Inspection Interval

A methodology that uses risk assessment to prioritize and plan inspection activities for pressurized and other equipment. RBI combines Probability of Failure (driven by damage mechanisms and equipment condition) with Consequence of Failure (driven by process hazards and safety impact) to rank equipment by risk and allocate inspection resources where they will have the greatest impact on risk reduction.

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.

The comprehensive plan that defines what to inspect, how to inspect, when to inspect, and what to look for on each equipment item. An inspection strategy integrates the RBI risk ranking, the identified damage mechanisms, the appropriate NDE techniques, the CML locations, the inspection interval, and the acceptance criteria into a coherent plan that can be executed in the field.

A categorical classification (typically A through E) derived from the risk matrix that represents the combined probability and consequence of failure for an equipment item. Risk ranking determines the urgency and extent of inspection, monitoring, and maintenance activities. Rankings are typically: A (lowest risk) through E (highest risk), each with prescribed inspection strategies and maximum interval limits.

The time between the point at which a potential failure is first detectable (P — the onset of detectable degradation) and the point of functional failure (F — the point at which the item can no longer perform its required function). The P-F interval determines the maximum inspection interval for condition-based maintenance — inspections must occur at intervals shorter than the P-F interval to detect the developing failure before functional failure occurs.