Article 4, API 579-1: Fitness-For-Service (FFS) - Pitting Assessment

By Stephen J. Gliebe, P.E.

Did a recent external inspection on an exchanger reveal pitting corrosion under the insulation? Did a unit upset result in internal pitting on the bottom head of the reactor? If you answered yes to these questions, or if pitting corrosion due to other factors is occurring in your plant, an API-579-1 FFS pitting assessment may provide an alternative to costly repairs.

In this article, we will take a look at evaluating widely scattered pitting by applying API 579-1 / ASME FFS-1 2007 Fitness-For-Service Level 1 assessment procedures.

What is Pitting?

Pitting is defined by API 579 as localized regions of metal loss characterized by a pit diameter on the order of the plate thickness or less. Widely scattered pitting is pitting that occurs over a significant region of the component.

Level 1 Pitting Assessment Limitations

Level 1 pitting assessments are permitted only if certain conditions are satisfied. The complete list of limitations in Parts 2 and 6 of API 579 should be reviewed before proceeding with a Level 1 pitting assessment. Some of the limitations are:

Level 1 Methodology

In a Level 1 assessment, surface damage, quantified by the pitted area and pit depth, is compared to standard pit charts to determine acceptability. If the depth of all the pits is less than the specified corrosion allowance, then a pitting assessment is not required.

However, if the pit depth exceeds the corrosion allowance, then a pitting assessment should be considered. Note that for a Level 1 assessment, the future pitting damage (FCA) is assumed to be zero, e.g., pitting corrosion due to corrosion under insulation that has been mitigated by application of an epoxy coating and permanent removal of the insulation. Below is a summary of the assessment process. Details of the process, as well as the nomenclature, can be found in API 579-1 Part 6.

Assessment Summary:

 

where: