Remaining Useful Life(RUL)
Definition
The estimated time from the present until an equipment item reaches a condition where it can no longer perform its intended function safely and must be repaired, replaced, or retired. For static equipment, RUL is typically calculated from the current measured wall thickness, the minimum required thickness, and the corrosion rate. For rotating equipment, RUL may be estimated from degradation models fitted to condition monitoring data.
Context
RUL is a critical output of integrity assessments that drives turnaround planning, capital budgeting, and replacement strategy. In Reliatic, RUL is calculated automatically from CML thickness data and corrosion rates, providing real-time remaining life projections on the asset detail page.
Formula
RUL = (Measured Thickness - Minimum Required Thickness) / Corrosion Rate
Examples
- A pipe elbow with measured thickness 8.2mm, minimum required thickness 4.8mm, and long-term corrosion rate 0.25 mm/year has RUL = (8.2-4.8)/0.25 = 13.6 years.
- Weibull-based RUL estimation for a pump bearing: based on vibration trend extrapolation, the bearing is predicted to reach functional failure in 45-60 days (80% confidence interval).
Applicable Standards
- API 510
- API 570
- API 579-1
Related Terms
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.
Corrosion RateThe 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.
Minimum Required Thickness(t_min)The minimum wall thickness required by the applicable design code to safely contain the operating pressure at the design temperature, considering the allowable stress of the material, weld joint efficiency, and applicable code margins. This is the absolute minimum thickness for pressure containment — below this value, the equipment exceeds its design basis and must be retired, repaired, or de-rated.
End of Life(EOL)The point at which an asset can no longer perform its intended function safely, economically, or in compliance with applicable standards, and must be decommissioned, replaced, or substantially rehabilitated. End of life may be determined by physical degradation (wall thickness below minimum), obsolescence (spare parts unavailable), regulatory changes, or economic factors (repair cost exceeds replacement cost).
Fitness-for-Service(FFS)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.
Where this appears in Reliatic
This concept is used in the Asset Register module.