CBM

CONDITION BASED MAINTENANCE PRINCIPLES, ECONOMICS AND APPLICATIONS

Dept of Mechanical Engineering, SJCE, Mysore

1. INTRODUCTION

Condition-based maintenance (CBM), shortly described, is maintenance when need arises.

In a continuous growing global market Productivity is playing a key role to stay competitive, for

any manufacturing company. Productivity can be achieved through availability and availability can

be increased through adopting the efficient maintenance practices, by focusing on different types of

maintenance and strategies. Condition based Maintenance or predictive maintenance, uses primarily

non destructive testing techniques, visual inspection, and performance data to assess machinery

condition.

It replaces arbitrarily timed maintenance tasks with appropriate maintenance task at only

when warranted by equipment condition. Condition-monitoring maintenance task intervals must be

properly understood and task intervals should be determined based on the expected P-F interval.

The P-F interval governs the frequency with which the predictive task must be done. Technological

advances are accepted and applied to CBM systems, which includes improved knowledge of failure

mechanisms, advancements in failure forecasting techniques, advancements in monitoring and

sensor devices, advancements in diagnostic and prognostic software, acceptance of communication

protocols, developments in maintenance software applications and computer networking

technologies.

This maintenance is performed after one or more indicators show that equipment is going to

fail or that equipment performance is deteriorating. This concept is applicable to mission critical

systems that incorporate active redundancy and fault reporting. It is also applicable to non-mission

critical systems that lack redundancy and fault reporting. Condition-based maintenance was

introduced to try to maintain the correct equipment at the right time. CBM is based on using real-

time data to prioritize and optimize maintenance resources. Observing the state of the system is

known as condition monitoring. Such a system will determine the equipment's health, and act only

when maintenance is actually necessary. Developments in recent years have allowed extensive

instrumentation of equipment, and together with better tools for analyzing condition data, the

maintenance personnel of today are more than ever able to decide what is the right time to perform

maintenance on some piece of equipment.

Ideally condition-based maintenance will allow the maintenance personnel to do only the

right things, minimizing spare parts cost, system downtime and time spent on maintenance.

http://en.wikipedia.org/wiki/Upkeep

http://en.wikipedia.org/wiki/Active_redundancy

http://en.wikipedia.org/wiki/Fault_reporting

http://en.wikipedia.org/wiki/Condition_monitoring



1.1 CONDITION MONITORING METHODS

There are two main methods for condition monitoring namely trend monitoring and condition

checking.

It involves the selection of some suitable and measurable indication of machine or component

deterioration, such as one of those listed in Fig 1.2 and the study of the trend in this measurement

with running time to indicate when deterioration is exceeding a critical state.

The principle involved is illustrated in Fig. 1.1 which shows the way in which such trend

monitoring can give a lead time before the deterioration reaches a level at which the machine

would have to be shutdown. This lead time is one of the main advantages of using trend

monitoring rather than the simple alarms of automatic shutdown devices used in permanent

monitoring. In some cases both methods are used together. Permanent monitoring can prevent

catastrophic failures. As soon as the parameter chosen as representative of the machine condition

exceeds a predetermined level, an alarm informs the personnel concerned and can even stop the

machine automatically. The preselected warning and alarm levels are based on the experience of

the users and in this way it is possible to optimize the life of wearing machine parts. With the

condition monitoring repairs are carded out only when the condition of the machine has

deteriorated to a predetermined level.

Thus, repairs or replacement of parts take place only when it has definitely been proved that a

fault exists and if left unrepaired would result in unsatisfactory operation, e.g. decrease of

production, or catastrophic breakdown with [possible damage to other machine parts and

disruption of production.

Condition monitoring includes three steps:

* DETECTION (when) of the developing fault at an early stage

*DIAGNOSIS (what) of its origin so that spare parts can be ordered

*PROGNOSIS (forecast) - subsequent measurement will then establish the trend and enable the

repair schedule to be planned.

