In actual pipeline installations, difficulty is encountered in identifying the leak when it is buried underground. This paper helps develop AE method to facilitate the analysis and characterization of pipeline defects. The results describe a common behavior of AE waveforms for each type of pinhole using FFT analysis. The pinhole diameter varies from 0.3 to 0.5 mm and from 0.5 to 1.85 mm depth for straight-type and stepwise-type, respectively. The stepwise pinhole is considered as converging-diverging nozzle, in which high-pressure air ranging from 100 kPa to 500 kPa flows over the sonic velocity. When air flows through the pinhole, instability of AE signals occurred above the critical value due to the burst pressure at the throat. We recorded the highest value ofAE magnitude as 153 mV at 280 kPa for straight pinhole with 0.5 mm diameter and as high as 37 mV at 290 kPa for 0.3 mm diameter. Here, the pressure shock occurred for all types of pinhole and the screech tone was generated specifically for straight-type and slit-type defects because of the large-scale instability of waves from airflow, in which the peak frequency decreases with increasing pressure. The critical pressure at 0.22 MPa was calculated based on the assumption that the flow velocity reached to its sound velocity. In the case of stepwise-type pinhole, different characteristics of AE waveforms were observed. Apparently, this was due to the inflow of air through the pinhole at various depths as well as the bursting of signal and unusual noise occurred beyond its critical pressure. Also, before reaching the critical value, a monotonic increase of amplitude took place since unsteady flow occurred at the onset along with the self-excited vibration of the generated AE. For stepwise pinhole with 1.0 mm depth, a disturbed flow was detected having a noticeable drop of AE amplitude from 13.2 mV to 8.9 mV over the critical pressure. Furthermore, experiment was conducted for slit-type pinhole, which shows that the rapid fluctuation of the waveforms was caused by stronger energy released so its amplitude became higher in magnitude. Clearly, the unstable AE signals above the critical pressure were due to effect of varied pinhole diameter since the greater the pinhole the larger the volume of air released while the pressure increases.
Acoustic emission waveform analysis of gas leakage on pipe with various types of artificial defects
2007
6 Seiten, 8 Bilder, 9 Quellen
Aufsatz (Konferenz)
Englisch
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