Optimise maintenance inspection frequency with predictive maintenance optimization model Excel based decision tool
Maintenance Optimization Model for inspection interval optimisation. Excel Spreadsheet maintenance optimisation tool identifies optimal maintenance inspection frequency
A simple MS Excel based maintenance optimization modelling spreadsheet program for selecting optimum condition monitoring inspection intervals. Using the maintenance optimisation tool to find the optimal inspection frequency lets you minimise maintenance costs and reduce plant and equipment failures.
This Microsoft Excel modelling spreadsheet is used to select the on-condition inspection period for plant and equipment. Condition based maintenance optimization is useful for on-condition maintenance strategy selection and to review your current PM inspection schedule to minimise maintenance costs while minimising equipment failures.
With the model your inspection and condition monitoring strategies no longer need to remain static after they are initially set, instead they can be optimized based on actual site equipment maintenance and failure data. The condition based maintenance inspection interval optimisation tool lets you use site specific information on failure mode risk, condition monitoring warning times and practical maintenance windows. You can do 'what-if' scenarios to find the optimal time between condition monitoring inspections.
Condition Maintenance Inspection Interval Optimization Modelling Tool (MS Excel Spreadsheet for selecting condition monitoring intervals that minimise
maintenance costs and the number of equipment failures)
How to Use the Modelling Software
For example, in the model below a gearbox breakdown costs the operation a $100,000 DAFT Costs (Defect and Failure Total Costs), but only $8,000 to repair if an emerging failure is found early. We know the gearbox fails about every 10 years, or 0.1 times per year. The cost for condition monitoring the gearbox using a combination of vibration analysis and oil analysis is $600 for each inspection, plus a site overhead cost of $200 per year for administration. It takes two weeks to get the inspection report back during which the gearbox condition is unmonitored.
This condition inspection regime we selected (as described above) will detect 80% of impending failures; 20% of failure modes will not be observed. We also know from historic records that 5% of inspection results are wrong. Once we find an emerging failure our maintenance history indicates there is a 5% chance the gearbox will fail in six weeks and a 50% chance it will last for 40 weeks. This information is entered in the table and the model produces the two cost curves below the table. The upper cost curve is a magnification of the left-hand side of the lower cost curve.
If we did nothing and let the gearbox run to failure we would have a breakdown every 10 years. So every ten years we would spend $100,000; an annualised cost of $10,000. The model informs us that the optimal period to do the CM inspection is every 28 weeks. This frequency produces an annualised cost (for monitoring, repairs and an occasional breakdown that is not prevented) of about $5,000. We have saved the business about $5,000 per year by doing the condition monitoring on the gearbox instead of letting it run-to-failure. If we were concerned that 28 weeks was too long and instead wanted to do it every 14 weeks, you would double the inspection cost, but still save the operation $4,400 annualised.
With the model we can do 'what-if' analysis of our options. What-if we could do condition monitoring coverage for 100% of failure modes by including wear particle analysis into the CM regime? Inspection costs and overhead costs rise to $1,000 and $400 respectively. The model now shows annualised savings of about $5,600 with the inspections still at 28 weeks. The model gives you confidence that improving the condition monitoring regime to include wear particle analysis is a good business decision.
'What-if' a situation arose where we could purchase a gearbox with half the rate of failure of the current unit (for example by getting an oversized gearbox) and we wanted to know the impact that would have on our maintenance costs? The large gearbox would cost more to repair and that is reflected in the model below. The unit of measure on the graphs has been changed to 2-weeks per division to show the longer 30-week optimal period between inspections.
The annualised operate-to-failure maintenance cost is $6,000 for a breakdown every 20 years; down from $10,000 and is an immediate $4,000 annualised saving on using the 10-year life gearbox. If we continue with the 100% failure mode detection condition monitoring regime on this long-lived gearbox we save about $2,400 per year compared to a run-to-failure strategy. Using a long-lived gearbox gives us a total annualised maintenance savings of $6,400, which when compared to the 10-year life gearbox savings of $5,600 is clearly a better outcome for the operation.
This condition based maintenance optimization modelling tool was developed by Howard Witt, a professional reliability engineer with over 25 years hands-on industry experience, including nuclear facilities and industrial process plants.
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