To Maintain or to Replace

Hi,

One question that plagues the mind of maintainers is the question of replacing equipment as they age, sometimes gracefully, sometimes catastrophically.

The current trend and availability of condition monitoring techniques can save the blushes for the maintainers as regards critical equipment.

Safety is the most convincing factor to seek replacement of equipment. Unsafe equipment / structures should be replaced without too much discussion since one safety incident could be more detrimental to the whole business than  the cost of replacement.

We could first categorise equipment using  a standard ABC analysis on “Cost aspects”:

  • Category A will make the bulk of low-cost items. Probably these could be replaced periodically as per some agreed parameters. Let us take the case of common battery chargers in service for charging various types of large storage batteries or traction batteries. We may replace these, whenever the items go faulty.
  • Category B items would be of medium cost, but would be lesser in numbers. This is the category that would cause some head ache in the “Maintain or Replace Conundrum”.  Let us consider the large number of motor driven pumps of 5 HP rating and above. The purchase cost is sufficiently high to call for some deliberation on the maintain or replace question, but not critical enough to take too  much of management time. If the population of such pumps are high in a facility, It may be worthwhile holding “hot spare switchgear-motor-pump assemblies” that may be used to replace a faulty assembly.
  • Category C list will contain the lowest numbers, but of high cost items. Much thought will be required in making the decision to replace in view of the capital expenditure involved. Let us consider a 1000 KVA Diesel Generator. Barring a catastrophic failure, it will have a standard life expectancy depending on the usage pattern and conditions of use. Simple condition monitoring techniques such as visual inspection, parameter variation analysis, oil spectrometry analysis and touch & feel of the equipment to understand changes in vibration levels would help in understanding the changes in conditions. Use of vibration monitoring and analysing equipment and thermal imaging will help in pinpointing problem areas and deciding on maintenance requirements. Planning for a replacement over a period of a few years is possible in this case.

Now, categorising equipment using  a standard ABC analysis on “Criticality aspects”:

  • Category A will again make the bulk of the list, made up of low criticality items such as small pumps, electrical switchboards, battery chargers, small capacity window type air conditioners, small capacity UPSs, small battery banks etc.
  • Category B will be the medium criticality items. They may not be show stoppers, but still could impact operations in a way. Multiple conveyor lines in which even if one fails, the product line does not stop.
  • Category C will be the high criticality items or a “Show stopper”. These need not necessarily be the most capital-intensive equipment in the facility. For example, a 50T EOT may not be as expensive as the sheet metal  presses and hammers in an automobile body parts stamping shop. The EOT could be a show stopper if it is not able to move the required pressing and hammering dies to and from the presses and hammers.

Combining the above two aspects, we will get a well categorised list of equipment that will help us in deciding on the maintenance and replacement policies.

Obsolescence

Electronic / Microprocessor based equipment go through obsolescence cycles periodically. Even core electrical and mechanical equipment may go through this process, but less frequently. The OEM will replace existing equipment with new designs time-to-time and declare the existing equipment not maintainable. This is another aspect to be considered in the replacement decision.

Take the case of UPS equipment. Every 5 to 8 years, such equipment may become obsolete. The batteries connected to the UPS will follow a different replacement cycle – more dependent on its own efficiency, discharging – charging  pattern and operating temperature.

Reliability

Reliability of equipment and systems is another aspect to be considered in the replacement decision.

The bath tub curve giveBath Tub Curve for Reliabilityn on the left  will provide some general guidelines to follow on the reliability aspects.

Apart from the infant mortality failure, equipment failure rates stabilise over a period and then start to increase with age.

Periodic maintenance scheduled at the right intervals will increase the stable period of operations.

With the current modular designs, changing of critical, comparatively shorter-life sub assemblies would stabilise the overall equipment / system reliability.

Mean Time Between Failures (MTBF)

This could be a part of the reliability study. For critical and capital-intensive equipment and systems, the analysis of MTBF could give indications on whether an equipment or sub-assembly is reaching the replacement threshold.

This study could be extended to those equipment with large population also. The analysis would aid in deciding on the make and model of equipment to choose as replacement for the failing ones.

Total cost of maintenance

Every maintenance activity consumes resources in the form of material, labour and overheads. Life cycle costing of equipment will provide insight into whether we need to re-think on the maintenance strategy.

  • In the case of low-cost items, it may be better to choose a work to failure strategy. Replace the item, every time it fails. Try and reduce the instances of failure by choosing a better make or model.
  • In the case of all other items preventive / predictive / Condition based maintenance strategies could be used. The conditioned based maintenance strategy will have some added monitoring costs, hence may be limited to critical equipment.

As and when the total maintenance cost gets close to the cost at purchase of the equipment / system, analyse all other aspects and decide on the replacement.

Benefits of having a good CMMS

The discussion above has indicated analysis of a variety of data while making the “Maintain or Replace” decision. A good CMMS suite will help the maintenance team to record all possible data on maintenance cost, breakdown, failure pattern, inventory cost, inventory carrying cost, equipment replacement cost, total maintenance cost etc. Analysis of the data will help in making an intelligent decision.

Thanks

KayCee

 

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