Maint w Reliability Concept

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MAINTENANCE MANAGEMENT WITH

RELIABILITY CONCEPT

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15/11/2011

TABLE OF CONTENT page

number

INTRODUCTION.3


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CHAPTER ONE Reliability main term definition.4

CHAPTER TWO Data collection and treatment5

CHAPTER THREE Reliability prediction and modelling7

CHAPTER FOUR Availability and maintainability plans.10

CHAPTER FIVE FMEA and FMECA14

CHAPTER SIX Spare parts specification and management15

SUMMARY16

REFFRENCE17


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INTRODUCTION

Maintenance is a large element of manufacturing costs and today many companies are paying

increased attention to capabilities of their maintenance function. The Traditional approach to

maintenance management has been to focus on reducing costs and head count. In leading

companies, however, maintenance is viewed as a high leverage contributor to the business

profitability through its impact on equipment capacity, product quality, safety and production

costs. An investment in reliability improvement will, however, ensure that costs

improve along with safety, production capacity and production quality.

This paper tends to present the application of reliability concept to industrial facility

Management Company and this will be specifically on building HVAC and BMS (building

management system). This is commencing by given clear definition of reliability main terms


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follow by (ii) data collection and treatment, (iii) Reliability prediction and model, (iv)

Availability and maintainability plans, (v) FMEA and FMECA, (vi) spare parts specification and

management and summery.

Reliability main term definitionReliability is simply the probability that an item or a system perform its

stated function without a failure within a stated condition. This usuallydenoted with a symbol R (t). F (t) is failure rate = 1 - R (t).

MTTF: - Mean time to failure; this term is used for non-repair item. This is the

average operative life of nonrepair item and the term use to measure the life vary. This could

be time , revolution etc.


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MTBF: - Mean time between failure; this term is used for repairable item. This simply

means the average of time interval between successive failures.

MTBMA: - Mean time between maintenance actions. This includes average downtime

on both schedule and unscheduled maintenance action

MTTR : - Mean time to repair. This simply refer to the time spent in restoring the

down tool machine back to operation

MDT; - Mean downtime. The average time use on logistic, waiting for parts and

administrative downtime

MAMT: - Mean active maintenance time. This includes the average

corrective maintenance time and the average time to perform preventive

maintenance.

FMEA: - Failure mode and effect analysis

FMECA: - Failure mode effects and critical analysis

FTA: - Fault tree analysis

RCM: - Reliability catered maintenance

Data collection and treatment


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1.0 Sauces of data

At the beginning of facility equipment for maintenance contract some

procedure are meant to be carried out. Some of these procedures are listed

below.

Auditing for operation level, safety and capacity. In the process or afterauditing some data will be collected and this will assist the maintenance

company in carrying out some aspect reliability analysis.

Installation and commissioning documents will also be requested fromthe client.

Operation data. These are the data collected during normal running of themachine and it usually consist of running time, breakdown period,

breakdown nature, periodic service program, operation procedure,

equipment log sheet etc.

Carrying out of failure mode analysis.Since the equipment in question is HVAC and BMS, such data will include

supply chilled water temperature, return chilled water temperature, deferential

chilled water temperature, chilled water flow rate, condenser water treatment

characteristics, condenser supply &return water temperature etc.

Data treatment.This will entail data analysis, evaluate and summarize data using techniques

such as trend analysis, weibull, graphic representation etc.

The first steps in summarizing data are construct a histogram and a control

chart. This will help to determine if the equipment performance is stable as well as

providing estimates of the mean and standard deviation.

The control chart can be helpful in spotting a trend line. Linear regression may

be used to fit a line to the data.


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Once failure data have been collected it is often useful to display them in

graphical format.

Key reliability metrics includes:

Probability density function (PDF) can be plotted as a histogram showingthe number of failure in each time block.

The hazard function shows the failure rate as a function of time. It isconvenient to use the following formula for hazard function

(t) = Fraction of failure during the time period/Amount of time during the

period.

The reliability function plots reliability as a function of time. The formula isR(t) = Number surviving at the end of the period/Number unit tested


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Reliability prediction and modellingA system can be model for reliability analysis using block diagrams. A

system consists of subsystems connected to perform given functions. A

mathematical solution can also be used to complex system to a graphical

representation of the interconnection of its subsystems. The math model use

to assist in making changes to the system for reliability improvement. The

model can be used to determine test and maintenance procedure.

fig. 3.1

The general approach is to capture the modelling effort with the use of Reliability

Block Diagrams.


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The model could be concerned with just showing the critical functions and the

associated failures modes, as derived in the FMECA. This information may further

be used in the FTA of the safety engineering.

Redundancy or back-up mechanisms will enhance the reliability of a system, but

augment the Life Cycle Support (LCC). Questions that would need to be

considered, is whether the system should employ "active redundancy or standby

redundancy ".

