Reliability, Availability, Maintainability

Value Improving Practices

The Approach

The Approach

The Approach

The Approach

The Approach

Risk

Risk

Risk Analysis

Probabilistic Risk Analysis

Reliability

Reliability

• Everyone has own notions about what the term “reliability” means - measure of performance.

• The role of reliability is observed daily by all of us.• Reliability is one of the most important quality

characteristics of components, products and systems.

• Does reliability add value to a product or a system?

• Historical perspective– Starting from aircraft industry in World War I -

qualitative.– 1940s, since the number of aircraft grew, quantified

measures such as mean failure rate, number of failures have been used.

– 1950s, nuclear industries started to develop. Reliability theory and maintenance technology were being increasingly used in the design of nuclear power plants and their control system.

Definisi

• Reliability– The ability of an item (product, system) to operate

properly without failure under designated operating conditions (such as temperature or volt) for a designated period of time or number of cycles.

– Cumulative probability of survival.– Reliability may be used as a measure of the system’s

success in providing its function properly.

Definisi

• Reliability as Probability• The measurement of reliability depends on the use of

statistics, and on the definition of reliability as a probability.

• Reliability : The probability that an item will perform a required function under stated conditions for a stated duration of operating life.

• Probability theory will be used frequently in reliability.

Definisi

• Mathematical definition– Let t be the time to failure of a item, a r.v.– T be the time designated to the item to ensure

the item being functional– Cj, i=1,2,... be the designated conditions

R(t) = Pr (t ≥T|C1,C2,…)If remove the conditionR(t) = Pr {t ≥ T}

Reliability Engineering

Reliability Engineering provides thetheoretical and practical tools for studyingand specifying the reliability of components,products and systems so that these canperform their required functions in specifiedenvironments for the desired period withoutfailure.

Reliability Engineering

• Engineers carry out reliability predictions, FMEA (Failure Mode and Effect Analysis) or FMECA, design testing programs, monitor and analyse field failures, and suggest design or manufacturing changes.

• Reliability engineering can be done by reliability engineers, design engineers, quality engineers, or system engineers.

• The overall goal of reliability engineering is to make your process and product more reliable in order to reduce repairs, lower costs, and to maintain your company's reputation and profitability.

Reliability Engineering

• Reliability Engineering tools can be used to optimise the maintenance strategy so that maintenance and inspection tasks are effective and done at an optimal interval, to ensure:

– minimise cost of maintenance and operation over the lifecycle of the plant,

– minimise cost of failure,– minimise safety and environment risks,– minimise operational risks.

• Component Reliability (1)• Basic Statistics

Basic Functions

• Reliability Function - R( t )• From Mean time to Failure (MTTF) - Ө

• MTTF can be generalised by allowing n tobecome extremely large, then we can obtain asmooth curve - R(t)

Basic Functions

• R(t) is the Probability of a component survival to time t.

• Properties:

Failure Function:

Basic Functions

F(t) = 1 – R(t)Properties:F(0) = 0, F(∞) = 1The failure function (or cumulative distributionfunction c. d. f.), is the probability that an itemfails at or before age t.

• Probability Density Function - f(t)

The area under the curve between any two agest 1 , t 2 , gives the probability that a new item willfail in that age interval. The total area under thecurve adds up to 1, because every item iscertain to fail at some time.

Probability Density Function, f( t)

Basic Functions

• To prove f(t) is a density function:– Since R(t) has negative slope, f(t) is always

positive

– the integral

Basic Functions

• Failure rate - l(t):

• f(t) is a measure of the overall speed at which failures are occurring.

• l(t) is a measure of the instantaneous speed of failure.

• Also called “Hazard rate” with bathtub shape

Bathtub Curve

Importance of the Failure RatePattern

• Identification of the failure pattern is an important factor in maintenance decision making.

• It helps identify the root cause of failure. There is a tendency to assume that wear –out causes most failures, but in fact, burn- in and random failures are more common. The identification of the failure pattern or patterns will give a useful indicator of how to find, and hence eliminate the physical cause of failure.

Failure patterns and causes offailureFailure Rate Pattern: Clue to Cause of Failure:

Burn in Manufacturing defect or fault installation

Random Operational problems; External factors; Aggregate of many causes.

Gradual wearout CorrosionPremature wearout Product insufficiently robustMature wearout Normal wear

Six Failure Rate Patterns

Bathtub curve for human

Failure Rate with/without maintenance

Failure Rate Patterns with maintenance

Relationships

• R(t), f(t) and λ(t) are related.• If any one of these functions are known, the

other two can be derived to help the analysis.

• If R(t) is known:

• If f(t) is known:

• If λ (t) is known:

The relationship between R(t), F(t) and f(t)

END

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