CONDITION-BASED MAINTENANCE: A PREFERRED MAINTENANCE TACTIC Mikal van Dyk Research and Development, Pragma

How can the proper application of condition-based maintenance benefit your organisation?

Investigating the monitoring of critical assets to minimise functional failure, downtime and the cost of maintenance while maximising asset lifespan and return on investment.

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Contents1. Executive summary........................................................................................2. Introduction..............................................................................................3. What is condition-based maintenance?...........................................................4. Applications.............................................................................................5. What differentiates condition-based maintenance from other maintenance

tactics?.....................................................................................................6. Asset health..................................................................................................7. CBM – the wrong way: missed opportunities.................................................8. CBM – the right way: critical success factors................................................9. Case studies................................................................................................10. Concluding summary....................................................................................

Through the use of condition-based maintenance methodologies and applications, the risk of functional asset failure can be minimised.

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1 Executivesummary

The failure of critical assets at a facility can be catastrophic. Today few organisations can

afford even a few hours of cease in operation every time an asset malfunctions. It leads to high repair or replacement costs and a loss of operation time, which could result in the late delivery of goods or services. These factors could have a long lasting negative effect on an organisation’s reputation and are usually the result of a lack of tactical maintenance activities or incorrect tactics being in place.

Through the use of effective preventive maintenance tactics, specifically condition-based maintenance methodologies and applications, the risk of functional asset failure can be minimised.Condition monitoring provides the earliest likely indication of asset health deterioration. This produces the longest possible time between potential failure and functional failure. Preventive actions can take place before failures occur, which would otherwise have resulted in a facility shutdown. This ensures that critical assets are kept at an optimal operational level which diminish the risk of failure. However, applied incorrectly – it can result in no benefit being derived from the investment.

In this white paper, we look at why condition-based maintenance is relevant, how to correctly incorporate condition-based maintenance into the current maintenance plans at any facility and how to ensure the expected return on investment.

How to correctly incorporate condition-based maintenance into the current maintenance plans at any facility and how to ensure the expected return on investment.

Fig. 1. Motor current signal analysis

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The health of assets should be a priority in any facility, as it cannot function without its

assets performing their required tasks. Condition monitoring provides an accurate asset health diagnosis which leads to preventive actions being taken as soon as a potential failure is detected.

2 Introduction

It is best practice to have a combination of maintenance methods to effectively manage assets and guarantee the best possible performance and lifespan from each one.

It is however imperative that if capital is invested into a maintenance system, that there are processes in place to ensure that it is applied to the right assets and implemented correctly.

For example condition-based maintenance being applied to a low criticality asset, or an asset that has reached the end of its economic life cycle or even the incorrect application to a vital asset could result in a waste of capital investmentand time.

Condition monitoring provides an accurate asset health diagnosis which leads to preventive actions being taken as soon as a potential failure is detected.

Fig. 2. Infrared inspection

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3 What is condition-based maintenance?

Fig. 3. The P-F curve

Condition-based maintenance is a preventive maintenance tactic which utilises condition

monitoring as a data gathering technique to assess the health of critical assets at a facility. As the name suggests, condition monitoring is the process of monitoring the condition of an asset by using a wide range of technologies which include but are not limited to:

• vibration analysis• oil analysis• ultrasonic analysis• infrared thermography• partial discharge detection

This is done to ensure that all components of an asset are correctly and accurately assessed by identifying potential failures before they can develop into functional failures and attempting to create the largest possible potential to functional failure (P-F) interval in which maintenance can be scheduled and completed.

The P-F curve shown below, illustrates the condition or performance of an asset in comparison to time or usage. The potential failure (P) and the functional failure (F) have been clearly marked, and show the P-F interval in which condition monitoring activities can identify issues which need to take place.

Condition-based maintenance can be applied to any asset which has a failure mode that satisfies the following criteria:

1. The potential failure provides a warning sign 2. The warning sign can be detected 3. The condition of the asset can be assessed

at an interval which is significantly less than the time between potential and functional failures, usually less than half of the P-F interval

4. The interval between detectable potential failure and functional failure is long enough for preventive actions to take place

Com

pone

nt e

vide

nt

cond

ition

or p

erfo

rman

ce

PF interval

Functional failure

F

Point where failure is detectable

Potential failure

P

Condition when new

Point of failureThe PF interval affects the time

available to initiate and perform a

maintenance task

Time / Usage

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The table below contains the five most common condition monitoring technologies, as well as some examples of applications:

4 Applications

Table 1. Condition monitoring technologies and applications

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An example of this would be changing a vehicle’s oil and oil filter every 5000 km or every 6 months or changing the bearings in a pump every 12 months. This tactic is based solely on the probability of failure, and thus has flaws in application. It could be applied far too early and result in the replacement of perfectly good parts before they are required, increasing maintenance costs unnecessarily. Alternatively, it could be applied far too late, resulting in functional failure. Usage-based maintenance does not allow risk to be completely mitigated.