Where the most important economic factor is damage to the machine permanent monitoring

should be employed. Where the most important economic factor is loss in production, CM is

most suitable. Some situations may well best be served by both methods used in parallel.



Condition checking is where a check measurement is taken with the machine running, using

some suitable indicator such as, again, one of those listed in Fig1.2 and this is then used as a

measure of the machine condition at that time. To be effective the measurement must be accurate

and quantifiable and there must be known limiting values which must not be exceeded for more

than a certain number of permitted further running hours.

To fix these values requires a large amount of recorded past experience for the particular type of

machine and this makes the method less flexible then the trend monitoring, particularly if it is

required to give lead time as well as machine knowledge. It can be particularly useful however,

in a situation where there are several similar machines operating together as in this case.

Comparative checking can be done between the machine which is monitored and other machine

which are known to be in new or good condition.

A comparison of methods of Condition Monitoring and Failure Diagnosis are shown in Table

1.1.

In view of the definition of monitoring, this term is redundant. Use of the adjective “trend”

implies that measurements will be made at regular, well-spaced time intervals in order to

determine the long-term trend in a particular parameter.

TREND MONITORING CONDITION CHECKING PROBLEM OR FAILURE DIAGNOSIS

TIMING OF MEASUREMENTS

Readings taken at regular time intervals while the machine is running

Readings taken at one time while the machine is running

When the problem has become manifest or after failure has occurred

QUALITATIVE MEASUREMENTS

Skilled operators can do subjective trend monitoring if they are close enough to their machines

Typical activity of an engineer when checking a machine during operation

When machine is stopped, inspection of components can indicate the cause of problem

QUANTITATIVE MEASUREMENTS

The taking of regular measurements and their recording and analysis gives a lead time on machine problems

Numerate values allow comparison with established standards or other similar machines to give knowledge of machine condition

Measurements may be analyzed in considerable detail to provide guidance on possible causes of the problem

CONDITION MONITORING

TABLE 1.1

A COMPARISION OF METHODS OF CONDITION MONITORING AND OF FAILURE DIAGNOSIS



FIG 1.1 TREND MONITORING



FIG 1.2

THE MATCHING OF METHODS TO THE MONITORING OF MACHINES AND

COMPONENTS



1.2 Benefits of condition Based Maintenance

Maintenance costs are a major part of the operating costs of all manufacturing or production

plants. Depending on the specific industry, maintenance costs can represent between 10 and

40 per cent of the costs of goods produced. For example in food related industries, the average

maintenance cost represents about 15 per cent of the cost of goods produced; while in iron and

steel, pulp and paper and other heavy industries maintenance represents up to 40 per cent of the

total production costs. Discounting the cost of fuel for nuclear units, most power production

facilities in the United States of America also fall into the upper range of costs.

Recent surveys of maintenance management effectiveness in the US manufacturing industry

indicate that one third, 33 cents out of every dollar, of all maintenance costs is wasted as the

result of unnecessary or improperly carried out maintenance. When the high percentage of

operating cost attributed each year to maintenance of plant equipment and facilities is considered

the impact on productivity of maintenance operation becomes obvious.

The main reason for this ineffective and/or inefficient management is the lack of factual data

that quantifies the actual need for repair or maintenance of plant machinery, equipment and

systems. Maintenance scheduling has been, and in many instances is predicted on statistical trend

data or on the actual failure of plant equipment.

Until recently, middle and corporate level management have ignored the impact of

maintenance on product quality, production costs and more importantly on bottom line profit.

The general opinion has been that „maintenance is a necessary evil‟ or „nothing can be done to

improve maintenance costs‟. Perhaps these were true statements ten or twenty years ago when

many of the diagnostic technologies were not fully developed.

However, the developments of microprocessor or computer based instrumentation that can be

used to monitor the operating condition of plant equipment, machinery and systems have

provided the means to manage the maintenance operation. They have provided the means to

reduce or eliminate unnecessary repairs, prevent catastrophic machine failures and reduce the

negative impact of the maintenance operation on the profitability of manufacturing and

production plants.



