API 653

Pressure Vessel Inspection Code — covers in-service inspection, repair, alteration, and rerating of pressure vessels. It establishes requirements for maintaining the integrity of pressure vessels after they have been placed in service, including inspection intervals based on corrosion rates and remaining life calculations.

Fitness-For-Service standard — provides quantitative engineering assessment procedures for evaluating flaws, damage, and degradation found in pressurized equipment. It covers assessment levels (Level 1 screening through Level 3 detailed analysis) for general metal loss, local thin areas, pitting, blisters, weld misalignment, crack-like flaws, creep damage, and fire damage.

An engineering assessment methodology used to determine whether equipment with identified flaws, damage, or degradation can continue to operate safely and reliably for a defined future period. FFS evaluations quantify the remaining structural capacity of degraded equipment using the procedures in API 579-1/ASME FFS-1, considering the specific type and extent of damage relative to the design margins.

The rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.

The calculated time until the wall thickness at a CML or the minimum measured thickness on a component reaches the retirement thickness, based on the current measured thickness and the applicable corrosion rate. Remaining life is the inspection-data-driven equivalent of Remaining Useful Life (RUL), specifically calculated from thickness data for static equipment.