External, UT thickness monitoring, and internal inspection intervals
◉ API 653 §6 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. Formulas and Table 6-1 are sourced directly from API 653 Section 6 (Inspection) as provided. The Risk-Based Inspection (RBI) alternative in §6.4.3 is a qualitative engineering assessment process, not a formula, and isn't computed here — an RBI assessment can extend intervals beyond what this tool shows, but requires a qualified engineer's review, not a calculator. Jurisdictional regulations may also set stricter intervals than API 653's own defaults (§6.2.3) — this tool doesn't know your local requirements. Confirm this is your current edition of API 653 before relying on this for a real assessment.
Identification
Research entries stay private to you for practice/testing. Switch to Official once this is a real assessment your company should see.
External Inspection Interval (§6.3.2.1)
Every 5 years or RCA/4N years, whichever is less. RCA = measured shell thickness − minimum required thickness. If the corrosion rate isn't known, the interval defaults to 5 years since RCA/4N can't be computed.
Result
RCA (remaining corrosion allowance)—
RCA/4N—
External inspection interval—
UT Thickness Monitoring Interval (§6.3.3.2)
If corrosion rate unknown: max 5 years. If known: smaller of RCA/2N years or 15 years. Reuses the RCA above.
Result
RCA/2N—
UT monitoring interval—
Internal Inspection Interval (§6.4.2, Table 6-1)
Interval is set so bottom plate thickness at the next inspection is not less than Table 6-1's value for your bottom/foundation design — in no case exceeding 20 years. If the bottom corrosion rate isn't known and there's no similar-service data to estimate it, the interval is capped at 10 years instead.
If you have anticipated/estimated rates from similar-service tanks, enter that here even without direct measurement — §6.4.2.1 allows this.