API 579 FFS - Pitting Assessment
ASME Pressure Vessels
The objective of this blog is to provide fundamental information and comprehension of the ASME code concerning the design of pressure vessels for the chemical and process industry in the United States and a majority of North and South America.
API 579-1 Fitness-For-Service (FFS) - Pitting Assessment
Have you recently discovered pitting corrosion beneath the insulation during an external inspection of an exchanger? Did an internal pitting occur on the bottom head of the reactor due to a unit upset? If your answer is yes to any of these questions, or if your plant is experiencing pitting corrosion caused by other factors, conducting an API-579-1 FFS pitting assessment might offer a cost-effective alternative to extensive repairs.
This blog explores the evaluation of widely scattered pitting by applying the assessment procedures outlined in API 579-1/ASME FFS-1 2007 Fitness-For-Service Level 1.
Definition of Pitting
Pitting, as defined by API 579, refers to localized areas of metal loss characterized by pit diameters approximately equal to or smaller than the plate thickness. Widely scattered pitting involves pitting that occurs over a significant region of the component.
Limitations of Level 1 Pitting Assessment
Level 1 pitting assessments can only be performed under certain conditions. It is crucial to review the complete list of limitations in Parts 2 and 6 of API 579 before proceeding with a Level 1 pitting assessment.
Some of these limitations include:
The pitting damage must be:
Located on a single surface of the component (internal or external)
Composed of numerous pits
The component must be:
A type A component subjected to internal pressure, such as cylindrical and conical shell sections of pressure vessels, which have a design equation specifically relating pressure or liquid fill height, and other loads, to the required wall thickness
Not in cyclic service
Not operating in the creep regime
Considered to possess sufficient material toughness
Methodology of Level 1 Assessment
In a Level 1 assessment, the surface damage, characterized by pit area and depth, is compared against standard pit charts to determine acceptability. If the depth of all pits is smaller than the specified corrosion allowance, a pitting assessment is not necessary.
However, if the pit depth exceeds the corrosion allowance, a pitting assessment should be considered. It is important to note that for a Level 1 assessment, the future pitting damage (FCA) is assumed to be zero. For example, pitting corrosion due to insulation corrosion mitigated by epoxy coating and permanent insulation removal. Below is a summary of the assessment process. Detailed information and nomenclature can be found in API 579-1 Part 6.
Review the limitations for a Level 1 assessment
Characterize the pitting damage (diameter, area, depth)
Evaluate the component as described in API 579 Paragraph 6.4.2
Determine the uniform measured thickness (trd) away from the pitted area
Calculate the Maximum Allowable Working Pressure (MAWP) using the measured thickness
Identify the component area with the highest density of pitting damage
Obtain photographs and rubbings of the damaged areas
Determine the maximum pit depth (wmax) and calculate the ratio below. If Rwt < 0.2, the Level 1 assessment is not acceptable.
Compare the surface damage depicted in the photographs and rubbings
Determine the remaining strength factor (RSF) from the table related to the pit chart selected. A typical RSF table is shown below.
If RSF > RSFa, then the pitting damage is acceptable for the MAWP calculated above. RSFa is the allowable remaining strength factor. It is typically set at 0.90 for ASME Section VIII Division 1 equipment.
If the pitting is unacceptable:
Repair, rerate or replace.
Lower the FCA.
Conduct a Level 2 assessment.
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