By Stephen J. Gliebe, P.E.
FFS assessments are quantitative engineering evaluations that are performed to demonstrate the structural integrity of in-service equipment that contain flaws or are otherwise damaged. They may be used in conjunction with a plant’s Risk Based Inspection (RBI) and Risk Based Maintenance Programs to enhance their effectiveness.
FFS assessment techniques have been around for a long time. The procedures used to perform FFS assessments were formalized in the First Edition of API Recommended Practice 579, Fitness-For-Service, released in January 2000. API RP 579 provides a common general approach that uses the assessment levels described below. It also provides specific component assessment procedures that are appropriate for particular degradation or damage mechanisms (e.g., general or local metal loss, brittle fracture, pitting, fire damage, deformation and cracks).
Since each of these mechanisms requires a different assessment methodology, I have decided to write a series of articles on how FFS techniques may be applied for several of the most commonly seen mechanisms. The articles will focus on Level 1 and Level 2 assessments since they are sufficient to establish that the equipment is suitable for continued service in most cases.
Each damage mechanism included in API RP 579 may be evaluated using three possible assessment levels.
Did a recent inspection find general, local or pitting corrosion in excess of the corrosion allowance on a vessel component? Will the component thickness be less than the minimum required thickness before the next scheduled maintenance opportunity? Are the flaws found unacceptable for continued service according to API 510, Pressure Vessel Inspection Code, guidelines? If you answered yes to any of these questions, a FFS assessment should be considered since it may justify continued service without a costly shutdown or repair.
This first in a series of FFS articles will focus on general metal loss. General metal loss (GML) is defined as relatively uniform thinning over a significant area of the equipment, or an area of metal loss that greatly exceeds its depth. An FFS assessment for GML would typically be considered when the remaining thickness is below the minimum required thickness. Since the minimum required thickness is not always known for all equipment components, some plants use metal loss in excess of the corrosion allowance to trigger a discussion on applying FFS techniques. Or sometimes, repairs may be recommended whenever there is metal loss in excess of the corrosion allowance. This second approach may be overly conservative. Section 4 of API RP 579 provides additional guidance for performing Level 1 FFS assessments for GML.
GML Level 1 assessments are permitted only if certain conditions are satisfied. The complete list of limitations in Section 4 of API RP 579 should be reviewed before proceeding with a GML Level 1 FFS assessment. Some of the limitations are:
A Level 1 GML assessment may be performed using individual point thickness readings or thickness profiles. Point thickness readings would be used if there are no significant differences among the values obtained at the inspection monitoring locations. Thickness profiles (i.e., thickness readings on a prescribed grid) are required if there is too much variation in the point thickness readings. Guidance on grid spacing is provided in API RP 579.
The accuracy of the FFS assessment is a function of the inspection data quality. Any uncertainties in the measurement data should be considered when performing an assessment. Below is a summary of a Level 1 GML assessment methodology using individual point thickness readings.
Point Thickness GML Level 1 Assessment Summary:
Article 2 in this series will cover API RP 579 General Metal Loss Level 2 assessments.