Ge Corrosion gageing Theory

8/11/2019 Ge Corrosion gageing Theory

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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



Corrosion Thickness Gaging

• Uses dual element transducers

• Erosion/Corrosion

• Typically on metal

• Irregular/Pitted reflecting surface



Dual Element Transducer

External

Housing

Connector Acoustic

Barrier Transmitting

Element

Receiver

Element

Delay

Material

Angular

Sound

Path

Test Sample



Dual Element Transducer on Co rrodedMaterial

• Roof angle focuses sound at the base of pits

TX RX



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



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



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



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



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



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



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



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

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