1.3 Setting up a cm activity

When an industrial establishment is considering whether to apply cm to its plant machinery or

equipment there are a number of aspects which should be taken into account and the following

suggested procedure may be helpful

The management of the establishment should become familiar with the principles of cm

and its possible benefits

They should then consider the various factor which affect the application of cm and

which may be relevant to their situation and also the actual benefits which they hope to

realize (Table 1.1)

The possible financial savings which they might obtain should then be assessed on the

basis of the general financial statistics of the establishment while making a general

financial assessment they also should determine what general organizational

arrangements appear you be appropriate.

After considering the various aspects 1 to 4 the management should have discussion with

their senior engineers who are concerned with the operation and maintenance of their

paint and equipment. the engineers should be asked to make a more precise estimate of

the likely savings and recommend an appropriate level of monitoring

If these more precise figures appear satisfactory, a decision should be made to go ahead decision

to go ahead should involve a commitment for at least three years. The management and staff

should be involved with this project are concerned about its success.

A senior engineer should then be asked to survey the plant and equipment, recommended which

items should be monitored are by what methods and then prepare cost estimates.

A final decision should then be made on the actual expenditure to be allowed and approval given

to order the equipment or services needed and to train the staff required. A particularly suitable

time to start CM is when ne plant or equipment is being introduced. CM can then be brought in as

a complete package.

A considerable amount of management support and administrative work on setting-up appropriate

procedures will be necessary during the first years of operation. This ill be necessary to ensure the

smooth integration of CM into existing operation and the most effective use of the machine data

that is collected.



1.4 IMPLEMENTATION OF CONDITION BASED MAINTENANCE

Implementation of CBM involves:

1. Listing and numbering all machines; in order to have their identification and location.

2. Selecting critical machines-. We decide critical machine when its shutdown would cause

production losses or danger to personnel, this distinction does not depend on quantity but also on

product quality. The degree to which a machine is critical is one of the factors influencing

examination frequency.

3. Establishing programmed and specifying the parts to be examined. The reliability of CBM

depends not only on instruments but mostly on the skill and sense of responsibility of the

examiner who is entrusted with a group of machines.

4. Establishing for each part of the machine the severity limits of the machine condition

parameter (vibration, sound, temperature, contamination etc.) to be measured.

5. Selecting proper examination frequencies.

To arrive at the best frequency of examination, we must consider for each machine-

Its criticality in the process flow chart

The availability of the standby machines

The standardization of items

The operating conditions

The failure statistics (MTBF – MTTR)

The cost of examination

The overall cost of failure

The cost of maintenance

The following formula has suggested in order optimizing examination frequency

e۸t

– ۸t = 1+۸ (ci/cq)

Where,

۸ = failure rate (FR)

T = examination interval

Ci = examination cost

Cq = unitary down – time cost (maintenance cost plus production losses)



This formula is valid only in the cases of a constant FR while in the case of a variable FR the

formula is more involved. Actually empirical methods are applied.

6. Recording data.

7. Training examiner.

The examiners must have a high standard of experience, a deep knowledge of machinery and

particularly endowed with analytical skill however it is necessary to train the personnel for

this job by:

Making clear the aims CBM

Illustrating the principles and working of the instruments

Going through or teaching mathematics or principles of physics essential for the

correct performance of job.

Attending refresher courses on professional matters, like lubrication, bill bearing as

simply, vibration analysis etc.

1.5 CONSEQUENCES OF THE INTODUCTION OF CBM

The role of the maintenance chief changes. We can say that while his most important

duties so forward the coordination and technical aid of his assistants, the planning of

maintenance through a agues work now he becomes really the manager of the service. The

examination team supplies him with all information conceding the conditions of the machine

while other offices will supply him with the necessary information about cost versus budget.

Under CBM regime the maintenance chief takes decisions and coordinates the maintenance

teams on the basis of data.

