External and internal inspection intervals for storage tanks (OISD-STD-129, 2006 Revised Edition, §8.4)
◉ OISD-STD-129 §8.4 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only, and distinct from the existing (API 653-based) Inspection Interval calculator. This is a different standard with a genuinely different formula and different fixed intervals by fluid category — the two tools aren't meant to be reconciled into one number; check both if your governing basis requires it. Formulas and Tables 8.1/8.2 verified directly against the standard's page images. Where your site's Quality Control Manual prescribes its own intervals, that overrides Table 8.2 (§8.4.c).
Identification
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External Inspection Interval (§8.4.b, Table 8.1)
Formula: DT/4N, where DT = measured shell thickness − minimum required thickness (mm), N = shell corrosion rate (mm/year). Result is the lesser of this and Table 8.1's value when using corrosion-rate-based assessment.
Result
DT/4N (calculated)—
Table 8.1 value for this fluid category—
External inspection interval (governing)—
Internal Inspection Interval (§8.4.c, Table 8.2)
Formula: (measured − minimum/retiring bottom plate thickness) / corrosion rate. When using corrosion-rate-based assessment, the interval is capped at 15 years regardless of fluid category (a uniform cap, unlike the External table's fluid-specific cap). Without corrosion-rate-based assessment, use the fixed value from Table 8.2.
Routine in-service inspection: not to exceed 1 month, by operations personnel. Annual visual external inspection: at least once a year, by a qualified and experienced authorized person — independent of the detailed external inspection interval calculated above.