API 579 FFS Level 1
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API 579 FFS Level 1 — General Metal Loss & LTA

Fitness-For-Service Screening per API 579-1 / ASME FFS-1 Parts 4 & 5

Level 1 FFS screening for corroded pressure equipment. Implements Part 4 (General Metal Loss) and Part 5 (Local Thin Area) of API 579-1/ASME FFS-1 for cylindrical shells under internal pressure. If Level 1 passes, the equipment is fit for continued service at MAWP. If Level 1 fails, escalate to Level 2.

Equipment

in
in
psig
psi
SA-516 Gr 70 @ ≤100°F = 17,500
ASME VIII / B31.3 only. For B31.8, S = SMYS and derating uses F/E/T below.
Class-location design factor (B31.8 Table 841.1.6-1). Only used for B31.8.
Longitudinal joint factor (B31.8 Table 841.1.7-1). Only used for B31.8.
Temperature-derating factor (B31.8 Table 841.1.8-1). Only used for B31.8.

Inspection Data

in
Thinnest point in CML / inspection area
in
Arithmetic average over CML
in
Corrosion rate × interval to next inspection
in
To nearest nozzle / weld / head

Local Thin Area (Part 5)

in
Longitudinal extent of thin region (used in λ, Part 5 Eq 5.6)
in
Sound metal away from LTA

📚 Learn the Theory

FFS framework, Level 1 vs 2 vs 3, RSF derivation, LTA acceptance criteria, MAWP reduction.

Read Engineering Guide →

Key Equations

Required wall thickness (ASME VIII UG-27(c)(1) / B31.3, inside-radius form):

tmin = P · R / (S · E − 0.6 · P)   R = (OD − 2·tnom) / 2

Corroded remaining wall (tc):

tc = tmm − FCA

General-loss acceptance (Part 4 §4.4.2 — NOT an RSF check):

tam − FCA ≥ tmin   AND   tmm − FCA ≥ max(0.5·tnom, 0.1·tnom + 0.1 in)   AND   LMSD ≥ 1.8·√(D·tc)

LTA reference wall (Part 5 Eq 5.3/5.4) & remaining-thickness ratio (Eq 5.5):

tc = trd − FCA    Rt = (tmm − FCA) / tc

LTA shell parameter (Part 5 Eq 5.6, D = inside diameter, tc = corroded reference wall):

λ = 1.285 · s / √(D · tc)    Mt = f(λ) per Table 5.2 (λ ≤ 20)

LTA Remaining Strength Factor (Part 5 Eq 5.12, RSTRENG-Modified-B31G):

RSF = Rt / (1 − (1/Mt)·(1 − Rt))   [accept RSF ≥ 0.90]

Level-1 flaw-depth gate (Part 5 §5.4.2.2):

Rt ≥ 0.20   AND   (tmm − FCA) ≥ 0.10 in (0.05 in piping)   else → Level 2/3

Reduced MAWP if RSF < RSFallow:

MAWPr = MAWP · (RSF / RSFallow)

Related Calculators

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B31G / Modified

Remaining strength for pipeline metal loss — companion FFS.
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MAOP / MAWP

Re-calc allowable pressure at reduced thickness.
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Class Location / HCA

FFS in HCA segments has tighter intervals.
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Material Selection

Allowable stress at temperature.