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P-F interval represents the second (middle) phase in a
three-phase life cycle
model and deterioration
model of assets.
The three phases are identified as follows
Also sometimes referred to as the leading indicators to failure of an asset or the failure development period.
The phrase P-F interval was coined by J. Moubray.
A type of deterioration model that utilizes a graph to represent the relationship between potential failure ("P") and functional failure ("F"). P-F curves are used to map and avert failures. An understanding of the P-F curve helps the owner determine which types of asset replacement policy is most appropriate to their tolerance for risk.
Examples of P-F Intervals for AssetsListed below are some examples to illustrate the application of the P-F interval:
Identifying the Points on the P-F IntervalP-F intervals are determined through a variety of techniques to gather empirical data on the condition of an asset, including:
Establishing the Patterns of the P-F Interval
The longer the P-F interval, the more time the owner has to make a good decision by planning an appropriate course of action and raising the necessary funds for renewal of the asset. It is therefore helpful to understand the pattern of the interval in order to map and avert failures.
For more information about the identification of the points along the P-F interval, refer to:
Application of the P-F interval:
The P-F interval is a useful concept for establishing appropriate asset management strategies at different stages in the lifecycle of an asset. It provides insight for the physical analysis of a reserve study.
Included below are three key concepts.
A. Shift from Time-Based Maintenance (TbM) to Condition-Based Maintenance (CbM)
Reliability-centered maintenance principles state that the frequency with which a predictive maintenance task should be performed on an asset is determined by the P-F Interval for that asset.Three types of indicators major maintenance activities that are driven by the P-F Interval
B. Shift from Fixed Intervals to Variable Intervals
C. Development of Replacement Policy
An understanding of the P-F interval helps the owner determine which types of asset replacement policy is most appropriate to their tolerance for risk.
The P-F Interval Relative to the Forces of Retirement:
Listed below are some examples of how the P-F interval can be mapped against the six replacement drivers of assets:
Fig. Decisions should consider the whole life of assets, including the I-P interval and the P-F interval.
Fig. The whole life of assets can be divided into life stages - early life, mid-life and late-life.
Fig. An example of the progression of deterioration from potential failure ("P") to functional failure ("F") along the P-F interval.
Fig. The P-F interval illustrated with asphalt paving along the curve.
Fig. The risk spectrum extending along the P-F interval to illustrate the varying strategies in the Pre-P and Pre-F periods. For example, a shift from Time-Based Maintenance (TbM) to Condition-Based Maintenance (CbM).
Fig. Fixed interval events mapped onto the P-F curve to illustrate the relationship between time and resistance to failure.
Fig. Variable interval events mapped onto the P-F curve to illustrate the relationship between time and condition.
Fig. The forces of retirement mapped onto the P-F curve.
Fig. Asset replacement policies mapped onto the P-F interval to illustrate their relationship to functional failure ("F").
Fig. Proximity to Potential Failure ("P") and Functional Failure ("F") along the P-F curve.
Fig. Left: multiple points P on the P-F curve; and Right: Leading indicators, lagging indicators and coincident indicators on a PF Curve (right).
Fig. Premature failure on the P-F interval, where both potential failure ("P") and functional failure ("F") occur earlier than intended.
Fig. A generic survivor curve overlaid with the points of potential failure (P) and functional failure (F) of the same asset at two buildings to draw the correlation between the global maxima (the class) and the local minima (the site conditions).
Fig. Comparison of the PF Curves of two assets.
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