Fitness for service
API 579-1 / ASME FFS-1: fitness for service, explained
API 579-1 / ASME FFS-1 is the joint American Petroleum Institute and ASME standard for evaluating whether pressure equipment with a flaw or damage can stay in service, and for how long. It covers pressure vessels, piping and tanks built to recognized codes, and organizes assessments by damage type across its parts: general and local metal loss, pitting, hydrogen damage, cracks, creep, fire damage, dents and gouges, and laminations. It does not replace the inspection codes, which decide when to look, and it does not itself authorize a repair method, which ASME PCC-2 covers.
What the standard requires
Assessment levels
Every part offers three levels. Level 1 is a conservative screening an inspector or engineer can run with measured data and tables, Level 2 is a more detailed engineering calculation, and Level 3 is a full analysis, typically numerical, reserved for an engineer experienced in the method.
Part 4, general metal loss
Level 1 compares the measured average and minimum thickness in the corroded region to the required minimum and a future corrosion allowance. Pass means the component stays in service to the next inspection; fail means Level 2, a rerate, or a repair.
Part 5, local metal loss
Local thin areas, grooves and pits are assessed with a remaining strength factor derived from the flaw's dimensions and the component's geometry, with acceptance criteria that depend on the flaw's extent and location relative to structural discontinuities.
Remaining life and future corrosion
Every assessment carries a future corrosion allowance and a remaining life estimate, so the result is a date and an inspection plan, not just a pass. The corrosion rate used has to be the one the inspection record supports.
Data quality
The standard is explicit that the assessment is only as good as the thickness grid, the material properties and the loading data behind it. Thickness readings must be taken on a defined grid with the spacing the part specifies.
Documentation and responsibility
The assessment record states the level performed, the inputs, the equations or method used, the result, the remaining life and who performed and reviewed it. Level 2 and Level 3 results are the responsibility of a qualified engineer.
What the record must carry
- Component identification, construction code, material and design conditions
- Damage type and the API 579 part applied
- Assessment level performed and why that level was chosen
- Thickness grid readings with spacing, date, method and gauge
- Required minimum thickness and future corrosion allowance
- Result: acceptable, rerate, repair or escalate to the next level
- Remaining life and the next inspection date it implies
- Engineer of record, reviewer and acceptance signatures
How intelliSPEC handles it
intelliINTEGRITY runs the API 579-1 Part 4 Level 1 screening by comparing measured thickness to tMin, with tMin routed by equipment type to ASME B31.3, ASME VIII Div 1 or API 653 and returned with the formula. Remaining life from the corrosion summary feeds the future corrosion allowance, and ASME B31G remaining strength for piping is available in the original and modified forms across the 1991 and 2012 editions, held as provisional until an engineer verifies it and never auto-derated. Damage mechanisms are first-class records, so the flaw, the readings, the screening result and the engineer's signature stay linked in a hash-chained audit trail.
Answers
API 579-1 ASME FFS-1 questions
What is the difference between Level 1, 2 and 3 in API 579?
Level 1 is a conservative screening using measured data and the standard's tables, meant to be run without specialist analysis. Level 2 is a detailed engineering calculation that accepts more components than Level 1 with the same data. Level 3 is a full analysis, usually finite element, for cases the lower levels cannot resolve or where the geometry or loading is outside their limits.
What does a Part 4 Level 1 assessment actually compare?
It compares the average and minimum measured thickness in the corroded region against the required minimum thickness plus a future corrosion allowance, using a thickness grid taken at the spacing the part requires. If both criteria pass, the component is acceptable to the next inspection; if not, the options are Level 2, a rerate to lower pressure, or a repair.
Who is qualified to perform an API 579 assessment?
Level 1 can be performed by an inspector or engineer familiar with the standard, but Level 2 and Level 3 are the responsibility of an engineer experienced in the assessment methods. Whatever the level, the record should name who performed it and who reviewed it.
Does a passing FFS assessment mean the equipment never needs another look?
No. Every assessment produces a remaining life based on the corrosion rate and the future corrosion allowance, and that sets the next inspection. If the rate changes or new damage appears, the assessment has to be redone.
How does API 579 relate to API 510, 570 and 653?
The inspection codes decide when and where to inspect and set the acceptance basis for normal corrosion. When an inspection finds damage that fails those simple criteria, API 579 is the referenced path to decide whether the equipment can keep running. The FFS record then becomes part of the equipment's permanent file under the inspection code.