On-Condition Maintenance Using P-F Interval or Failure Detection Threshold (FDT)
[Editor's Note: This article has been updated since its original publication to reflect a more recent version of the software interface.]
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++, 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 on condition tasks for use in the analysis of repairable systems in BlockSim or RCM++.
P-F curves and P-F Intervals
Failure Detection Threshold (FDT)
Estimating the P-F Interval or FDT
Many failure mechanisms can be directly linked to the degradation of part of the product. Weibull++'s 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 analysis folio.
The first step in this analysis is to specify a degradation model to use to fit the observed data. For this type of failure mode, it was determined that the exponential degradation model is an appropriate model (the choice of degradation model comes from a physics of failure understanding of how the degradation of the performance/quality progresses over time). After the parameters of the degradation model are calculated for each of the observed units, the models can be used to estimate the times that correspond to the warning limit of thickness. This is done by setting the Critical Degradation field on the Main page of the control panel to 3.5. Using the fitted degradation model, the time values equivalent to the warning limit of thickness are calculated. (To see these time values, after calculating, click the ... icon in the Degradation Results frame on the control panel and view the Extrapolated Failure-Suspension tab in the Results window.) A plot of degradation versus time with the failure thickness and warning limit labeled is shown next.
Table 1 summarizes the estimated "P" and "F" times in addition to the P-F interval or FDT values for each observed unit. The P-F interval or FDT values for each observed unit use the following equations:
FDT = P/F
Table 1 P and F results for each observed pipe along with the calculated P-F Interval and FDT
The P-F interval and FDT average values shown above can be used as the final P-F interval and FDT estimates (you can also use median values).
Using P-F Interval and FDT in System
The next figures show how P-F intervals or FDT values can be specified in the properties of an on condition task, which can be used for repairable systems analysis in RCM++ or BlockSim.
Note that on condition tasks include both an inspection portion, which dictates when the component or system will be inspected and uses either a P-F interval or FDT value to describe the detectability of failure, and a preventive portion, which specifies the preventive maintenance that occurs if and when it is triggered by the detection of failure during inspection.
Estimating Efficient Inspection
BlockSim and RCM++ 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.
1. Moubray, John, Reliability-Centered Maintenance, Industrial Press, Inc., New York City, NY, 1997.
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