Maximizing the P-F Interval Through Condition-Based Maintenance


By extending the P-F Interval, Condition-Based Maintenance (CBM) methods provide the earliest possible prediction of equipment failure, with maximum benefit: minimum production loss, reduced maintenance labour and materials costs, extended equipment life, and reduced capital expenditures.

Dale R.Blann

Dale R. Blann


Marshall Institute Inc.


As a maintenance professional, unless you have been sleeping under a rock these last ten or so years, you will have at least heard of the “P-F Curve”, even if you don’t understand it, or use it to your advantage. The P-F Curve (Figure 1) is a common way to represent the behaviour of an asset (pump, motor), or asset component (belt, bearing) before actual functional failure has occurred.

There is a sense in which failure is a process, not an event. We can say that (most) failures take time, and for that we can be thankful. It’s really the causes of failure that take time, of course. Common causes leading to failure like wear, stretch, deterioration, misalignment, contamination, etc. fortunately do not lead to failure immediately, or all at once. Though failure may inevitably occur from any or all of them, many of them generally take time to develop to a level or degree that actually causes the failure. Better yet, that time interval preceding failure is often long enough to be useful. We can often rely on this time interval to detect or correct processes leading to failure, and if not avoid failure altogether, at least be able to reduce or avoid the consequences of failure.

Remember, our job in maintenance is not to “fix it when it fails”. Our job is not to avoid failure altogether. Sometimes failures are due to normal causes, and cannot be avoided. For example, belts and bearings can fail due to normal wear alone, such failure can’t actually be avoided. Rather, our job is to maintain physical assets in such a way that we can avoid the consequence of failure, whatever its cause.

A little wear, deterioration or contamination is not necessarily a problem; a lot of wear, deterioration or contamination is almost always a problem. The good news is that when these kinds of failure causes take significant amounts of time to actually create loss of function, we can often choose from alternative courses of action. If the consequence of failure is small or negligible, we may choose to do nothing, especially if doing something costs more or has more negative consequences than doing nothing. However, if the consequence of failure is not negligible, for example when production loss or costs of repair are high, then it will almost always be worth the effort to detect the failure “process” as early as possible before the failure, so that we may take whatever corrective action required to either avoid the failure, if possible, or reduce its consequence.


Figure 1. P-F Curve as it is usually presented. The curve represents the behaviour of an asset (pump, motor), or asset component (belt, bearing) before functional failure has occurred. The key is the P-F Interval.


Figure 2. Inspection intervals must be less than the P-F Interval. In practice, inspection intervals equal to ½ the P-F Interval is usually acceptable. (NOTE: P-F is NOT necessarily related to MBTF.)


Figure 3. Maximizing the P-F Interval, which can be days, weeks, or months depending on Inspection Methodology!

The P-F Curve and the P-F Interval

The horizontal (X) axis of the P-F Curve represents time-in-service for an asset, or asset component. The vertical (Y) axis represents some measure of performance, rate, condi- tion or suitability for purpose. The curve shows that the performance or condition of an asset or component declines over time leading to functional failure, i.e. loss of function for which it was intended. The curve may take various shapes, linear or exponential, but is generally represented as exponential as shown in Figure 1.

The P-F Interval is the key. It may be days, weeks or months, but whatever it is, it must provide a sufficient “window” of time between the latest inspection and functional failure to prioritize, plan and execute the necessary maintenance activities before the failure (Figure 2). Obviously the longer the P-F Interval, the better it will be.

Maximizing the P-F Interval Using Condition-Based Maintenance Technologies

The method and frequency of inspection makes a difference in the length, and therefore the utility of the P-F Interval. Generally, technologically-based Condition-Based Maintenance (CBM) methods (on-line inspections) provide the greatest P-F Intervals (Figure 3), and are less disruptive than alternative time-based, shutdown inspections. The avoidance of off-line inspections, which often cause loss of production, can make CBM methods economically feasible.

