Performance Based Maintenance

Objectives, Strategies & Processes

December 1998

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What is maintenance ?

• Work that restores an assets’ lost capacity for production, or work done with a view to preventing loss of production capacity (preventive maintenance).

• Loss of capacity may occur due to wear, rust, rot or a wide variety of other physical and chemical processes.

What is maintenance ?

• Loss of capacity may occur independently of operation eg. rust.

What isan asset ?

• An asset is a physical object that is made up of components that must interact to achieve an objective.

• Components may have specific tasks eg. provide electrical insulation, transmit rotary motion, contain pressure etc.

What isan asset ?

• These components will need to be maintained to replace the loss of ability to perform their specific tasks eg. insulating oil needs to be filtered to restore any loss of insulation.

Why performmaintenance ?

• Helps prevent an asset breaking down, which usually interrupts production.

• Helps extend the life of the asset, meaning that purchasing a new asset can be deferred.

When shouldmaintenance

be performed ?

• Ideally, just before the assets’ performance drops below a minimum required performance level (this level may include “break down”).

When shouldmaintenance

be performed ?

• Most maintenance is based on implying a components ability to continue performing from an easily observable parameter such as breakdown, calendar hours, running hours, number of starts etc.

• This has given rise to a wide variety of maintenance strategies.

Maintenancestrategies

• Breakdown.

• Time based.

• Fixed interval.

• Combination.

• Condition based.

• Design out.

• Opportunity based.

Breakdownmaintenance

• Occurs when a component breaks and the asset ceases to function.

• Need to perform maintenance is obvious.

• Scheduling work may be difficult depending on when breakdown occurs.

Breakdownmaintenance

• Scope of work may include repair of associated catastrophic damage.

• Probably will involve an unplanned asset outage.

Time basedmaintenance

• Performed at fixed calendar intervals.

• Implies component condition from elapsed calendar time.

• Scheduling work is simple.

• Scope of work involves rejuvenating or replacing components.

Time basedmaintenance

• Breakdown may still occur between maintenance intervals.

• May result in assets being “over-maintained”.

Fixed intervalmaintenance

• Performed after a fixed number of events eg. trips, starts, running hours.

• Implies component condition from elapsed events.

• Scheduling work becomes more difficult.

Fixed intervalmaintenance

• Scope of work involves rejuvenating or replacing components.

• Breakdown may still occur between maintenance intervals.

• Less likely to result in assets being “over-maintained” than time based maintenance.

Combinationmaintenance

• Performed on a combined time and fixed interval basis eg. annually or after 500 trips.

• Implies component condition from a combination of elapsed calendar time and events.

• Scheduling work becomes more difficult.

Combinationmaintenance

• Scope of work involves rejuvenating or replacing components.

• Breakdown may still occur between maintenance intervals.

• May result in assets being “over-maintained” because of time based driver.

Condition basedmaintenance

• Performed when component conditions reach a predefined trigger level rather than implying component condition from elapsed time or events.

• Assessing component condition adds an extra layer of work to the process.

• Scheduling work can be difficult.

Condition basedmaintenance

• Scope of work involves rejuvenating or replacing components.

• Breakdown unlikely to occur between maintenance intervals.

• Less likely to result in assets being “over-maintained” due to condition assessment preceding maintenance.

Design outmaintenance

• Eliminates maintenance by replacing components with different components that require less maintenance.

• Scope of work involves design, specification, purchase and installation.

• May involve lengthy asset outages.

Design outmaintenance

• Likely to be run as a capital project rather than a maintenance exercise.

• May involve a rigorous cost-benefit analysis before approval is given.

• Will significantly alter future maintenance requirements.

Opportunity basedmaintenance

• Maintenance that can be performed during an unplanned outage eg. car hitting a pole requires a section of 11 kV line to be isolated. Work such as replacing insulators can be piggy-backed into this unplanned outage.

• Usually restricted to minor maintenance or defect repairs.

Opportunity basedmaintenance

• Requires all opportunity based work to be readily identifiable when an unplanned outage occurs.

• Requires resources to be available at short notice.

• Overall philosophy is “buying” higher system availability by avoiding planned outages.

The costs ofmaintenance

• Cost of actually doing the work.

• Value of lost production or customer good will if outages are required.

• Value of component life discarded if maintenance is performed too soon.

The benefits ofmaintenance

• Minimises unplanned lost production or loss of customer good will.

• Helps prevent catastrophic failure.

• Allows work to be performed under planned, controlled circumstances.

