Corrosion Rate Selection After a Service Change to Wet Sour
A carbon steel knockout drum has a nominal wall thickness of 19.05 mm and a required minimum of 9.5 mm. UT readings are 17.8 mm in 2014, 17.2 mm in 2019, and 16.9 mm in 2024 at the same four points. In 2022 the service was changed to wet sour gas containing 4,500 ppm H2S, and a water wash was added but is sometimes bypassed. The 2024 UT was taken at the same four points as earlier; no cracking inspection has been performed. The unit runs continuously, and the next turnaround is in 2028.
1. Calculate the long-term and interval corrosion rates and the remaining life to minimum thickness. What do the numbers tell you?
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Using the four-point average readings:
- 2014 to 2019: (17.8 - 17.2) / 5 = 0.12 mm/yr
- 2019 to 2024: (17.2 - 16.9) / 5 = 0.06 mm/yr
- Long-term 2014 to 2024: (17.8 - 16.9) / 10 = 0.09 mm/yr
Remaining thickness above minimum is 16.9 - 9.5 = 7.4 mm. At 0.09 mm/yr that is about 82 years; at 0.12 mm/yr it is about 62 years. Thinning is not the controlling risk on this timescale.
The important judgement is that the 2024 rate does not yet reflect the 2022 change to wet sour service. A low recent thinning rate can coexist with a high cracking risk, which UT thickness readings do not detect. Treating the 0.06 mm/yr rate as proof of low risk would be a mistake.
2. Which corrosion rate should feed the probability-of-failure assessment, and why?
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Do not automatically use the lowest recent interval. The rate must represent the current and future service, and the 2024 interval covers only about two years of the new wet sour condition. Because the water wash is sometimes bypassed, there is no documented reason to believe the lower rate is sustainable.
A defensible approach is to use the higher of the long-term rate and the most recent interval rate (0.12 mm/yr) as a conservative planning rate unless a documented process review supports a lower value. State the basis and sensitivity in the assessment: at 0.12 mm/yr the remaining life is still long, so the thinning calculation is not what drives inspection. The rate choice mainly prevents a false sense of low risk while the cracking mechanism is unaddressed.
3. What additional damage mechanisms and inspection data are needed before the PoF is finalized?
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Wet sour service at 4,500 ppm H2S introduces cracking mechanisms that are not evaluated by thickness UT: sulfide stress cracking, hydrogen-induced cracking, and stress-oriented hydrogen-induced cracking. The absence of cracking inspection is a major data gap, not evidence of absence.
The assessment should add a cracking damage factor and require an effective cracking inspection method, such as wet fluorescent magnetic particle testing or ultrasonic shear-wave examination at selected welds and susceptible areas. It should also document water-wash reliability, process upset history, H2S partial pressure, pH, and any hardness or weld condition data. Until those data exist, the PoF should carry a conservative uncertainty allowance and the inspection plan should address cracking, not only wall loss.