Reliability HotWire Issue 76, June 2007 Hot Topics On-Condition Maintenance Using P-F Interval or Failure Detection Threshold (FDT) On-condition maintenance relies on the capability to detect failures before they happen so that preventive maintenance can be initiated. Many failure modes exhibit signs ofwarning as they are about to occur. If, during an inspection, maintenance personnel can find evidence that the equipment is approaching the end of its life, then it may be possible to delay the failure, prevent it from happening or replace the equipment at the earliest convenience rather then allowing the failure to occur and possibly cause severe consequences. This article explains a methodology, using Weibull++ 7, to estimate the P-F interval or Failure Detection Threshold (FDT), which are two typical ways to describe the detectability of a failure. In addition, this article shows how to use the detectability information in the analysis of repairable systems using BlockSim 7 orRCM++ 4. Background In the arena of Reliability Centered Maintenance (RCM) or repairable system analysis, one of the strategies for failure management is on-condition maintenance, also called predictive or condition-based maintenance. This strategy relies on the capability ofmaintenance personnel to detect potential failures in advance in order to take appropriate actions. Examples of failure signs that can be detected are vibrations, cracks, particles in oil, temperature, noise, viscosity, color, etc. Many technologies have been developed to monitor failure characteristics such as vibration analysis, X-ray radiography, ultrasonics, infrared thermography, oil analysis, acoustic emission, etc. P-F curves and P-F Intervals A common curve that illustrates the behavior of equipment as it approaches failure is the P-F curve. The curve shows that as a failure starts manifesting, the equipment deteriorates to the point at which it can possibly be detected (P). If the failure is not detected and mitigated, it continues until a "hard" failure occurs (F). The time range between P and F, commonly called the P-F interval, is the window of opportunity during which an inspection can possibly detect the imminent failure and address it. P-F intervals can be measured in any unit associated with the exposure to the stress (running time, cycles, miles, etc). For example, if the P-F Interval is 200 days and the item will fail at 1000 days, the approaching failure begins to be detectable at 800 days.
On-Condition Maintenance Using P-F Interval or FailureDetection Threshold (FDT)On-condition maintenance relies on the capability to detect failures before they happen
so that preventive maintenance can be initiated. Many failure modes exhibit signs of
warning as they are about to occur. If, during an inspection, maintenance personnel
can find evidence that the equipment is approaching the end of its life, then it may be
possible to delay the failure, prevent it from happening or replace the equipment at the
earliest convenience rather then allowing the failure to occur and possibly cause severe
consequences. This article explains a methodology, using Weibull++ 7, to estimate
the P-F interval or Failure Detection Threshold (FDT), which are two typical ways to
describe the detectability of a failure. In addition, this article shows how to use the
detectability information in the analysis of repairable systems using BlockSim
7 orRCM++ 4.
Background
In the arena of Reliability Centered Maintenance (RCM) or repairable system analysis,
one of the strategies for failure management is on-condition maintenance, also called
predictive or condition-based maintenance. This strategy relies on the capability of
maintenance personnel to detect potential failures in advance in order to take
appropriate actions. Examples of failure signs that can be detected are vibrations,cracks, particles in oil, temperature, noise, viscosity, color, etc. Many technologies
have been developed to monitor failure characteristics such as vibration analysis, X-ray
radiography, ultrasonics, infrared thermography, oil analysis, acoustic emission, etc.
P-F curves and P-F Intervals
A common curve that illustrates the behavior of equipment as it approaches failure is
the P-F curve. The curve shows that as a failure starts manifesting, the equipment
deteriorates to the point at which it can possibly be detected (P). If the failure is not
detected and mitigated, it continues until a "hard" failure occurs (F). The time range
between P and F, commonly called the P-F interval, is the window of opportunity
during which an inspection can possibly detect the imminent failure and address it. P-F
intervals can be measured in any unit associated with the exposure to the stress(running time, cycles, miles, etc). For example, if the P-F Interval is 200 days and the
item will fail at 1000 days, the approaching failure begins to be detectable at 800 days.
