Reliability

Reliability

Probability that the product continues to meet the specification over a given time period subject to given environmental conditions

Factors Influencing Reliability

• Quality• Temperature• Environment• Stress• Complexity

Modifying Factors

πλ λEXPECTED

=STANDARD

Design for Reliability

• Element/component selection • De-rating• Environment• Minimum complexity• Redundancy• Diversity• Calculation of overall system reliability

Design Tools for Reliable Products

FTA – Fault Tree Analysis

PFMEA – Potential Failure Mode & Effect Analysis

Reliability in Maintenance

Preventive MaintenanceTotal Productive MaintenanceReliability Centred Maintenance

Evolution of Maintenance Practices

Pre 1950 - Breakdown Maintenance

1950’s 1970’s - Preventive Maintenance

1980 onwards - Condition Based or PredictiveMaintenance

Reliability Centred Maintenance

Aims to plan scheduled preventive maintenance effectively and efficiently

ObjectivesTo reflect on the role of maintenanceTo appreciate use of models to ‘optimise’ preventive maintenanceTo understand methodology underpinning Reliability Centred Maintenance (RCM)To assess uses and limitations of maintenance programmes in practice

Why Maintain Equipment?

Maintenance Impact on Reliability?

Time

Failure Rate

Desired impact of maintenance

How Maintain Equipment?Maintenance tasks– action(s) required to achieve a desired

outcome which restores an item to (or maintains and an item in) serviceable condition

Maintenance programme– methods, procedures and resources

required for sustaining the support of an item throughout its life cycle

Maintenance

Preventive Corrective

Time/cycle based On-Condition monitoring

Diagnosis and repair as

soon as failure occurs

Activities on non-faileditems to avoid or reduce

probability of failure

To restore non-faileditem at scheduled

time e.g. age,routine servicing

To restore non-faileditem when condition

assessment failsdefined acceptance

criteria

= INITIAL COST(purchase,delivery,installation,commissioning)

+ RUNNING COST OVER LIFETIME(fuel,energy,services)

+ COST OF FAILURES AND MAINTENANCE OVER LIFETIME

- NET SALVAGE VALUE

Net Salvage Value = Salvage Value - Disposal Cost

Total Lifetime Operating Cost

Amount of PM

Costs

Failure Costs

PM CostsTotal Costs

Optimum level of PM

Components of Downtime

Passive Active Active Passive Active Active

(a)realisation

(b)access

(c)diagnosis

(d)logistics

(e)repair/replace

(f)check

(b) (c) (e) (f)

Repair time

Downtime

Cost Components of Downtime

Materials CostRepair Labour CostAvailability Cost with– breakdown– preventive replacement– on-condition replacement– standby redundancy

Maintenance Options

Corrective or breakdownPreventive– Maintenance or replacement by time or

cycle– On-condition with periodic testing or

continuous monitoring– Standby with routine testing for unrevealed

faults

Preventive Maintenance

Theory

Predict when failure is going to occur and carry out preventive repair or replacementprior to failure

Preventive Maintenance Performance Measures

MTTF - Mean Time to Failure

MTBF - Mean Time Between Failures

Device number

1

23

N

Up Up Up

Up Up

Up Up

Up Up

Up Up

Down Down

Down

Down

Down

Down

Test time T

Failure number

234

j

NF

TD1 TD2

TD3

TD4

TDj

TDNF

MTBF - Mean time between failures

Service age

Probabilityof failure

Safe lifelimit

Economiclife limit

Average ageat failure

Cumulativepercentfailure

Time (hours)

100

75

50

25

1000 2000 3000 4000 5000 6000

Product life characteristics curve

Failure Rate

Time

Earlylife Useful life

Wear out

(bathtub)

Analytical Tools in Reliability Engineering

•Weibull Failure Rate Model•Constant Failure Rate Model

Preventive Maintenance

Practice

A wide range of failure patterns exist and it is difficult to predict the age at which failure will occur - ‘90%’ of items fail randomly

4%

2%

5%

7%

14%

68%

A

B

C

D

E

F

On Condition Maintenance

Theory

Monitor equipment and detect early signs of failure then carry out preventive maintenance prior to failure

On Condition MaintenanceProblem

•There must be a detectable warning sign of incipient failure - the potential failure point•There must be sufficient time interval to carry out repair before failure - potential failure interval•The failure time must be reasonably predictable

Condition

Potentialfailure

Functionalfailure

Potential failureinterval(PFI)

Logistics(d)

Activemaintenance(b)+(e)+(f)Time

Continuous Condition MonitoringUsed when:• PFI small - days , hours• Cost due to lost availability high

Temperature -bearings, lubricating oilStrain - pressure vessels, boilersVibration - pumps, compressorsPressure - oil supply systemsDifferential pressure - filtersElectric current - motors, pumps

Standby Maintenance

Active redundancyStandby redundancyTesting

Suitability of PM tasks

Task must be technically applicable

and

Task must be worthwhile

Benefit of On-ConditionMonitoring

Removing components of downtime reduces availability related costs

Maintenance Programmes

Reliability Centred Maintenance (RCM)Total Productive Maintenance (TPM)Breakdown Maintenance

RCM Definition 1 (MSG)– A disciplined logic or methodology used to

identify PM tasks to realise the inherent reliability of equipment at least expenditure of resources

RCM Definition 2 (IEC)– A method for establishing a scheduled PM

programme which will efficiently and effectively achieve the inherent reliability and safety levels of equipment and structures

History of RCM1967 - Boeing 747 purchasers, produced Maintenance Steering Group handbook (MSG-1)1969 - FAA approval of MSG-11970 - MSG-2 for DC10, L10111983 - MSG-3 for B757, B7671984 - Electric Power Research Industry1986 - MIL-STD-2173 for Naval Aircraft, Weapons and Support Equipment

RCM Basic ConceptsIdentifies important functions and consequences for safety, operations and economy of itemsIdentifies dominant failure modes through feedback of operating experienceIdentifies applicable and effective PM tasksDocuments decision-making processProvides ‘living’ maintenance programme

System operatingrequirements

Functionalanalysis

FMEA etc

Criticalitems

Maintenancetask selection

Systemevaluation

PM programme

PMjustified?

Systemre-design

y

n

Maintenance Task Selection

Could you develop a decision strategy for selecting between different types of maintenance tasks?

Reported Benefits of RCM

RCM provides a rational basis for maintenance planningRCM aims to optimise maintenance costs for a given reliability levelRCM optimises process resources and organisationRCM is a proven approach

Issues-Applying RCM in PracticeSystematic application of RCM involved creation of detailed databases and maintenance schedulesClose co-ordination is required between operations and engineering maintenance personnel for accessResource planning to reduce delays is often done with software systems designed to create valid maintenance plans