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Maintenance Policies in
BlockSim
BlockSim
includes a number of maintenance policies to model 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 group
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 an "upon inspection" policy on the Corrective
Maintenance tab of the Block Properties window. The frequency of
the inspections can be specified using an "upon system age"
inspection policy (on the Inspections tab) if the inspections
occur periodically based on calendar time.
For example, consider the
failure of the wiring of an aircrafts 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 hrs and that
inspections are carried out every 50 hrs, the appropriate block
properties to be used to model this scenario in BlockSim
are described below.
As shown next, the corrective
maintenance properties for the Fire Extinguisher Wiring block
should be set using an "upon inspection" policy. It is assumed
that the duration of the corrective action is 3 hrs.
The periodic inspections
carried out to detect the failure of the Fire Extinguisher
Wiring block are modeled using an "upon fixed time interval
based on system age" inspection policy. As shown next, the
frequency of the inspections is specified to be every 50 hrs. It
is assumed that the inspections last 1 hr 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
hrs and a seed of 1.
The plot shows the inspections
on the Fire Extinguisher Wiring block occurring every 50 hrs.
The block fails at 125.529 hrs but does not undergo repairs
until the next inspection at 150 hrs detects the failure. A
corrective action triggered by this inspection at 151 hrs fixes
the failed wiring.
Maintenance of an Item Upon
the Maintenance of Another Group Item
The "Upon Maintenance of Another Group Item" policy in
BlockSim is used to model preventive maintenance actions
performed on a component when another component undergoes a
corrective or preventive maintenance. To accomplish this, the
components should be identified as belonging to the same group.
This is done in BlockSim
using the Item Group Number field. For example, consider
a valve (Weibull distribution with beta = 1.5 and eta = 1000
hrs) 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 hrs). 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 group item" policy would be the same even
if the two items were in series).
The following figure shows the
maintenance properties on the compressor module. Note that an
item group number of 1 is specified.
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. These preventive
maintenance actions are modeled using the Upon Maintenance of
another Group Item option. Note that the item group number
of 1 used for the compressor module is also used on the valve.
Also notice that if the valve fails, no preventive maintenance
is carried out on the compressor module, since no such policy
has been set for the compressor module in this example. In other
words, just because the compressor module and the valve belong
to the same group and 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
177.766 hrs and undergoes corrective maintenance with the
duration of 3 hrs. At the same time, a preventive maintenance is
also carried out on the valve at 177.766 hrs that is 1 hr long.
Conclusion
This article discussed two of the maintenance policies available
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.
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