The Study On The Acoustic Emission Testing Of The Inner Leakage Of Valve

Valve is an indispensable part of petrochemical system device which is used with high frequency. The reliability and security of valves is always the focused as most of the medium in the process of petrochemical production is corrosive, toxic, flammable, explosive and other characteristics. While in the practical use, the valve leakage accident s occur frequently and the vast majority of the leakage is difficult to find. Therefore, it will not only cause huge waste of raw materials and products, but also irreversible damage on the surrounding environment, worst it may even causes catastrophic accident. So, It has an important application prospect to study the effective and accurate valve leakage detection technology. In this paper, acoustic emission is applied to the detection of valve internal leakage and its characteristics are studied theoretically and experimentally.Firstly, this paper is based on the theory of sound mechanism of the valve leakage which is when the valve leaks, the medium in the valve body will jet out from the leak hole with high-speed forced by pressure difference before and after of the leak.The leakage hole multiphase turbulent jet flow field is formed by producing pressure fluctuation and the acoustic emission signal will come out.According to the principle of river closure of the gate and its internal structure, this paper simplifies the internal leakage of valve port processing, using FLUENT software to simulate gate valves under different opening of leakage flow field, and analyses the influence of inlet pressure and different opening leakage mouth on the fluid. When the valve opening is fixed, with the inlet pressure increases, pressure distribution in the corners of the place of leak and leakage flow channel is higher, and it gradually declines to the center, but the pressure gradient is not obvious. when the inlet pressure is fixed, with the valve opening degree increases, the higher pressure distribution area is also in the place of leak and leakage flow channel edge angle, and it gradually declines to the center. But the change of pressure gradient is clear and hierarchic.Secondly, taking butterfly valve and gate valve as the research object, the fault simulation experiment station and corresponding acoustic emission detection system of the gas and liquid medium in the valve leakage are designed and constructed. Through the different pressure, opening and leakage rate of acoustic emission monitoring experiment, it obtains the variation rules of the change of the sound leakage emission signal amplitude with the duration and RMS voltage with the valve inlet pressure and opening which will determine the optimal location according to the acoustic emission parameter variation of sensor above. At the same time, the intensity of the acoustic emission signal, the distribution of the frequency spectrum of the inlet pressure and the change rules of the opening degree of the valve is known. And fitted the leakage rate of acoustic emission signal effective value voltage according to the double logarithmic relationship through the MATLAB then obtained the quantitative relationship between the leakage rate and the acoustic emission signal effective value of voltage under different inlet pressure.Finally, it processes the collected acoustic emission using the spectrum analysis and wavelet packet analysis method. The effect of the frequency of the valve internal leakage AE signal frequency is determined by spectral analysis method. The energy distribution of the signal energy in each band is obtained by the wavelet packet decomposition knowing the main band of the energy of AE signal. Using FFT power spectral density and the signal energy distribution of acoustic emission obtained by wavelet packet decomposition to establish corresponding relationship between the degree of valve internal leakage and the frequency of signal which is used to characterize the condition of the valve l leakage.According to the valve inner leakage acoustic emission signal characteristics and detection conditions under different conditions in the laboratory, it makes a set of applicable detection scheme for the valve leakage in the field which provides effective guidance for on-site detection.