surface of the equipment beingmonitored.The sound generated by a leak canpropagate through the walls of a vessel aswell as through any liquid inside. Ingeneral, it can be said that liquid inside avessel or pipe will assist in thepropagation of sound while liquid outside(such as moisture in the soil) has atendency to reduce the detectable signal.Prediction of the actual acousticwaveform generated by a leak is verydifficult. An example of a point leak in aburied pipeline has been reported.7The frequency of the leak signal can beconsidered broadband at the source.Various applications have been developedusing a variety of sensors with sensitivitiesin the range of 1 to 400 kHz. Using lowerfrequencies implies that the leak can bedetected from greater distances althougheffects of environmental backgroundnoise are more pronounced.A typical low frequency applicationwould be leak testing for buried pipelineswhere sensors are mounted no more than15 to 30 m (50 to 100 ft) from anypotential leak. A typical high frequencyapplication would be that of internal leaktesting for flare gas valves where a sensoris mounted in a location less than about0.3 m (1 ft) from any potential leaks.Feasibility of AcousticEmission LeakageMonitoringAs discussed above, various applicationsrequire different frequency responses.Leak testing for buried pipelines requiresthat a maximum sensor spacing beachieved. This drives the applicationtoward the low frequency range whereattenuation of the acoustic emissionsignal is not as severe as it is at higherfrequencies. However, as the frequency islowered, the effects of background noisebecome more pronounced, indicating thata compromise is required.Internal leakage detection andassessment is performed on valves usingacoustic emission testing. In thisapplication, a sensor is placed on thevalve so that it is less than about 0.3 m(1 ft) from any leak site. Because thesource-to-sensor relationship is so small,attenuation does not become a factor,thus allowing sensors that operate in thehigh kilohertz range, where backgroundnoise is minimized. By taking ameasurement at higher frequencies, thecontent of the signal is dominated moreby the leak than by background noise.This allows an accurate assessment of theleak rate to be made using the acousticsignal.The effectiveness of the applicationand the design of the acoustic emissiondetection/monitoring system (for a givenfrequency range) depends on thefollowing factors: (1) the amplitude of theleak signal at the leak source, (2) thebackground noise level, (3) theattenuation of the signal from the leaksource to the detection sensor and (4) theneed to characterize and separate the leaksignal from other signals.Factors 1 and 4 can be investigated tosome degree in the laboratory throughsimulation. If possible, it is best toinvestigate under field conditions becausethis will provide the only opportunity forinvestigation of factors 2 and 3.
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