CBM is involving the lubrication team, in fact owing to its own function, lubrication

service is another very useful source of information as lubrication team systematically controls

the machinery examination and lubrication therefore must be strictly connected as they may

transfer each other information on the conditions of the machine. Flow of information under

CBM is detailed in the Upper side of Fig.1.3

On the whole the organized structure of maintenance because more flexible as it based on

a daily input of date while the short term programming because more important than the long



term one. Besides estimation instead of it on a rigid PM program. Therefore the budget can be

based only on statistical data.

Fig 1.3 CONDITION BASED MAINTENANCE INFORMATION SYSTEM



1.6 THE DESIGN OF INFORMATION SYSTEM

Fundamental aspects of information system (show in Fig 1.3)

With the introduction of CBM are:

Collection and processing of a great quantity of information (never available before)

regarding the condition of each part of a machine.

Necessary to carry out maintenance works within lead time (i.e. in a period of time,

which may be very short, elapsing between the alarm and emergency shutdown)

The above aspects are strictly bounded either to the optimization of frequency of examination of

to the scheduling and programming of maintenance works.

As can be seen from Fig 1.3 the examiner can meet two different situations:

The condition of the machine is near to the lead time; the normal procedure will be

followed through planning office (work requisition – work order with specification

programming carrying out work)

The condition of the machine is already within the lead time (next to the shutdown), the

information will be directly passes to the Forman for emergency maintenance.

In order to let CBM operate correctly it is necessary that the following information are given

to the maintenance foreman:

Condition of machine

Probable part of defective machine

Probable defect

Time durations which the fault to be repaired.



Job report is very important and it is fundamental in order to assure the success of the

new philosophy in fact through the scrutiny and correction of diagnosis versus actual

work it will be possible:

To control the examiners training or his eventual deviations from instructions

To improve the correlation between parameters chosen for condition

measurements and real detects.

To get severity curves specific for each machine.

1.7 SELECTION METHODS OF MONITORING

Inspire of the large number of techniques and the amount of instrumentation that is available,

there are really only five main techniques of condition monitoring and these are:

1. Visual monitoring which involves inspection and recording of surface appearance this

may involve the use of bioscopes to see inaccessible places or the use of photography or

surface imprinting for record purposes.

2. Vibration monitoring which usually involves the attachment of a transducer (velocity

probe accelerometer or proximity probe) to a machine to record its vibration level.

Special equipment is also available for using the output from the sensor to indicate the

nature of the vibration problem and even its precise cause.

3. Wear debris monitoring which works on the principle that the working surfaces of a

machine are washed by their lubricating oil, and damage to them should be detectable

from particulars of wear debris in the oil. If the debris consists of relatively large ferrous

lumps such as those generated by the fatigue of rolling element bearings and gears or the

pitting of cams and tappets, these can be picked up by removable magnetic piles inserted

in the oil return lines. For smaller debris particles, spectrographic analysis or microscopic

ex animation of oil samples after magnetic separation are commonly uses techniques

(SOAP)

4. Performance and behavior monitoring involves checking the performance of a machine

or components to see whether it is behaving correctly. This may, for example, involve

monitoring the performance of a bearing by measuring its temperature to see whether it is



carrying out its function of transmitting load between moving surfaces with the minimum

of friction.

5. Corrosion monitoring which is usually applied to fixed plant containing aggressive

materials is intended to monitor the rate of internal corrosion of the walls of the paint.

This may be done by drilling sentinel holes partway through the wall, which the

corrosion rate is assumed to relate to that of plant.

In the case of the first four methods of monitoring which are applied primarily to various

farms of rotating plant the way in which they are used is that the existence of a problem is

usually detected from the general level of the measurements and its rate of change, while the

nature of the problem can generally be determined from a more detailed analysis of the

measurements obtained this is outlined in Table 1.2.

The methods of monitoring described in table are in effect a mechanism of communication

between a machine and a monitoring engineer. It will be observed that the visual method of

monitoring in relation to the others requires negligible technological backup. This is because

the necessary analytical facilities already exist within the human observer. The lack of these

natural analytical facilities in the other three methods of monitoring is the various techniques

within the main categories of monitoring methods need to be applied. The various techniques

within the man categories of monitoring method are listed in Table 1.3A & Table 1.3B as a

guide to their general selection.



