API 579-1/ASME FFS-1 · 2016 edition

Tank Shell Fitness-for-Service — User Manual

Oct 19, 2026@Shamit
tmm
Contents

    About this tool

    The Tank Shell FFS calculator applies API 579-1/ASME FFS-1 to a corroded shell course of an atmospheric storage tank: Part 4 (general metal loss), Part 5 (local thin areas and grooves) and Part 6 (pitting), Levels 1 and 2, with a reduced maximum fill height when the course does not pass, and a remaining life.

    API 653 4.3 allows metal loss beyond its own rules to be assessed by API 579-1; the Shell Evaluation tool does the API 653 checks, and this tool takes over from there.

    Edition and scope

    It follows the 2016 edition, the text on file; a 2021 edition is believed current and has not been compared. It covers Type A components only: a cylindrical course under liquid head, away from the shell-to-bottom joint, nozzles and rings by at least Lmsd. Cracks (Part 9), pitting on both surfaces and Level 3 are outside it. An FFS assessment is made and signed by a qualified engineer.

    Quick start

    Three samples: widespread pitting on a 24 m tank’s second course, a local thin area in the bottom course of a 30 m tank that fails Part 4 but passes Part 5, and general thinning of a 40 m tank’s second course from 20 point readings that passes Part 4.

    1. 1
      Section 1: D, the fill height you need, G, the height of the bottom of the course, tnom, course position, material (API 653 Table 4.1) or Sa, stress basis, joint efficiency, RSFa.
    2. 2
      Section 2: confirm the four applicability conditions (recognized code, smooth contours, not cyclic, toughness).
    3. 3
      Section 3: trd, FCA, and the vertical thickness profile (height, thickness); or point readings for Part 4. Lmsd to the nearest major discontinuity.
    4. 4
      Sections 4 and 5: read the Part 4 and Part 5 results; the verdict at the top names the first level that accepts, or the fill height to rerate to.
    5. 5
      Section 6: if there is pitting, its type, the matching pit chart and deepest pit (Level 1), and the pit-couples (Level 2).
    6. 6
      Section 7: corrosion and pit growth rates for the remaining life and the next inspection.

    Part 4 or Part 5

    • Start with Part 4 (4.1.2, 5.1.2). It averages thickness: over all point readings, or over the length L of the profile where the average is lowest. It is quick and conservative for local metal loss.
    • Use Part 5 when the loss is local and a profile exists. It credits the stronger metal around the thin spot through the Folias factor Mt and the remaining strength factor RSF.
    • Point readings: at least 15 (4.3.3.2 b). If the coefficient of variation is over 10 %, take profiles instead (4.3.3.2 c).
    • For a tank the critical inspection plane is vertical (4.3.3.3 b 3) and Part 5 needs only the longitudinal (vertical) CTP (5.3.3.2 a 1). Enter the thinnest reading at each height across the parallel planes, starting and ending where the thickness is back to trd.

    Part 4: thickness averaging (Table 4.6)

    StepRuleLTA sample
    tmin4.9 D G (H − 0.3) / (Sa E), Eq 2C.177; H above the bottom of the course4.9 × 30 × 0.9 × 13.7 / 172 = 10.54 mm
    Averaging lengthtml = tnom − FCAml; Rt = (tmm − FCAml)/tml; L = Q √(Dml tml), Table 4.8Rt 0.552 → Q 0.657 → L 433 mm
    Averagelowest average over L along the profile, minus FCAml, at least tmin10.44 − 1.5 = 8.94 mm < 10.54: fails
    Thinnesttmm − FCAml ≥ max(0.6 tmin, tlim), tlim = max(0.2 tnom, 2.5 mm)8.0 ≥ 6.32: meets
    MFHrthe fill height the average thickness supports, Eq 2C.17811.93 m against 14 m

    Level 2 uses Sa × Hf: Hf = 1.0 for API 653 allowables, 1/RSFa for API 650 or API 620 allowables (Table 4.6 note 1). General sample: tam 14.31 mm, tmin 13.00 mm, MFHr 16.82 m ≥ 16.5 m: accepted at Level 1.

