WRITEN BY: Erik Hupje, Roadtoreliability.com
Whether you are
developing a new maintenance program. Or improving the maintenance program for
an existing plant. All reliable maintenance programs should be based on the
following Principles of Modern Maintenance:
Principle
#1: Accept Failures
Principle
#2: Most Failures Are Not Age-Related
Principle
#3: Some Failures Matter More Than Others
Principle
#4: Parts Might Wear Out, But Your Equipment Breaks Down
Principle
#5: Hidden Failures Must Be Found
Principle
#6: Identical Equipment Does Not Mean Identical Maintenance
Principle
#7: “You Can’t Maintain Your Way To Reliability”
Principle
#8: Good Maintenance Programs Don’t Waste Your Resources
Principle
#9: Good Maintenance Programs Become Better Maintenance Programs
As a Maintenance &
Reliability professional, you must understand these principles.
You must practice them.
Principle
#1: Accept failures
Not all failures can be
prevented by maintenance. Some failures are the result of events outside our
control. Think lightning strikes or flooding. For events like these, more or
better maintenance makes no difference. Instead, the consequences of events
like these should be mitigated through design.
And maintenance can do
little about failures that are the result of poor design, lousy construction or
bad procurement decisions.
In other cases the
impact of the failure is low so you simply accept a failure (think general area
lighting).
So, good maintenance
programs do not try to prevent all failures. Good maintenance plans and
programs accept some level of failures and are prepared to deal with the
failures they accept (and deem credible).
Principle
#2: Most failures are not age-related
As explained above the
research by the airline industry has shown that 70% – 90% of failure modes are
not age-related. Instead, for most failure modes the likelihood of occurrence
is random. Later research by the United States Navy and others found very similar
results.
This research is
summarised in the six different failure patterns shown below:
Apart from showing that
most failure modes occur randomly. These failure patterns also highlight that
infant mortality is common. And that it typically persists. That means that the
probability of failure only becomes constant after a significant amount of time
in service.
Don’t interpret Curves
D, E, and F to mean that (some) items never degrade or wear out. Everything
degrades with time, that’s life. But many items degrade so slowly that wear out
is not a practical concern. These items do not reach wear out zone in normal
operating life.
So what do these
patterns tell us about our reliable maintenance programs?
Historically
maintenance was done in the belief that the likelihood of failure increased
over time (first generation maintenance thinking). It was thought that
well-timed maintenance could reduce the likelihood of failure. Turns out that
for at least 70% of equipment this simply is not the case.
For the 70% of
equipment which has a constant probability of failure, there is no point in
doing time-based life-renewal tasks like servicing or replacement.
It makes no sense to
spend maintenance resources to service or replace an item whose reliability has
not degraded. Or whose reliability cannot be improved by that maintenance task.
In practice, this means
that 70% – 90% of equipment would benefit from some form of condition
monitoring. And only 10% – 30% can be effectively managed by time-based
replacement or overhaul.
Yet most of our PM
programs are full of time-based replacements and overhauls.
Principle
#3: Some failures matter more than others
When deciding on
whether to do a maintenance task consider the consequence of not doing it. What
would be the consequence of letting that specific failure mode occur?
Avoiding that
consequence is the benefit of your maintenance.
The return on your
investment.
And that is exactly how
maintenance should be seen: as an investment. You incur a maintenance cost in
return for a benefit in sustained safety and reliability. And as with all good
investments, the benefit should outweigh the original investment.
So, understanding the
consequences of failures is key to developing a good maintenance program. One
with a good return on investment.
Just as not all
failures have the same probability, not all failures have the same consequence.
Even if it relates to
the same type of equipment.
Consider a leaking
tank. The consequence of a leaking tank is severe if the tank contains a highly
flammable liquid. But if the tank is full of potable water the consequence
might not be of great concern.
Easy, right?
But what if the water
is required for fire fighting?
The same tank, the same
failure but now we might be more concerned. We would not want to end up in a
scenario of not being able to fight a fire because we had an empty tank due to
a leak.
Apart from the
consequence of a failure you also need to think about the likelihood of the
failure actually occurring.
Maintenance tasks
should be developed for dominant failure modes only. Those failures that occur
frequently and those that have serious consequences but are less frequent to
rare. Avoid assigning maintenance to non-credible failure modes. And avoid
analyzing non-credible failure modes. It eats up your scarce resources for no
return.
