Article 2: API RP 579 Fitness-For-Service

By Stephen J. Gliebe, P.E.

The first installment of the Fitness-For-Service (FFS) series of articles introduced the concepts contained in API RP 579. The types of assessments (i.e., Level 1, Level 2, Level 3) and degradation mechanisms (e.g., general metal loss, pitting) were described, and plants were encouraged to consider how FFS assessments could be used to reduce maintenance expenditures in their facility.

The discussion was limited to assessing general metal loss using individual point thickness readings applied to components subjected to pressure loading ONLY. The procedures and limitations for assessing general metal loss under pressure loading conditions were provided.

In this second installment, we will take a look at a general metal loss (GML) Level 2 assessment in a region located away from structural discontinuities, but where supplemental loads must be considered.

GML Level 2 Limitations

GML Level 2 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 2 FFS assessment. Some of the limitations are:

GML Level 2 Methodology

A Level 2 GML assessment may be performed using individual point thickness data or thickness profiles as described in Article 1. Thickness profiles are required if there is too much variation in the individual point thickness readings. Below is a summary of the Level 2 GML assessment methodology using thickness profiles where supplemental loading must be considered. Guidance on the use of thickness profiles can be found in API RP 579 Section 4.

GML Level 2 Assessment Summary For Cylindrical Components:

 

Article 3:

Article 3 in this series will cover API RP 579 General Metal Loss Level 2 assessments for reinforced nozzles in a cylindrical shell using the area replacement method.