Post on 03-Jun-2018
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orrosion
Gaging
Theory
GE Panametrics
GE Power Systems
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Ultrasonic Thickness Gaging
• Uses high frequency sound waves
–Typically 0.5MHz thru 20.0 MHz
–Longitudinal sound energy
• Thickness measurement from one side
• Non-destructive
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Long i tud inal Sound Propagation
WAVE DIRECTION
Partical Motion
• Particle motion is perpendicular to the wave direction
–Example: Billiard balls
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Shear Sound Propagat ion
d
WAVE DIRECTION
Particle
Motion
• Particle motion is perpendicular to wave direction
–Example: Ocean wave
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Thickness Gaging
• Precision thickness gaging
–Uses single element transducers
• Corrosion thickness gaging
–Uses dual element transducers
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Precis ion Thickness Gaging
• Uses single element transducers
•Provides high degree of accuracy
• New material for quality control
–Metals, plastics, glass and composites
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Sing le Element Transducer
External Housing
Connector Electrical Leads
Inner Sleeve
Backing
Active
Element
Wear Plate Electrodes
Electrical
Network
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Corrosion Thickness Gaging
• Uses dual element transducers
• Erosion/Corrosion
• Typically on metal
• Irregular/Pitted reflecting surface
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Dual Element Transducer
External
Housing
Connector Acoustic
Barrier Transmitting
Element
Receiver
Element
Delay
Material
Angular
Sound
Path
Test Sample
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Dual Element Transducer on Co rrodedMaterial
• Roof angle focuses sound at the base of pits
TX RX
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Dual Element Transducer Advantages
• Roof angle provides pseudo-focus at the base of pits
•
High temperature capabilities
• Separate transmitting and receiving elements
–Use higher initial system gain
–Better near surface resolution
–Stable readings on rough entry surfaces
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Auto Probe Recogni t ion
• Optimizes setup and receiver gain
• Transducer V-Path correction
• Accurate measurements over large thickness ranges
True
Thickness
Sample
Angular Sound Path
TX RX
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Auto Zero Compensat ion
• Uncouple and press zero key to:–Measure time of flight through transducer
–Track transducer wear
–Compensate for thermal drift at elevated temperatures
Rx
Delay
Tx
Delay
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Zero Offset Error Caused by Built In Test Block
• Incorrect zero offsets
– With worn transducers
–On rough surfaces
–On curved surfaces
Zero Block
Worn Probeon ZeroBlock
RoughSurface
ZERO
OFFSET
ZERO
OFFSET
ZERO
OFFSET
ZERO
OFFSET
Worn Probeon CurvedPipe
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Two Point Cal ibrat ion
• Calibration on actual samples having:
–The same surface conditions
–Same geometry
–Same material
Cal Velocity
Cal Zero
Enter Min Sample ThicknessEnter Max Sample Thickness
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Veloc ity Calibrat ion
• Calculates sound velocity of the material
• Factors that effect sound velocity are:
–Material density and elasticity–Material composition
–Grain structure
–Temperature
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Zero Calibrat ion
• Compensates for time of flight through:
–Transducer
–Cable
–Couplant
–Electronics
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Veloc ity Calibrat ion
• Couple to a sample representing your maximum thickness
• Enter known thickness
• Gage measures time of flight
• Gage solves for velocity of material
Velocity = (Thickness)( 2 ) Time
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Zero Calibrat ion
• Couple to a sample representing your minimum thickness
• Enter known thickness
• Gage measures actual time of flight
• Gage calculates theoretical time of flight
• Gage subtracts theoretical time of flight from measured time of flight
Time = (Thickness) (2) Velocity ( Theoretical )
Time = Total Measured
Time of Flight ( Actual )
Zero Offset = (Time Actual ) -(Time Theoretical )
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Cal ibrat ion Errors
• Velocity errors:
–If thickness is incorrect and error increases as thickness increases then the error is most likely in the
velocity value
• Zero offset errors:
–If thickness is incorrect by a fixed amount on multiple steps then the error is most likely error in the
zero calibration
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Doubl ing
• Definition:
–Gage detects second back wall reflection and misses first back wall
• Causes:
– With duals, the first back wall echo is not always the highest amplitude signal due to the roof angle
and focus of the transducer
–Transducer wear causes signals to be lower in Amplitude
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Signals from Thick and Thin Samples
At thickness greater than the focuspoint the first back wall is thehighest amplitude echo
At thickness below the focus pointthe first back wall is not the highestamplitude echo
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Decreased Signal mplitude on Thick and Thin Sample
Signal amplitude decreased but
first back wall still abovedetection point
Due to wear in the transducer, the
first back wall echo is below thedetection point and the gage readsthe second back wall, or doublethickness
Detectionthreshold
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Thick Sample Less Likely to Produce Doubling
First reflection off back
wall
Second Multiple reflection off
back wall
RxTx
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Doubling on thin Sample
RxTx
First reflection off back
wall
Second Multiple reflection off
back wall
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Doubling During Calibration
• Gage uses input thickness values to calculate velocity and the zero offset value:
–Doubling most likely occurs on thin sample
–If gage measures time to second back wall reflection and user inputs thickness of material, the
zero offset is set too high to compensate for first round trip in the material
–Gage will read correctly on thick and thin sample but will not be linear.
