Research On Acoustic Emission Feature Extraction Of Gap Leakage And Its Application In Leaking Fault Diagnosis

The heat ex-changers are important auxiliary equipment in thermal power plant. Internal leakages are common and multiple faults in heat ex-changers, which has an important impact on the safety and economic operation of the whole unit under the normal operation. Due to the harsh environment of the heat ex-changers(the high-pressure heater work in thermal power plant especially), there are multiple factors can affect its operation, which created great difficult on diagnose the internal leakage is exist or not. At present, the internal leakage detection we used has some limitations so that it is difficult to accurately detect out the leakage. Thus, there is a quite important engineering value in application of acoustic emission testing on the heat ex-changers to detect leakage.By using the method for combining numerical simulation and experimental research, comprehensively considering practical typical leakage accidents, the flow field and acoustic emission signal under the conditions of the leakage and non-leakage of the tube gap in the heat ex-changers were studied in this dissertation.Firstly, the main reason of gap leakage of heat ex-changer in power plant is analyzed in this dissertation, and the mechanism of acoustic emission arising from leakage is elaborated. Meanwhile, the data acquisition system of acoustic emission is also introduced in this dissertation; the wavelet decomposition theory is explained, and the related parameters of acoustic emission signal are confirmed; the model of heat ex-changer with gap leakage is established.After mesh model and reasonable resetting of boundary conditions, we simulated the gap leakage heat ex-changer accident with different entrance pressure, and found the corresponding gap leakage flow. According to the study on the leakage data, we found that the entrance pressure and the stagnation pressure value on the gap is linear, what's more, the leak speed and energy loss increases exponentially with entrance pressure.Secondly, the heat ex-changer leakage acoustic emission detection system had be set up and the AE wavelet decomposition procedure was programmed. Based on the experimental station, we have studied the gap leakage under all circumstances and measured the corresponding leakage acoustic emission signal. Analyzing the signal data by wavelet decomposition procedure, we've found that after wavelet noise reduction processing, the noise signal in the collected acoustic emission signals can basically eliminate, the real leakage signals can be well preserved, and the signal peak under the leakage mode keep stable, does not vary with the tube entrance pressure. After the signal wavelet decomposition under these two modes, the difference of each layer signal is very great,which can be a basis used to judge whether there is a leakage in a heater.Finally, based on the comparison with experimental data and simulated data, it can been found that the numerical simulation energy loss value and the value of energy after the experimental signal decomposed by the wavelet had the same trend with the change of working condition; at the same time, according to the study on numerical simulation data of energy loss value, leakage volume changes with the experimental signal energy parameters, we had found the quantitative relation of that three parameters, it provides reference for the leakage detection in practical application.