Remaining life means nothing without a target. The minimum required thickness is the wall the pressure design actually needs — not the nominal wall, and not the nominal wall minus the corrosion allowance, which is a purchasing decision rather than a design limit.

For a cylindrical shell under internal pressure, ASME VIII Division 1 UG-27 gives two thicknesses and the larger governs. Circumferential stress, resisted by the longitudinal seam, needs t = PR/(SE − 0.6P). Longitudinal stress, resisted by the circumferential seam, needs only t = PR/(2SE + 0.4P) — roughly half — which is why a cylinder that fails by internal pressure splits along its length. R is the inside radius in the corroded condition, so it grows as the wall thins.

ASME B31.3 works from the outside diameter instead: t = PD/(2(SEW + PY)), where Y is a coefficient from Table 304.1.1 that accounts for the through-wall stress distribution, W is the weld joint strength reduction factor for elevated temperature, and E is the quality factor for the pipe itself. The pressure design thickness is then increased by mechanical, corrosion and erosion allowances to give the minimum wall to order, but for a fitness assessment it is the pressure design thickness that the measured wall must beat.

Two things dominate the answer and neither comes from ultrasonics. The allowable stress S must be read from ASME Section II Part D at the design temperature, and the joint efficiency E must match the actual radiography of the seam. Both are engineering inputs, and a minimum thickness quoted without stating them is not a number anyone can check.

The formulas here cover thin-wall internal pressure only. External pressure, structural minima, discontinuities, nozzle reinforcement, wind and seismic loading and localised metal loss all sit outside them and are handled by their own rules or by a fitness-for-service assessment.

Worked example

Design rule	b313_straight
Design pressure	3 MPa
Outside diameter	168.3 mm
Measured wall thickness	7.11 mm
Allowable stress	137.9 MPa
Joint efficiency / quality factor E	1
Weld joint strength reduction factor W	1
Coefficient Y	auto
Material class for Y	ferritic
Design temperature	100 °C
Manual Y	0.4
Corrosion and mechanical allowance	1.6 mm
Inside radius (corroded)	77.04 mm
Coefficient Y in use	0.4
Pressure design thickness	1.81 mm
Minimum wall including allowance	3.41 mm
Wall above the minimum	3.7 mm
MAWP at the measured wall	12.06 MPa

NPS 6 Schedule 40 pipe, 168.3 mm OD measured at 7.11 mm, 3.0 MPa design, S = 137.9 MPa, E = W = 1.0, ferritic at 100 °C so Y = 0.4 from Table 304.1.1. t = 3.0 × 168.3 / (2 × (137.9 + 3.0 × 0.4)) = 504.9 / 278.2 = 1.81 mm. With a 1.6 mm allowance the minimum wall is 3.41 mm, leaving 3.70 mm of margin. Rearranged for pressure, the measured 7.11 mm wall is good for 2 × 137.9 × 7.11 / (168.3 − 2 × 0.4 × 7.11) = 12.06 MPa.

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