Minimum acceptable shell thickness, hydrostatic test height limit, corrosion rate/remaining life, and bottom plate MRT
◉ API 653 §4 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. Formulas, Table 4-1 (allowable shell stresses), and Table 4-2 (welded joint efficiencies) are sourced directly from API 653 Section 4 (Suitability for Service) as provided. Covers welded tank shells only, for tanks ≤200 ft diameter (the variable design point method required above 200 ft per §4.3.3.4 isn't implemented here). Riveted tank shells (§4.3.4) and the annular plate ring thickness table (Table 4-4, §4.4.8) aren't included either. Corrosion rate/remaining life uses standard linear-rate arithmetic applied to the computed minimum thickness, not a separate standard-specific formula. Confirm this is your current edition of API 653 before relying on this for a real assessment.
Identification
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Corrosion Area & Critical Length (§4.3.2)
L = 3.7√(D×t2), capped at 40 in. Enter at least 5 equally-spaced thickness readings along the vertical profile plane over length L to get the averaged thickness t1.
#
Thickness Reading in
Averaged Thickness t1
Average over L (t1)—
Shell Minimum Thickness — Welded Tank (§4.3.3.1)
S and St auto-fill from Table 4-1 based on material + course position, but remain editable. Y default 30,000 psi and T default 55,000 psi if unknown, per the standard's own defaults.
For the entire-course method, H is measured to the bottom of this course. For the local-area method, H is measured to the bottom of L or the point of interest.
Result Not evaluated
Minimum acceptable thickness tmin—
t1 (avg) ≥ tmin?—
t2 (least) ≥ 60% of tmin?—
Hydrostatic Test Height Limit (§4.3.3.2)
Uses the same tmin, E, and St from above. Ht = (St×E×tmin)/(2.6D) + 1 for an entire course, or without the "+1" for a local area/point of interest.
Result
Hydrostatic test height Ht—
Corrosion Rate & Remaining Life (shell)
Generic linear-rate calculation using the computed tmin above as the retirement thickness — not a separate formula from the standard itself, just straightforward arithmetic applied to it.
MRT = min(RTbc, RTip) − Or×(StPr + UPr). Use StPr = 0 for coated top-side areas expected to outlast Or; use UPr = 0 for areas with effective cathodic protection.