Condition Monitoring Methods and Economics - Brüel & Kjær

[Pages:15]Condition Monitoring Methods and Economics

A paper presented by M J . Neale and B.J. Woodley at the Symposium of the Society of Environmental Engineers held at Imperial College, London, September, 1975. It is based on the much fuller report prepared for the Department of In dustry in 1975, which has now been revised to include the latest 1 9 7 8 data and is to be published by HMSO during 1 9 7 8 for public pur chase.

Condition Monitoring Methods and Economics

M.J. Neale

B J . Woodley

Principles and methods

As a starting point for any discussion on condition monitoring it is useful to define what is meant by the term, and to describe how it relates to other techniques used in the operation and maintenance of machines, such as alarm and shut down systems or methods for failure and problem investigation.

The crudest method for operating machines is to run them until they fail, and then to try and repair them in order to make them fit for further service. This method of operation can be very expensive in terms of lost output and machine destruction, and in addition can in volve hazards to personnel. It is now well recognised that, particularly in the case of large and expensive plant, it is more economical and operationally satisfactory to carry out regular maintenance. This involves the maintenance of the machine or its various components at re gular intervals, to reduce the likelihood of failure during a time when the machine is re quired to be available for use. The problem in planning this type of maintenance lies in the choice of an appropriate maintenance interval for the machine, because the actual running time before maintenance is really needed is not constant, but varies from one occasion to another, due to differences in the operation of the machine in the behaviour of its compo nents. Fig. 1 shows how the running time to failure of a typical machine would be likely to vary if no preventive maintenance were carried out. The vertical line in this diagram repres ents the safe time interval between preventive maintenance work which could catch all the failures before they occurred. If this safe overhaul interval is chosen, however, there will be many occasions when the machinery will be overhauled long before it is really necessary, such as in those cases at the right hand side of the curve where it could have run on for much longer without failing. This situation wastes production time, and by increasing the frequency of maintenance operations increases the incidence of human errors on reassem bly of the machine.

A more satisfactory compromise in terms of maintenance strategy is to carry out preventive maintenance at what may be irregular intervals, but to determine these intervals by the ac tual condition of the machine at the time. For such condition-based maintenance to be possi ble, it is essential to have knowledge of the machine condition and its rate of change with time. The main function of condition monitoring is to provide this knowledge.

There are two main methods used for condition monitoring, and these are trend monitoring and condition checking. Trend monitoring is the continuous or regular measurement and in terpretation of data, collected during machine operation, to indicate variations in the condi tion of the machine or its components, in the interests of safe and economical operation. This involves the selection of some suitable and measurable indication of machine or compo nent deterioration, such as one of those listed in Fig.2, and the study of the trend in this measurement with running time to indicate when deterioration is exceeding a critical rate. The principle involved is illustrated in Fig.3, which shows the way in which such trend moni toring can give a lead time before the deterioration reaches a level at which the machine would have to be shut down. This lead time is one of the main advantages of using trend monitoring rather than simple alarms or automatic shut down devices.

1

ro

Number of Breakdowns

average running time between breakdowns longest recorded running time before breakdown

totally safe regular time interval between preventive maintenance work

Running Time Until Breakdown Occurs

Fig. 1. The typical pattern of machinery failure with no Preventive Maintenance

STRUCTURAL COMPONENTS

DETECTING CHANGES IN THE COMPLETE MACHINE

DETECTING DEFECTS IN THE MACHINE COMPONENTS

I FIXED JOINTS

SEALS

Boroscopes Acoustic

emission Resonance

change Strain gauges

and brittle coats Crack detection

Ultrasonics Mag.flux Penetrants

Rattle/Noise Staining

Fretting Leaks

Visual inspection Noise - sonic

ultrasonic Snifting

Wear debris

Noise

Leaks

PERFORMANCE TRENDS OVERALL VIBRATION AND NOISE LEVELS

PERFORMANCE MEASUREMENT

Power Loss Friction Temperature

Noise Vibration

spectra analysis

MOVING JOINTS

CHANGES IN COMPONENT SURFACES

1 1

Surface casts Witness indents Movement or

clearence change Shock pulse

measurement Vibration

signal averaging

1DETECTING DEBRIS

LOST FROM COMPONENT

SURFACES

1

1

i 1

La rge

Small and

particles

dissolved

particles

Oil analysis

Ferrous chip detectors

Non-ferrous filter cYleeks

Fig. 2. The indications of Machine or Component Deterioration

Condition checking is where a check measurement is taken with the machine running, us ing some suitable indicator such as, again, one of those listed in Fig.2, and this is then used as a measure o 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 than 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 mahines oerating together as in this case comparative checking can be done between the machine which is monitored, and other machines which are known to be in new or good condition.

