A local thin area is metal loss small enough that the surrounding sound shell still carries load around it. Unlike general metal loss, where the whole section is thinner, an LTA is a dimple in an otherwise sound cylinder, and it fails by bulging: the thin patch deflects outward under pressure, the membrane stress in it rises above nominal, and the ligament tears. API 579-1 Part 5 quantifies that with two dimensionless numbers.

The first is the remaining thickness ratio, R_t = (t_mm − FCA)/t_min, which says how much of the required wall survives at the worst point. The second is the longitudinal flaw length parameter, λ = 1.285·s/√(D·t_min), which compares the axial extent of the flaw to the characteristic shell decay length √(Dt). A short flaw is restrained by the surrounding shell; a long one is not. From λ comes the Folias-type bulging factor M_t, taken from the Annex 2C closed-form fit for a cylindrical shell with a flaw of longitudinal extent, which is close to 1 for a very short flaw and rises steadily as the flaw lengthens. The circumferential-extent fit in the same annex is a different and much weaker curve — using it for an axially oriented thin area under hoop stress over-states the remaining strength.

Those combine into a remaining strength factor, RSF = R_t / [1 − (1 − R_t)/M_t], the ratio of the collapse load of the damaged component to that of the undamaged one. If RSF is at least the allowable value RSF_a — 0.90 for most pressure equipment — the component is acceptable at its existing MAWP. If it falls below, the MAWP is reduced in proportion: MAWP_r = MAWP · RSF/RSF_a. This calculation uses the conservative rectangular metal-loss idealisation; a Level 2 assessment using the measured thickness profile will generally return a higher RSF.

Two Level 1 gates must also be satisfied and are reported separately. R_t must be at least 0.20 — below that the ligament is too thin for the method and the flaw must be treated as a crack-like defect — and the remaining wall t_mm − FCA must be at least 2.5 mm. The flaw must also stand clear of any major structural discontinuity by at least 1.8·√(D·t_min).

For pitting, Part 6 permits a conservative screening in which the deepest pit is treated as an LTA: use the pit diameter as the flaw length s and the thickness at the pit bottom as t_mm. If that passes, the pitting is acceptable without further work. If it fails, a full Part 6 assessment against the standard pit charts, or a Level 2 pit-couple evaluation, is needed — widely scattered pitting is often acceptable under Part 6 when the single-pit-as-LTA screen has rejected it.

Worked example

Inside diameter	500 mm
Required minimum thickness t_min	8 mm
Minimum measured thickness in the flaw	4.5 mm
Longitudinal extent of the flaw	60 mm
Future corrosion allowance	0.5 mm
Allowable remaining strength factor RSF_a	0.9
Current MAWP	3.5 MPa
Remaining wall less FCA	4 mm
Remaining thickness ratio R_t	0.5
Flaw length parameter λ	1.219
Bulging factor M_t	1.2748
Remaining strength factor RSF	0.8226
Minimum clearance to a discontinuity	113.8 mm
Gate - R_t ≥ 0.20	1
Gate - remaining wall ≥ 2.5 mm	1
Permitted MAWP	3.199 MPa

t_mm - FCA = 4.5 - 0.5 = 4.0 mm, so R_t = 4.0/8.0 = 0.500. sqrt(500 x 8) = 63.2456, so lambda = 1.285 x 60/63.2456 = 1.21906, lambda^2 = 1.48610 and lambda^4 = 2.20848. Numerator = 1.02 + 0.4411 x 1.48610 + 0.006124 x 2.20848 = 1.689045; denominator = 1.0 + 0.02642 x 1.48610 + 1.533e-6 x 2.20848 = 1.039266; M_t = sqrt(1.625234) = 1.27484. RSF = 0.5/(1 - 0.5/1.27484) = 0.5/(1 - 0.392205) = 0.5/0.607795 = 0.82265 -> 0.8226. That is below RSF_a = 0.90, so the MAWP is reduced: 3.5 x 0.82265/0.90 = 3.1992 -> 3.199 MPa. For comparison, Modified B31G on the same geometry (z = 60^2/(500 x 8) = 0.90) gives a Folias factor of 1.25, which is the cross-check that the longitudinal-extent M_t is the right one. Both Level 1 gates pass (R_t 0.50 >= 0.20 and 4.0 mm >= 2.5 mm) and the flaw must sit at least 1.8 x 63.2456 = 113.8 mm from any structural discontinuity.