Preventive Maintenance vs Corrective Maintenance
At the top level, I see maintenance being either preventive or corrective:
- When we do preventive maintenance we are doing a task before a failure has occurred. That task can be aimed at preventing a failure, minimising the consequence of the failure or assessing the risk of the failure occurring.
- When we are conducting corrective maintenance the failure has now occurred and we are basically reinstating equipment functionality. To be clear, corrective maintenance can be the result of a deliberate run-to-failure strategy.
Preventive maintenance can be defined as “an equipment maintenance strategy based on replacing, or restoring, an asset at a fixed interval regardless of its condition. Scheduled restoration tasks and replacement tasks are examples of preventive maintenance tasks.”
Preventive maintenance (or preventative maintenance) is basically a type of maintenance that is done at a regular interval while the equipment is still functioning with the objective of preventing failure or reducing the likelihood of failure.
Preventive maintenance can be time based i.e. every week, every month or every three months. But preventive maintenance can also be based on usage e.g. every 150 cycles, every 10,000hrs or like your car: service every 10,000km.
Apart from the regular interval approach (time based maintenance) there are also other types of maintenance that fall within the category of preventive maintenance:
In the following paragraphs, I will explore each of these types of maintenance in more detail including when you should consider using them.
Time Based Maintenance (TBM)
Time Based Maintenance refers to replacing or renewing an item to restore its reliability at a fixed time, interval or usage regardless of its condition. This is what Moubray calls Scheduled Restoration or Scheduled Discard tasks in his RCMII book.
I limit the use of that phrase as for some reason people then jump to the conclusion that other maintenance is not scheduled. When in fact of course all maintenance should be scheduled through our Weekly Schedule. The only exception would be Emergency Maintenance, which due to its very nature of requiring immediate attention cannot be scheduled.
The purpose of Time Based Maintenance is to protect yourself against the failure of known wearing parts which have predictable Mean Time Between Failure (MTBF) i.e. Time Based Maintenance assumes that the failure is age related and a clear service life can be determined. Or, that it’s simply not worth the effort to assess the condition and a time based replacement is more economical and still (reasonably) effective.
Time Based Maintenance can never effectively manage non-age related failure modes and therefore should only form a small part of your overall maintenance program as >70% of the failure modes in your plant are not age related (refer to the article 9 Principles of Modern Maintenance).
Failure Finding Maintenance (FFM)
Failure Finding Maintenance tasks are aimed at detecting hidden failures typically associated with protective functions. Think pressure safety valves, trip transmitters and the like.
This type of equipment won’t be required to function until something else has failed. That means that under normal operating conditions you will not know whether this equipment is still functional i.e. the failure modes are hidden.
And since these failures are hidden, you’ll need to find them before you are relying on that equipment to protect you.
It’s important to realise that failure finding maintenance tasks do not prevent failure but simply detect it. And once detected you’ll have to repair the failure you found. Failure Finding Maintenance is conducted at fixed time intervals typically derived from legislation or risk based approaches.
Risk Based Maintenance (RBM)
Risk Based Maintenance (RBM) is when you use a risk assessment methodology to assign your scarce maintenance resources to those assets that carry the most risk in case of a failure (remembering that risk = likelihood x consequence).
As a result, equipment that has a higher risk and a very high conseauence of failure would be subject to more frequent maintenance and inspection. Low risk equipment may be maintained at a much lower frequency and possibly with a much smaller scope of work.
When you implement a Risk Based Maintenance process effectively you should have reduced the total risk of failure across your plant in the most economical way.
Risk Based Maintenance is essentially preventive maintenance where the frequency and scope of the maintenance activities is continuously optimised based on the findings from testing or inspection and a thorough risk assessment. Examples of Risk Based Maintenance would be Risk Based Inspection as applied to static equipment like vessels and piping or even pressure relief valves.
Condition Based Maintenance (CBM)
Most failure modes are not age related. However, most failure modes do give some sort of warning that they are in the process of occurring or are about to occur.
If evidence can be found that something is in the early stages of failure, it may be possible to take action to prevent it from failing completely and/or to avoid the consequences of failure. Condition Based Maintenance as a strategy therefore looks for physical evidence that a failure is occurring or is about to occur. Thinking of CBM in this way shows its broader applications outside condition monitoring techniques often only associated with rotating equipment.
An important concept within Condition Based Maintenance is the P-F curve shown in the figure below:
The curve shows that as a failure starts manifesting, the equipment deteriorates to the point at which it can possibly be detected (point “P”).
If the failure is not detected and mitigated, it continues until a functional failure occurs (point “F”). The time range between P and F, commonly called the P-F interval, is the window of opportunity during which an inspection can possibly detect the imminent failure and give you time to address it.
It is important to realise that CBM as a maintenance strategy does not reduce the likelihood of a failure occurring through life-renewal, but instead is aimed at intervening before the failure occurs, on the premise that this is more economical and should have less of an impact on availability.
In other words: condition monitoring does not fix machines and condition monitoring does not stop failures. Condition monitoring only lets you find problems before they become a failure.
A common rule of thumb is that the interval between CBM tasks should be one-half or one-third of the P-F interval.
How much more effective CBM is above breakdown maintenance depends on how long the P-F interval is. With plenty of warning the rectification can be planned, materials and resources can be mobilised and breakdown prevented (though production is still stopped for the maintenance duration). When the P-F interval is only a few days the resulting organisational and workplace actions are much like a breakdown and the value of CBM is largely lost.
For CBM to be effective as a strategy, early intervention is essential. This requires an efficient and effective process for data gathering, data analysis, decision making and finally intervention.
For failure modes where the P-F interval shows a large variability, condition monitoring is not an effective strategy.
If you're interested to find more about how to best manage failure modes don't forget to check out my article Reliability Centered Maintenance - 9 Principles of Modern Maintenance.
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