Condition Based Maintenance Strategy
Most equipment failures have no relationship to length of time in-service. Most failures are unpredictable. But if you detect a future failure early, you can handle it most cost effectively before it becomes a breakdown.
Abstract:
Condition Based Maintenance Strategy. With only about 15% to 20% of your equipment
failures being age related, and the other 80% to 85% being totally time-random
events, how can you improve the uptime of your plant and facility? This article explains how to detect the
random failures that make-up the vast majority of maintenance expense and production
downtime by using simple, low cost condition monitoring methods.
Keywords: equipment condition monitoring, random equipment failure, equipment failure patterns
Equipment Failure Probability Curves Showing The Six Time Related Patterns of
Failure
With the introduction in the
1960’s of the Boeing 727 questions were raised about the sense of continuing
with maintenance requirements based on the traditional ‘bath-tub curve
maintenance paradigm present at the time.
Investigations were conduced of
past aircraft maintenance history. It was found that all failures fitted one of six probability (or likelihood of occurrence) failure curves. The USA navy conducted similar investigations and confirmed the findings of the airline industry. The six failure patterns discovered are shown in the Figure 1 below. The traditional
paradigm (Pattern ‘A’) explained 3% - 4% of failures.
Whenever these results have been tested by other parties, their findings have confirmed the validity of the original investigations. It seems clear that with the equipment technologies available in the early 21st Century, equipment failures fit one of the six time–related failure curves in
Figure 1.
Here was definite proof that most failures were not age-related, where the equipment failed because of length of use. It meant that time-based preventative maintenance was pointless in most cases. 'Age-related use' includes fatigue failures (e.g. shafts breaking, springs breaking, boiler tubes leaking), erosion/corrosion failures (e.g. material erosion, metal corrosion), wear-out failures (e.g. car tyre tread wear, packed gland leaks) and other such failures where the length of operating time contributes to the eventual failure.
Non-time related failures were unpredictable! Time in service had no influence on 77% to 89% of the failures. This is not the same as saying that there as no reason for the failure. There will definitely be reasons for a failure, but you cannot predict when there will be a failure based only on the age of the equipment. For the vast majority of equipment you need to base maintenance on non-age related factors.
Most equipment assemblies and components eventually settle into a long period of chance failure. About 15% to 20% of maintenance will repeat based on age-related factors. You will see these in work requests for the same repair again and again over a period of years.
You find time-related failures by looking at your work orders on each item of equipment for as far back as you can and creating a Pareto Chart of its failure history. You can also get good answers by asking your long-serving maintainers and operators what keeps failing on each piece of equipment.
About 80% - 85% of your work orders will happen randomly. You cannot predict a date when they occur. But you can detect that they have started. It is possible to use the changed condition of the equipment to tell when a failure is due.
Equipment Condition Monitoring
Starting from new, a part
properly built and installed into equipment without any errors, will operate at
a particular level of performance, which ideally is at its design
requirement. As its operating life progresses degradation occurs. Please do
not assume degradation is normal and nothing can be done about it. This is not the case. In fact equipment failure should never happen! The acceptance of equipment failure as normal is a total lie.
Regardless of the reasons for
degradation, the item can no longer meets its original service requirements and
its level of performance falls. By detecting the loss-in-condition of the item you have advanced warning that
degradation has started. If you can detect this change in performance level you have a means to forecast a coming failure.
Figure 2 below represents the
‘typical’ degradation process experienced by equipment. Following a period of normal operation, where
the item has been running smoothly, a change occurs that affects its
performance. This change gradually, or rapidly in some cases, worsens to the point that the equipment cannot reliably and safely deliver its duty. If it continues in operation the part will fail and the equipment will stop working.
By using the ‘tell-tale’ evidence
of changing equipment performance due to degradation, you can detect a failure
and act to address it before an unplanned production disruption occurs.
There are many ways to identify a
change in equipment condition. Some
commonly used ones are changes in vibration, changes in power usage, changes in
operating performance, changes in temperatures, changes in noise levels,
changes in chemical composition, increase in debris content and changes in volume
of material. You can be as creative as
you want in developing ways to warn you of future problems.
The most important issue is to spot the tell-tale signs early so that you have time to plan and prepare an organised and least cost correction. Once the equipment is broken you will have to spend whatever time, money and resources it takes to get it back in operation fast.
This explains why the leading companies have created a
‘condition monitoring technician’ position in their organisation and, like the
Oiler and Greaser long used to lubricate equipment and stop bearing failures,
they get the ‘condition monitoring man’ out amongst the equipment looking for
tell-tale signs of coming failure. Such a person will save you a great deal of lost production and frustration.
It is not necessary to spend vast
amounts of money on oil analysis programs, thermography cameras, state of the
art vibration analysis equipment, ultrasonic listening devices and the
like. It is wise to use such technologies selectively when accuracy of results is critical. But you can do a great deal of condition monitoring of mechanical equipment accurately enough yourself with a laser gun
to tell temperature, an automotive stethoscope to hear noise, a low-cost
bearing vibration detector to note change in bearing vibration, laboratory
filter paper to separate debris in oil and a magnifying glass and magnet to
check the debris content plus your own five senses.
When you need expert help for
more accurate results, or a measured opinion on the implications to continued
operation, or the equipment is particularly critical to your business and you
do not have the necessary expertise and skills in-house, sub-contract those
specialities in at the time.
Condition Based Maintenance Strategy
With around 80% of equipment
failures being totally unpredictable based on the equipment’s age, you must
have a maintenance strategy to deal with them.
The around 20% time-based
repetitive failures are addressed by doing preventative maintenance and planned
replacement maintenance. But non-time
related failures cannot be addressed by renewal-based maintenance strategies,
they require different solutions.
If you apply renewal based maintenance
strategies to non-time related failures you will waste about 80% of your money,
time and effort!
With time-random failures the simplest
(but not the only) management strategy to use is to inspect your equipment and
look for evidence of degraded conditions.
You can use a continuous means of monitoring condition by trending an equipment’s
performance graphically (e.g. power use verses throughput), or you can
introduce periodic inspections of equipment condition through observation and
data measurement (e.g. lubrication sampling, temperature measurement, etc).
If condition monitoring is based
on timed inspections, you must set the time periods at a frequency that will
let you spot the change well before the impending failure. Figure 3 shows a frequency inspection period
that will detect the degrading performance well before the failure.
Having discovered the start of a
failure you can prepare for its repair, or put into place strategies and make
changes in its use, to extend the time to failure.
But doing condition based
maintenance will only marginally reduce your maintenance costs. The main thing condition monitoring does for
you is to tell you that you have a problem in time to deal with it in a low
cost way. It does not stop the problem!
There is one more step that you
must now do to drastically reduce your maintenance costs. You must remove the failure mode. Having discovered a cause of failure through
condition monitoring, you must now get rid of it, or else it can randomly occur
at anytime in the future after it is repaired.
Only by removing failure modes will you significantly reduce your future
maintenance.
Please click this link if you
want to read more about ‘defect elimination strategy’.
Further Strategic Maintenance Planning Assistance
Please contact me if you wish more information on any questions
you may have from this article. I can be contacted on the email address found
in the 'Contact Us' page.
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Best regards,
Mike Sondalini
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