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The AE phenomenon
Materials and structures that undergo stress, deformation and cracking
processes, emit sound and ultrasound waves i.e. acoustic emission signals.
These signals can be acquired by sensitive sensors that are mounted throughout
the entire length of the line. The unique QAE NDI analysis of these signals is
used for the identification of flaws’ location, their type, danger level and
the dynamics of their development.
Monitoring System
Measurement points are typically spaced 15 ft (~5 meters) from each other
and in potential high stress zones, closer. The average spacing is 10ft (~3
meters) for Hot-Reheat (HRH) and Main Steam (MS) lines and 8 ft (~2.5
meters) for Cold Reheat (CRH) lines. Each measurement point includes:
sensor (A), preamplifier (B) and waveguide (C).
The Analysis
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Margan developed a unique statistical approach to the AE signals analysis. This
includes, among variety of statistical tools, the ellipse of dispersion, which
is used for:
1) Creation of laboratory -made flaw-material and flaw-stage specific AE "finger
prints".
2) Characterization and classification of different groups of AE Signals.
3) Characterization of the background noise.
Ellipses of dispersion are used in the analysis process for accurate
separation and filtration of the background noise, friction noise and other
non-related signals, already in the field. The remaining signals which are
related to flaw-development (crack propagation, creep development, fracturing
of hard inclusions, etc.) are analyzed to determine flaw-types and their danger
level. This is done by comparing their ellipses (the solid ellipse in the
example) to the lab-made AE "finger-prints" (the transparent ellipses).
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The Report and Results
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Results and recommendations include:
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Locating zones of active flaw development in the line (marked yellow).
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Flaw 'type'
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Determining their danger level in terms of fracture mechanics criteria
(J-integral value).
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Recommendations for further monitoring intervals.
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Hangers and Supports Performance
Proper performance of hangers and supports plays a crucial role
in the line integrity. Improper performance of hangers and supports can create
overstressed zones, increase the risk of flaw nucleation and accelerates
development of existing flaws.
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In this HRH line,the AE activity at the hangers had changed significantly
between the inspections of 7/2000 (green bars) and 1/2001 (blue bars). The
analysis showed indications of overstresses in the piping between hangers No 4
and No 5 due to malfunctioning of hanger No 4. This hanger was repaired and the
line was inspected again 2 years later. The results of the inspection (pink
bars) show more uniform distribution of stresses along the line. Consequently,
the development of flaws that had been identified in the previous inspection,
was slowed down.
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