Determination of Component Functional Importance

Function Importance determines the criticality of the component within the overall system. The reason for carrying out a functional importance evaluation is that maintenance should be focused on preserving critical system, structure and component functions. Thus, in order to achieve this, it is important to identify which systems, structures and components support critical functions [3] [13]. It is also important to note that some components may not support critical functions, but can have serious consequences if they fail in a certain manner (e.g. barring gearbox coupling engages while mill is in service), and this must be considered.

Based on this, components are classified into three categories:

• Critical: Critical components are those where the consequences of failure are serious and where the aim must be to defend against all plausible failures. • Non Critical: Non-critical components are still important components, but are

those where we can tolerate a failure.

• Run to failure (RTF): Components are Run-to Failure where there are no effects beyond repair of the actual failure itself.

These categories are based on the potential risk would be incurred if the component failed. Critical components are those components that would pose the following:

• Safety, health or environmental incursion • Have a statutory impact

• Incur a production loss of greater than 5% of the Power Station output over a period greater than 8 hours

A Case Study on the Development of and Asset Management Process within the Eskom Fossil Fired Power Stations with Emphasis on Reliability Basis Optimisation

45 • Components that have a hidden failure. These are components that serve a protective or redundancy function and the failure is only discovered when the component is called upon to carry out its specified function (e.g. a smoke detector is only discovered to have failed when there is a fire in a building).

Non critical components are those components for which failure would lead to significant costs being incurred or secondary damage to plant equipment.

A run to failure strategy is applied to those components where there are no effects beyond the repair of the actual failure itself.

As the success of the resulting Maintenance Strategy relies on accurate component categorisation, it is important to ensure that staff with the appropriate experience review the functional importance is therefore used to perform this review.

5.3.2.5.1 Functional Importance Categories

Table 3 summarises the functional importance categories based on the potential failure consequence [18].

Table 3: Function Importance Categories

Hidden consequence failures are categories into two sub-categories:

Potential Safety or Environmental Consequences:

• Hidden loss of a safety function (the component provides a safety function and failures are only evident when the safety function is used)

• Hidden loss of a protective device (potential safety or environmental impact that occurs when the function is used)

A Case Study on the Development of and Asset Management Process within the Eskom Fossil Fired Power Stations with Emphasis on Reliability Basis Optimisation

46 • Erroneous information to operators (potential safety or environmental impact).

Incorrect information given to the operator could lead to a potential safety or environmental excursion.

Potential Operational (Production) Consequences

• Hidden loss of a protective device in an operational system (operational impact only)

• Erroneous information to operators (potential operational impact)

Evident Consequences are those failures that can be picked up when inspections or testing is carried out and are classified as follows:

Safety or environmental impact:

• Failures having either safety or environmental consequences. Safety consequences include personnel hazard, injury or death.

• Environmental impacts include any environmental excursions that would result in failure.

Operational (Production) Consequences:

• Turbine trip (auto/manual) • Unit off-line (Islanded)

• Reduced power operation (>30% for < 2hrs)

• Sustained reduced power operation (>5% for >8hrs) • Delay in outage (> 1day)

• Reduced plant efficiency

• Degraded transient response capability • Adverse effect on water chemistry • Impaired ability to operate the plant

• Impaired ability to perform regulatory surveillance, testing or inspection

Economic Consequences

• Significant repair/renewal costs (failure cost is greater than the PM cost over a period of time)

• Significant consequential damage to plant equipment • Plant transient

A Case Study on the Development of and Asset Management Process within the Eskom Fossil Fired Power Stations with Emphasis on Reliability Basis Optimisation

47 • Evident loss of redundancy or protective device in an operational system (No

operational consequences) • Long-term plant health impact

• Significant impact on station resources

• Impairment of routine operating, inspection or preventive maintenance activities

• Excessive corrective maintenance

Finally for a component failure to be categorised as not having a significant consequence, it is implied that the failure would have no effect on safety, health, environmental excursions, cost or production impacts and simple routine tasks may be required to ensure intrinsic reliability (e.g. routine greasing tasks or visual inspections).

5.3.2.5.2 Duty Cycle

The duty cycle of the component will affect the periodicity at which a maintenance task will be executed, it is therefore important to determine the Duty Cycle during the analysis. For components with a High Duty cycle the maintenance tasks will be execute at a more frequent interval as compared to a component with a low Duty Cycle. Duty Cycle is not limited to running hours only but could include factors such as the number of starts or stops of a particular component. It is therefore important that the team involved in the optimisation analysis understand the Duty Cycle concept and how to apply it to the components within the system being analysed.

5.3.2.5.3 Environmental Factors

The environment in which the component is functioning is also important.

Components that operate in harsh environments like a coal or ash system with require maintenance interventions on a more frequent interval as compared to components that may be operating in an environment that is mild. Environmental factors are considered by the analysis team during the optimisation process.

A Case Study on the Development of and Asset Management Process within the Eskom Fossil Fired Power Stations with Emphasis on Reliability Basis Optimisation

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