Optimisation Analysis

If a component strategy has been classified as a run to failure strategy then the reason for using a RTF strategy must first be validated using the Run to Failure Checklist (refer to Appendix 9.2). The RTF checklist takes into account various criteria such as safety, health, environmental impacts, costs as well as obsolescence. If the answer to any of the criteria is “Yes” then the RFT strategy cannot be applied and the component has to either be reclassified as critical or non critical.

If the RTF criteria are met then the strategy followed applies. The components need to be further analysed to identify any statutory requirements that could pertain to the components. The Power Station GMR 2 then reviews all RTF components to ensure that no statutory components have been included as part of the RTF strategy (a GMR 2 is a person that that been delegated by the Chief Executive of Eskom in terms of the General Machinery Regulation paragraph 2).

For components that are classified as critical or non critical two scenarios exist within the optimisation process, the application of the Generation Generic Component Strategy (GGCS) or in the absence of the GGCS, a formal Failure Modes Analysis (FMA) that would include the Task Selection, Interval and Packaging thereof.

5.3.2.6.1 Application of the Generation Generic Component Strategy

The Generation Generic Component Strategy is a strategy that is developed for identical components across the generation fleet. The strategy takes into account the environment that the component is functioning in, the running cycle of the component as well as the criticality of the component. This is a generic model, and therefore cannot be immediately applied to all Power Stations. The Power Station will assess the Generation Generic Component Strategy and enhance the Component Strategy to conform to the needs of the specific Power Station. Refer to section 5.4 on the development of the GGCS.

In applying the GGCS the following must be taken into consideration:

• Basic design of the component: Is the component of similar design in that the component will display failure characteristics as those covered in the GGCS? • Functional importance: if the component critical or non critical?

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49 • Operating environment: Is the environment in which the component operating harsh or mild (e.g. motor in the administration builder compared to the same motor in the ash plant)?

• Duty cycle: Is the duty cycle high or low?

Once the above criteria have been dealt with, it is necessary to then validate the maintenance tasks associated with the GGCS to ensure that the task are applicable to the component in terms of any plant specific considerations. These considerations could include modifications done to the component or specific operating practices performed on the component.

As the GGCS’s are developed using leading practices and Eskom experience, it is necessary to justify and deviations from the preventive maintenance practices contained within the GGCS’s. If any plant specific requirements exist they are added to the component maintenance strategy. By applying the GGCS’s to each component a list of Component Maintenance Strategies are then developed.

5.3.2.6.2 Failure Modes Analysis

A failure modes analysis is carried out on the system components in the absence of GGCS’s. This is done by identifying all the failure modes of a component that would lead to a functional failure of the component being analysed [13] [15]. A failure mode is defined as the manner in which equipment or a component fails to perform a required function. For example:

• Seizing of a ball bearing • Boiler tube rupture

• Overheating of circuit breaker contacts • Valve stem leakage

• Drive belt breakage • Pump impeller worn out

A functional failure is defined as the inability of equipment or component to fulfil its design intent e.g. a pump designed to pump water at a flowrate of 500l/min but is either pump at a rate less than 500l/min or not pumping any water at all.

Once identified, the failure modes are then listed alongside the failure locations (the part at which the actual failure occurs). This is done to ensure that the failure modes are grouped appropriately.

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5.3.2.6.2.1

Task Selection

For each failure mode, the team then identifies the possible failure causes. For each failure mode and failure cause combination the team then identifies the most applicable and effective maintenance task to defend against the failure. The preventive maintenance tasks are selected in the following order of priority:

5.3.2.6.2.1.1

Condition Monitoring Tasks

Condition monitoring tasks are aimed at detecting the onset of failure or potential failure condition in order to prevent a functional failure or to avoid the associated consequences [18].

• Failure patterns are usually random

• There must be a measurable parameter whose change over time can be correlated to failure onset.

• There must be a clear potential failure condition or value of the parameter, when action can be taken to prevent the potential failure or the associated consequences.

• The failure development period should be long enough to allow appropriate action to be taken.

• The task should be non-intrusive.

5.3.2.6.2.1.2

Time Directed Tasks

Time Directed tasks are aimed directly at failure prevention or retardation [18]. • The task is carried out at the preset interval without any further input.

• The task action is known to directly provide failure prevention of retardation benefits.

• There is usually a specific age where the conditional probability of failure increases.

• The task usually entails some form of intrusion into the equipment (e.g. six yearly overhaul of a gearbox).

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5.3.2.6.2.1.3

Failure Finding Tasks

Failure Finding Tasks are aimed at discovering a hidden failure condition before an operational demand [18].

• Task is performed to detect whether something has already failed, in order that action can be taken to prevent the multiple failure.

• The hidden failure condition does not become evident during normal plant operation.

• There is usually a “protective device”10 and a “protected function”.11

• No consequences are evident until the “protective device” is called upon to operate.

Using a set of specific criteria (refer to Appendix 9.5) the selected tasks, whether they are condition directed, time directed or failure finding tasks, as assessed to technical feasibility and effectiveness. If the tasks after found to be technically feasible and effective then they are recorded as the Maintenance Strategy for the component being analysed.

5.3.2.6.2.2

Task Interval Selection

Once the task for the component has been developed, a suitable frequency at which the tasks needs to be carried out must be determined. In identifying the frequency the following rules are adhered to:

• For Condition Monitoring tasks, the periodicity should be based on the failure development period.

• The task interval is based on the failure progression period (period between the point at which the potential failure condition can be detected and the point at which the functional failure would occur). The task interval is set at less than half this failure progression period. Koeberg operating experience has shown that a task interval of 1 monthly (while in service) has been successful in reducing the failure rate to an acceptable level.

10

An example of a protective device would be a trip wire on a conveyor belt

11

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52 • For Time Directed tasks, the periodicity should be based on the useful life. • The task interval is based on the age at which the component shows a rapid

increase in the conditional probability of failure. In many cases, due to the lack of historical failure data, this age is estimated based on maintenance history and international experience (Eskom experience and EPRI PM Templates).

• For Failure Finding tasks, the periodicity should be based on the risk of the multiple failures.

• The task interval is based on an assessment of the reliability of the protected function, the availability of the protective device and the acceptable level of risk associated with the multiple failure. Due to the lack of available reliability data, specialist opinion and international experience are also used to determine a suitable task interval. This task should be performed concurrent with any operating periodic or surveillance testing.

5.3.2.6.2.3

Task Packaging

Once the tasks have been identified and the appropriate interval at which the task is to be executed has been determined, then the task has to be packaged. There is no exact science when it comes to task packages and hence task packaging is based on the team experience. Tasks can be packaged as single tasks or multiple tasks can be packaged with multiple operating groups responsible for these tasks, this is referred to as a multiple disciplinary task package. The tasks are packaged based on:

• Periodicity

• Responsible execution group (maintenance mechanical; maintenance electrical; control and instrumentation; unitised maintenance; auxiliary maintenance)

• Impact on other components: will wok carried out on one component affect the operating of another component?

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5.3.2.6.3 Component Strategy Compilation

The component maintenance strategy is then compiled based either on the use of the GGCS or the Failure Modes Analysis process. All statutory or Eskom specific requirements are identified and included in the component strategy. The Power Station GMR2 then reviews the component maintenance strategy.