5 What differentiates condition-based maintenance from other maintenance tactics?

5.1 Usage-based maintenance 5.2 Run-to-failure

5.3 Condition-based maintenance

5.4 Selecting the right tactic

Usage-based maintenance is another preventive maintenance tactic which replaces components within an asset, based on time in operation or usage, regardless of whether the component has entered a mode of failure or not. The time between maintenance tasks is determined by assessing the probability of failure based on past failure events. The graph below illustrates the probability of failure against time or usage. Once the asset has been in operation for a time period of ‘x’ or has a usage of ‘x’, the probability of failure will increase dramatically and the component will be replaced to avoid functional failure.

Run-to-failure is an alternative maintenance tactic, which allows an asset to operate until it fails. It is a predetermined tactic, not a ‘do nothing approach.’ It is only applied to assets which experience a constant failure rate. This tactic is only feasible if the consequences of failure are less than the cost of prevention. An example of an asset to which you would apply the run-to-failure tactic is a lightbulb. This tactic does not mitigate risk, but rather accepts and manages it.

Condition-based maintenance is the preferred maintenance tactic, with the above mentioned tactics being implemented if condition-based maintenance is not feasible or if the asset does not have a high criticality.

The following flowchart shows the process of selecting which maintenance tactic to apply:Fig. 4. Probability of failure over time / usage

Fig. 5. Maintenance tactic flowchart

Time / Usage

Probability

Con

ditio

nal

prob

abilit

y of

failu

re

X

TacticCondition-based maintenance

No No

NoNo No No

Yes Yes

Yes

Yes Yes Yes Yes

Usage-based maintenance Run-to-failure Design

Improvement

Review alternatives

Apply design Improvement tactic

Apply run-to-failure tactic

Apply usage-based maintenance tactic

Apply condition-based maintenance tactic

Technicalfeasibility

Decision

Are warning signs detectable

Are warning signs detectable

Is there adequate warning to react?

HSSErequirements Are the HSSE

aspects addressed?

Are the HSSE aspects addressed?

Are the HSSE aspects addressed?

Are the HSSE aspects addressed?

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The benefits of condition-based maintenance cannot be achieved if the tactic is not

properly implemented. Positive results or the expected return on investment cannot be realised if the technologies are not used appropriately and to their full potential.

Each asset type is unique and therefore must be monitored according to a unique schedule utilising methods applicable to it. When planning the frequency of assessments, the P-F curve of the specific asset as well as its history of failure, the environment in which it is operating and its current health level should all be considered.

6.1 Condition monitoring methods

be conducted. By utilising multiple methods of analysis, the exact health status of each component within the pump system can be seen. It also guarantees the accuracy of the assessment results, allowing for optimum performance through follow-upmaintenance tasks.

However, if the pump is not of a high criticality to the facility’s operation or if the pump is at the end of its life cycle – the question must be asked whether it is worth the capital investment and time to implement all of the above mentioned processes? This can be answered by determining the asset care maturity level of the facility and maintenance team. In addition the cost of condition monitoring activities compared to the losses incurred if the pump were to fail also need to be considered

When selecting the condition monitoring methods to apply to an asset, great care must be taken in choosing the appropriate method. For example if we look at a pump system, vibration analysis on the bearings is a good starting point, but will only provide partial insight into the asset’s overall health picture. It could be compared to a person going to a doctor for a comprehensive medical check-up, but only have their blood pressure tested which would not result in a complete health assessment. Assets are similar in their need for multiple methods of diagnoses. In reviewing the example of a pump system - thermographic analysis can be used to find any hotspots on the pump train - both electrical and mechanical, as well as an ultrasound inspection to check lubrication, friction and impacting along with identifying cavitation or impeller issues. Oil analysis for internal wear detection should also

6 Assethealth

Fig. 6. Ultrasound inspection

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Condition monitoring utilises a wide variety of technologies, all of which require proper

training of resources in the implementation, application and the interpretation of the results. The following are examples of occurrences found at organisations that were trying to employ condition-based maintenance – the wrong way:

• Misinterpreting or unusable thermal images due to incorrect camera settings, both because of insufficient training of the maintenance team using the infrared technology.

• Defects detected in MV switchgear by condition monitoring technicians not being corrected because maintenance managers do not recognise signs of deterioration, resulting in functional system failure.

• Defects being found and no corrective steps being taken, as no formal planning and scheduling process follows the condition monitoring inspections.

