the increase in damping time and intelligent bubble noise processing can achieve large changes in the output, the latter is more conducive to the measurement of reaction speed during normal measurement. Â
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