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surface of the equipment being monitored. The sound generated by a leak can propagate through the walls of a vessel as well as through any liquid inside. In general, it can be said that liquid inside a vessel or pipe will assist in the propagation of sound while liquid outside (such as moisture in the soil) has a tendency to reduce the detectable signal. Prediction of the actual acoustic waveform generated by a leak is very difficult. An example of a point leak in a buried pipeline has been reported. 7 The frequency of the leak signal can be considered broadband at the source. Various applications have been developed using a variety of sensors with sensitivities in the range of 1 to 400 kHz. Using lower frequencies implies that the leak can be detected from greater distances although effects of environmental background noise are more pronounced. A typical low frequency application would be leak testing for buried pipelines where sensors are mounted no more than 15 to 30 m (50 to 100 ft) from any potential leak. A typical high frequency application would be that of internal leak testing for flare gas valves where a sensor is mounted in a location less than about 0.3 m (1 ft) from any potential leaks. Feasibility of Acoustic Emission Leakage Monitoring As discussed above, various applications require different frequency responses. Leak testing for buried pipelines requires that a maximum sensor spacing be achieved. This drives the application toward the low frequency range where attenuation of the acoustic emission signal is not as severe as it is at higher frequencies. However, as the frequency is lowered, the effects of background noise become more pronounced, indicating that a compromise is required. Internal leakage detection and assessment is performed on valves using acoustic emission testing. In this application, a sensor is placed on the valve so that it is less than about 0.3 m (1 ft) from any leak site. Because the source-to-sensor relationship is so small, attenuation does not become a factor, thus allowing sensors that operate in the high kilohertz range, where background noise is minimized. By taking a measurement at higher frequencies, the content of the signal is dominated more by the leak than by background noise. This allows an accurate assessment of the leak rate to be made using the acoustic signal. The effectiveness of the application and the design of the acoustic emission detection/monitoring system (for a given frequency range) depends on the following factors: (1) the amplitude of the leak signal at the leak source, (2) the background noise level, (3) the attenuation of the signal from the leak source to the detection sensor and (4) the need to characterize and separate the leak signal from other signals. Factors 1 and 4 can be investigated to some degree in the laboratory through simulation. If possible, it is best to investigate under field conditions because this will provide the only opportunity for investigation of factors 2 and 3.
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  • Fall '19
  • Acoustic Emission

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