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	System Improvements

Optimum Replacement Window

The Optimum Replacement window allows you to specify costs for planned (i.e., preventive) and unplanned (i.e., corrective) replacement of the component(s) of interest in the current diagram. It then uses this information to determine the most cost-effective time to replace the components, either individually or as a group.

Preventive maintenance is a logical choice if, and only if, the following two conditions are met:

Condition #1: The failure rate of the component in question increases with time, implying wearout.
Condition #2: The overall cost of the preventive maintenance action must be less than the overall cost of a corrective action. (The overall cost for a corrective action should include ancillary tangible and/or intangible costs, such as downtime costs, loss of production costs, lawsuits over the failure of a safety-critical item, loss of goodwill, etc.)

If both of these conditions are met, then preventive maintenance makes sense. The Optimum Replacement window then helps you to determine the appropriate time frame for the preventive maintenance.

What’s Changed? In BlockSim 7, optimum replacement calculations were performed for individual blocks. The utility in BlockSim 8 allows you to perform calculations either for an individual block or for multiple blocks in a diagram simultaneously. When working with multiple blocks, the calculations can be for individual blocks or a group of blocks.

To access the Optimum Replacement window for an entire diagram, choose [Diagram/Fault Tree] > Calculation > Optimum Replacement.

To access the Optimum Replacement for a single block, click the Optimum Replacement icon in the Block Properties window. This functionality is available for standard blocks (including contained blocks) in RBDs and for events (including dependent events) in fault trees.

If you are working with a diagram instead of with a single block, the Available Blocks area on the left side of the window lists all of the standard blocks (in RBDs) or the events (in fault trees), as well as the switches for any standby containers. The following requirements must be met for an item to be available for inclusion in the optimization calculations:

Blocks must have a failure distribution.
Switches must have a failure model assigned. Note that in the standby configuration, the switch is the item that is optimized, not the container or the contained blocks/dependent events.
The failure model must meet the following requirements.
- Fixed reliability and exponential distributions are not allowed.
- Weibull distributions must have beta > 1.
- Gamma distributions must have k > 1.
- Loglogistic distributions must have std < 1.
- For generalized gamma distributions and mixed Weibull distributions, there must be an increasing failure rate within at least one time period. A local optimum can then be found within intervals with an increasing failure rate.

You can select an individual item's check box or drag the block into the table on the right side of the window to include the item in the optimization calculations. You can also select the check box in the Available Blocks header to select all check boxes. To remove items from the optimization calculations, you must clear the check box(es); you cannot drag items out of the table.

For each item that is included in the optimization calculations, you will need to specify the cost for a planned replacement and the cost for an unplanned replacement. This is done by calculating the costs for replacement based on the item settings and then, if desired, manually entering any additional costs for either type of replacement in the corresponding columns of the table. You can calculate the replacement costs in either of two ways:

Click Estimate Costs to calculate the costs for each item based on its associated URD.

Note: For costs associated with planned replacements, only preventive tasks scheduled to be performed based on item age or system age (at either fixed or dynamic intervals) are considered. If more than one such preventive task is assigned, only the first (i.e., highest priority) is considered.

To simulate each item as if it were a system and then use the results from the simulation in the table, click Estimate Costs Using Simulation. The Simulation Settings window will appear. Specify the desired settings and click OK to perform the simulation. For details on simulation settings, see Maintainability/Availability Simulation Window.

Note: For costs associated with planned replacements, all preventive tasks based on item age or system age (at either fixed or dynamic intervals) are considered. Because each item is simulated as a system (i.e., in isolation from any other item), tasks triggered in other ways are not considered.

The results will be used to populate the Calculated Planned Replacement Cost and the Calculated Unplanned Replacement Cost columns. Starting in version 8.0.5, if you subsequently decide that you do not want to use the calculated replacement costs, you can clear these columns by clicking Clear Values.

If you are working with a diagram instead of with a single block, when you click Calculate, you can choose from the following options:

Calculate the individual optimum replacement time for each item.
Calculate a common optimum replacement time, which is the most cost-effective time to replace all of the selected items concurrently.
Calculate clustered optimum replacement times, which allows you to specify a number of times to replace groups of the selected items. For example, if you enter 3 in the Number of clusters field, the software will determine the three best times to replace groups of items such that all of the selected items are replaced at one of the three times.

If you are working with a single block, no choice is necessary.

BlockSim will calculate the optimum replacement time for the items based on their failure characteristics and on the defined costs. This value will be displayed in the Optimum Replacement Time column of the table. In addition, the Cost per Unit Time column will display the estimated cost per unit time for a replacement performed at the calculated optimum replacement time. Multiply the replacement time by the cost per unit time to obtain the cost of replacing the item at the optimum replacement time.

If you have accessed the Optimum Replacement window from a simulation diagram or a block within a simulation diagram, you can click Create Maintenance Task(s) at Replacement Time to create a preventive task for one or more of the items. The window that appears allows you to select which items to create tasks for. The preventive tasks created in this way restore the item to as good as new condition and are scheduled at fixed intervals based on item age, equal to the optimum replacement time.

You can click Cost vs. Time Plot to show a plot of the results. If you have calculated individual optimum replacement times, the plot will show the cost for each item as a function of time. If you have calculated a common optimum replacement time, the plot will show the common cost as a function of time. If you have calculated clustered optimum replacement times, the plot will show the cost for each cluster as a function of time.

The ReliaWiki resource portal provides more information on preventive maintenance and the Optimum Replacement window at: http://www.ReliaWiki.org/index.php/Introduction_to_Repairable_Systems.