The most commonly used techniques of condition monitoring are:

  1. Lubricant Sampling and Analysis
  2. Corrosion Monitoring
  3. Motor Current Analysis
  4. Acoustic Emissions Detection (e.g., ultrasound)
  5. Vibration Measurement and Analysis
  6. IR Thermography
  7. Process Parameter Trending (e.g., flows, rates, pressures, temperatures, etc.)
  8. Process Control Instrumentation (measurement and trending)
  9. Visual Inspection (look, listen and feel).

Types of Condition- Monitoring Systems

Condition-monitoring systems are of two types: periodic and continuous. Periodic monitoring systems acquire measurements from equipment at selected time intervals; the analysis of the collected data is generally performed in the office or laboratory. The measurements may be on-line or offline, depending on what is being measured and the access required. Continuous monitoring systems collect almost always on-line the measurement values at selected points on the equipment and this data is continuously compared with pre-established levels or criteria. “Set-points” are usually established for the purpose of providing automatic warnings and/or equipment shutdown, depending on the consequence of the impending failure.

Selection of one or the other type of condition monitoring is usually a function of time limitations for conducting periodic; on- or off-line monitoring and a function of hardware and installation costs for continuous, on-line monitoring.

Available Condition- Based Technologies

Table 1 shows the variety of conditionbased measurement techniques that have been developed for applications to a wide range of mechanical and electrical equipment. More are being developed all the time, technologies are being improved, usability (user-friendliness) is being enhanced, product features are being added and prices/performance points are becoming more attractive. Selection of appropriate techniques for your particular applications will of course be a function of several variables:

  • Applicability of the CBM technology to the potential failure modes to be detected
  • Applicability of the CBM technology to the equipment design itself
  • Operational features of the measurement equipment
  • User qualification and training requirements (principles of both the technology and measurement equipment)
  • Pros and cons of using the technology for the selected failure mode in your specific equipment context
  • The economics of purchase, operation, training, and usability of the selected technology.

If the consequence of failure is high, there is almost always a relevant measurement technology and a vendor in the market with equipment to measure it. Economics notwithstanding, there will be measurement products out there that will allow you to see critical failures in progress giving you the opportunity to take corrective action at the earliest, most advantageous time possible, with least hassle and lowest cost.

Table 1. Available Condition-Based Technologies and Their Applications.

Table 1
Figure 4

Figure 4. Building a Condition-Based Maintenance Program.

Building a Successful CBM

Program Here are some of the major steps in establishing an effective, sustainable Condition- Based Monitoring program (shown in Figure 4):

  1. Select the Assets A good place to start is with your critical assets, those with substantial safety, production and cost risks.
  2. Identify Known and Probable Failure Modes Apply Reliability-Centred Maintenance (RCM) principles and identify failure modes for which CBM strategies are appropriate.
  3. Select CBM Technologies Select technically feasible and economically viable CBM measurement technologies for the identified CBM-appropriate failure modes.
  4. Establish Baseline Measurements for Selected CBM Technologies Set acceptable condition limits, alarms and set-point triggers.
  5. Establish and Execute the CBM Program Develop the program, establish responsibilities, assign resources (train as necessary) and conduct the program. Collect and record the measurements, trend and analyze the data.
  6. Act! Take appropriate action based on the analysis.

The Benefits of Condition-Based

Maintenance When implemented properly, Condition- Based Maintenance (CBM) has the ability to reduce both the time between maintenance activities (TBM) and time to repair (MTTR) with resultant savings in labour costs and reduction of downtime production losses. These benefits are achieved from predicting potential failures ahead of time, providing the maintenance staff with time to prioritize, plan and execute the work most efficiently and economically. Spare parts inventories and/or expediting costs are reduced as well when there is sufficient lead-time to order parts on an “as needed” basis.

Furthermore, prevention of catastrophic failure eliminates excessive damage to equipment, both direct and collateral, potentially increasing the useful operating life of equipment with subsequent reduction in capital replacement costs. For large capital- intensive assets this can be a significant benefit.

Extending the P-F Interval using CBM technologies may just be the strategy you should consider, if you want to realize these benefits.