1. Scope
This procedure covers encoded phased-array ultrasonic testing (PAUT) of in-service piping, pressure vessels, and storage-tank components for the purpose of corrosion mapping and remaining-wall-thickness assessment to feed API 579-1 / ASME FFS-1 Fitness-for-Service Level 2 evaluation. Materials: carbon steel and low-alloy steel; thickness range 3 mm to 75 mm. Component geometries: straight pipe, elbows (long-radius and short-radius), tank shells, and pressure vessel walls.
2. References
- API 579-1 / ASME FFS-1 — Fitness-for-Service, current edition
- ASME BPVC Section V, Article 4 Mandatory Appendix IV (PAUT)
- ASNT SNT-TC-1A or CP-189 plus ASNT ILI-PQ where applicable
- API 510 / 570 / 653 — for re-inspection trigger
- Customer Written Practice
3. Personnel Qualification
PAUT examination shall be performed by personnel certified to ASNT NDT Level II in UT with documented PAUT-specific training (minimum 40 contact hours theory + 80 hours application) per the employer's Written Practice. Procedure approval and interpretation against FFS Level 2 criteria shall be by a Level III in UT with API 579 familiarity.
4. Equipment
- PAUT instrument: 32:128 (32 active, 128 element addressable) or larger, e.g., Olympus OmniScan X3 / Sonatest Veo+ / Eddyfi M2M Gekko. Calibrated linearity (vertical, horizontal, control) within ±5% per ASME V Mandatory Appendix IV.
- Probe: 5L64-A12 or equivalent — 5 MHz, 64 elements, 12 mm pitch — coupled to a 0° wedge (compression-wave) for thickness mapping or a shear-wave wedge (typically SW45 / SW60) for flaw screening adjacent to corroded areas. For corrosion mapping, 0° compression with electronic scanning is the typical configuration.
- Encoder: optical or magnetic-wheel encoder with resolution ≤ 0.5 mm and a verified accuracy of ±1% over 1 m of travel.
- Couplant: glycerin-based, water-based, or low-sulfur cellulose paste. Halogen-free for austenitic stainless.
- Reference block: step wedge cut from the same material as the production component, with steps spanning the expected thickness range; calibrated by an accredited cal lab within 12 months. Side-drilled holes per ASME V Mandatory Appendix IV.
- Calibrated thickness comparator (micrometer or 0° UT thickness gauge) for verification.
5. Surface Preparation
Examination surface shall be free of loose scale, weld spatter, paint scale that exceeds 0.25 mm, and surface irregularities that impair coupling. Surface roughness Ra ≤ 12.5 µm (500 µin) for encoded scanning; tighter where DAC sensitivity is required.
Coated surfaces: where coating < 0.5 mm is uniform and bonded, scanning may be performed through coating provided a coating-compensated thickness reference is established on a representative coupon. Where coating is loose or exceeds 0.5 mm, removal is required.
6. Calibration
6.1 Zero / Velocity Calibration
0° focal law calibrated for zero offset and material velocity using the reference step wedge. Velocity tolerance: ±0.3% of nominal (e.g., 5920 m/s ±18 m/s for carbon steel).
6.2 Encoder Calibration
Encoder pulses-per-mm calibrated on a 1 m reference straight-edge. Tolerance: ±1%.
6.3 Sensitivity Calibration
For thickness mapping, sensitivity set so that the first back-wall echo at maximum thickness reaches 80% ± 5% FSH. DAC or TCG curve built from SDH responses across the thickness range of interest.
6.4 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, or cable
- At end of scan campaign
A change in back-wall echo amplitude > 2 dB or in measured thickness > 0.2 mm requires re-calibration and re-examination of all scans since the last valid verification.
7. Scan Plan
7.1 Coverage
Encoded raster scans with index increment of 1 mm or finer in the axial direction. Scan speed: ≤ 50 mm/s for raster; ≤ 100 mm/s for linear. Overlap between adjacent scans: ≥ 10% of probe aperture.
7.2 Data Storage
Full waveform data (A-scans plus encoder position) stored for every scan. File format: native to the PAUT instrument (e.g., .nde, .opd) for re-analysis. Backup copy stored within 24 hours.
7.3 Resolution Requirements
Pixel resolution in the resulting C-scan: axial 1 mm × circumferential 1 mm or finer. For pit-detection scans, 0.5 × 0.5 mm.
8. Analysis
8.1 Thickness Determination
Minimum remaining wall thickness (t_mm) determined from the C-scan as the lowest valid value within a defined assessment volume. Edge effects (probe pulse near-surface) and lateral wave artefacts excluded.
8.2 Local Thin Area (LTA) Mapping
LTAs identified per API 579 Part 5 — flaw length s, width c, and R_t = t_mm / t_nom recorded for every contiguous region where t_mm < t_nom - 0.5 mm OR per the customer corrosion-rate-action threshold.
8.3 Output
PAUT analysis output for FFS evaluation:
- Minimum remaining-wall-thickness contour map (C-scan)
- LTA geometry table (length, width, R_t, axial / circumferential position)
- Corrosion-rate calculation against the last-inspection record
- Remaining-life calculation per API 579 Part 4 Level 1 screening criteria
9. Acceptance / FFS Routing
Results route to API 579 evaluation as follows:
- R_t ≥ 0.8 AND LTA length s ≤ s_max per Table 5.4 → Level 1 acceptable, document.
- R_t < 0.8 OR s > Level 1 limit → escalate to Level 2 numerical evaluation.
- R_t < 0.5 OR through-wall → immediate isolation, repair planning per customer Mechanical Integrity program.
10. Reporting
Per ASME V Mandatory Appendix IV §IV-T-490 — minimum: customer and asset ID; circuit / line ID; material and nominal thickness; probe and wedge identity; encoder resolution; software and software version; scan-plan diagram; raw data archive identifier; reduced thickness map; LTA table; corrosion-rate and remaining-life calculation; examiner name and certification; Level III sign-off and API 579 evaluation routing.
11. Safety
Standard plant safety rules apply. PAUT-specific: ergonomic management of overhead encoded scanning, eye protection for above-shoulder work, and confined-space entry procedures for in-vessel scanning. Coupling-fluid disposal per plant environmental procedures.
This procedure is a published reference. Production use for FFS feed requires Level III approval and API 579 SME review. For a tailored scan plan covering your asset geometry, expected corrosion morphology, and FFS routing, use the free AI Procedure Generator.