General metal loss is corrosion spread over an area large enough that the component behaves as if it were uniformly thinner, rather than as a shell with a local dimple in it. API 579-1 Part 4 handles it, and the central idea is thickness averaging. A single low ultrasonic reading does not by itself condemn a vessel, because a pressure boundary carries load by membrane action over a finite length. What matters is the average wall over the length that actually shares the load, together with a floor on how thin any single point may be.

The length over which averaging is permitted is L = Q·√(D·t_min). The √(D·t) group is the characteristic decay length of a shell — the distance over which a local disturbance in a cylinder dies away — and Q scales it according to how thin the worst point is relative to what is required. Q comes from Table 4.4 and follows Q = 1.123·√{[(1−R_t)/(1−R_t/RSF_a)]² − 1}, where R_t = (t_mm − FCA)/t_min is the remaining thickness ratio and RSF_a is the allowable remaining strength factor, 0.90 for most pressure equipment. As R_t approaches RSF_a the permitted averaging length grows without limit, because the wall is barely below requirement anywhere. As R_t falls, Q shrinks and the averaging window closes down onto the defect.

Level 1 then applies two acceptance criteria, and both must pass. First, the average measured thickness less the future corrosion allowance must be at least the required minimum: t_am − FCA ≥ t_min. Second, the single minimum measured thickness less FCA must be at least the greater of half the required thickness and 2.5 mm: t_mm − FCA ≥ max(0.5·t_min, 2.5 mm). That second criterion is the guard against local perforation and against a very thin spot being hidden by a favourable average.

Two things sit outside this calculation and must be checked separately. Level 1 in Part 4 assumes the metal loss is remote from major structural discontinuities — nozzles, head-to-shell junctions, stiffeners — and does not apply where supplemental loads such as wind, weight or thermal expansion are significant. Where the loss is clearly local rather than general, Part 5 for a local thin area is the correct route and will usually be less conservative. And the future corrosion allowance is an engineering judgement about remaining life, not a measurement: it is the corrosion rate multiplied by the intended interval to the next inspection.

Worked example

Inside diameter	500 mm
Required minimum thickness t_min	8 mm
Average measured thickness t_am	9.5 mm
Minimum measured thickness t_mm	7.2 mm
Future corrosion allowance	1 mm
Allowable remaining strength factor RSF_a	0.9
Allowable stress	118 MPa
Weld joint efficiency	1
Remaining thickness ratio R_t	0.775
Averaging parameter Q	1.431
Length for thickness averaging L	90.5 mm
Averaged wall less FCA	8.5 mm
Minimum wall less FCA	6.2 mm
Minimum permitted local wall	4 mm
Check 1 - average thickness	1
Check 2 - minimum thickness	1
MAWP from averaged wall	3.93 MPa

R_t = (7.2 - 1.0)/8.0 = 0.775. (1-0.775) = 0.225; (1 - 0.775/0.90) = 0.138889; ratio 1.62; 1.62^2 - 1 = 1.6244; sqrt = 1.274520; Q = 1.123 x 1.274520 = 1.431285 -> 1.431. This agrees with API 579 Table 4.4, which gives Q = 1.680 at R_t = 0.80 and 0.579 at R_t = 0.50 from the same equation. L = 1.431285 x sqrt(500 x 8) = 1.431285 x 63.24555 = 90.52 -> 90.5 mm. Check 1: 9.5 - 1.0 = 8.5 >= 8.0, pass. Check 2: 7.2 - 1.0 = 6.2 >= max(4.0, 2.5) = 4.0, pass. MAWP = 118 x 1.0 x 8.5/(250 + 0.6 x 8.5) = 1003/255.1 = 3.932 -> 3.93 MPa.

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