June 1, 2017
Across industry, many definitions are used when it comes to the different types of maintenance. It can quickly get confusing when people talk about preventive maintenance, condition based maintenance or predictive maintenance but actually have something else in mind than you do. Some people get very excited about these definitions and can spend a lot of time on for example disagreeing with what is and what isn’t preventive maintenance. Let’s not do that, instead, I’ll offer you my view of the different types of maintenance and more importantly, when to use them.
9 TYPES OF MAINTENANCE: FROM PREVENTIVE MAINTENANCE TO CORRECTIVE MAINTENANCE AND EVERYTHING IN BETWEEN
As far as I am concerned terminology is not important. Other than making sure we are talking about the same thing. If what I consider to be condition-based maintenance you call predictive maintenance that doesn’t really matter. As long as we can sensibly talk about the underlying principles.
When to use condition based maintenance. And how to use it.
However, as I’m often asked questions about the different types of maintenance I decided to put a quick overview together of the types of maintenance. At least, the way I see it:
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:
– Time Based Maintenance (TBM)
– Failure Finding Maintenance (FFM)
– Risk Based Maintenance (RBM)
– Condition Based Maintenance (CBM)
– Predictive Maintenance (PDM)
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.
PREDICTIVE MAINTENANCE (PDM)
Up until recently when people spoke about Predictive Maintenance (PDM) this was essentially as a synonym for Condition Based Maintenance. But in my view with the advent of Artificial Intelligence, much lower costs of equipment sensors (IIoT) and machine learning there is clearly a difference appearing between Predictive Maintenance (PDM) and Condition Based Maintenance (CBM), at least in my view.
I see Predictive Maintenance as an extension, a more advanced approach to CBM where we use potentially many process parameters gained from online sensors to determine if our equipment is moving away from stable operating conditions and is heading towards failure.
There are a lot of (very large) companies actively moving into this space and it is certainly a fast-moving and exciting part of our discipline as Maintenance & Reliability professionals. However, I do still believe that even the most advanced Predictive Maintenance approaches need to be underpinned by sound reliability principles and understanding.
CORRECTIVE MAINTENANCE (CM)
A Run to Failure or Corrective Maintenance strategy only restores the function of an item after it has been allowed to fail. It is based on the assumption that the failure is acceptable (i.e. no significant impact on safety or the environment) and preventing failure is either not economical or not possible.
Apart from being the outcome of a deliberate Run to Failure strategy Corrective Maintenance is also the result of unplanned failures which were not avoided through preventive maintenance.
A run to failure strategy can effectively be used for general area lighting, smart process instrumentation (without trip functionality) etc. where the consequence of failure is limited and would not necessitate a need for an urgent repair.
When opting for corrective maintenance as a strategy it is essential to ensure that the failure modes under consideration do not have the potential to become Emergency Maintenance. You see, if you adopt run-to-failure for equipment that once it has failed must be restored immediately to have doomed your organisation to a reactive maintenance environment. A reactive maintenance environment is not where you want to be. It is more expensive, less efficient, and less safe.
So although a run-to-failure strategy can be a good option, make sure you decide wisely.
DEFERRED CORRECTIVE MAINTENANCE
In the chart of maintenance types I broke ‘corrective maintenance’ into two sub-types:
– Deferred corrective maintenance
– Emergency maintenance
And that was very deliberate because it is so essential that we absolutely minimize the amount of Emergency Maintenance we allow into our organisations. As I already pointed out above Emergency Maintenance is expensive, various sources have suggested that Emergency Maintenance is 3 to 5 times as expensive as ‘normal’ preventive maintenance. Emergency Maintenance typically leads to longer equipment outages and more production impact. And it is less safe. So when a corrective maintenance work request is raised it is essential that you prioritise it properly to make sure that where possible you defer the work request and give your team the time to properly plan and schedule the work.
EMERGENCY MAINTENANCE (EM)
Emergency Maintenance is corrective maintenance that is so urgent that it breaks into your Frozen Weekly Schedule (you do have one don’t you?). It upsets your plans and schedules and typically throws everything into disarray. Some people thrive in this type of environment and often get heralded as heroes when they’ve worked 16hrs non-stop to get production back online. But when it comes to the Road to Reliability it is a dead end.
So Emergency Maintenance is the one and only maintenance type that we really want to avoid as much as possible. In fact, World Class organisations ensure that less than 2% of their total maintenance is Emergency Maintenance. How much Emergency Maintenance do you have?
TYPES OF MAINTENANCE: A COMPARISON CHART
The Table below shows a brief summary of:
– the different types of maintenance;
– what type of tasks are involved;
– the objective of the task;
– and how the interval between the tasks is determined.
An efficient and effective Preventive Maintenance Program will have a mix of all these different types of maintenance.
SO WHAT IS BREAKDOWN MAINTENANCE?
And frequently asked question is ‘what is breakdown maintenance’ and as it’s not in my explanation I thought I’d just covered it here briefly. As far as I am concerned, breakdown maintenance is simply corrective maintenance and not another type of maintenance in itself. In the case of breakdown maintenance you’ve had a failure and so now it needs to be fixed. And depending on the risk associated with that breakdown it could be urgent or less urgent.
But, in many people’s mind, breakdown maintenance is urgent maintenance, maintenance that needs to be done right now i.e. Emergency Maintenance. And if that’s the case for you, you know what to do: get rid of it!
WHAT IS THE DIFFERENCE BETWEEN PREVENTIVE MAINTENANCE AND PREDICTIVE MAINTENANCE?
I think I have covered this in the article, but as it’s such a frequently asked question I’ll just summarise the key differences here:
- Preventive maintenance covers multiple types of maintenance that are used before a failure has occurred. Predictive maintenance is a form of preventive maintenance.
- When most people talk about preventive maintenance they really mean Time Based Maintenance which is a repair or replacement on a fixed interval irrespective of the condition of the equipment. The interval can be time-based (days, weeks or months) or usage-based (operating hours, cycles or km).
AND WHAT ABOUT AUTONOMOUS MAINTENANCE?
The above table of types of maintenance does not include Autonomous Maintenance or Autonomous Care (also referred to as Front Line Maintenance in other organisations). The CLAIR (Clean, Lubricate, Adjust, Inspect and Repair) activities conducted under Autonomous Care are essentially a combination of the above strategies, but conducted on a higher frequency by frontline staff.
WHAT TYPES OF MAINTENANCE DO YOU USE?
Leave a comment below telling us what types of maintenance you use and why. Have you had great results with one specific type of maintenance let us know: