What the assessment answers
Construction codes such as the ASME Boiler and Pressure Vessel Code design new equipment; they do not say what to do with a vessel that has lost wall thickness, pitted or cracked after years of service. API 579-1/ASME FFS-1 fills that gap. An FFS assessment compares the damaged component, as measured, against acceptance criteria and returns one of a limited set of outcomes: continue operating, continue at a reduced maximum allowable working pressure or temperature, monitor at a shortened interval, repair, or replace. It also estimates remaining life, which sets the next inspection date.
The in-service inspection codes — API 510 for pressure vessels, API 570 for piping and API 653 for aboveground tanks — refer to API 579-1 when inspection finds damage beyond what their own simple rules cover.
Three levels of assessment
| Level | Who typically performs it | Data and method |
|---|---|---|
| 1 | Inspector or plant engineer | Conservative screening criteria, minimal inspection and component data |
| 2 | Engineer experienced in FFS | More detailed calculations with better data; less conservative |
| 3 | Specialist | Most detailed analysis, often numerical stress analysis such as finite element methods |
A component that fails Level 1 is not condemned; it moves to a higher level, where better data and analysis can remove excess conservatism. The trade-off is cost and time against margin recovered.
Organized by damage mechanism
The standard is divided into parts, each dedicated to one damage type with its own data requirements and criteria. General metal loss is assessed in Part 4, localized metal loss in Part 5, and pitting corrosion in Part 6. Other parts address hydrogen blistering and related damage, weld misalignment and shell distortion, crack-like flaws, creep, fire damage, dents and gouges, laminations and fatigue.
Choosing the right part starts with identifying the damage mechanism correctly. Pitting assessed as uniform thinning, or a crack-like flaw from stress corrosion cracking treated as metal loss, produces a result that answers the wrong question.
What it needs to work
FFS is only as good as its inputs: thickness data taken on a documented grid or at condition monitoring locations, flaw dimensions from qualified examination, material properties, design and operating conditions, and a defensible future corrosion allowance. Arrest the mechanism, or allow for its continuation, before the remaining-life figure means anything.