    Part 5: local thin areas (5.4.2, 5.4.3.2)

    StepRuleLTA sample
    tc, Rttc = trd − FCA (Eq 5.4); Rt = (tmm − FCAml)/tc (Eq 5.5)14 mm; 8/14 = 0.571
    λ1.285 s / √(D tc), s = length below trd (Eq 5.6)s 1250 mm → λ 2.478, Mt 1.846
    Flaw-size limitsRt ≥ 0.20; tmm − FCAml ≥ 2.5 mm; Lmsd ≥ 1.8 √(D tc); groove gr/((1−Rt)tc) ≥ 0.5Lmsd 1350 ≥ 1167 mm: met
    Figure 5.6on or above the curve: acceptable at the MFHRt 0.571 below the curve 0.805
    RSF, Level 1Rt / (1 − (1 − Rt)/Mt), Eq 5.120.744
    MFHrHf + (MFH − Hf) RSF/RSFa, Eq 2.4; Hf = bottom of the flaw1.15 + (18.50 − 1.15) × 0.744/0.9 = 15.50 m ≥ 14: accepted
    Level 2lowest RSF over subsections of the profile, Eq 5.18, using the metal-loss area0.896 → MFHr 18.42 m
    • The tool evaluates every contiguous subsection of the profile, which includes all those of the ranked starting points in STEP 8.
    • The circumferential extent (STEP 9) is not evaluated: an atmospheric tank needs only the longitudinal CTP, and the hoop stress from the liquid governs. This is the tool’s reading of 5.3.3.2 a 1.
    • The fill height is measured from the bottom of the course (one-foot method). Keeping the flaw height Hf in Eq 2.4 makes the reduced height fall to the bottom of the flaw as RSF falls to zero.

    Part 6: pitting (6.4)

    Type of pitting (Figure 6.2)How the tool assesses it
    WidespreadLevel 1 pit chart (arrested pitting) and Level 2 pit-couples; RSFpit is the average RSFk (Eq 6.21)
    LocalizedLevel 2: equivalent thin area teq = RSFpit × tc (Eq 6.22) over the pitted length s, then Part 5 Level 1
    Thin area within widespread pittingRSFcomb = RSFpit × RSFLTA (Eq 6.24), RSFLTA from the Part 5 profile
    Pits inside a thin areapit depths measured from teq = RSFLTA × trd (Eq 6.23), then Eq 6.24
    StepRulePitting sample
    Thickness under the deepest pittmm = trd − wmax (Eq 6.3), at least 2.5 mm; Rwt = tmm/tc ≥ 0.20 (Eqs 6.4, 6.6); pits on the opposite side from the general corrosion subtract the FCA (Eqs 6.7, 6.9)9.6 − 4.0 = 5.6 mm; Rwt = 5.6/9.1 = 0.615
    Pit diameterd ≤ Q √(D tc) (Eq 6.10); a wider pit is a thin area12 mm ≤ 363 mm
    Level 1 RSFfrom the table under the matching pit chart, cylinder column, interpolated in Rwt; beyond Grade 8, RSF = Rwt (Eq 6.11)Grade 4: 0.904 ≥ 0.90
    Level 2 RSFk1 − (1 − Rwt,avg)(1 − Eavg), Eavg = min(Φ/Ψ, 1) from the ligament efficiency μ = (P − davg)/P and the stresses (Eqs 6.13 – 6.20)10 couples, RSFpit 0.930
    MFHrEq 2.4 from the bottom of the pitted regionRSF ≥ 0.90: MFH at tc = 15.31 m ≥ 12 m
    • The pit charts of Figures 6.6 – 6.13 are not reproduced: compare a scaled photograph or rubbing of the most densely pitted area with the charts in your copy of API 579-1 and pick the grade.
    • Level 1 needs arrested pitting on one surface; Level 2 is needed for active pitting, localized pitting and pitting combined with a thin area (6.2.5.2).
    • Pick at least ten pits across the area and pair each with its nearest neighbour (6.3.3.2 b, c). θ is the couple angle from the longitudinal stress; 0° is conservative. For a tank shell under liquid head the longitudinal stress is about zero, so σ2/σ1 = 0.
    • The FCA is not applied to the pit depths (6.2.8). With an FCA a shallow pit can give Rwt above 1; the tool caps the couple average at 1.0, which is conservative.
    • The course must pass for both the general or local metal loss and the pitting; the lower reduced fill height governs.
    • Remaining life grows the pits at the pit depth and diameter rates (Eqs 6.33, 6.34) while trd thins at the rate away from the flaw. New pits forming (6.5.2 STEP 4) are not modelled: re-inspect.

    Remaining life (4.5, 5.5)

    • The profile is thinned at the flaw rate, trd at the rate away from it, and the FCA is replaced by the corrosion over time (Eqs 5.36 – 5.39). The life is when MFHr falls below the fill height you need, or a flaw-size limit stops being met.
    • General sample by Part 4: (tam − tmin)/Crate = (14.31 − 13.00)/0.12 = 10.9 years (Eq 4.7).
    • Without the FCA the limit Lmsd ≥ 1.8 √(D tc) uses a thicker tc, so it can fail at year 0 even when the assessment passes: give enough distance.
    • Next inspection: half the remaining life, at most 15 years, as API 653 6.3.3.2 sets for UT intervals.