In addition to P-F intervals, the indication of when the approaching failure will become
detectable during inspections can be specified using a factor called the FailureDetection Threshold (FDT). FDT is a number between 0 and 1 that indicates the
percentage of an items life that must elapse before an approaching failure can be
detected. For example, if the FDT is 0.9 and the item will fail at 1000 days, the
approaching failure becomes detectable after 90% of the life has elapsed, which
translates to 900 days in this case (09.*1000=900).
Estimating the P-F Interval or FDT
Estimation of the P-F interval or FDT can be achieved using the judgment and
experience of the people who design, manufacture and/or operate the equipment. Note
that estimating the P-F Interval or FDT should be done on one failure mode at a time.
Many failure mechanisms can be directly linked to the degradation of part of the
product. Weibull++ 7s Degradation Analysis Folio enables the analysis of
degradation data. Degradation analysis involves the measurement of the degradation
of performance/quality data that can be directly related to the presumed failure of the
product in question. Assuming such data can be obtained, the FDT or P-F Interval can
be estimated using this technique.
To illustrate the use of this method, we use an example from an oil refinery company
that performed a study on the clogging problem in a type of pipes in its refinery. A
type of inspection equipment that uses gamma rays to measure the thickness of
clogging is passed outside of the pipe. This is a reliable non-intrusive method. The pipe
is considered to be failed if the thickness of clogging exceeds 5 inches (this is
equivalent to the "F" point in the P-F curve). Also, a "warning" thickness degradation
level of 3.5 inches has been identified. If the clogging thickness increases above 3.5
inches, this is considered to be an obvious sign of imminent failure (this is equivalent
to the "P" point in the P-F curve).
The following data set shows the thickness measurement over time at different
inspection times. The failure times for each observed unit were also recorded (if the
failures are not actually observed, they can be estimated using degradation analysis).
The figure below shows the measurements (in months) entered in the Degradation
Note that in order to use on-condition maintenance in BlockSim, you need to perform
the following steps:
•
Specify an Inspection Policy that dictates when the block will be inspected.• Specify the P-F interval or FDT value that describes the detectability of failure
for the block.
• Specify a Preventive Maintenance Policy so that preventive maintenance can be
performed on the block. Given that no other preventive maintenance is required in
this example, it is necessary to create a Preventive Maintenance Policy that
ensures that the preventive maintenance does not occur unless and until is
triggered by the detection of failure during inspection. Some options for creating
such a policy include specifying that preventive maintenance will occur at one of
the following times:
o Upon maintenance of another group item-- in this case, leave the default
of 0 for the block's Item Group Number. A group number of 0 indicates thatthe block is not part of a group, so there are no other group items and the
preventive maintenance will therefore not occur independently.
o Upon start of a maintenance phase-- because this example includes only
an operational phase, this will not occur independently.
o Upon fixed time interval based on system age-- in this case, specify an
interval larger than the simulation time to ensure that the preventive
If inspections are done frequently, the costs due to inspections will be high, but so will
be the likelihood of catching potential failures. On the other hand, if inspections are
performed rarely, the costs due to inspections will be lower but so will be the likelihood
of detecting potential failures. In practice, inspection intervals that are equal to half of
the P-F interval are considered to be adequate. [1] The time necessary to take anaction also needs to be considered to ensure that ample time is allocated for repairs.
The severity of the failure also weighs into the decision on the frequency of the
inspection interval.
BlockSim 7 and RCM++ 4 allow the analyst to evaluate the impact of the use of a
certain inspection period on the component and the system. The impact can be
described based on different criteria such as availability, throughput, uptime and profit.
Various inspection intervals can be compared and the optimum inspection interval can
be determined.
Conclusion
This article explained how the detectability of failures as expressed by P-F intervals or
FDT can be estimated based on degradation data. It also showed how such information
can be incorporated into repairable system analysis in BlockSim or RCM++.
References
1. Moubray, John, Reliability-Centered Maintenance, Industrial Press, Inc., New York
