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Advanced Ultrasonic Backscatter Technique (AUBT)

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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, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.

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 the velocity ratio, because of the lack of both speed and sensitivity, is used only when backscatter shows uniform microstructure (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.

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Articles about Advanced Ultrasonic Backscatter Technique (AUBT)
May/June 2021 Inspectioneering Journal

Hydrogen-induced damage mechanisms have been studied endlessly, leading to changes in equipment design and development of new inspection practices. However, the highly localized nature and difficulty of detection of HTHA add additional complexities.

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HTHA of hydrogen-containing equipment can be prevented with appropriate material selection and fabrication, appropriate welding procedures, regular inspection of equipment using proven, effective technologies and equipment operated by qualified...

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High Temperature Hydrogen Attack (HTHA) is a long known and still occurring degradation issue for fixed equipment construction materials in the hydrocarbon process industry where hydroprocess plants (hydrogen plus hydrocarbons) are in service....

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