Precursor: As with all new initiatives,
the basics of change management need
to be in place prior to and during
the process of introducing RCM. Please refer to the relevant section
on this website.
At a fundamental level, the introduction
of RCM within your organization is
best carried out by answering seven
questions about the asset and/ or
its system under review. They are
as follows:
Functions and Performance
Standards [F]: Question 1
The first thing to do is ensure that
the physical asset continues to do
whatever its users want it to do in
its present operating context; by:
• Determining what its users want
it to do.
• Ensuring that it is capable of doing
what its users want.
This is why the first step in the
RCM process defines the functions
of each asset in its operating context,
together with the associated desired
performance standards. There are two
types of functions;
• Primary functions - why the asset
was installed in the first place [speed,
output, carrying or storage capacity,
product quality, customer service
etc].
• Secondary functions - which recognize
that every asset is expected to do
more than simply fulfill its primary
functions [safety, control, containment,
comfort, structural integrity, economy,
protection, efficiency of operation,
compliance with environmental regulations,
appearance etc]
Functional Failures [FF]:
Question 2
The only factor stopping any asset
performing to the standard required
by its users is some kind of failure.
This means that maintenance achieves
its objectives by adopting an effective
strategy to failure avoidance. However,
firstly we need to identify what failures
can occur.
In the world of RCM, failed states
are known as functional failures because
they occur when an asset is unable
to fulfill a function to a standard
of performance which is acceptable
to the user.
Failure Modes [FM]: Question
3
Once functional failures have been
identified, the next step is to identify
all the events which are reasonably
likely to cause each failed state.
These events are known as failure
modes. "Reasonably likely" causes
include;
• those which have occurred on the
same or similar equipment operating
in the same context
• failures which are currently being
prevented by existing maintenance
procedures
• failures which have not happened
yet but which are considered to be
real possibilities
And these will include;
• failures caused by deterioration
or normal wear and tear.
• failures caused by human errors
(on the part of operators and maintainers)
• design flaws
Failure Effects [FE]: Question
4
The fourth step is to listing failure
effects that describe what happens
when each failure mode occurs. This
should include all the information
needed to support the evaluation of
the consequences of the failure, such
as:
• what is the evidence that the failure
has occurred
• in what ways does it poses a threat
to safety or the environment
• in what ways does it affect production
or operations
• what physical damage is caused by
the failure
• what must be done to repair the
failure.
The process of identifying functions,
failures, causes and their effects
generate surprising and often very
exciting opportunities for improving
performance [reducing cost], safety
as well as eliminating waste.
Failure Consequences [FC]:
Question 5
The strength of RCM is that it recognizes
that the consequences of failures
are far more important than their
technical characteristics.
In fact, it recognizes that the only
reason for doing any kind of proactive
maintenance is not to avoid failures
per se, but to avoid or at least to
reduce the consequences of failure.
The RCM process classifies these
consequences into the following groups
Hidden Function – failure
will NOT become evident to operators
under normal circumstances if it occurs
on its own
Evident Function – failure
will become evident to operators under
normal circumstances with four types
of consequences:
• Safety: a failure mode has safety
consequences if it causes a loss of
function or other damage which could
injure or kill someone.
• Environmental: a failure mode has
environmental consequences if it causes
a loss of function or other damage
which could lead to the breach of
any known environmental standard or
regulation.
• Operational: a failure has operational
consequences if it has a direct effect
on operational capability
• Non-operational: any evident failure
not included above.
RCM says the assets’ failure consequences
need to be described for every failure
mode and states in what way the failure
matters in the areas of;
Consequence Risk Assessment:
It is important to be able to assess
the strength and importance of the
consequences of failure. This is done
in an easy analytical way by using
an RPN [risk priority number]
This attaches a numerical value to
the probability, relative to the size
and priority of each significant failure.
These are categorised as; Occurrence,
Severity and Detection and are rated
on a scale 1 to 10.
RCM aims to reduce the RPN as part
of its objective.
This is an example of what the RCM
team could use to identify priorities
alongside consequence, RPN and PCM
[planned condition monitoring] maintenance
policy
Proactive
Tasks: Question 6
Many people still
believe that the best way to optimize
plant availability is proactive maintenance
on a routine basis. Second Generation
wisdom suggested that this should
consist of overhauls of component
replacements at fixed intervals.
This diagram shows the fixed interval
view of failure.
The assumption
is that most assets function reliably
for a period of time, and then wear
out. Classical thinking suggests that
extensive records about failure will
enable us to determine this life and
so make plans to take preventive action
shortly before the item is due to
fail in future.
But this model
is only true for certain types of
simple equipment, and some complex
items with dominant failure modes.
In particular, wear-out characteristics
are often found where equipment comes
into direct contact with the product.
Age-related failures are also often
associated with fatigue, corrosion,
abrasion and evaporation.