A maintenance program
should consider both the consequence and the likelihood of failures. And since
Risk = Likelihood x Consequence we can conclude that good maintenance programs
are risk-based.
Good maintenance
programs use the concept of risk to assess where to use our scarce resources to
get the greatest benefit. The biggest return on our investment.
Principle
#4: Parts might wear out, but your equipment breaks down
A ‘part’ is usually a
simple component, something that has relatively few failure modes. Some
examples are the timing belt in a car, the roller bearing on a drive shaft, the
cable on a crane.
Simple items often
provide early signals of potential failure, if you know where to look. And so
we can often design a task to detect potential failure early on and take action
prior to failure.
For those simple items
which do “wear out” there will be a strong increase in the probability of
failure past a certain age. If we know the typical wear outage for a component,
we can schedule a time-based task to replace it before failure.
When it comes to
complex items made of many “simple” components, things are different.
All those simple
components have their own failure modes with its own failure pattern. Because
complex items have so many, varied failure modes, they typically do not exhibit
wear outage. Their failures do not tend to be a function of age but occur
randomly. Their probability of failure is generally constant as represented by
curves E and F.
Most modern machinery
consists of many components and should be treated as complex items. That means
no clear wear outage. And without clear wear out age performing time-based
overhauls is ineffective. And wasteful of our scarce resources.
Only where we can prove
that an item has wear outage does performing time-based overhaul or component
replacement make sense.
Principle
#5: Hidden failures must be found
Hidden failures are
failures that remain undetected during normal operation. They only become
evident when you need the item to work (failure on demand). Or when you conduct
a test to reveal the failure – a failure finding task.
Hidden failures are
often associated with equipment with protective functions. Something like a
high-high pressure trip. Protective functions like these are not normally
active. They are only required to function by exception to protect your people
from injury or death. To protect the environment from a major impact or protect
our assets from major damage. This means we pretty much always conduct failure
finding tasks on equipment with protective functions.
To be clear, a failure
finding task does not prevent failure. Instead, a failure finding task does
exactly what its name implies. It seeks to find a failure. A failure that has
already happened, but has not been revealed to us. It has remained hidden.
We must find hidden failures
and fix them before the equipment is required to operate.
Principle
#6: Identical equipment does not mean identical maintenance
Just because two pieces
of equipment are the same doesn’t mean they need the same maintenance. In fact,
they may need completely different maintenance tasks.
The classic example is
two exactly the same pumps in a duty – standby setup.8 Same manufacturer, same
model. Both pumps process the exact same fluid under the same operating
conditions. But Pump A is the duty pump, and Pump B is the standby. Pump A
normally runs and Pump B is only used when Pump A fails.
When it comes to
failure modes Pump B has an important hidden failure mode: it might not start
on demand. In other words, when Pump A fails or under maintenance, you suddenly
find that Pump B won’t start. Oops.
Pump B doesn’t normally
run so you wouldn’t know it couldn’t start until you came to start it. That’s
the classic definition of a hidden failure mode. And hidden failure modes like
this require a failure finding task i.e. you go and test to see if Pump B will
start. But you don’t need to do this for Pump A because it’s always running
(unless when it’s off or failed).
So when building a
maintenance program you must consider the operating context.
A difference in criticality
can also lead to different maintenance needs. Safety or production critical
equipment will need more monitoring and testing than the same equipment in low
criticality service.
It’s important to
reinforce that identical equipment may need different maintenance requirements.
This is far too often forgotten or simply ignored for convenience. But you
could find yourself facing critical failures by ignoring this basic concept.
Especially if you use a library of preventive maintenance tasks.
Principle
#7: “You can’t maintain your way to reliability”
I love this quote from
Terrence O’Hanlon and it’s so very true. Maintenance can only preserve your
equipment’s inherent design reliability and performance.
If the equipment’s
inherent reliability or performance is poor, doing more maintenance will not
help.
No amount of
maintenance can raise the inherent reliability of a design.
To improve poor
reliability or performance that’s due to poor design, you need to change the
design. Simple.
When you encounter
failures – defects – that relate to design issues you need to eliminate them.
Sure, the more
proactive and more efficient approach is to ensure that the design is right, to
begin with. But all plants startup with design defects. Even proactive plants.
