development of a pulsed eddy current method for corrosion
TRANSCRIPT
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A Novel Self Calibrating Pulsed Eddy Current Probe
for Defect Detection in Pipework
S.Majidnia ,J.Rudlin, R.Nilavalan
Malaysia NDT Conference November 2015
Malaysia International NDT Conference & Exhibition 2015 (MINDTCE-15), Nov 22-24 - www.ndt.net/app.MINDTCE-15
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Corrosion under Insulation for pipes (CUI)
Measurement of Scab corrosion (apparently surface but can be deep)
Other methods to assess exist but both still “hot topics”
PEC Applications
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Introduction
Concept
Early Work (2006-2012)
Modelling
Applications
Results
Current Status
Content
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• The rate of decrease of the eddy currents is monitored by the PEC probe and is used to determine wall thickness and corrosion
Principles
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Encircling coil with sensor array enables large area coverage of pipe and gives fairly rapid inspection
Concept
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Original system with a thyristor discharge circuit and fluxgate sensors positioned at minimum applied field positions – flaws then produced a deviation from this
Was shown to work through coverings and detect the pipe
Original work 2006-9
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Sensors and deployment
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Equipment
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Site trials
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Not very portable and difficult to manoeuvre
Uncertain maximum currents
Complex software to handle fluxgate sensor outputs. Too slow.
Difficulties
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Theoretical work by PhD student, modelling the effects and assisting equipment design
Controlled amplitude step down
Replace fluxgates with GMR’s to simplify receiver electronics
Re-write software and displays.
Continuation/revision
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2D and 3D Models were developed.
Sensor positions and type of sensors were identified.
Research work
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3D Defect modelling.
Wall thinning under insulation and covering.
Coil shielding.
Modelling work
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Magnetic Fields during the pulse
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Induced current flow
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Probe
PC with Matlab DC Power Supply
DAQ Card
Driver
Pipe
Probe
Pipe
PEC Experimental Platform
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Defect Signals 4 sensors
Defect Name Depth
(mm) Length (mm)
Width (mm)
A 7 100 65 B 5.2 80 50 C 6.2 50 80
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Indications on all 4 channels of one flaw
Effectively an encircling coil eddy current test
Gives an idea of
Total current change from reference
Any imbalance between coils (provided only one flaw)
Lift off effects (ie closer coil amplifies indication)
Depth Sizing implications based on indication amplitude
Features of Indications
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Covering effect on PEC Signal
Air
Alu
min
ium
G
alv
anis
ed S
teel
Sta
inle
ss S
teel
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Covering effect on PEC Signal
Model was solved for 2 scenarios
1. Cladding only (air inside ) 2.Cladding and pipe inside
Differential Magnetic field
signal (Z component)
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CUI detection – insulation only and SS covering
Wall thinning from outside 50mm insulation
Reference (no flaw) was subtracted from test signal of different remaining wall thickness(large area)
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Aluminium and Galvanised Covering
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Some practical validation
Experiments for 2 scenarios
1. Cladding only 2.Cladding and pipe
Differential sensor signal
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New prototype
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New prototype
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Conclusion
Continued development of PEC system
Comsol software was successfully used to determine the output of a PEC system for simulated corrosion under insulation situations for
Variations in wall thickness.
Different cladding materials (Stainless Steel, Aluminium , galvanised steel)
Validations were carried out for some available samples.
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New prototype planned to be completed for end of year for TWI use
2 power sources, 2 coil sizes (initially).
Software (producing C-scan type display) complete for 16 channels
Current Status
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Future work
Testing to obtain calibration for sizing
Modelling of corrosion patches under conductive cladding.
Equipment Development for different sizes