These two methods of condition monitoring are compared in greater detail in Table 1 , and the resulting advantages in terms of the provision of lead time and better machine know ledge are shown in Table 2.

The economics of condition monitoring

The main savings which can be made by the application of condition monitoring to industrial machinery arise by avoiding losses of output due to the breakdown of machinery, and by red ucing the costs of maintenance.

Output related losses can be estimated from the number of days output lost multiplied by the added value output per day. The maintenance costs which can be saved are rather more difficult to quantify, but are likely to relate mainly to the labour costs of breakdown mainte nance. Both these forms of saving have been studied in greater detail in a recent survey car ried out for the Department of Industry (Refs.), and a figure of the order of ? 7 5 0 million per year has been estimated for the maximum conceivable saving which could be obtained by applying condition monitoring across the whole of British Industry. This is shown in Fig.4, which also gives the contributions to this total figure which could be made by the various in dustrial sectors. This suggests that the savings which might be made, on this basis, amount to an average of about 1 % of added Value output with a range for various sectors of from 0,5% to 3%.

Of the total sum of ? 7 5 0 million per year, 65% arises from output related savings and 35% from maintenance related savings. Unfortunately this figure of ? 7 5 0 million per year is not really obtainable as a real saving, because not all industrial plant, processes and establish ments are suitable for the application of condition monitoring.

One method of obtaining a more realistic figure for the likely savings is to identify the indus trial sectors which operate suitable plant machinery, and then to take the savings from these sectors only, in order to reach a more realistic total.

Industrial sectors which rely on machinery rather than on manual work to produce their out put will be particularly appropriate for condition monitoring, and sectors of this type will have a high value of:

Annual capital invested in plant and machinery per employee.

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TIMING OF MEASUREMENTS

QUALITATIVE MEASUREMENTS

*--

QUANTITATIVE MEASUREMENTS

TREND MONITORING

CONDITION CHECKING

Readings taken at regular time intervals while the machine is running.

Readings taken at one time while the machine is running.

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.

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

CONDITION

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

MONITORING

PROBLEM OR FAILURE DIAGNOSIS

When the problem has become manifest or after failure has occurred.

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

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

Table 1. A comparison of Methods of Condition Monitoring and of Failure Diagnosis

1

1

SAF

1

1

ADVANTAGES OBTAINED

METHODS BY WHICH CONDITION MONITORING GIVES THESE ADVANTAGES

Lead Time

Better Machine Knowledge

Reduced injuries and

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LU

Fatal Accidents to

Personnel caused by

Machinery.

Enables plant to be stopped safely when instant shut down is not permissible.

Machine condition, as indicated by an alarm, is adequate if instant shut down is permitted.

1ncreased

More Running

Time

Enables machine shut down for maintenance to be related to required production or service, and various consequential losses from unexpected shut downs to be avoided.

Allows time between planned machine overhauls to be maximised and, if necessary, allows a machine to be nursed through to the next planned overhaul.

Mn nachi ine

Enables machine to be shut down without

Less

destruction or major

damage requiring a long

Availability

repair time.

Maintenance

Enables the maintenance

H a.

1--

Time

team to be ready, with spare parts, to start work as soon as machine

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is shut down.

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Increased Rate of

Reduces inspection time after shut down and speeds up the start of correct remedial action.

Allows some types of machine to be run at increased load and/or speed.

Net Output

Improved Quality of Product or Service

Allows advanced

planninq to reduce the

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effect of impending

breakdowns on the

customer for the product

or service, and thereby

enhances company

reputation.

Can detect reductions in machine efficiency or increased energy consumption.

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Can be used to reduce the amount of product or service produced at sub-standard quality levels.

Table 2. The advantages obtained by the use of Condition Monitoring

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