Remaining life of a thinned white liquor accumulator
White liquor accumulator V-210 is a vertical pressure vessel, 3.6 m inside diameter, design 0.85 MPa(g) at 120 C, SA-516 Gr. 70, original shell thickness 20 mm, full radiography, E = 1.0. A recent UT grid found a localized area 300 mm by 400 mm with minimum 14.1 mm; the previous survey four years ago read 15.5 mm in the same area. The area straddles a longitudinal weld. General corrosion elsewhere is 0.10 mm/y. The mill needs five more years before replacement and asks whether it can keep the 0.85 MPa rating. Allowable stress at 120 C is 138 MPa. There is no external corrosion, but the support skirt has 2 mm of rust scale.
1. Calculate the minimum required thickness for the current rating and the remaining life at the measured corrosion rate; state assumptions.
Read the worked answer
Use R = 1,800 mm. t_required = P R / (S E - 0.6P) = 0.85 x 1,800 / (138 x 1.0 - 0.6 x 0.85) = 1,530 / 137.49 = 11.13 mm. Full radiography gives E = 1.0, so the longitudinal weld does not reduce E. Corrosion rate = (15.5 - 14.1) / 4 = 0.35 mm/y. Remaining life to t_required = (14.1 - 11.13) / 0.35 = 8.5 years. If the thinned zone were in a weld with E = 0.85, t_required = 13.1 mm and life would be 2.9 years. Assumptions: 14.1 mm is a general thin area, not a crack or isolated pit; the 0.35 mm/y rate is linear; no future corrosion allowance is required for a remaining-life calculation; and the measured area controls.
2. Is the five-year run acceptable without rerating, and what conditions would change your answer?
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Conditionally yes, because 8.5 years exceeds five years, but only if E = 1.0 is confirmed for the longitudinal weld, the UT minimum is not a localized pit or crack, the corrosion rate is based on comparable locations, and the skirt is inspected. The 0.10 mm/y general rate is not controlling. If E = 0.85 applies because the weld is not fully radiographed or the thin area is in the weld, life is 2.9 years and the vessel must be rerated or repaired. Rerun UT in 12 months; if the local rate exceeds 0.6 mm/y, reassess. Do not commit to five years without a second survey.
3. What NDE and inspection plan would you require before return to service?
Read the worked answer
Run a UT thickness grid at 300 mm spacing, plus 100% UT or RT of the longitudinal weld in the thinned band. MT or PT the weld for cracks, and check for pitting and under-deposit corrosion. Perform an external inspection of the skirt and supports, including the rusted area. If the thin area is not general, assess it as a local thin area under API 579. Mark a corrosion-monitoring location and set a 12-month UT interval.