Maintenance Policies in BlockSim
[Editor's Note: This article has been updated since its original publication to reflect a more recent version of the software interface.]
BlockSim includes ways to model a number of maintenance policies to perform corrective, preventive and inspection activities on repairable systems so that the operation and maintenance of systems can be simulated as accurately as possible. This article discusses two of these policies: "Corrective maintenance upon inspection" and "Maintenance of an item upon the maintenance of another item." The implementation of these policies in BlockSim is illustrated using simple examples. These policies also can be combined to model other complex maintenance procedures, such as the scenario in this month's Hot Topics article.
Corrective Maintenance Upon Inspection
In many industries, periodic inspections are used to identify components that may have failed but are not immediately detected because the impact of the components failure on the system performance is minimal or because the item is not in continuous use. Such failures are often called "hidden failures," "benign failures" or "degraded failures." The hidden failures are subjected to corrective maintenance actions only when a periodic inspection finds the failure. The case of hidden failures is easily modeled in BlockSim using a corrective task that is configured to be performed when the item is found failed during an inspection. The frequency of the inspections can be specified using an inspection task set to occur periodically based on calendar time.
For example, consider the failure of the wiring of an aircraft's fire extinguishing system. The failure can be detected only through an inspection; it otherwise remains hidden during the normal system operation. Assuming that the times-to-failure follow the Weibull distribution with beta = 1.5 and eta = 100 hours and that inspections are carried out every 50 hours, the appropriate properties to be used to model this scenario in BlockSim are described below.
As shown next, the corrective maintenance task that is used by the Fire Extinguisher Wiring block should be set to be performed when the block is found failed during an inspection. It is assumed that the duration of the corrective action is 3 hours.
The periodic inspections carried out to detect the failure of the Fire Extinguisher Wiring block are performed at fixed intervals based on calendar time. As shown next, the frequency of the inspections is specified to be every 50 hours. It is assumed that the inspections last 1 hour and the block does not operate during the inspection.
With the above settings, the operation of the Fire Extinguisher Wiring block can be simulated. The Block Up/Down plot in BlockSim can be used to check whether the model performs as desired. The next figure shows the Block Up/Down plot using a simulation end time of 200 hours and a seed of 1.
The plot shows the inspections on the Fire Extinguisher Wiring block occurring every 50 hours. The block fails at 112.891 hours but does not undergo repairs until the next inspection at 150 hours detects the failure. A corrective action triggered by this inspection at 151 hours fixes the failed wiring.
Maintenance of an Item Upon the Maintenance of Another Item
Maintenance groups can be used in BlockSim to model preventive maintenance actions performed on a component when another component undergoes corrective or preventive maintenance. For example, consider a valve (Weibull distribution with beta = 1.5 and eta = 1,000 hours) that is serviced every time a maintenance action is carried out on the adjacent compressor module (Weibull distribution with beta = 1.5 and eta = 500 hours). The compressor module is corrected upon failure. This scenario can be modeled in BlockSim using the reliability block diagram and the maintenance properties shown below. (Note that in this case we consider the valve and the compressor module to be in a parallel configuration, but the analysis and application of the "upon maintenance of another item" setting would be the same even if the two items were in series).
The following figure shows the compressor module's block properties. Note that the block is assigned to Maintenance Group 1. The corrective task used by the block is assumed to last 3 hours.
The maintenance properties for the valve are as follows. The valve gets a corrective replacement on failure but also gets preventive maintenance every time the compressor module fails. This preventive task is set to be performed when corrective maintenance starts for any block in Maintenance Group 1. Note that, unlike in previous versions of BlockSim, in the Synthesis version the valve does not need to belong to the maintenance group in order for this to work. Also notice that if the valve fails, no preventive maintenance is carried out on the compressor module, since no such task has been set up for the compressor module in this example. In other words, just because the valve is set to be maintained when the compressor is maintained, the opposite will not occur unless it is specifically set this way by the user.
The Block Up/Down plot shown next illustrates how this maintenance policy works.
The compressor module fails at 116.533 hours and undergoes corrective maintenance with the duration of 3 hours. At the same time, a preventive maintenance is also carried out on the valve at 116.533 hours that is 1 hour long.
This article discussed two of the maintenance polices that can be modeled in BlockSim. Simple examples were used to show the implementation of these policies. The policies can be combined to model other maintenance scenarios, as illustrated in this month's Hot Topics article.