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“Predictive Maintenance (PdM)” or “Preventive Maintenance (PM)”

The proponents of PM and PdM have been debating for long over which is more beneficial and effective at reduction in maintenance costs while fulfilling the main goal of maintaining equipment reliability. Let us look at some of the advantages:

·         Reduction in unplanned outages

·         Reduction in failure rates

·         Increased availability

·         Better reliability

·         Saving in maintenance costs (mainly outage costs)

These benefits are not marketing jargons but has time and again been proved and several statistics are already available over different surveys conducted by eminent bodies (eg. CIGRE, IEEE etc.). However, theyseem to accrue from both types of maintenance.

Predictive Maintenance (PdM) is generally taken to mean “Assessing machine condition through acquiring/analysing data on key parameters, during its operating condition and anticipating a failure”. So, the key in above definition is “during its operating condition” and most of the time this is to mean “Online” assessment while the equipment is running, and you will find several articles on the Net, where PdM is mostly about vibration and noise, thermography and possibly extending into lube oil analysis.

Primarily this is the most “visible” area in PdM which the maintenance guys can relate to. Alignment with load, soft foot corrections, bearing replacements are most common maintenance activities and hence vibration and noise are well understood. So, many plants have their own in-house teams for such PdM programs (vibration analysis). These teams conduct measurements on a periodic basis and survey a wide range of mechanical equipment. Also, now with better development in wireless and internet technology/IoT, most industries either have upgraded or are contemplating upgradation to a continuous monitoring system with mobile alarms/alerts.

So, the role of PdM then is to highlight machines or components that show a defect that can manifest into a failure. But PdM by itself will not help with above benefits. It is just an observation and a recommendation! In most scenarios, unfortunately, these recommendations occupy the shelves in Managers cabin. In good scenario, the recommendations are used to determine a PM schedule. In best scenario, these go into AMPs (Asset Management Program) or CMMS (Computerized Maintenance Management System)where there are deeper learnings, knowledge sharing about equipment or the systemand is linked to business objectives, spare procurement /inventory management, generating purchase requirements/work orders etc. Importantly, they can quantify all these efforts into monetary value.

What is ignored is that PdM can also tell you which machines are in good condition and does not merit any service. What does this mean? It means “Money saved”. But ironically many maintenance managers fail to realize the importance of a report that says, “Your machine is absolutely in good condition”. They feel sceptical - all that money spent on PdM has been down the drain – the results should have pointed out some defect – ain’t that an objective of PdM?

It’s like going to a doctor saying, “I kind of feel low, give me some medicine” and the doctor inspects you (even takes a good fee) and says, “There’s nothing wrong with you. Just go home and sleep and you are fit”. So, the mind says, “What! If I am good, why did I pay that fee for?”. It’s time to rejoice! Your machine is in good condition. Not only you defer its maintenance and save on cost, you can use that budget for other priorities. You can showcase “money saved” by not taking an outage or not having failures. You can highlight that your maintenance staff is very effective and deserve a bonus! Ok if not bonus, at least a pat on the back.

Preventive Maintenance (PM) is performed while the equipment is under normal operation to avoid unexpected breakdowns and the associated downtime and costs. During the specified date and time, equipment is shut down and PM tasks on that piece of equipment are performed. Maintenance is set on a schedule based on calendar dates or usage, often at the recommendation of the manufacturer.

So, the important aspect is the warranty issues, spare or service support issues with manufacturer or legalities of the insurance in the sad circumstances of failure backed up by proof that PM was prescribed by the manual, but was not conducted in the given schedule – blasphemy! So, if the bearing needs to be changed after x hours, change it. The new bearing will definitely outlast the old one, which may be in good condition presently. This may be a good rationale and justified too, but definitely, it is a factor that makes PM a costlier proposition.