• Assists in preserving future earnings for asset owners.

Performance basedmaintenance

• Objective is to not only perform maintenance based on the condition of components, but to consider the performance requirements of the asset in deciding when and how to maintain.

Case study - wooden poles

• A hard-wood pole in a 110 kV line would be inspected and maintained differently to the same pole in a remote rural 11 kV line.

• Even though both poles are physically identical and subject to the same degradation processes, the pole in the 110 kV line is obviously more critical to the owners business.

Case study - transformer oil

• Oil in a 350 MVA generator transformer would be DGA tested monthly because its performance is critical to the owners business.

• The same oil in a 10 kVA distribution transformer would be replaced every 12 to 15 years without any inspection when the transformer is bought in for an overhaul.

Performance basedmaintenance

• Key processes are:• Developing a comprehensive component

register.• Defining each components’ objectives, its

performance requirements and it’s the relevant mode of performance degradation.

• Defining suitable tests and inspections for assessing component condition.

• Performing the inspections and tests.

Performance basedmaintenance

• Comparing the actual condition to the trigger level at which maintenance must be performed.

• Performing the maintenance if required.• Recording the results of inspections and tests,

and the scope of work performed.• Modify the inspection and test requirements as

required.• Repeating the cycle.

Developing thecomponent register

• Augment the asset register to the level of individual asset components.

• Identify each components’ objective eg. provide electrical insulation between live components and the metal case.

• Identify the relevant mode of performance degradation eg. loss of dielectric strength due to oxidation.

Developing thecomponent register

• Identify the components’ performance requirements eg. minimum dielectric strength of 55 kV/cm.

• Identify a suitable trigger level at which maintenance will be performed eg. 60 kV/cm for a an 11.5 MVA transformer, but 70 kV/cm for a 350 MVA generator transformer.

Setting thetrigger level

• Depends on the criticality of the asset and the consequences of getting it wrong eg. the oil dielectric strength in a 350 MVA generator transformer is more critical than in a 10 kVA distribution transformer in a remote rural area, and merits a higher trigger level to avoid failure.

Setting thetrigger level

• Super-critical applications such as military and aviation may permit the discarding of unconsumed life in order to stay well above the trigger levels.

• This may be done on a time or fixed interval basis.

Defining suitabletests & inspections

• Requires measurement of the relevant mode of performance degradation.

• Requires the frequency of measurement to be defined.

Comparing actualperformance with

trigger levels

• Requires comparison of the actual component performance with the predefined trigger level eg. the dielectric strength of the latest oil sample is 72 kV/cm, which is much greater than the 60 kV/cm trigger level for an 11.5 MVA transformer.

Performingmaintenance

• Requires the defined scope of work to be performed, and the asset to be confirmed as suitable for return to service.

• Requires information to be recorded, such as scope of work performed, any defects noted, modifications made etc.

Changingthe culture

• Like any organisational change, changing maintenance strategies requires a thorough change management process.

• May well be entrenched attitudes that will covertly resist any change in strategy, particularly any that reduce overtime payments.

Is it the beststrategy ?

• May not always be the best strategy because of the expense of assessing component condition.

• Requires the criticality of the asset and the consequences of breakdown to be quantified.

• Perform cost-benefit analysis.

Maintenancestrategy selection

• If criticality is very low, use Breakdown.

• If criticality is medium, and cost of condition inspections is low, use Performance Based.

• If criticality is medium, and cost of condition inspection is high, use Time, Fixed Interval or Combination.

Maintenancestrategy selection

• If criticality is high, and cost of condition inspection is low, use Performance Based.

• If criticality is high, and cost of condition inspection is high, use Design Out.

• If work scope is minor and criticality is low, use Opportunity Based.

Life cycleplanning

• Important that the chosen maintenance strategy integrates into the overall life cycle plan.

• Most asset owners have a capital development plan that considers changing asset requirements over time, whereas the maintenance plan may only consider maintaining status quo performance.

Life cycleplanning

• Important that the maintenance plan (regardless of which strategy is chosen) reflects the requirements of the capital development plan eg. a transformer that will removed and scrapped in the 2007/08 financial year should not be given a full overhaul in 2005/06 (filtering the oil and minor testing should suffice).

Conclusions

• Maintenance strategies must be justified on a cost-benefit basis.

• Performance based maintenance may always not be the best strategy.

• Consideration must be given to the possibility of catastrophic failure and the loss of customer service in the event of breakdown.

Conclusions

• Maintenance plans must reflect the requirements of the capital development plan.

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