advantages of ultrasonic thickness gages over flaw detectors for corrosion thickness measurements
TRANSCRIPT
Advantages of Ultrasonic Thickness Gages over Flaw Detectors for Corrosion Thickness Measurements
Greg Bauer | Product Specialist, Ultrasonic Thickness Gages January 11, 2017
Advantages of Flaw Detectors § More versatile
– Straight beam, angle beam, scanning
§ Includes specialized software for sizing defects: – Distance Amplitude Correction (DAC) – Time Varied Gain (TVG) – Distance/Gain/Size (DGS)
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Instrument Priority § Flaw detectors
– Accuracy of signal amplitude
§ Thickness gages – Consistency and accuracy of the time of flight measurement
§ Use the right tool for the job
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Why were flaw detectors used for measuring thickness? § History of ultrasonic (UT) equipment
– Ultrasonic instruments were released in the late 1940s – Digital thickness gages released in the 1960s
– No waveform = less confidence – Thickness gage with on-board waveform released in 1980s
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Additional Information from Live A-Scan
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Waveform 1 Waveform 2
Detection of backwall Detection of noise
Additional Information from Live A-Scan
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Waveform 1 Waveform 2
Heavily corroded backwall Cleaner backwall
Advantages of Thickness Gages over Flaw Detectors § Size and cost
§ Ease of calibration and operation
§ More consistent and accurate time of flight measurement
§ Specialized features or software for special applications
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Thickness Gage Measurement Advantages § Auto zero compensation
§ V-path correction
§ Auto probe recognition
§ Detection algorithms and digital signal processing (DSP)
Auto Zero Compensation
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Why are dual element transducers used for corrosion applications?
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V-Path Correction When Using Thickness Gages
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Sample
TX RX
Angular Sound Path
True Thickness
Thickness Measurement Error When Using a Flaw Detector
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Auto Probe Recognition § Optimizes setup and receiver gain
– Easier and faster setup for the user
§ Recalls transducer V-path correction – Accurate measurements over large thickness ranges
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V-Path Builder on Olympus 38DL PLUS® Thickness Gage
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Step 3 Step 4
Step 1 Step 2
Flaw Detector Detection Mode — Flanking/Edge Gate
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Detection point at 60 dB Detection point at 54 dB
§ Thickness reading changes 0.011 in. (0.280 mm) from a 6 dB change
Flaw Detector Detection Mode — Peak Gate
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Signal from the pit Majority of signal not from thinnest area
Corrosion Thickness Gages Use Algorithms and Digital Signal Processing (DSP)
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Detection Threshold
Thickness Gage Application Advantages § Temperature compensation
§ Electromagnetic acoustic transducer (EMAT)
§ Internal oxide
§ Encoded B-scan
§ Echo-to-echo measurement
§ THRU-COAT® technology
High-Temperature Applications
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§ Setup adjustments – Increase gain – Highest update rate – Auto zero – Temperature compensation
Olympus 38DL PLUS Thickness Gage Temperature Compensation § Sound velocity changes with temperature
– Approximately 1% for every 100 °F (55 °C) change in temperature for carbon steel
– V0: Sound velocity at calibration – T0: Temperature at calibration (user inputted) – T1: Temperature at measurement (user inputted) – k: Temperature coefficient (user inputted)
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Olympus 38DL PLUS Thickness Gage Temperature Compensation
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Boiler Tube Applications § Two solutions
– Measuring tube wall thickness using EMAT – Measuring tube wall and internal scale thickness using 38DLP-OXIDE
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Olympus 38DL PLUS Thickness Gage — Measurements with the E110-SB EMAT § Electromagnetic acoustic transducer (EMAT)
– Requires external oxide scale to measure tube wall thickness – No couplant required – Fast and easy
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§ Measure boiler tube wall thickness and internal scale thickness – Minimum scale with M2017: 0.010 in. (0.250 mm) – Minimum scale with M2091: 0.006 in. (0.150 mm)
– Shear wave couplant (SWC-2) required
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Olympus 38DL PLUS Thickness Gage — Optional 38DLP-OXIDE Software
§ Enables the 38DL PLUS thickness gage to perform encoded B-scans – Data is transferrable – Less expensive
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Olympus 38DL PLUS Thickness Gage — Optional 38DLP-EBSCAN Software
Distance traveled
§ Echo-to-echo – Used to ignore coatings and measure only the true metal thickness
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Measuring Thickness on Painted Metal Applications
Auto echo-to-echo Manual echo-to-echo
§ Measuring thin materials – About 0.080 to 0.100 in. (2.0 to 2.5 mm) minimum
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Echo-to-Echo Limitations
0.080 in. (2.0 mm) sample 0.040 in. (1.0 mm) sample Valid echo-to-echo measurement No separation, false reading
§ Samples with a heavily corroded back surface – No multiple backwalls
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Echo-to-Echo Limitations
§ THRU-COAT technology – Used to measure paint thickness and steel thickness at the same time
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Measuring Thickness on Painted Metal Applications
§ Measure thinner samples – Minimum of about 0.040 in. (1.0 mm)
§ Measure thicker samples § Measuring painted samples with corroded back surfaces
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Advantages of THRU-COAT Technology
Echo-to-echo THRU-COAT technology
§ Used in standard, THRU-COAT, and echo-to-echo mode § Limitations
– Rough or textured surfaces – Coating has to be at least 0.005 in. (0.127 mm) – Cannot be used for high temperatures
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THRU-COAT Transducers (D7906-SM, D7906-RM, D7908)
Thickness Gage Feature Advantages § Doubling warning
§ Gain optimization
Thickness Gage Feature — Doubling Warning § Definition of doubling
– Gage detects the second backwall reflection and misses the first backwall signal
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Dual Element Transducers — Measuring Thick and Thin Samples
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Thick sample Thin sample
Calibration Doubling Warning
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Potentially wrong echo detected!
CAL VEL on thick CAL ZERO on thin
Doubling Issue with Dual Element Transducers § How to correct the doubling issue
– Increase gain so the first backwall is above the 20% detection threshold
Waveform 1 Waveform 2
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Gain Optimization Calibration § Advantages
– Increase useful life of transducer – Help prevent calibration and measurement errors
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Default gain of 66 dB for this D790 Default gain of 59 dB for this D790
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