TOFD Procedure for Process Piping Girth Welds (ASME B31.3)

1. Scope

This procedure covers Time-of-Flight Diffraction (TOFD) examination of process-piping girth welds and longitudinal seam welds in carbon-steel and low-alloy-steel piping fabricated to ASME B31.3 Process Piping. It is applied alongside or in lieu of radiography per ASME B31.3 §344.6.2 with Owner approval, qualified per ASME BPVC Section V, Article 4 Mandatory Appendix III. Wall thickness range: 6 mm (¼ in) to 75 mm (3 in). Outside diameter range: NPS 4 (114 mm) and greater.

2. References

• ASME B31.3 — Process Piping, current edition
• ASME BPVC Section V, Article 4 Mandatory Appendix III (TOFD)
• ASME BPVC Section V, Article 4 Mandatory Appendix VIII (TOFD Demonstration)
• ASTM E2373 / E2373M — TOFD Examination Practice
• ASNT SNT-TC-1A or CP-189 plus instrument-specific TOFD training (minimum 40 hr theory, 80 hr application)
• Customer Written Practice

3. Personnel Qualification

TOFD examination shall be performed by an ASNT NDT Level II in UT with documented TOFD training and demonstrated competence per Mandatory Appendix VIII. Procedure approval and final interpretation shall be by a Level III with TOFD-specific certification.

4. Equipment

• TOFD instrument: digital, gated A-scan with B-scan and D-scan display. Examples: Olympus OmniScan X3 (TOFD module), Sonatest Veo+, Eddyfi M2M Gekko.
• Probe pair: two longitudinal-wave compression-mode transducers, frequency 5 MHz or 10 MHz selected per thickness band (10 MHz for ≤ 25 mm; 5 MHz for 25 mm – 75 mm), element 6 × 6 mm to 10 × 10 mm. Beam-spread angle (refracted longitudinal angle) selected to give a single TOFD coverage zone — typically 60° or 70° for ¼ wall thickness.
• Wedges: matched pair, refracted-angle selected per the thickness band; wedge delay verified on the calibration block.
• Encoder: optical or magnetic-wheel, resolution ≤ 0.5 mm.
• Couplant: low-sulfur cellulose paste (or glycerin / water-based per material compatibility).
• Calibration block: per Figure T-434.1 with side-drilled holes; supplemental block with through-wall thickness and a known notch or SDH to verify TOFD diffraction signature.

5. Surface Preparation and Geometry

Scan surface roughness: Ra ≤ 6.3 µm (250 µin). Weld cap shall not exceed 3 mm reinforcement for the scan plane. Where reinforcement exceeds this, grinding flush is required for the scan area.

Probe Centre Spacing (PCS) — determined from a single-zone or multi-zone coverage diagram:

PCS = 2 × (Wall Thickness) × tan(refracted angle)

For 12 mm wall × 60° refracted angle: PCS = 41.6 mm.

6. Calibration

6.1 Wedge-Delay Calibration

Performed on a reference block of the same material and thickness as the production weld, using the side-drilled holes and the calibration-block back-wall. Wedge delay set such that the lateral wave appears at the correct time-of-flight per the geometry.

6.2 Sensitivity Calibration

Lateral wave amplitude set to 70% to 100% FSH of full screen. Sensitivity gain noted; scan gain typically 6 dB above this reference.

6.3 Verification Frequency

System verification on the reference block:

• Before scan campaign
• After every 4 hours of continuous use
• After any change of operator, probe, wedge, cable
• At end of scan

A change of > 20% in lateral wave amplitude or > 6 dB in DAC across the back-wall response requires re-calibration and re-scanning since the last valid verification.

7. Examination

7.1 Coverage

Single-zone or multi-zone coverage diagrams generated for the specified wall thickness, weld preparation, and probe geometry. Mandatory Appendix III Table III-432.1 governs multi-zone schemes for wall thickness above 50 mm. For each circumferential pass, a near-side and far-side scan is recorded — the dead zones (lateral wave dead zone near the surface, and the back-wall dead zone) are addressed by complementary pulse-echo angle-beam scans per Mandatory Appendix III §III-460.

7.2 Scan Speed

Maximum encoded scan speed: 100 mm/s to ensure data point density and signal-to-noise.

7.3 Coupling Verification

Continuous monitoring of lateral wave amplitude during the scan — loss > 6 dB sustained over 25 mm requires re-scanning.

8. Data Analysis

Analysis performed offline in the manufacturer's analysis software (e.g., Olympus TomoView, M2M Capture).

8.1 Flaw Detection

Diffraction tip-signal pairs (top and bottom tips of an embedded planar reflector) located in the time-of-flight domain. Through-wall extent measured from the time-of-flight separation between tip signals.

8.2 Sizing Accuracy

TOFD sizing accuracy per Appendix VIII demonstrated:

• Through-wall extent: ±1 mm for thickness ≤ 25 mm; ±1.5 mm for thickness 25 – 75 mm
• Length: ±10 mm or 10%, whichever is greater

8.3 Dead-Zone Compensation

Lateral-wave dead zone and back-wall dead zone addressed by:

• Complementary pulse-echo scans at 45° and 60° angle-beam
• Or twin-probe TOFD with offset PCS to overlap dead zones

9. Acceptance Criteria

Per ASME B31.3 Table 341.3.2 and Mandatory Appendix III §III-465 — typical for Severe Cyclic / Category M service:

• Cracks, lack of fusion, lack of penetration: rejectable regardless of size.
• Other linear indications: rejectable if through-wall extent > 1/3 × wall thickness OR length > 12 mm OR amplitude > 100% of reference (Mandatory Appendix III §III-465.1.1).
• Volumetric (slag, porosity): rejectable if through-wall extent > 1/3 × wall thickness OR aggregate length > wall thickness in any 12 × wall thickness length.

Customer acceptance criteria (e.g., Owner-imposed B31.3 Severe Cyclic) may be more stringent.

10. Reporting

Per Mandatory Appendix III §III-490 — examination data archive (raw and reduced), instrument settings, probe and wedge identification, encoder calibration record, scan-plan diagram with multi-zone coverage, flaw table with through-wall extent and length, lateral-wave and back-wall reference traces from start and end of campaign, examiner name and certification, Level III sign-off, and disposition per the acceptance criteria.

11. Safety

Standard plant safety. TOFD-specific: pinch-hazard awareness during encoder scanning around piping; eye and back protection for elevated or overhead scans; coupling-fluid disposal per plant environmental procedures.

This procedure is a published reference. Production use requires Owner Inspector approval against the current ASME B31.3 edition and the customer Mechanical Integrity program. Generate a custom multi-zone TOFD plan including PCS, focal laws, and acceptance criteria at the free AI Procedure Generator.