The actual decision for the system redundancy could also be dictated by other

engineering constraints, for example the safety requirements might mandate a 2 out

of 3 voting redundancy for critical system components.

3.1 Active Redundancy

The approach here would have redundant elements that would support a fault

tolerant architecture. In this case, the active redundancy, all of the redundant

elements are utilised by the system, e.g. they are powered up. However, in the

event that one (or more) element fails, the system is capable of performing itsrequired function and operation. The redundant elements incorporated into a design

could be a simple affair or consist of very complex elements. With a more complex

configuration the architecture could consist of a combination of elements having

no redundancy, a couple of elements having dual redundant, to several elements in

parallel etc.

The final system configuration would be influenced by the actual required

reliability and availability requirements, which takes into consideration whether

the system is repairable or non-repairable.

In the following given example is a simple Dual Redundant Configuration. The

system's reliability can be calculated as given. This calculation assumes that the


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systems are identical, other words each having the same failure rate and that the

failures follow an exponential distribution pattern.

fig. 3.2

3.2 Standby Redundancy

There might be instances where a system must achieve an operational availability

and itself cannot afford an extend downtime. With repair actions of a failed unit

being possible, but the implementation of an active redundant configuration not

considered feasible, due to economic or operational reasons. It may be more

appropriate to utilize a standby redundant configuration. An example of this could

be a field power generation plant. The power distribution configuration would

consist of two 800 KVA diesel generators. One would be online (running)

continuously and the other would be in a standby state (not running). In the event

that the operational generator experiences a failure (or where the need to perform

preventative maintenance exists), the standby generator would be brought on-line.

This would then permit a repair action to be implemented on the failed generator

set.

3.3 Reliability Prediction

This is the process used to determine the MTBF of an item. This is achieved by

performing a prediction analysis.


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Maintainability and Availability Planning.

4.1. Reliability and Maintainability Engineering Program Approach


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The approach to a reliability and maintainability program is dependent upon many

factors that include the customer's requirements, the business strategy of the

company, and the size of the project etc. The effective implementation of an R&M

Engineering Program must take into consideration these and other factors. Detailed

in Mil-Std-785 and Mil-Std-470 are various tasks associated to the reliability and

maintainability engineering program. Careful task selection must be made for each

particular program, to ensure that the reliability and maintainability requirements

and objects are achieved. How do you determine the reliability of a system, taking

into consideration the mission operation profiles? How do you optimize the

reliability (and availability) of a system with respect to the life cycle cost? Where

should you focus your engineering efforts to minimize program cost? These are

just a few of many questions that need to be asked and answered prior to

implementing a reliability and/ or maintainability program.

Prescribed maintenance actions will maintain a system in operating condition and

reduce the probability of failure due to wear.

The two main general classification of maintenance actions are :-

1. Preventive Maintenance (PM) :- This include the actions performed to

keep the system in an operating state by preventing wear-out failures. It

does not reduce the constant failure rate that is inherent to the system but

tends to keep the system operating at that level of failure probability. This

can be planned and it can be carried out when the system is not being used.

2. Corrective Maintenance (CM):- The actions require returning the

system back to operating state whenever failure has occurred. This cannot be

planned, and most be performed when the system fail. This usually demands

for prompt response.


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4.2. Availability is a measure of likelihood that a system will be ready to operate

when is called on to operate. Factors affecting availability are (a) Failure

occurrence, (b) Lengthy corrective actions, (c) lack of effective preventive

maintenance and other logistic reasons like late arrival of spares, (d)

Administrative Delay in making decision

Availability is a function of the number of maintenance actions necessary

and the time it takes to complete the actions.

MTTR: - Mean time to repair

MTBF: - Mean time between failures.

Availability = MTBF / (MTBF+ MTTR)

eq. 4.1

The availability of a system will increase when MTBF increases and MTTR

decrease.

C. To obtain increase in MTBF and decease in MTTR. The following must

be achieved :- (a) Availability of spares to reduce meantime to repair, (b)

provision of effective training on the equipment in question to increase

mean time between failure.

4.3. MAINTENANCE STRATEGIES.

The RCM analysis is a systematic approach for identifying preventative

maintenance tasks or scheduled maintenance tasks for an equipment end item and

establishing necessary preventative (or scheduled) maintenance task intervals. In

essence a maintenance task would be implemented prior to the failure occurrence

of the component in question.


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The consolidated results from the RCM analysis process forms the basis of a

Preventive Maintenance (PM) program for the system. A RCM analysis is

conducted to determine which PM tasks would provide increased equipment

reliability for the life cycle. The RCM analysis would use the information

generated by the FMEA to identify which hardware components have the greatest

effect on the equipment reliability and availability, by identifying probable failure

modes.

Using the decision tree process of RCM analysis, a complete analysis of each

Functional Significant Item and their assigned failure modes can be conducted.

(i) Reliability- Centered maintenance and (ii) Predictive maintenance.

