Advanced Ultrasonic Backscatter Technique (AUBT)

Last update: Jan 13, 2017

Advanced Ultrasonic Backscatter Technique (AUBT) involves a collection of ultrasonic techniques used for detecting High Temperature Hydrogen Attack (HTHA) in process equipment. AUBT is well-documented in API RP 941.

Key steps in the AUBT procedure include:

  • A pattern-based backscattering technique is used as the initial screening method on the external surface that has been flapper-wheel (200 grit) prepared.
  • Depending on the backscatter pattern observed, one of several follow-up techniques, including frequency dependent backscatter, velocity ratio, spectral analysis, and spatial averaging, can be used to determine the cause of backscattering signals.
  • Note that velocity ratio, because of the lack of both speed and sensitivity, is used only when backscatter shows uniform microstucture (i.e. this indicates either no damage or near through-wall damage), to determine if damage really exists.
  • Spectrum analysis uses Fast Fourier Transform (FFT) to convert the signal readout from amplitude versus time to amplitude versus frequency. This technique compares spectra of back-wall echoes measured from suspect locations with a "clean" reference spectrum taken from a "no damage" area. This follow-up technique is used because of its speed and sensitivity to HTHA. 
  • A spatial averaging technique may be used where velocity ratio and spectral analysis have difficulty determining whether nearly through-wall HTHA exists (e.g. for clad nozzles, etc.)
  • For weld and heat affected zone (HAZ) inspections, angle-beam techniques based on backscatter and spectral analysis principles are used. These angle beam techniques do not require flush grinding of weld crowns for inspection.

 

Recommend changes or revisions to this definition.

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