Also, PM advocates claim that “Condition Monitoring is all gimmick and fancy” or “Flavour of the month”. These sentiments are often nurtured as some of the technologies are far too advanced and complex –Fourier Transform, Demodulation, Neural networks, fuzzy logic, AI, Machine Learning. Wow! The reports, however graphicalthey may be, are beyond the realms of the person who changes bearing or does alignment. Many maintenance personnel argue, “What is the use of incurring additional costs in PdM when the recommendations will be to change the bearings or overhaul the motor. We were going to do the same anyway”. How many of you just said “Bingo!”?

PdM advocates say that it has an edge over PM as the costs are much lower than PM while the benefits are further fine-tuned. For e.g. If the condition of component or asset is good, it need not be repaired/replaced, resulting in more savings. “If ain’t broken, why fix it?”. The other common sentiment is “Machine was running nicely but now after assembly, something has been messed up and machine has higher vibrations/noise”. Now how many of you said “Bingo!”

But on a happy note, everyone does agree that “Breakdown Maintenance” is a strict NO, except for small value assets where replacement is much easier and less costly option than maintenance.

When it comes to electrical equipment maintenance – say a motor, generator or transformer, PdM with only vibration/noise scope is only a part of the story. Consider a scenario, where the main motor fails after having all the relevant processes, tools, manpower (read that as investment in PdM) in place. To the top management, the failure has resulted in loss of production plus losses in present batch – sudden unexpected outflow of cash, lost revenue and profitability - even after allocation of maintenance budget. To the credit of Maintenance Manager – he had conducted the measurements and analysis at correct schedules and there was no cause of alarm from the reports. On further investigation, one sees a Stator bar failure! So obviously, this may not reflect in vibration signals.

So, would a PM help in this case or PdM?

For such insulation diagnostics, PdM is available in form of conventional IR-PI checks, which if done as spot measurements are not very effective. Tan Delta & Capacitance measurements with tip-up calculations, partial discharge measurements, surge comparison, balance of resistance/impedance, DC HV Step voltage tests, Charge-Discharge Current Profiling (better than IR/PI) are some tests that can give effective assessment as a package of tests and it is not prudent to skip one due to reasons like lack of budget.

But these tests are mainly outage based which further intensifies the debate.

Is it wise to plan an outage for doing these tests and later another planned outage for PM? For plants that have standby motors, it is possible to do a PdM and maintain a tab, especially as condition of machine can be affected even when it is not operational.

Combining the power of PdM and PM

I guess is the answer and a change of perspective. If PdM, by itself, cannot increase reliability, why is it considered as a separate maintenance approach? Shouldn’t PdM be considered as an integral part of or as a subset of PM?

This works wonders for a plant that has several assets but limited outage times and budget per year. So, while a PM activity must be scheduled, do it around the first time as per the manufacturers recommendation. Also do the offline PdM assessments like test mentioned earlier or say a visual inspectionin same outage. Conduct the online assessment before and/or after the outage. Consolidate the results to determine the frequency of next PdM schedule and base the next PM schedule based on the condition of equipment or the component, instead of time.

So, if a motor is say “clean” and bearing condition is “good” (as confirmed from reports) defer the PM activity for that motor to later than that indicated by time based maintenance. Conversely, if the results indicate any anomaly, pre-pone the PM activities.

The other advantage of doing the PdM with PM i.e. say doing insulation diagnostics in offline condition after the cleaning activity, can have multiple benefits too.

·         It will help assess if the PM activities were done effectively. E.g. Say a drop in PD levels can help assess that the overhang clean-up was effective.

·         Any defect shrouded by previous condition can be revealed during PM or with PdM measurements. For e.g. Often ageing conditions is difficult to diagnose for a contaminated machine. So, after removal of contaminants, the measurements can help in correct assessment of insulation degradation.

·         The reverification by PdM may be several times less costly than opening the motor for cleaning only to find that it is clean.

So, while the PM activity saves cost by way of reducing failure, the PdM integrated with PM further fine tunes or optimises the maintenance costs by focusing only on machines or components that need attention.

Thus, PdM and PM are not rivals but pitched together can make a winning team.

Ameet Choughule

www.powerelectronical.com