Mining Equipment Maintenance

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    Mining EquipmentMaintenance

    Fundamental Concepts

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    Old Underground Coal Mines

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    Modern Longwall Mine

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    The effect of Mechanisation on Productivity

    0

    5000

    10000

    15000

    20000

    25000

    1916

    1920

    1925

    1930

    1935

    1940

    1945

    1950

    1952

    1955

    1960

    1969

    1972

    1975

    1980

    1985

    1988

    1995

    2003

    Tonnes/Man-Year

    Produced in QueenslandCoal Mines

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    Maintenance Costs

    30-50% of the operating costs Annual bill is about $10 billion

    Equally significant is the cost of lostproduction when the machine is down

    Every 1% improvement in equipment

    availability or productivity improves the

    company profits by up to 3.5%

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    Objective Function forMaximising Company profits

    The Annual Production for a given

    investment is a suitable objective function

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    Total Annual Production

    Is this total annual production ?Tonnes/Hour x Total Hours

    More realistic prediction is

    Tonnes/Hour x (TotalHours - Downtime) Probably the following is more illuminating

    Tonnes/Hour x (TotalHours -

    -Scheduled Downtime

    - Breakdown maintenance)

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    Total Hours of PossibleProduction

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    Subtract PlannedMaintenance

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    Subtract BreakdownsThis is the time available

    for production. Obviously,you want to maximise thistime. That is why the ratioof this time to the totaltime is a very importantKPI.

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    Availability

    TH - PM - BMA= TH

    TH Total HoursPM Planned Maintenance

    BM Breakdown Maintenance

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    Availability

    !Operating TimeTH - PM - BM

    A= TH Total Time

    Is also expressed in terms of MTBF and MTTR as

    MTBFA=MTBF+MTTR

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    Availability

    !Operating TimeTH - PM - BM

    A= TH Total Time

    Is also expressed in terms of MTBF and MTTR as

    MTBFA=MTBF+MTTR

    This is the textbookdefinition of AVAILABILITY

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    AvailabilityOperating Time = N x MTBF

    Downtime = N x MTTR

    Total Time = N x (MTBF+MTTR)

    Operating Time MTBFA=

    Total Time (MTBF

    N x

    N )x +MTTR

    !

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    Maximise the Return to the Company

    The aim for the company is to maximise the return from its

    productive assets.

    Let us keep it simple and express this as maximising theproduction through the year.

    To achieve this,

    (a) The machine availability must be high

    (b) The machine must be producing at a high rate when it is

    operating

    The solution is a compromise between the two.

    We will see this in an example.

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    Optimum Bucket SizeThe recommended suspended load for your dragline is 150

    tonnes. A competitor put a larger bucket on a similar unitand they are running it at a suspended load of 170 tonnes.

    Your Manager wants you to install an even bigger bucket,

    taking your suspended load up to 210 tonnes. You know

    that the safe static load for this dragline is 250 tonnes.

    -Does a larger bucket necessarily mean higher production?

    -Why not increase it to 250 tonnes if this is the safe load?

    -How would you determine the optimum bucket size?

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    Question TH - PM - BMA= THPresent Production Po = 9000 t/h.

    Total Mine Operating Time in a Year = 8640h

    12-hour planned maintenance shift every month.Breakdown maintenance downtime is expected tovary with the production rate as

    3336o

    PBM wR RhereP

    ! " !P is the production rate with the bigger bucket.

    Find R that maximises the annual production.

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    Solution

    L

    ostpr

    oduction

    due

    to

    Breakd

    ownm

    aintenanc

    e

    Optimum Point

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    Dependence on MTBF

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    Maintenance

    Maintenance must be considered in thecontext of asset utilisation

    Mining is an asset-rich industry

    Optimum utilisation of these assets is the

    only way a company will stay competitive

    This is a task for both production and

    maintenance engineers.

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    Some Basic Concepts

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    FailureLoss of abilityof an item to

    perform itsrequiredfunction

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    Failure

    Broken teethon shearerdowndrive gear

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    Failure

    Cost associated with

    this failure:

    Lost production whenthe machine was down

    Replacement gear

    Maintenance labour

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    Failure Rate, MTBF, MTTRMTTR MTBF MTTR MTBF

    0 100 200 Hours

    R

    epair

    Repair

    Repair

    Failure

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    Failure Rate, MTBF, MTTRMTTR MTBF MTTR MTBF

    0 100 200 Hours

    R

    epair

    Repair

    Repair

    MTBF = Mean Time Between Failure (100 h)

    Failure Rate = Number of failures per unit time (0.01 h-1)

    MTTR = Mean Time To Repair (20 h)

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    Historical RecordsFailures occur randomly. The repair time is also not constant.How do we find MTBF and MTTR?

    If we treat failure as a random event, then we can use the

    well-established tools of probability and statistics to modelthe uptime, downtime and availability for our equipment.

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    Random Failure?

    All nature is but art, unknown to thee

    All chance, direction thou canst not see

    All discord, harmony not understood

    Alexander Pope, Essay on Man

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    Poisson ProcessPoisson distribution is commonly used in forecasting to

    represent the number of occurrences of a specific event ina given continuous interval.

    Ships arriving at a dock on a given day

    Traffic accidents on the SE freeway in a month

    Mad cow disease breakouts in the world in one year

    Typos per page in a long report typed by Hal Gurgenci

    Cable shovel failures in one day of operation

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    Poisson Distribution

    # $( ) !

    xte tp x

    x

    % %&

    !

    This is the probability distribution of the Poisson random

    variable X representing the number of outcomes occurring in

    a given time interval t. The parameter % is the averagenumber of outcomes per unit time.

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    ReliabilityAssume failure events follow a Poisson distribution.What is the probability of having NO FAILURES in a

    given time interval t?

    This can be found by substituting x=0 in the Poisson

    distribution function:

    # $0

    (0)0!

    t

    te t

    p e

    %

    %%& &! !

    This is referred to as the survival probability or the

    reliability

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    Reliability( ) tt e %&!

    Reliability is the probability that a product will

    operate throughout a specified period without failure

    when maintained in accordance with the manufacturer's

    instructions; and

    when not subjected to the environmental or operational

    stresses beyond limits stipulated by the manufacturer

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    The value of e

    Two centuries ago, a Polish Statistician, Ladislaus

    Bortkiewicz, investigated the Prussian army fatalities

    caused by horse kicks. According to army reports, the rate

    was about one fatality every 1.64 years. Ladislaus

    collected the reports for one year. These were 200 reportsand 109 recorded no deaths at all.

    Can you estimate the value of

    eusing the above data?

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    ExampleA major piece of equipment fails twice a day on average.

    Consider its reliability over a period of month.

    What is the probability of failure at any time during that

    month?

    R li bilit

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    Reliability

    2t

    e

    &

    !

    Z I

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    Zoom In

    2t

    e&

    !

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