    When Levels 1 and 2 are not enough: the Level 3 data pack

    A Level 3 assessment is a stress analysis to API 579-1 Annex 2D, usually finite elements; a web form cannot do it. The tool collects what the analyst needs. Level 3 brief opens a printable page, and Download Level 3 data (JSON) saves the same data as a file. Both are always in SI, whatever units are on screen.

    Part of the packWhat it holds
    Why Level 3the reasons from this assessment, such as a flaw within Lmsd of the bottom joint, unconfirmed applicability, or a reduced fill height below the one required; each Level 1/2 result with its RSF and MFHr, and the best reduced fill height
    Component and materialdiameter, fill height, G, course and its bottom height, nominal and uniform thickness, FCA, joint efficiency, design temperature, Lmsd; grade, allowable stress, minimum yield and tensile; what is still needed (stress-strain curve, 2D.1.6)
    Damagethe vertical profile, flaw extent, groove radius, point readings, pitting data and pit-couples, corrosion and growth rates; the circumferential extent is listed as still to be measured
    Loadsliquid head (also with P = 0), hydrotest, dead and roof loads, wind, seismic, settlement and the fill cycles (2D.1.7, 2D.1.8)
    Failure modesplastic collapse (2D.2), local failure (2D.3), buckling at the end-of-life flaw size (2D.4) and fatigue (Part 14)
    Deliverablesmethod and model, validation, results per failure mode, the acceptable fill height or repair, remaining life and next inspection, assumptions and sensitivity

    The pack is a brief, not an assessment. Wind and seismic loads can be checked first in the Wind Design and Seismic tools with the inspected thicknesses.

    The checks

    CheckClauseResult
    Creep rangeTable 4.1fails above 343 °C
    Applicability4.2.6, 5.2.5a Note until the four conditions are confirmed
    Reading spacing; profile endsEq 4.1, Figure 4.10Notes
    Point readings4.3.3.2a Note for COV over 10 % or fewer than 15 readings
    Part 4 Levels 1 and 2Table 4.6fails when the average or the thinnest reading is short
    Part 5 flaw-size limitsEqs 5.7 – 5.11fails when not met
    Part 5 Levels 1 and 25.4.2, 5.4.3.2, Eq 2.4fails when MFHr is below the fill height you need

    How the numbers were checked

    • Tables 4.6, 4.8 and 5.2, Figure 5.6, the Figures 6.6 – 6.13 RSF tables, Eqs 6.1 – 6.24, Eqs 2.4, 4.1 – 4.7, 5.3 – 5.18, 5.36 – 5.39 and 2C.177 – 2C.180 were read from the 2016 text. API 653 Table 4.1 stresses are shared with Shell Evaluation.
    • The test suite (57 checks, plus 12 for the Level 3 data pack) reproduces Table 5.2 and Table 4.8 rows, Figure 5.6, both samples by hand, an independent Level 2 subsection search and sliding average, the Lmsd and groove limits, the API 650 Hf factor, the pit charts and pit-couple equations, the four pitting categories, the USC rule set, reopen, the report and the 390 px layout.

    Saving, reopening and reports

    • Inputs, including the profile and point readings, are kept on this device as you type. Calculate & Save stores the check in the ffsShellCalcs collection; Show saved checks lists them for the tank.
    • Level 3 brief and Download Level 3 data: see the Level 3 section above.
    • Print / PDF report: verdict, checks, the profile table and chart, Part 4 and Part 5 tables, Figure 5.6, remaining life, checklist and inputs.

    Limits and interpretations

    • Built on the 2016 edition; the 2021 edition has not been compared.
    • Wind, seismic and other supplemental loads (2C.2.7), external pressure, nozzles and the shell-to-bottom region are not assessed: these need Level 2 supplemental-load rules or Level 3.
    • Liquid-level cycling can make a tank cyclic service; check Part 14 before relying on Levels 1 and 2.
    • SI and USC use their own printed equations and Table 4.1a/b stresses, so results differ slightly between unit systems.

    Glossary

    CTPCritical thickness profile: the thinnest reading at each position, projected from parallel inspection planes.
    RSF / RSFaRemaining strength factor of the damaged shell, and its allowable value (0.90 recommended).
    MFH / MFHrMaximum fill height, and the reduced fill height for the damaged course.
    LmsdDistance from the flaw to the nearest major structural discontinuity.
    FCA / FCAmlFuture corrosion allowance, away from and within the region of metal loss.