Use verified UT readings, corrosion rate and construction-code minimum thickness. Results are screening values, not a final engineering disposition.
A qualified API 579 assessor must confirm applicability, data quality and Level 2/3 requirements before continued operation or rerating.
Engineering Tips:
- General Metal Loss (API 579 Part 4) is for uniform corrosion. High COV may indicate local thinning.
- Local Metal Loss (API 579 Part 5) evaluates localized thinning using the Remaining Strength Factor (RSF).
- Always identify the damage mechanism before selecting the assessment type.
- Use the governing corrosion rate from history if it exceeds the short-term rate.
Recommended values
- Corrosion rate: Use ≥ 2 inspections; take the more conservative of short-term and long-term CR.
- Inspection interval: Set to ≤ half the remaining life (API 570), capped by the applicable code.
- Data scatter: Reading COV > ~10% points to localized thinning — assess as local metal loss (Part 5).
- Minimum thickness: Base t_min on the current pressure-design plus code corrosion allowance, not the original wall.
Limitations
- Level 1 screening: API 579 Part 4/5 Level 1 — heavy scatter or complex geometry needs Level 2/3 with FEA.
- Not included: Fatigue, creep, and environmental cracking (SCC/HIC) are separate damage mechanisms.
About the standards
- API 579-1 / ASME FFS-1: Fitness-For-Service.