This two are proactive strategies to encourage replacement before failure

occurrence. Hence Preventive Maintenance (PM).

Corrective Maintenance (CM):- Condition based maintenance is reactive.

Replacement is made after Failure occurrence.

Factors to be considered for selecting replacement strategies are:-

The failure distribution of the unit.

The cost associated with the failure of the unit. Any safety issue associated with the failure.(iv) The cost of the replacement unit.

(v) The cost associated with scheduled replacements.

(vi) The cost of inspection or test.

Maintainability Apportionment and Allocation.Reliability is attending to system failure reduction and thus reducing the

frequency of unscheduled maintenance actions.


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Maintainability is attending to reduction of downtime duration of both

scheduled and unscheduled maintenance actions.

NB:- MILHDBK472 gives following definition of maintainability.

Maintainability is the ability of an item to retain in or restored to specified

conditions when maintenance action is performed by personnel having a

specified skill levels and using prescribed procedures and resources at each

prescribed level of maintenance and repair.

Maintainability objectives are best accomplished if an effort is made to

eliminate unscheduled downtime and reduce the duration of scheduled downtime.

A systemlevel maintainability requirement may need to be allocated to lower

levels of the system.

Maintainability allocation is a continuing process of apportioning requirement at

the system level to subsystem levels.

AVAILABILITY TRADEOFFS.System availability (A), is a function of its reliability (R), and Maintainability

(M).

Availability is the measure of the time the system is in an operating state,

compared to the total time. The various availability depend on the downtime

that are included in the total time.

With constant system rate of failure assumption we have:-

Inherent Availability: - This includes downtime on preventivemaintenance and logistics.

Ainh =MTBF /(MTBF +MTTR)..eq. 4.2


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Achieved Availability; - This include downtime on corrective andpreventive maintenance action and it also includes any delay in

logistic and administration. Hence, the (MTBMA) which means time

between maintenance actions. This includes both schedule and

unscheduled actions.

MTBMA is a function of the failure rate () and preventive maintenance rate (u).

MTBMA = 1 /(+u)

Aach =MTBMA /(MTBMA +MAMT).eq.4.3

MAMT:- Mean active maintenance time.

Operational Availability:- This includes all down time. The meandown time (MDT) includes logistic time, spares part delay time and

administrative downtime

Aop =MTBMA /(MTBMA +MDT)..eq. 4.4

FMEA and FMECAThis Failure Modes and Effect Analysis. It is one of the most utilized methods for

conducting reliability analyses. The Failure Modes and Effects Criticality Analysis (FMECA) is

really an extension of the FMEA, focusing on the quantitative parameters for a criticality

assigned to each probable failure mode,. A widely accepted military standard for conducting

FMEAs is Mil-Std-1629. This military standard details the specifics in conducting a FMEA.

These procedure will be carried out on all newly acquire equipment to be

maintain and the result will be used for taking appropriate maintenance

management decision.


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The steps of FMEA:-

Select the equipment to analysed and gendered associated data List all possible failure modes. Calculate a risk priority number (rpn) for each mode. Assigning a

value of from one to ten for each of these categories: S = severity; O=

occurrence; D= detection and rpn = S X O X D.

Develop a corrective action plan for the risk or failure Document and report the result

6. Spare parts specification and management


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The require number of spares necessary for a given period of time is a

function of the expected number of failures and the number of spares required for

preventive maintenance. The number of failures in the function of the unit

operating time and the failure rate. If any spares are required for preventive

maintenance, the number is a function of operating time and the preventive

maintenance circle.

The MTTR calculation assumes that spare parts are readily available. This

means that spares must be in inventory when they are needed. If there is waiting

time to obtain a spare, the unit downtime will increase and availability will suffer.

If the unit has a constant failure rate, the probability of requiring no more than r

replacement unit can be found using cumulative Poisson distribution.

If preventive maintenance require spare parts, the n umber of spare

necessary is a function of the operating time and the maintenance circle. The

number of spares required for preventive maintenance is the product of the

maintenance circle and the total operating time. The total number of spares that

must be kept in inventory is a function of delivery time, the cost of maintaining

the inventory, and the availability requirement.


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SUMMARY

All the above highlighted procedures are meant to be reliability quality

process to be followed by the concern company for maintenance processes.

These are meant to solve the following inherent problems:- i. low equipment

availability; ii. High maintenance cost; iii. Downtime due to shortage of

spare parts; iv. Logistics delay; v. low technical skill etc.


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REFRENCE

THE CERTIFIED RELIABILITY ENGINEER HANDBOOK. By DONALD W.BENBON and HUGH W. BROOME. Pub: - ASQ. 2009

Maintenance Management by L. Pinetelon, L. Gelders and F Van Puyvelde published byacco 1997.

Maintenance By Jasper L. Coetzee, 1997. Practical Reliability Engineering 4th Edition, By Patrick D. T,. O`CONNOR, 2010 .

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Maint w Reliability Concept