• High criticality assets being taken off-line, causing planned facility shutdown, to remedy a defect found by vibration analysis, only to find that it was misdiagnosed and the outage was unnecessary. If follow-up inspections had been done on-line utilising other condition monitoring technologies for corroboration, this would have been realised and the shutdown could have been avoided.

• Maintenance managers not understanding the findings in the condition monitoring reports, either resulting in no or inappropriate action being taken.

• Assets which should be replaced being maintained and monitored constantly incurring large expenditure beyond end of economic life.

• Assets found to be in good health still being maintained utilising unnecessary usage-based maintenance measures which is wasteful and creating a potential scenario for erroneous reassembly after maintenance (eg shaft misalignment).

• Execution of interventions triggered by condition monitoring, which are not re-assessed or verified, allowing for the same issue to recur or a new issue to arise undetected.

• Assets only assessed for reliability, ignoring life status, efficiency and performance.

All of the above examples show missed opportunities for full return on investment, wasting the capital investment and time invested into applying condition monitoring inspections.

Condition monitoring utilises a wide variety of technologies, all of which require proper training of resources in the implementation, application and the interpretation of the results.

7 CBM – the wrong way: missed opportunities

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Purchasing the equipment is only the first step in the process towards an effective

condition-based maintenance implementation. Some critical success factors organisations should be cognisant of are listed below.

• Keep an adequate asset register to collect the condition monitoring data of each asset for trending purposes and ensure that it is regularly updated.

• Choose the condition monitoring method by evaluating all asset components and corroborate results with alternate methods to prevent unnecessary shutdowns.

• Ensure that the equipment is properly implemented and adequate inspection intervals have been scheduled.

• Personnel utilising the technologies should be properly trained in the application and interpretation of results.

• A system should be in place to guarantee that inspection tasks are executed on time and that the results are analysed, followed-up on and stored.

• If potential failures or inefficiencies are detected, a system should be in place to: plan, schedule and execute follow-up maintenance tasks including feedback to verify intervention.

• A post-intervention inspection of the asset should always be done to ensure that the potential failure has indeed been remedied and to establish a new assessment baseline. It is also done to ensure that a new issue has not arisen during the intervention process.

8 CBM – the right way:critical success factors

These critical success factors show how to appropriately apply and follow-up on condition-based maintenance allowing for optimal return on investment.

Keep an adequate asset register to collect the condition monitoring data of each asset for trending purposes and ensure that it is regularly updated.

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9.1 Ultrasound leak survey

9.2 Discharge monitoring systemThe two case studies which follow are from organisations that commissioned Martec,

condition monitoring specialists, for a condition monitoring service. The first shows an easy and immediate ROI from which the organisation is still benefitting today and the second shows how the incorrect use of condition-based maintenance will render an investment void.

When conducting an ultrasound leak survey at a bottling facility in Pretoria, South Africa, Martec discovered that approximately 15% of the compressed air being generated at the facility was being wasted due to leakages. Although this is well under the global average of 31%, the savings per annum after being rectified were still substantial (to the effect of ZAR 189,561 in 2012). This saving is directly related to the energy costs of running the compressors in the facility. With South Africa’s high energy supply cost increases (41.6% tariff increase from 2012 to 2015*), the saving has increased annually along with the cost of electricity, equating to ZAR 268,418 in 2015. This is just one inspection method applied to one system in the facility, producing an immediate and recurring return on investment.

At an industrial gas production facility in Pretoria, South Africa, Martec was tasked with the installation of a continuous on-line partial discharge monitoring system on a 10 MW synchronous motor with a rated voltage of 11 kV. This was done utilising capacitive couplers mounted in the motor terminal box, permanently connecting one sensor per motor terminal, all supplying information to a monitor. However, after installation the client did not connect the monitor to an alarm system as suggested. They opted to rather periodically download and analyse the data. After some time, a fault occurred in the motor terminal box, as cables were crossed over which reduced the air gap.

This resulted in active partial discharge (PD) which was detected by the sensors, but not by the maintenance managers, as the downloading and analysis of the PD data was too infrequent. This resulted in the partial discharge destroying the cable insulation, causing a phase-to-phase fault. A perfectly good condition monitoring system was rendered completely useless due to sub-optimal implementation, which led to massive expenses which could have beeneasily avoided.

9 Casestudies

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Condition-based maintenance detects potential failure at the earliest possible stage,

allowing for preventive maintenance actions to take place without relying on probabilities, greatly decreasing the risk of functional failure. By assessing multiple components with different technologies, condition-based maintenance also provides for a holistic asset health analysis.

Condition-based maintenance is an exceptional tactic, which if available in an organisation’s repository can greatly facilitate in the quest for optimum asset health – but only if it is used on the right assets and only if it is correctly applied.

10 Concludingsummary