Pressure testing background noise was generally low

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Pressure Testing Background noise was generally low in all cases. Noise from the recirculation system and nitrogen flow was noted in some tests. Noise was reduced to acceptable levels by shutting off hydrofluoric acid vapor recirculation and reducing the nitrogen flow rate as appropriate. Each test followed the inservice test sequence shown in Fig. 4. The exception was in the first year. This ranged from 100 to 120 percent of the maximum operating pressure instead of the 90 percent to 110 percent range in the procedure. Test Results Analysis of acoustic emission data begins with any necessary data file processing, assessment of data quality and filtering of identifiable noise. The acoustic emission data from each transducer is then compared to the following evaluation criteria: (1) emission during load holds, (2) cumulative duration or count rate during loading, (3) number of hits above and below operating pressure, (4) large amplitude hits and (5) cumulative signal strength. Specific values for the criteria depend on the type of test, type of vessel and material. In most cases, the acceptance levels for acoustic emission during load holds and high amplitude hits are 275 Acoustic Emission Testing of Pressure Vessels, Pipes and Tanks F IGURE 3. Arrangement of transducers on sphere: (a) as seen from above; (b) as seen from south. 1 1 5 8 7 15 14 13 12 24 23 22 31 30 29 36 40 41 42 39 35 21 20 5 4 3 2 1 0 4 3 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 (a) (b) F IGURE 4. Inservice pressure vessel test sequence. 110 100 90 80 70 60 Percentage of maximum operating load Elapsed time (relative scale) Legend A. 90 percent, 10 min hold (background). B. 100 percent, 10 min hold. C. 105 percent, 10 min hold. D. 110 percent, 30 min hold. A B C D
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minimal. In all cases, increasing signal strength or amplitude will fail the fifth acceptance criterion, cumulative signal strength. If a vessel fails one or more of the criteria (and most do), intensity analysis is applied to help measure the significance of the acoustic emission data. In fact, common practice is to apply intensity analysis as a matter of course with the evaluation criteria and other measures of acoustic emission significance. Further filtering and evaluation may be needed especially when there is a noisy data set or questionable activity. Location plotting serves to narrow down areas for followup testing in this type of test. Test results are reported in terms of the location or zone of acoustic emission activity, its intensity and recommended action. Notes range from no further action through note for reference to immediate followup testing of the area. Initial and Followup Tests Table 2 summarizes the test results. Intensity analysis is used to compare tests but is not the sole measure of acoustic emission significance. Sometimes (and wrongly) seen as a method for automatic analysis of acoustic emission data, the processor simply measures acoustic emission intensity. It does not distinguish between acoustic emission from cracks and acoustic emission from fluid flow, rain or pebbles tossed on the vessel to see if the lights flash. The number of hits (in
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  • Fall '19
  • Nondestructive testing, Hydrostatic test, Acoustic Emission

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