–It is always recommended to have a Multi-Step calibration block to verify linearity
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Ways to Correct Cal ibrat ion Errors
• Perform a Measurement Reset
–Zero and Velocity will be set to default values
• Check thickness on thick and thin sample
• If the thickness on thin sample is doubled then increase the gain until reading is close
• Re-calibrate
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Factors for Choosing Transducers
• Material–Carbon steel
–Cast material
– Aluminum
•
Thickness Range–Min and Max thickness
• Geometry
–Min diameter
–Convex/Concave surface
–Surface condition
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Transducer Criter ia
• Frequency
–Lower frequency gives better penetration
–Higher frequency gives better minimum capability
• Roof Angle
–Steeper roof angle will have shorter focus
• Delay Material
–
High temperature ?
Advantages o f Thickness Gages over Flaw
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Advantages o f Thickness Gages over FlawDetectors
• Size and cost
• Ease of calibration and operation
• Auto probe recognition
• V-Path correction
• Auto zero compensation
• Greater datalogger capability
• Simple thru paint echo-to-echo measurements
• Better thickness accuracy
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Flank ing Gate Detect ion
• Accuracy Affected By:
–Coupling pressure
–Echo amplitude
–Leading edge shape
–Transducer alignment
–Front surface condition
–Backwall surface condition
–Material properties
SIGNAL
AMPLITUDE
AT 50dB
SIGNAL
AMPLITUDE
AT -6 dB
THRESHOLD
GATE
Detection 1
Detection 2
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Peak Detect ion
• Dual element signals have multiple peaks
• Peaks change due to:
–Transducer alignment
–Surface condition and coupling pressure
–Back wall surface condition
–Grain structure
• Peak detection is less sensitive to pits
TIME TO
PEAK
PEAK
SIGNAL
PEAK
GATE
PEAK
SIGNAL
PEAK
GATE
TIME TO
PEAK
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A lgor i thms and DSP
• Leading edge of echo is automatically detected
• Calibrated accuracy maintained when gain is adjusted
• System runs at lower gain and yields a cleaner waveform
Detectionthreshold
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Features For High Temp App l icat ion
• Gain adjust (add gain )
• Fast update rate (20 readings/sec)
• Freeze waveform
• Probe zero (to compensate for thermal drift)
• Save data
– Waveform
–Thickness
–Gain settings
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High Temperature Coup l ing Techniques
• Use appropriate couplant for temp range
–F-2 Medium temp. below 260o C (500o F)
–E-2 High temp for 260 - 500o C (500-1000o F)
• Apply couplant to transducer tip
• Use firm coupling pressure
• Limit contact time to five seconds
•
Wipe transducer and press zero key to compensate for transducer drift
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Steps To Ensure Transducer Longevi ty
• Limit transducer contact time to five seconds
• Never let transducer get too hot to hold
• If transducer gets hot
–Cool in air
–Dip tip in water
–Re-zero
• Avoid dragging transducer cable across pipe