And that’s why the most reliable plants in the world have an effective defect
elimination program in place.
Principle
#8: Good maintenance programs don’t waste your resources
This seems obvious,
right? But when we review PM programs we often find tasks that add no value.
Tasks that waste resources and actually reduce reliability and availability.
It’s so common for
people to say “whilst we do this, let’s also check this. It only takes 5
minutes.”
But 5 minutes here and
there, every week or every month and we’ve suddenly wasted a lot of time. And
potentially introduced a lot of defects that can impact equipment reliability
down the line.
Another source of waste
in our PM programs is trying to maintain a level of performance and
functionality that we don’t actually need.
Equipment is often
designed to do more that what it is required to do in its actual operating
conditions. As maintainers, we should be very careful about maintaining to
design capabilities. Instead, in most cases, we should maintain our equipment
to deliver to operating requirements. Maintenance done to ensure equipment
capacity greater than actually needed is a waste of resources.
Similarly, avoid
assigning multiple tasks to a single failure mode. It’s wasteful and it makes
it hard to determine which task is actually effective. Stick to the rule of a
single, effective task per failure mode as much as you can. Only for very high
consequence failure modes should you consider having multiple, diverse tasks to
a single failure mode.
Most organisations have
more maintenance to do than resources to do it with. Use resources on
unnecessary maintenance, and you risk not completing necessary maintenance. And
not completing necessary maintenance, or completing it late, increases the risk
of failures.
And when that unnecessary
maintenance is intrusive it gets worse. Experience shows that intrusive
maintenance leads to increased failures because of human error. This could be
simple mistakes. Or because of defective materials or parts, or errors in
technical documentation.
A lot of maintenance is
done with the equipment off-line. So doing unnecessary maintenance can also
increase production losses.
So make sure you remove
unnecessary maintenance from your system. Make sure you have a clear and
legitimate reason for every task in your maintenance program. Make sure you
link all tasks to a dominant failure mode. And have clear priorities for all
maintenance tasks. That allows you to prioritise tasks. In the real world, we
are all resource-constrained.
Principle
#9: Good maintenance programs become better maintenance programs
The most effective
maintenance programs are dynamic. They are changing and improving continuously.
Always making better use of our scarce resources. Always becoming more
effective at preventing those failures that matter to our business.
When improving your
maintenance program you need to understand that not all improvements have the
same leverage:
First, focus on
eliminating unnecessary maintenance tasks. This eliminates the direct
maintenance of labour and materials. But it also removes the effort required to
plan, schedule, manage, and report on this work.
Second, change
time-based overhaul or replacement tasks into condition-based tasks. Instead of
replacing a component every so many hours, use a condition monitoring technique
to assess how much life the component has left. And only replace the component
when actually required.
And third, extend task
intervals. Do this based on data analysis, operator and maintainer experience.
Or simply on good engineering judgment. Remember to observe the results.
The shorter the current
interval, the greater the impact when extending that interval. For example,
adjusting a daily task to weekly reduces the required PM workload for that task
by more than 80%.
This is often the
simplest and one of the most effective improvements you can make.
Reliability
Centered Maintenance references
I wrote this article
based on a number of key sources listed below (and throughout the article). I
strongly recommend getting yourself a copy of Moubray’s book if don’t already
own a copy. And I’d definitely get the NAVSEA RCM manual as it’s well written
and easy to understand:
- • Moubray, J. (1997) Reliability Centered
Maintenance Second Edition. Industrial Press. Available at: https://www.amazon.com/Reliability-Centered-Maintenance-Second-John-Moubray/dp/0831131462
- • NAVSEA (2007) Reliability Centered
Maintenance (RCM) Handbook [S9081-AB-GIB-010]. Available at:
- • Allen, T. M. (2001) ‘U.S. Navy Analysis
of Submarine Maintenance Data and the
Development of Age and Reliability Profiles’. Available at:
http://www.plant-maintenance.com/articles/SubmarineMaintenanceDataRCM.pdf.
- • White Paper (no date) ‘What is
Reliability Centered Maintenance?’ Available at:
https://www.mainsaver.com/pdf/Reliability_Centered_Maintenance_White_Paper.pdf.
- •Wikipedia (2017) Reliability-centered
maintenance. Available at: https://en.wikipedia.org/wiki/Reliability-centered_maintenance
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