Terminology Explained: What is Reliability-Centred Maintenance (RCM)?
In today’s Terminology Explained: What is Reliability-Centred Maintenance (RCM)?
Maintenance has changed over the past 30 years. This is due to a huge increase in the number and variety of physical assets that must be maintained, more complex designs, new maintenance techniques and changing views on maintenance organisation. Reliability-Centred Maintenance (RCM) has played a central role in the evolution of maintenance strategies.
In the past, preventive maintenance was considered the most advanced and effective maintenance technique available. A Preventive Maintenance (PM) program would assume a cause-and-effect relationship between scheduled maintenance and operating reliability. This assumption was based on the intuitive belief that because mechanical parts wear out, the reliability of any equipment is directly related to operating age. It therefore followed that the more frequently equipment was overhauled, the better protected it was against the likelihood of failure.
This strategy comes with a lot of shortcomings:
- Cost of downtime: shutting down systems all time results on loss of production and therefore loss of revenue
- Cost of Maintenance: utilizing the scarce resources to frequently perform unnecessary planned maintenance is expensive;
- Potential for serious safety or environmental consequences: shutting down and starting-up a facility is one of the riskiest operations thus intrusive PM can increase the risks
- More complex systems: the more complex systems became, the more difficult it was to perform Planned Maintenance
In addition to this, research has shown that failure patterns contradict this belief that there is always a connection between operating age and reliability i.e. the more often an item is overhauled the less likely it is to fail. Scheduled overhauls can actually increase failure rates by introducing new infant mortality failures into otherwise stable systems.
- Earliest view: as things get older they are more likely to fail
- 2nd generation: bathtub curve
- 3rd generation: 6 patterns of failure
We discussed these patterns in this other post – History of failure data. For this post, it is important to remember only the 3rd generation, where the 6 patterns are:
- Constant or slowly increasing failure ending in wear out
- Bathtub curve
- Slowly increasing failure probability but no identifiable wear out age
- Low conditional probability of failure when item is new then a rapid increase to a constant level
- Random failures (constant probability)
- High infant mortality dropping to constant or slowly increasing probability of failure
Mobray also refers to studies on civil aircraft to reconcile the number of times each one of these patterns occur:
- Constant or slowly increasing failure ending in wear out: 2%
- Bathtub curve: 4%
- Slowly increasing failure probability but no identifiable wear out age: 5%
- Low conditional probability of failure when item is new then a rapid increase to a constant level: 7%
- Random failures (constant probability): 14%
- High infant mortality dropping to constant or slowly increasing probability of failure: 68%
The book goes on and also states that as systems are getting more complex we should expect to see more equipment items falling in the category E & F. One obvious caveat is that we can’t simply assume that the same proportions will occur in the oil and gas industry. Unfortunately, I couldn’t find any research providing similar results to compare with these numbers.
Anyways, these findings required the industry to develop a technique which could systematically and rigorously assign the correct maintenance strategy to specific systems. That’s where Reliability-Centred Maintenance (RCM) comes to play.
Reliability-Centred Maintenance (RCM)
The objective of a successful maintenance program is to prevent or mitigate the consequences of failures, not to prevent the failures themselves. In simple terms: if the consequences of a failure do not have an adverse effect on safety; operations, environment or cost, then there is no need to carry out scheduled maintenance.
The consequences of failure differ depending on where and how items are installed and operated. A thorough review of failure consequences focuses attention on maintenance tasks that have most effect, and away from those which have little or no effect. In times where maintenance budget is being scrutinised, this helps to ensure that whatever is spent on maintenance strategy is allocated to the most effective tasks.
Reliability-Centred Maintenance (RCM) builds on these ideas to determine applicable and effective maintenance for each failure given the operational context of the equipment being assessed. The technique can be described as a process to help people determine the best policies for managing the functions of physical assets and for managing the consequences of their failures.
I personally like to use a silly example of my mother’s cooking skills. She is an amazing cook! But every time I ask for some help in the kitchen, she is of little assistance. She knows everything by heart – asking questions as she cook one dish is a systematic method of ensuring that I can become a great cook as well. RCM is very similar – we are systematically asking questions to ensure that we learn from other people’s experience and develop the best maintenance strategy.
The Seven RCM Question
The maintenance strategies are generated by answering seven clearly defined questions:
- What are its functions?
- In what conditions can it fail?
- What causes it to fail?
- What happens when it fails??
- Does it matter if it fails?
- Can anything be done to prevent the failure?
- What if the failure cannot be prevented?
The answers must consider the characteristics of the failure under review, the risks that might be involved and the costs that are incurred. RCM achieves greater safety and environmental integrity, improved operating performance, greater maintenance cost-effectiveness and better teamwork. An RCM review results in a comprehensive and fully documented record of the maintenance requirements.
RCM in Synergi Plant
The application supports the definition of many scopes and allow users to quickly identify tasks that are due to start. So RCM does not need to be a revolution, it can be an evolution by analysing packages of higher risk first and move along to next package:
Upon selecting a specific scope, the analyst must move to the screening process. The screening process starts by qualitatively assigning Consequences for 4 different areas: Safety, environment, asset or business loss and image. Then, the analyst must allocate the correct probability of occurrence.
One feature that I find cool is the ability to assign comments to each answer selected – this helps when sharing the analysis with colleagues that were not involved at first or when the analysis is revisited a year later and one tries to understand why a certain option has been selected.
Upon completion of screening process, Synergi Plant will display the risk matrix on the top right, with the associated risk for the package. After assigning the risks to all packages, the user must move to a detailed assessment which involves performing a FMEA/FMECA:
At this page, Synergi Plant will only display packages with risk categorised as Medium and High. This follows the RCM mindset of allocating the right resources to right maintenance tasks – if the risk is low, let’s not focus on that. We discussed FMEA/FMECAs in the context of Maros/Taro in this post – From FMECA to RAM studies.
The tool will load all functions associated with a specific equipment item and list the functional failures, failure modes and the impacts/effects. This is part of a library which helps to speed up the RCM process. In theory, the more RCM analyses you carry out, the more data you will have in the library and the faster you will be able to complete a package assessment.
The next step is then scrutinising each functional failure by following the process of:
- Assign a Criticality
- Using a Logic Tree Analysis (LTA) to assign the impact
- Using a Task Selection Road map Flow Chart (TSRM)
- Assigning a recommend PM, if necessary
Synergi Plant will finally link all this information to an approval process which is the final step of the analysis.
Once the process has been finalised, the results of the RCM is display in one of the dashboards available in Synergi:
RCM is a fantastic tool. As mentioned before, in times where maintenance expenditure is being constantly challenged, understanding where investment should be placed is fundamental to ensure that everybody goes home safely in the end of the day.
Would you like to read some more?
- SAE JA1011
- SAE JA1012
- NASA Guide for RCM
- ISO 14224:2016 – Petroleum, petrochemical and natural gas industries — Collection and exchange of reliability and maintenance data for equipment