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Attenuation is measured by applying an artificial

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Attenuation is measured by applying an artificial acoustic emission source at increasing distances from a transducer. Plotting the received amplitude values shows the loss of signal with distance. Attenuation should be measured in two 272 Acoustic Emission Testing F IGURE 1. Loading schemes: (a) new pressure vessel; (b) hybrid test of a column; (c) sphere fill and pressure test. (a) (b) (c) Percentage of test load Elapsed time (min) 0 50 100 150 200 250 300 350 126 115 110 105 100 95 90 85 Pressure in proportion to operating load (percent) Elapsed time (min) 0 100 200 300 400 500 600 700 800 900 Legend = fill test = pressure test Legend A. Top section test to 114 kPa (16.5 lb f ·in. –2 gage). B. Midsection test to 228 kPa (33 lb f ·in. –2 gage). C. Bottom test to 341 kPa (49.5 lb f ·in. –2 gage). D. Final test pressure to 469 kPa (68 lb f ·in. –2 gage), 90 percent of operating pressure. 120 100 80 60 40 20 0 50 65 85 100 percent hydrostatic test hold 30 min if final 50 65 85 98 percent 10 min hold Background Minimum time = greater of hydrostatic test hold or 600 s to maximum of three days First loading Second loading Background (10 min) Elapsed time (not to scale) 552 (80) 483 (70) 414 (60) 345 (50) 276 (40) 207 (30) 138 (20) 69 (10) (0) Pressure, kPa (lb f ·in. –2 gage) A B C D
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directions (horizontal and vertical) and both above and below the liquid level if applicable. The most common artificial acoustic emission source is a 0.3 mm (0.013 in.) diameter pencil graphite. Fracture of the graphite, using a consistent (3 mm or 0.13 in.) length, angle (0.52 rad or 30 deg) and distance from each transducer provides a reproducible, high amplitude signal. Some systems provide pulsing capabilities to calibrate the transducers remotely. Mechanical devices like spring loaded center punches are used to verify the system operation before and after the actual testing. Monitoring Background acoustic emission activity must be characterized before loading. Low level background noise such as flow noise may be eliminated with a small detection threshold increase. In more severe cases, it may be necessary to identify the source and take remedial action such as tightening a flange or valve. Each transducer’s acoustic emission activity is monitored throughout the pressure test. A test log is kept with notes on noise sources and acoustic emission activity. Periodic observation of the equipment and its surroundings is good practice and should also be logged. This and other information is necessary for proper evaluation of the test data. Acoustic emission activity that shows a departure from linearity (increasing signal strength, amplitude or activity rate with load) should be monitored closely. Continuing acoustic emission activity during load holds is also an indication that significant discontinuities may be present. Normally, the acoustic activity is recorded and saved for further analysis. On rare occasions, if the acoustic emission activity is of concern, then the loading shall be stopped and the load reduced until acoustic emission activity returns to acceptable levels.
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  • Fall '19
  • Nondestructive testing, Hydrostatic test, Acoustic Emission

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