However, equipment
in general is far more complex than
it was twenty years ago. This has
led to startling changes in the patterns
of failure, as shown in diagrams above.
…approx 82% of
failure conform to failure patterns
E and F – not age related!
Studies done on
civil aircraft showed that 4% of the
items conformed to pattern A, 2% to
B, 5% to C, 7% to D, 14% to E and
no fewer than 68% to pattern F
Pattern
A is the well-known bathtub
curve. It begins with a high incidence
of failure (known as infant mortality)
followed by a constant or gradually
increasing conditional probability
of failure, then by a wear-out zone.
Pattern
B shows constant or slowly
increasing conditional probability
of failure, ending in a wear-out zone
(the same as the above diagram).
Pattern
C shows slowly increasing
conditional probability of failure,
but there is no identifiable wear-out
age.
Pattern
D shows low conditional probability
of failure when the item is new or
just out of the shop, then a rapid
increase to a constant level
Pattern
E shows a constant conditional
probability of failure at all ages
(random failure).
Pattern
F starts with high infant
mortality, which drops eventually
to a constant or very slowly increasing
conditional probability of failure.
Proactive
Maintenance: Ideas and Guidance
Use the consequences
and risk facts to define the type
and frequency of proactive maintenance
tasks that must be carried out before
failure occurs. These include:
• Predictive maintenance:
i.e. on-condition based maintenance
– important to know what happens once
a failure has started to occur [use
the P-F Curve]
• Preventative maintenance: i.e. scheduled
repair, replacement and overhaul –
the relationship between age and likelihood
of failure
Although many
failure modes are not age related,
most give some warning of their occurrence.
Then we could use on-condition monitoring
to decide when to take avoiding action.
Predictive
Maintenance: Ideas and Guidance
Always choose
predictive before preventive maintenance
because it:
• Can be done
on-line without affecting operations
• Identifies corrective action before
preventive work starts
• Enables the asset to realize most
of its working life
This means carrying
out on-condition based monitoring
[so called because the equipment is
left in service after inspection on-the-condition
it will continue to perform. This
is technically feasible if
• It is possible
to define a clear potential failure
condition using the P-F Curve
• The P-F interval is reasonably consistent
• It is practicable to monitor the
equipment at intervals lees that the
P-F interval.
On-Condition
monitoring: Ideas and Guidance
There are several
hundred techniques available; each
designed to detect potential failures
such as leaks, vibration, temperature
changes, particles, etc.
On-condition monitoring
can be used every minute to several
months depending on the P-F curve
interval. Two major categories are
known as;
• Primary effects
monitoring – readings taken by the
operator or maintenance engineer on;
speed, temperature, flow rates, pressures,
power current and the actual readings
compared with the P-F interval
• Human effects monitoring – where
assessments are made based on the
senses; look, listen, feel, smell.
Scheduled
Repair / Replacement [incl. overhauls
/ turnarounds]
This work is done
at pre-set intervals to prevent age-related
failures.
It is used to
restore the initial capability at
or before a specified age, regardless
of its apparent condition at the time.
This must be applied to failure modes
conforming to patterns A, B and C.
This is technically feasible if there
is;
• an identifiable
age at which the equipment shows a
rapid increase in the conditional
probability of failure, and
• it restores the original resistance
to failure.
Failure modes
conforming to patterns; D, E and F
require caution as there is no relationship
between reliability and age. Scheduled
maintenance can actually increase
overall failure rates by introducing
infant mortality into otherwise stable
systems. It’s the operator who says;
“Its taken us till Wednesday to get
this machine going again”
Prevention
Tasks – Understanding Age and Deterioration
For any asset
to be maintainable the desired performance
of the asset must fall within the
envelope of its initial capability.
The stresses cause the asset to deteriorate
by lowering its resistance to future
stress and eventually the resistance
drops too low – below the desired
performance and the asset fails.
Default
Actions: Question 7
These deal with
the failed state and are the best
option when it is not possible to
identify an effective pro-active task,
and include;
Failure-finding
Fault finding
for hidden failures [proof testing]
means satisfying ourselves that a
proactive devise will work when required
to do so; i.e. checking a pressure
switch by dropping oil pressure and
seeing if the machine shuts down or
activate a fire alarm to check if
it has failed.
This is worth
doing if it reduces the probability
of the associated multiple failure
to a tolerable level. If a fault-finding
task can not be found and the multiple
failures do not affect safety or the
environment, then it is acceptable
to take no action.
Re-design
Re-design may
be justifiable if multiple failures
has costly consequences and is compulsory
for safety or environmental consequences
where proactive maintenance is not
possible.
Run to
Failure
Run to failure
as the name implies entails making
no effort to anticipate or prevent
failure modes to which it is applied
and so those failures are simply allowed
to happen and then repaired.
