1. Scope
This procedure covers manual ultrasonic thickness measurement (UTM) of aboveground storage tank (AST) shells, floors, and roofs constructed and inspected to API 653 (Tank Inspection, Repair, Alteration, and Reconstruction). Carbon-steel and low-alloy-steel tanks; thickness range 3 mm to 50 mm. Floor scanning when AST is empty and gas-freed; shell and roof scanning may be in-service depending on access and customer JSA.
2. References
- API 653 — Tank Inspection, Repair, Alteration, and Reconstruction, current edition
- API 575 — Inspection Practices for Atmospheric and Low-pressure Storage Tanks
- ASME BPVC Section V, Article 23 — UT thickness measurement
- ASTM E797 / E797M — Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo
- ASNT SNT-TC-1A or CP-189
- Customer Mechanical Integrity Program / Written Practice
3. Personnel Qualification
UTM shall be performed by personnel certified to ASNT NDT Level II in UT per the employer's Written Practice meeting SNT-TC-1A or CP-189. Tank Inspector responsible for the assessment shall hold a current API 653 certification (API Individual Certification Program).
4. Equipment
- Digital UT thickness gauge (e.g., Olympus 38DL Plus, GE DM5E, Krautkrämer DMS 2) with calibrated A-scan display preferred for accurate gating on corroded surfaces. Resolution: 0.01 mm. Linearity verified per ASTM E797 §6.
- Dual-element probe 5 MHz, 12.5 mm diameter for general scanning; 7.5 MHz, 6.4 mm for areas of high pitting or restricted access. Single-element 0° probes for areas where A-scan interpretation of subsurface laminations is required.
- Couplant: glycerin-based, water-based, or grease-type compatible with the tank service fluid. Halide content ≤ 25 ppm where austenitic floor strips are encountered.
- Calibration step block — material-matched to the tank steel — covering the thickness range of measurement. Block calibrated by an accredited cal lab within 12 months.
- Surface-preparation tools: power wire wheel, surface grinder, or sandblast as required by surface condition.
5. Surface Preparation
Coating removal where coating exceeds 0.25 mm or is loose. For epoxy-coated tank floors and corroded shells, grit-blast to SSPC SP-6 commercial blast or finer at each examination spot. Surface roughness Ra ≤ 12.5 µm.
For pit-prone areas, light grinding with a flap disc is preferred to preserve actual remaining wall — aggressive grinding falsifies the reading.
6. Calibration
6.1 Velocity / Zero Calibration
Calibrated on the step block at the start of each shift, after any probe or cable change, and at the end of each shift. Velocity tolerance: ±0.5% of nominal. Zero-offset tolerance: ±0.05 mm.
6.2 Verification Frequency
Verification on a known-thickness coupon of the tank material:
- Every 2 hours during continuous measurement
- After any temperature change > 10 °C from the calibration ambient
Drift > 0.1 mm requires re-calibration and re-measurement of all readings taken since the last valid verification.
7. Examination
7.1 Shell Scanning Pattern (Course Inspection per API 653 §4.3.3)
Minimum coverage per Table 6.1:
- Course thickness measurement: minimum 8 readings per course on each course, evenly distributed around the circumference, taken at the 1/4-height of the course
- Minimum-Thickness Readings (MTR): minimum at every observed corroded area plus the lowest of the eight readings per course
Pit measurement: A-scan UT to confirm pit depth, supplemented by pit gauge where the pit is open.
7.2 Floor Scanning
Magnetic Flux Leakage (MFL) is the standard floor-screening method; this procedure covers the UT follow-up of MFL anomalies and the verification of remaining floor-plate thickness. UT readings taken at every MFL indication where the indication mark-and-paint matches a thickness loss > 20% per the MFL crew's acceptance criteria.
7.3 Roof Scanning
Where the roof is suspect (cone, dome, internal floating roof), readings on a grid per the Tank Inspector's scan plan — typically 600 mm centres for the primary roof surface; 300 mm centres where coating wear or wet insulation is evident.
8. Corrosion-Rate Determination
Long-term corrosion rate (LT-CR) per API 653 §4.4.4:
LT-CR = (t_initial - t_actual) / (years in service)
Short-term corrosion rate (ST-CR) from the previous inspection:
ST-CR = (t_previous - t_actual) / (years between inspections)
Controlling rate: the higher of LT-CR and ST-CR per §4.4.4.2.
9. Acceptance / Minimum Acceptable Remaining Thickness
Per API 653 §4.3.3.1 and §4.4:
- Shell minimum thickness:
t_min = (2.6 × D × (H − 1) × G) / (S × E)- where D = nominal tank diameter (ft), H = height of fluid above the course bottom (ft), G = specific gravity, S = allowable stress (psi), E = joint efficiency.
- Bottom plate minimum: 0.10 inch (2.54 mm) per §4.4.5.1, OR per the corrosion-rate-anchored projection that gives a 10-year remaining life.
- Critical zone (3 inches from chime weld): 0.10 inch (2.54 mm) per §4.4.5.2.
A reading below t_min, OR a remaining life < 5 years, OR a critical-zone reading below 0.10 inch — escalate to the API 653 Tank Inspector for re-rate / repair planning.
10. Reporting
Per API 653 §6 — minimum: tank ID; date and inspector; equipment serial numbers (UT gauge, probe, cable); calibration block ID and calibration certificate number; couplant; surface preparation; full reading set with location identifier; minimum reading per course; LT-CR and ST-CR; t_min calculated for each course; remaining-life projection; signed by the Tank Inspector (API 653 certified).
11. Safety
Tank-entry permits per the customer Confined Space Entry program. Gas-freeing verified by competent person; LEL ≤ 10%, O₂ between 19.5% and 23.5%, H₂S < 10 ppm, benzene < occupational limit. Rescue plan with retrieval equipment in place before entry. Hot-work permits for any grinding inside the tank. Coupling-fluid disposal per plant procedure.
This procedure is a published reference. Production use requires API 653 Tank Inspector approval and customer Mechanical Integrity program alignment. Generate a custom tank-UT plan including grid layout and corrosion-rate trending at the free AI Procedure Generator.