Fitness-for-Service Assessment: Severe Local Corrosion

When t-min isn’t Really the Minimum

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By Greg Garic at Stress Engineering Services, Inc.. This article appears in the March/April 2013 issue of Inspectioneering Journal

Fitness-for-Service (FFS) Assessment

If an operator finds cracking in a furnace waste heat boiler, excessive thinning in an absorption tower, or severe bulging in a converter, FFS assessments—not standard code analyses—are needed to evaluate the unit’s mechanical integrity. FFS assessments, according to the American Petroleum Institute (API), are “quantitative engineering evaluations that are performed to demonstrate the structural integrity of an in-service component containing a flaw or damage.”

The potential benefits of applying FFS methods to assess equipment condition are obvious when it is recognized that most code-based analyses are very conservative. It has been reported that in older versions of the ASME Section VIII, Div. 1 Pressure Vessel Code, the actual safety factor at failure was near 10. Therefore, if an operator is willing to expend some extra effort to perform a more detailed FFS assessment, the equipment may be found to be able to be operated safely, even in a somewhat degraded condition.

In the 1980s, FFS assessment was performed in accordance with “Generally Accepted Good Engineering Practices,” and was highly dependent on the analytical skills of the engineer. At that time, ASME and API committees began developing a standard to address in-service equipment problems. With the strong support of major oil companies, API moved more quickly in this development and published API RP-579, “Recommended Practice for Fitness-for-Service,” in 2000.

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