|by Daan Burger, N.D. (Elect. Eng.)
|Implementing reliability-centered maintenance (RCM)|
|Decisions to implement RCM are based on the consequences of functional failures.||
Using RCM to develop an initial maintenance program for new equipment as well as scheduled maintenance for existing plant, involves a structured decision-making process based on the consequences of functional failure of this equipment. RCM analysis produces a program which includes all scheduled tasks - and only those tasks - necessary to ensure safety and operating economy. The decision-making logic behind RCM analysis applies to any complex equipment which requires a maintenance support program aimed at maximizing operating reliability at the lowest cost.
The objectives of an RCM maintenance program are:
Before examining the RCM decision-making logic, it would be useful to review some maintenance terms.
|Condition monitoring is characterized by the absence of preventive maintenance tasks.||Effectiveness
The success of scheduled maintenance in achieving reduced failures, plant availability or cost-effectiveness.
potential failures must be less than the combined cost of
the operational consequences plus the cost of repairing
the failed component.
The first step in developing an RCM program is to evaluate the consequences of equipment failure. This identifies significant items and determines the priorities of the maintenance effort. The different types of failure consequences include:
Failure consequences can be evaluated using the flow chart below:
The next phase of RCM analysis involves a systematic study of the different failure modes of each significant item to establish the benefits of scheduled maintenance. One needs to determine whether one of the basic maintenance tasks will satisfy both the criterion for applicability and the specific conditions for effectiveness. There is a specific order of preference to the use of preventive tasks as the Maintenance Task Flow Chart shows.
Scheduled on condition inspections directed at specific failure modes are the most desirable type of task. They are aimed at detecting reduced resistance to failure. The component remains in service until a potential failure is discovered. Where visual inspection is impractical, oil and filter analysis can detect abnormal wear and contamination.
On condition tasks include checking of pressures, temperatures, speeds, flow rates, output power, current draw, vibration, fluid levels, fluid leaks, cycle times, borescope inspections, and oil
filter analysis. These tasks discriminate between units
that require corrective maintenance and those that are
likely to survive until the next inspection (sample),
thereby allowing all units to maximize their useful life.
Thus, when on condition tasks are applicable and
effective the cost of both scheduled and corrective
maintenance is minimized.
If no applicable and effective on condition task can be found, scheduled rework or overhaul is the second choice.
discard tasks are economically least desirable. Although
lubricant change is considered a discard task, it can be
cost-effective because of its low relative cost. However,
in large recirculating systems the oil itself is a
significant and expensive item. Oil analysis can be used
to monitor the serviceability of the oil and lengthen oil
drain periods as well as to check for wear.
If no task or combination of tasks are applicable and effective, the unit cannot benefit from scheduled maintenance. Replacement or product improvement/redesign should be considered.
|Evaluating failure consequences helps to identify significant items and maintenance priorities.|
We are now ready to develop an initial maintenance program for plant from the research that has been completed. We have already listed all the significant items that could benefit from maintenance, and a list of all known and envisaged functional failure modes for each component has been compiled.
We now consider the failure consequences of each functional failure. If a failure is found to have
consequences with plant downtime and loss of production
predicted, the maintenance task flow chart is applied to
that failure mode to determine applicable and effective
maintenance tasks. The same logic is applied for safety,
non-operational and hidden consequences.
The maintenance schedule should now be drawn up using conservative task intervals. Job cards are prepared detailing what tasks are to be done at a service. Task cards can be compiled to indicate how inspection and servicing are to be done and to define limits and procedures. Each task card
|On-condition tasks allow all units to maximize their useful life.|
|Oil analysis is appropriate for detecting wear, deterioration and contamination in large gearboxes.||is
listed and signed for on the job card. Task cards are
reusable and may contain illustrations and maintenance
manual extracts. They should be sealed in plastic and
used on the job.
Lastly, optimum service and inspection intervals are developed with operating experience. Tasks must be scheduled frequently to minimize failures, with the overall aim of finding a reasonable balance between low maintenance costs and low failure rates.
RCM decision-making logic is applied to two common industrial components as examples of how scheduled maintenance can extend equipment life.
EXAMPLE 1: Industrial gearbox
Safety consequences? No
Function failure mode:
bearing, gear, shaft or thrust washer failure.
Is an on condition task
applicable and effective? Yes
In a large gearbox holding 400 litres of synthetic oil at
$45 per litre, the oil itself becomes a significant item
and a capital asset. In this case oil analysis is
warranted in testing for oil contamination and
deterioration, and it has the added benefit of monitoring
wear. The oil is changed only if it becomes unfit for
EXAMPLE 2: Industrial hydraulic system
Hydraulic systems and their controls can be very complex so only the basic power pack is considered here:
Safety consequences? Yes,
bursting hoses can cause injury and fire, and loss of
power can result in loss of control over the load in
Initial scheduled maintenance program
F.S. Nowlan, H.F. Heap - Reliability-Centered Maintenance.
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