Experimental Analysis Of Ultrasonic Testing For Level Of Material In Tubular Ball Mill Of Fossil-fuel Power Plant

At present, about 80%～85% of pulverizing mill in thermal power generating units of our country are tubular ball mill. In practice, it is difficult to measure the material level of the tubular ball mill accurately, which makes the performance optimization of tubular ball mill hard to realize, automatic control hard to achieve, failure like coal powder leakage and coal blockage often to happen, and results in the efficiency of ball mill unable to ensure, and the labor intensity of operator remains high etc. For the material level in the tubular ball mill, there are some measuring techniques now, such as differential pressure method, electric ear method, axes vibration method and electric current method etc. But these methods can not be used extensively because of the limit of their measuring principle.In the industrial production domain, ultrasonic testing (UT) is one of the five general Nondestructive Testing techniques, which has many advantages including testing reliably, admeasuring quickly, operating easily and doing no harm to people etc. So the UT technique was utilized in every industry or trade of national economy.In view of the above-mentioned status quo, the method of detecting the material level in the tubular ball mill by applying ultrasonic wave is brought up by the task members, and the measured value can be the basis of operational optimization for tubular ball mill system. This dissertation discussed the feasibility of the ultrasonic method experimentally.For the material level in the tubular ball mill, three testing projects were put forward, which are ultrasonic testing thickness, sonar and acousto-ultrasonic. In order to carry on the research conveniently, the material level in the tubular ball mill was simply regarded as the makeup of steel plate with some coal on it, and the experimental table and system were put up. Also, the processing of digital experimental signal was done, which included pre-processing, frequency spectrum of amplitude and power spectral estimation of signal, then the power spectral estimation of signal using different methods was compared.Based on the above-mentioned studies,the experiment of ultrasonic penetrating coal layer and acousto-ultrasonic were carried out. The ultrasonic penetration experiment showed that the more high the detector's working frequency was and big the coal's thickness was, the more great the attenuation of ultrasonic was and poor the penetrating capacity of ultrasonic in coal was, while the ultrasonic whose working frequency was 50KHz, can't penetrate the coal seam 20 centimeters thick. During the acousto-ultrasonic experiment, the extraction of efficient signal was studied, and the result was that under the two situations of steel plate without loading and with coal loading, the received signal contrasted greatly in the characteristic and waveform, in addition, the more small the distance between two detectors was, the more great the difference of energy difference of the received signal was.From the research of this dissertation, it can be concluded that the acousto-ultrasonic testing method is feasible to a certain extent, while the feasibility of ultrasonic penetrating testing method is poor.