P-F Interval

A graphical representation of the degradation trajectory of an item from the point of initial potential failure (P) to functional failure (F). The P-F curve shows how condition deteriorates over time and where different inspection or monitoring techniques can detect the developing failure. The area between P and F represents the window of opportunity for planned intervention.

The process of monitoring a parameter of condition in equipment (vibration, temperature, pressure, flow, oil quality, acoustic emission, electrical properties) to identify significant changes indicative of a developing failure. Condition monitoring is the technical enabler of predictive maintenance, detecting degradation at the P point on the P-F curve to allow planned intervention before functional failure.

A maintenance strategy that uses condition monitoring data, analytics, and failure prediction models to determine the optimal timing for maintenance intervention. Predictive maintenance aims to perform maintenance just before failure is expected, maximizing equipment life while minimizing unplanned downtime. It goes beyond condition monitoring by adding prediction — estimating the remaining useful life based on degradation trends.

The time period between successive inspections of an equipment item, determined by the RBI risk ranking, the remaining life, and applicable code requirements. API 510/570 set maximum intervals but allow extension based on RBI assessments. The inspection interval must not exceed half the remaining life (for general thinning) and should be set at less than half the P-F interval for condition monitoring tasks.