Air receiver with general thinning and a deep pit: thickness calculation, remaining life and derating

Use the thin-shell formula with the design values actually used for this vessel:

t_min = p·Di / (2[σ]t·φ − p) + C

With p = 1.6 MPa, Di = 1800 mm, φ = 0.85, C = 1.0 + 0.3 = 1.3 mm, and taking [σ]t for Q345R at the design temperature from the material table (about 170 MPa at this temperature, to be confirmed from the applicable standard):

t_min = (1.6 × 1800) / (2 × 170 × 0.85 − 1.6) + 1.3 = 2880 / 287.4 + 1.3 = 10.0 + 1.3 = 11.3 mm

The measured 11.6 mm is only 0.3 mm above this calculated minimum, so general thinning is technically acceptable but with almost no margin. The six-year deferral is not justifiable.

Corrosion rate from the two surveys: (12.6 − 11.6) / 6 = 0.17 mm/year, consistent with the earlier trend. Remaining margin ÷ rate = 0.3 / 0.17 ≈ 1.8 years. Allowing for measurement error and for the pit discussed separately, the next internal inspection should be set at about 1.5 years, with an external inspection and thickness check within 12 months. The interval must be stated in the report with the calculation and the survey data attached, not simply copied from the previous cycle.

The pit is below the calculated minimum of 11.3 mm by about 1.5 mm, so it cannot be accepted by general-thickness logic and it cannot simply be watched for six years.

Dispositions, in order of preference:

1. Repair. Grind the pit to sound metal, assess the resulting depth against the minimum, then weld-repair with a qualified procedure and qualified welders, using preheat/drying appropriate to Q345R, followed by surface NDT (MT or PT) and volumetric NDT where geometry allows, and a hydrostatic test at the required test pressure. If grinding alone removes the defect and thickness remains above minimum, a blend-and-inspect disposition may be acceptable with the profile checked for smooth transition.
2. Fitness-for-service assessment. A local-thinning assessment (for example by the appropriate FFS procedure) may show the pit is tolerable at a reduced pressure, but this must be a documented engineering evaluation, not an inspector's opinion, and it must account for the pit being on the lower shell where water collects.
3. Derate. Recompute the allowable pressure using the measured 9.8 mm: p ≈ 2[σ]t·φ·(t − C) / (Di + t − C) = 2 × 170 × 0.85 × 8.5 / 1808.5 ≈ 1.36 MPa. A rating of about 1.3 MPa would require a new nameplate, re-setting the safety valve to the new pressure, written approval and notification of the user.

Whichever route is chosen, the pit must be mapped, photographed and recorded with its location and dimensions, and the cause (condensate, draining practice) addressed.

The report must be a decision document, not a data dump:

• The thickness calculation with every input stated and traceable to the material standard, not a bare result.
• The survey data for both years, the corrosion rate, the calculated remaining life and the inspection interval selected, with the reason the requested six years was refused.
• A defect record for the pit: location, size, remaining thickness, disposition chosen, the repair procedure and NDT results if repaired.
• Confirmation that the safety valve is set consistently with whatever pressure is finally certified, that the pressure gauge is calibrated and in range, and that the drain and support/settlement checks were done.
• The certification decision and its limits: full design pressure only if the pit is repaired and the vessel re-tested; otherwise operation at the derated pressure after approval and re-marking.

Tell the owner plainly: continued operation at 1.6 MPa with a 9.8 mm pit is not certifiable, and the six-year interval is not available on these readings. If the owner wants the deferral, the route is repair followed by a fresh survey and a re-calculated interval. Record the refusal in writing, because an oral 'no' will be replaced by a new inspector and a new request next month.