field implementation of a transient eddy current system
TRANSCRIPT
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Field Implementation of a Transient Eddy
Current System for Carbon Steel Pipe
Thickness MeasurementsJeremy A. Buck, Colin Kramer, Jia Lei, and Brian A. [email protected] 613-584-3311 ext. 42359
Canadian Nuclear Laboratories, Chalk River LaboratoriesInspection, Monitoring and Dynamics Branch
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• Tile hole storage arrays
• Inspection head and probe design
• Transient eddy current (TEC) testing theory
• Analysis methods and results
• Voltage-threshold analysis
• Power-law analysis
• TEC multi-frequency analysis
• Comparison to ultrasonic testing (UT) results
• Summary and conclusion
Outline
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• Engineered concrete structure for radioactive waste storage
• Multiple vertical tile holes
• Each tile hole has a carbon steel liner:
• 5 m long, and surrounded by concrete
• 250 mm inside diameter Schedule 40 pipe
• 9.3 mm nominal wall thickness
• Carbon steel is a common waste container material:
• Good structural properties
• Neutron absorber
Tile Hole Storage ArraysBackground
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Tile Hole Storage Arrays
• Inspection requirements:
• Determine overall condition of carbon steel pipes
• Report thickness to ±1 mm
• Flag if thickness <50% nominal
• Sample 10 tile holes from four separate arrays
• Challenges:
• Unknown pipe surface conditions
• Moderate liftoff (~13 mm)
• Ferromagnetic material
• No prior inspection data
Inspection Requirements and Challenges
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Inspection HeadDesign Approach
TEC Sensors
UT Transducers
• Inspection head combining TEC and UT probes
• Complimentary techniques
• 8 UT transducers
• 8 TEC coil pairs
• Sensors evenly spaced in 45° intervals
• Couplant not required for TEC inspection
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Inspection Head
• Two TEC sensor designs:
• “Reflection” coil pairs (T1R1, T2R3, T3R5, T4R7)
• “Transmit/Receive” coil pairs (R2, R4, R6, R8)
• Sensor designs alternated around the probe
• Two scans of each tile hole were performed, rotating the inspection head 45°, for full coverage with both probe types
• TEC measurements collected every 100 mm translating down the pipe
TEC SensorsT1 R2
R1
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Transient Eddy Current (TEC) Testing
• Eddy currents generated via Faraday’s Law:
휀 = −𝑁𝑑𝜑
𝑑𝑡• Broadband voltage pulse used to induce eddy currents
• Eddy currents decay following a diffusion process
• TEC testing much less susceptible to skin effects
Theory
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Analysis Methods
• Signal-to-noise ratio of T/R data hindered analysis, so reflection data was exclusively examined.
• Three analysis techniques were applied to the TEC data:
• Voltage-threshold analysis
• Power-law analysis
• TEC multi-frequency analysis
Overview
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Voltage-Threshold
• Channels calibrated independently for each scan to compensate for thickness variations around the calibration pipe and slight differences in probe response
• A linear interpolation was used to estimate unknown thickness
%𝑊𝑇 = 100 −𝑊𝑇𝑁𝑂𝑀 −𝑊𝑇𝐼𝐷𝐶𝑁𝑂𝑀 − 𝐶𝐼𝐷
∗ 𝐶𝑁𝑂𝑀 − 𝐶𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡
• Assumed thickness and voltage-threshold relationship was linear
Calibration
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Voltage-ThresholdResults
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270°
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Angular Position
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Axial Position [mm]
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Power-Law Analysis
• Decay of TEC signals can be described as a sum of decaying exponentials
• To first order, the rates of decay are related to a diffusion time:
𝜏𝐷~𝜇𝜎𝓁2
• A sum of exponentials can be approximated by a power-law fit:
𝑦 = 𝐴𝑥−𝐵
• Power coefficients B are expected to correlate to remaining wall thickness
Method
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Power-Law Analysis
• Each channel calibrated independently
• Data were windowed from 3 to 30 ms before power fit after window optimization based on lab data
• Analysis was performed post-acquisition using MATLAB
Calibration
y = 574.88x-3.49
y = 580.41x-3.498
y = 380.51x-3.434
y = 354.26x-3.3860.001
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Log(
Vo
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V]
Log(Time) [ms]
T1R1
T2R3
T3R5
T4R7
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Power-Law AnalysisResults
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Angular Position
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TEC Multi-Frequency Analysis*
• Requires a Fourier Transform be applied to the data
• Fourier components normalized through complex division of reference point components at each frequency
• Liftoff Fourier components normalized and subtracted from measurement components
• Analytic approximations of
Skin Depth: 𝛿 = 50𝜌
𝑓𝜇
Amplitude: A = s(δ/2)(1+p)(e−2x/δ−e−2w/δ)
Phase: φ = φ0 + φl + 1 + 2/δ(we−2w/δ − xe−2x/δ)/(e−2w/δ − e−2x/δ)*presented in greater detail by Dag Horn at this conference.
Method
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TEC Multi-Frequency AnalysisResults
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Angular Position
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Ultrasonic TestingResults
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Comparison of Results
• UT thickness estimates accepted as accurate measurements
• TEC analysis-method means compared to UT means
• On average, TEC multi-frequency analysis outperformed the other two methods
Tile Hole
UT Voltage Threshold Power Law TEC MFAMean Mean SD % diff Mean SD % diff Mean SD % diff
1 9.3 8.2 0.5 12 8.8 0.3 5 9.1 0.4 2
2 9.2 7.4 0.9 19 8.8 0.4 4 9.5 0.4 -33 9.8 8.2 0.6 16 9.2 0.4 6 9.6 0.4 24 9.6 7.6 0.7 20 9.4 0.4 3 9.4 0.3 25 9.6 8.7 1.1 10 9.2 0.3 5 9.3 0.4 3
6 9.5 8.5 1.1 11 8.7 0.3 8 9.3 0.4 27 9.3 7.8 0.9 16 9.5 0.2 -2 9.7 0.3 -5
8 9.3 7.7 0.7 17 9.5 0.4 -2 9.7 0.3 -4
9 9.6 8.4 0.5 13 9.2 0.3 5 9.6 0.4 0
10 9.4 7.4 0.7 21 9.3 0.3 1 9.6 0.2 -2
Average 9.5 8.0 0.8 15 9.2 0.3 3 9.5 0.4 0
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Conclusions• Inspection requirements met by TEC and UT:
• No regions of wall thinning beyond nominal tolerance reported
• Good overall agreement with UT measurements qualitatively and quantitatively
• TEC method sensitive enough to identify pilger manufacturing process in ferromagnetic pipes (~0.5 mm amplitude ripples)
• Liquid couplant not required for TEC inspection
• Voltage-threshold most sensitive to low-voltage noise
• Power-law and TEC multi-frequency analysis methods are more closely related to electromagnetic phenomena, and produced more accurate results
• Electromagnetic inspection of ferromagnetic pipe has been successfully demonstrated