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P-F Interval
The 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

The time, or cycles, from when a "Potential Failure" ("P") is first detected on an asset, or component, until the asset or component has reached "Functional Failure" ("F").  

Also sometimes referred to as the leading indicators to failure of an asset or the failure development period.

The pattern of the P-F Interval is reflected on a P-F Curve. and is used to express the probability of failure (PoF) of an asset within a particular calender year.

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 Assets

Listed below are some examples to illustrate the application of the P-F interval:
  • Functional Failure (F) is a bearing seazing in a fan motor and causing the fan to stop. Partial failure,  or potential failure (P), is a worn impeller in a pump that still pumps fluid but not to the required level.
  • Potential Failure (P) is the loss of some protective granules from the cap sheet of an SBS roof. Functional Failure (F) is the leakage through a  roof system into the occupied space below as a result of systemic delamination and blistering of the waterproofing membrane.

Identifying the Points on the P-F Interval

P-F intervals are determined through a variety of techniques to gather empirical data on the condition of an asset, including:
The inspection/testing methodology will depend on where along the interval the asset is assumed to lie. The following concepts illustrate the complexities of making these types of professional judgments:


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 Some examples of 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:
  • 1.  Physical Deterioration:  The blisters on a roof assembly (“P”) and the eventual leaks (“F”) are mapped along the P-F Interval to help owners find an appropriate balance of time-based maintenance and condition-based maintenance strategies during the pre-P and post-P periods.
  • 2.  Technological Obsolescence:  Knowledge of the emerging difficulties in procuring replacement parts (“P”) for certain assets (such as fire alarm panels and elevator controls) helps owners understand the justification for modernization projects (“F”) and to plan accordingly.
  • 3.  Economic Obsolescence: The arrival of more efficient  products in the local market (“P”), such as pumps with variable frequency drives and the pre-emptive steps to receive approval from the local utility to implement energy efficiency measures and receive a rebate through a product incentive program before funding expires (“F”). 
  • 4.  Legal Obsolescence:   The announcement of a recall (“P”) of a certain deficient asset, such as certain sprinkler heads, leads to the negative impact on insurance coverage (“F”) and necessitates renewal. The efficacy of statutory maintenance on fixed-time intervals during the P-F interval is also considered.
  • 5.  Style/Aesthetic Obsolescence:  For example: “When we replaced our lobby floor (“P”) we also replaced the lobby furniture (“F”) as it appeared dated and did not match the new aesthetics”.  
Decisions should consider the whole life of assets, including the I-P interval and the P-F interva
Fig. Decisions should consider the whole life of assets, including the I-P interval and the P-F interval.



The whole life of assets can be divided into life stages - early life, mid-life and late-life.
Fig. The whole life of assets can be divided into life stages - early life, mid-life and late-life.



example of the progression of deterioration from "P" to "F" along the P-F curve
Fig. An example of the progression of deterioration from potential failure ("P") to functional failure ("F") along the P-F interval.


P-F interval illustrated with asphalt paving
Fig. The P-F interval illustrated with asphalt paving along the curve.



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. 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).


fixed interval events mapped onto the P-F curve to illustrate the relationship between time and resistance to failure
Fig. Fixed interval events mapped onto the P-F curve to illustrate the relationship between time and resistance to failure.



Variable interval events mapped onto the P-F curve to illustrate the relationship between time and condition.
Fig. Variable interval events mapped onto the P-F curve to illustrate the relationship between time and condition.


The forces of retirement mapped onto the P-F curve
Fig. The forces of retirement mapped onto the P-F curve.


Asset replacement policies mapped onto the P-F interval to illustrate their relationship to functional failure ("F").
Fig. Asset replacement policies mapped onto the P-F interval to illustrate their relationship to functional failure ("F").

Proximity to Potential Failure ("P") and Functional Failure ("F") along the P-F curve.
Fig. Proximity to Potential Failure ("P") and Functional Failure ("F") along the P-F curve.

1 Leading indicators, lagging indicators and coincident indicators on a PF 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).


Premature failure on the P-F interval, where both potential failure ("P") and functional failure ("F") occur earlier than intended.
Fig. Premature failure on the P-F interval, where both potential failure ("P") and functional failure ("F") occur earlier than intended.


Survivor curve with an overlay of the PF interval to correlate the generic class to the local circumstances
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).

See also:
Compare with:

Comparison of the PF Curves of two assets
Fig. Comparison of the PF Curves of two assets.



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