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:
- The original design criteria must be in accordance with a
recognized code or standard (e.g., ASME Code Section VIII or ASME
- The component must have either a design equation that specifically
relates pressure (or liquid fill height for tanks) and/or other loads,
as applicable, to a required wall thickness (e.g., pressure vessel or
storage tank cylinder), OR must be one of the following components:
- Pressure vessel nozzle, tank nozzle, or piping branch
- Reinforcement zone of conical transitions
- Cylinder-to-flat head junctions
- Integral tubesheet connections
- Piping systems
- Supplemental loads that impact the thickness of the component
being evaluated must be considered. Some examples include loads due
- Weight of the component including contained fluid, insulation,
- Wind, earthquake, snow and ice
- Constraint of free thermal expansion
- Process upset conditions
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:
- Review the inspection history of the equipment.
- Calculate the required minimum thickness due to circumferential
stress (e.g., caused by internal pressure or liquid fill height).
- Determine the supplemental loads (e.g., weight of the component
and insulation, wind, earthquake). Refer to API 579 Appendix A for
additional details on loads.
- Calculate the required thickness due to supplemental loads.
- Calculate the required minimum thickness due to longitudinal
stress (e.g., consider thickness required for internal pressure,
external pressure, and weight loads).
- Obtain the thickness profiles for the thin areas.
- Determine the critical thickness profiles for the circumferential
and longitudinal directions (CTPC and CTPL).
- Select the minimum measured metal thickness from the lower of CTPC
- Specify the Future Corrosion Allowance (FCA). The FCA is the
amount of metal loss anticipated before the next scheduled inspection.
- Compute the remaining thickness ratio. The remaining thickness
ratio is based on the minimum measured thickness, the FCA, and the
minimum required thickness as noted below. It is one of the inputs
needed to establish the length over which the thickness data may be
- Compute the maximum length that may be used for thickness
- Determine the axial and circumferential extent of the flaws using
the tables and equations in API RP 579 Section 4.
- Measure the distance from a major structural discontinuity (e.g.,
nozzle, support skirt).
- Determine the average and minimum thicknesses by numerically
averaging over the length of the flaw. For cylinders, this must be
done in both the longitudinal and circumferential directions.
- Compare the calculations for the remaining thickness ratio,
minimum measured thickness minus the FCA, average measured wall
thickness, circumferential extent of the flaw, and the distance from a
major structural discontinuity, to the API RP 579 acceptance criteria.
- If the flaw is acceptable, develop an inspection strategy for the
- If the flaw is unacceptable:
- Repair the flaw, rerate or replace the component
- Lower the FCA (e.g., by reducing the inspection interval or
mitigating the corrosion)
- Conduct a Level 3 FFS assessment. Note that a Level 3
assessment is not required in most situations, and it requires
more data and more time to conduct.
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