Tank Shell Fitness-for-Service (API 579-1)

Tank Shell Fitness-for-Service (API 579-1 Parts 4 & 5)

Fitness-for-service of a corroded atmospheric tank shell course to API 579-1/ASME FFS-1: general metal loss by thickness averaging (Part 4, Table 4.6) and local thin areas or grooves by the remaining strength factor (Part 5, Levels 1 and 2), pitting by pit charts or pit-couples (Part 6), with the reduced maximum fill height (Part 2, Eq 2.4), the Figure 5.6 screening curve and a remaining life

New here? Read the User Manual — when to use Part 4 or Part 5, how to take the thickness profile, and worked examples.

◉ API 579-1/ASME FFS-1 (2016) · PART 4 TABLE 4.6 · PART 5 LEVELS 1 & 2 · PART 6 PITTING · ANNEX 2C.6 · ATMOSPHERIC TANK SHELLS

⚠ Indicative tool only. Follows API 579-1/ASME FFS-1, 2016 edition, the text on file; a 2021 edition is believed current and has not been compared. Covers Level 1 and Level 2 for cylindrical atmospheric tank shell courses under liquid head only (Type A components). Not covered: metal loss within the distance L_msd of the shell-to-bottom joint, nozzles or stiffening rings, wind or seismic loads, pitting on both surfaces, crack-like flaws (Part 9) and Level 3. An FFS assessment is made and signed by a qualified engineer (Part 1, 1.4.3).

Identification

Company

Tank ID

Location

Record Type

Units

Each unit system uses its own printed equations (Annex 2C, 2C.6.3) and API 653 Table 4.1a or 4.1b stresses, so SI and US results differ slightly. Records are kept in SI.

Summary of checks

Enter the tank and the course, then the thickness data.

1. Tank and Shell Course (Annex 2C, 2C.6)

Nominal diameter, D m

Required fill height, MFH m above the tank bottom

Specific gravity, G largest stored

Bottom of the course m above the tank bottom

Nominal (furnished) thickness, t_nom mm

Course

Material (API 653 Table 4.1)

Allowable stress, S_a MPa

Allowable stress basis (Table 4.6 note 1)

Joint efficiency, E (2C.2.5)

E entered

Allowable RSF_a recommended 0.90 (2.4.2.2 d)

Design temperature °C

Joint efficiency: a damaged region counts as at a weld if any part of it is within the weld band, centred on the weld and the greater of 50.8 mm (2 in.) or twice the plate thickness wide (2C.2.5 a); otherwise E = 1.0 may be used. The fill height is measured from the bottom of the course (one-foot method) and reported above the tank bottom.

2. Applicability of Levels 1 and 2 (4.2.6, 5.2.5)

4.2.6 a) Designed to a recognized code or standard (API 650, API 12C, or equal). 4.2.6 b) The metal loss has relatively smooth contours, without notches. 4.2.6 c) Not in cyclic service: fewer than 150 pressure or temperature cycles in its past and planned life, or the Part 14 screening is met. 5.2.5 b) Material toughness is adequate (Part 3, or the API 653 Section 5 brittle fracture screening).

A tank whose liquid level is cycled many times may count as cyclic service; check it with Part 14 before relying on Level 1 or 2. The Brittle Fracture tool walks the API 653 screening.

3. Thickness Data (4.3.3, 5.3.3)

Data for Part 4

Uniform thickness away from the flaw, t_rd mm · blank = t_nom

Future corrosion allowance, FCA mm

FCA in the region of metal loss mm · blank = FCA

Metal loss on

Flaw (5.2.1)

Groove base radius, g_r mm

Distance to the nearest major discontinuity, L_msd m · Figure 4.6

For an atmospheric tank the critical inspection plane is vertical (4.3.3.3 b 3), and Part 5 needs only the longitudinal CTP (5.3.3.2 a 1). Enter the thinnest reading at each height across the parallel vertical planes (Figure 4.10). Start and end the profile where the thickness is back to t_rd; space readings no more than min(L, 2 t_rd) apart, at least five per plane (Eq 4.1). L_msd is from the nearest edge of the metal loss to the nearest weld of the shell-to-bottom joint, a nozzle or a ring.

#	Height m	Thickness mm

Point thickness readings mm · separate with commas or spaces; at least 15 (4.3.3.2 b)

Flaw length s override m · blank = from the profile

Bottom of the flaw override m above the tank bottom

4. Part 4: General Metal Loss (4.4, Table 4.6)

5. Part 5: Local Metal Loss (5.4.2, 5.4.3.2)

6. Part 6: Pitting (6.4)

Type of pitting (6.2.1, Figure 6.2)

Pits are on

Bottom of the pitted region m above the tank bottom · blank = course bottom

6.2.5.1 e) Pitting is on one surface only (pits on both surfaces need Level 3). 6.2.5.1 f) Many pits: single pits or isolated pairs are assessed as thin areas by Part 5. 6.2.5.1 g) Pitting is arrested (needed for Level 1).

Level 1: pit charts (6.4.2)

Pit chart that matches the damage

Deepest pit, w_max mm

Largest pit diameter mm

Compare a photograph (with a scale) or a rubbing of the most densely pitted area with the pit charts in Figures 6.6 to 6.13 of your copy of API 579-1 (6.3.3.1); the charts are not reproduced here. Measure the deepest pit with a pit gauge (6.3.4.1).

Level 2: pit-couples (6.4.3)

Stress ratio σ₂/σ₁ 0 for a tank shell under liquid head

Height of the pitted region, s m (Figure 6.15)

Select at least ten pits across the area and pair each with its nearest neighbour (6.3.3.2 b, c). Depths are below t_rd (for pits inside the thin area: below its equivalent thickness t_eq = RSF_LTA × t_rd, Eq 6.23); diameters and the centre spacing P in the same unit. θ is the angle of the line joining the pits from the σ₂ (vertical) direction; 0° is conservative. For a tank shell σ₁ is the hoop stress and σ₂ the small vertical stress.

#	d_i mm	w_i mm	d_j mm	w_j mm	P mm	θ °	RSF_k

7. Remaining Life (4.5, 5.5, 6.5)

Corrosion rate in the flaw mm/yr

Corrosion rate away from the flaw mm/yr · blank = same

Growth of the flaw length mm/yr · optional

Pit depth growth, PPR mm/yr (Eq 6.33)

Pit diameter growth mm/yr (Eq 6.34)

Remaining life is the time until the reduced fill height falls below the required one, with the future corrosion replacing the FCA (Eqs 4.7, 5.36 – 5.39); pits deepen and widen at the pit growth rates (Eqs 6.33, 6.34), without new pits forming. The profile is thinned uniformly at the flaw rate; the flaw length grows only if a growth rate is given.

Inspection and Documentation Checklist (5.3.4.3, 2.8)

Tick each item once it is confirmed. Items that don’t apply are hidden.

Save & Report

Inputs are kept on this device as you type and offered back if the page is closed before saving. The API 653 checks of the whole shell (4.3, including local thin areas by 4.3.2) are in the Shell Evaluation tool; this tool is the API 579 route that 4.3 allows for metal loss beyond those rules. Where Levels 1 and 2 do not accept the course, the Level 3 brief and Level 3 data collect what an analyst needs for an Annex 2D assessment (geometry, damage, material, loads, failure modes, deliverables), in SI. Saved results feed the Seismic and Wind Design tools, which can load the inspected course thicknesses for the same Tank ID.