Acoustic Emission Testing – Acoustic Sensors and Instrumentation

Most acoustic sensors used for acoustic emission testing of pressure vessels employ piezoelectric ceramic elements that convert the stress waves (i.e., acoustic emissions) propagating through the vessel shell into electrical impulses. These sensors are usually attached to the vessel shell with magnets or an epoxy glue. Several sensors are positioned on the shell in a geometric pattern that is based on the size and configuration of the vessel, to be certain that significant emissions from any major component of the vessel will be detected by at least two of the sensors. Multiple sensors are also required for the location of the acoustic emission sources by triangulation.

The electrical outputs of the acoustic sensors are sent separately to a preamplifier that contains a band pass filter to cut off frequencies below 50 kHz. Acoustic emissions with lower frequencies are predominantly mechanical and hydraulic “noises” (such as movement of insulation, vessel supports, internals, and flange connections), and they could seriously confuse the interpretation of emissions from flaws. Some emissions from these mechanical and hydraulic sources can have higher frequencies that coincide with those that originate at flaws, which must be taken
into consideration when analyzing the data. The filtered signal from each sensor is subsequently sent to a main amplifier, and then on to various multichannel recording and monitoring instruments.

Recording of the acoustic emission data for permanent retention is usually accomplished with a magnetic disk. The amplitude of every emission detected by each acoustic sensor is recorded on a time base. Audio and visual monitors can be provided for “real-time” observation of the test results. X-Y recorders are used to plot the acoustic emission data as a function of some test parameter, such as hydrostatic test pressure. These plots can be made during the test, but they are more commonly made immediately after the test for a more detailed analysis of the data. X-Y recorders can also be used to plot on a “map” of the vessel the locations of the flaws revealed by the acoustic emissions.

Pressure transducers are used to determine the hydrostatic test pressure, which is also recorded on the magnetic disk on a time base.

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