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10 min read•ASME BPVC Section V Mandatory Appendix III / ISO 10863
TOFD • Ultrasonic SizingTarget: Advanced UT Level II/III Inspectors, Plant Asset Integrity Specialists
TOFD Flaw Height Sizing & Diffraction Tip Analysis Guide
Field Procedure for ASME Sec V Mandatory Appendix III & BS 7706 Accuracy
Applicable Standards:ASME Sec V App IIIISO 10863BS 7706API 510
Key Technical Takeaways & Execution Rules
- TOFD relies on diffracted tip signals rather than specular reflection, providing height sizing accuracy of ±1mm.
- Lateral wave (surface) and backwall echo define the depth inspection window.
- Upper diffracted tip signals are phase-inverted relative to the lateral wave, whereas lower tip signals match lateral wave phase.
- Dead zones near the OD surface (due to lateral wave pulse width) require supplementary high-frequency PAUT or creep wave inspection.
Step-by-Step Field Execution Procedure
1
PCS (Probe Center Separation) Calculation
Calculate PCS based on target depth (typically 2/3 wall thickness). Set PCS to focus beam intersection at the critical zone.
2
Time Window & Gain Calibration
Adjust time window to display lateral wave at ~20% screen left and backwall echo at ~80% screen right. Set lateral wave amplitude to 50-80% FSH.
3
Flaw Tip Signal Identification
Locate parabolic diffraction hyperbolas on D-scan image. Identify upper tip (phase-inverted) and lower tip (in-phase) arrivals.
4
Depth & Height Sizing Measurement
Apply hyperbolic cursor fitting to extract exact depth of top tip (d1) and bottom tip (d2). Flaw height = d2 - d1.
Probe & Wedge Selection Specs
Code Acceptance Criteria
ASME Sec V Appendix III / ISO 15626 Level 1: Planar flaws with height (h) > 2mm or length (l) > 12mm in 25mm wall pipe are rejectable.
Apply This Technique in the Field