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Best Practice for thickness data statistical analysis?

Best Practice for thickness data statistical analysis?

Best Practice for thickness data statistical analysis?

We manufacture and store sulfuric acid on site in tanks built to either API 620 or API 650. All tanks have been modified to NACE SP0294 (Sulfuric Acid Tank standard). Both API 653 and NACE require an internal inspection at 1/2 the remaining life of the tank bottom.

Is there a best practice on how to determine the 1/2 life (and retirement date) when I have non-uniform corrosion on the tank bottom? We typically have > 60 data points on the tank floor (thickness monitoring locations). We've recently started doing Magnetic Flux Leakage testing to get a 100% floor scan. Regardless the corrosion is non-uniform and depending on how we average the data we can end up with widely varying retirement dates and corresponding 1/2 life inspection dates.

Is it best practice to base the next inspection date on the single TML with the earliest retirement date?
Is it more common to base the next inspection date on some type of averaging or statistical analysis? If so, what is the preferred way to analyze the data?

Any insight would be appreciated.

Peter Morenc
Sr. Reliability Engineer

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