TABLE 1.3A SELECTING METHODS OF MONITORING



TABLE 1.3B SELECTING METHODS OF MONITORING



1.8 FORMALISED ASSMENT OF MONITORING TECHNIQUES

The choice of the monitoring techniques to be applied in a particular plant is difficult by such

factors as skills available effect of the environment reliability of the machines and the monitoring

method etc. At present this decision tends to be made on an abhor base of personal experience or

advice there is however a clear requirement for a formulized approach to be developed.

In the field of NDT where a variety of competitive techniques for surface crack detection

have long been established, the LEO approach has been put forward by Birchon as a simple

formalized way of selecting the appropriate technique by assessing effectiveness. Three separate

factors are considered and then combined to provide a measure of the size of the fault that must

be detected for the technique to be effective.

1. Critical parameter size, L which will cause the particular failure being considered. In the

context of cracks this could be crack depth determined by calculation, experiment or

studies of failures. If the failure were seizure of a rolling bearing it is unlikely that the

size of the damage in the races could be considered. More appropriately it would be a

correlation between vibration or shock pulse level and occurrence of seized bearings.

Thus, L would be the Vibration/Shock pulse level at which experience has shown the bearing

will seize.

2. The experience factor, E, to take account of:

The amount of knowledge possessed of the cause and mechanism of the failure and

The rate of progression of the failure in relation to monitoring frequency.

Both these points of experience amount to a measure of the extent of previous

trouble encountered.



3. The monitoring operational efficiency factor, O, which takes account of:

The sensitivity and reliability of the method and the equipment in relation to the

characteristics of the machine and its environment and

The skill of the monitoring personnel.

0 would vary from one, where failures had been thoroughly monitored, to zero where it had

never been possible to detect the fault in the past.

The product L X E X O provides a measure of the monitored parameter level or change

which the monitored technique must be able to detect to be certain of providing sufficient

warning of impending failure. Unfortunately, although data have been collected for NDT

applications the approach has not yet been taken up for general CM and no exact information is

available on L, E or 0 factors applicable to behavior, wear debris or vibration monitoring.

However, the maintenance engineer will find it useful to develop his own factors for the

technique he has at hand. Some proposals for monitoring operational efficiency factors, derived

from Birchon are shown in Table 1.4

SUBTRACT FROM 0 - 1

Experience of monitoring personnel in applying the technique

None Some considerable

0.2 0.1 0

Conditions of access In relation to the requirements of the machine

Poor Fair good

0.2 0.1 0

Conditions of the environment affecting the technique (noise, dirt, fumes, heat, electrical, Interference, use of protective clothing)

Bad Average good

0.2 0.1 0

Availability of direct comparisons (standard Lubricant debris, vibration Spectra, temperature graphs etc.)

None Some sufficient

0.2 0.1 0

TABLE 1.4 MONITORING OPERATIONAL EFFICIENCY FACTORS



1.9 CASE EXAMPLES

Introduction of vibration monitoring and reduction in the number of pump repairs:

Fig.l.4 shows monitoring effect at a large refinery where there has been a 40% reduction in the

number of pumps undergoing workshop overhaul since 1970. There has been a similar drop

reported in the number of electric motors undergoing workshop overhaul. Such a significant drop

in overhaul work is clear evidence of a reduction in the number of catastrophic failures and

supports the thesis that because of vibration monitoring, more remedial maintenance work is

being carried out in the field. Clearly it must be far less expensive to replace worn or damaged

parts in a machine in situation than it is to carry out major repair work to a machine that has

failed catastrophically. The difference in cost for a typical process centrifugal pump would be

several thousand rupees. For an electric motor the difference in cost could be a lakh or more.

FIG 1.4 EFFECT OF VIBRATION MONITORING ON PUMP REPAIRS

CBM

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