2. Functions of maintenance
• To maintain
• Keep in existing condition
• Preserve, protect
• Keep away from failure or decline
3. Classification of maintenance
Preventive maintenance –
Corrective maintenance
Function of preventive maintenance
Prevent/ Mitigate the failure occurrence
Detect onset of failure
Discover the hidden failure
Do nothing, because of valid limitation
4. Preventive maintenance categories
Time directed- The equipment is restored periodically at fixed time intervals.
The intervals may be specified like clock time, cycles, and calendar days,
seasons or prior to some events. Overhauls are preferred methods for this.
Condition directed-find some parameter which co-relates to incipient failure.
By measuring this parameter over the period of time onset of the failure is
detected. Vibration, temperature etc
Failure finding- Many equipments or systems may experience the failure and
in normal course of operation no one would know that failure has happened.
Backup system, emergency systems and infrequently used systems may feel this.
These are called hidden failures. These systems are periodically operated just to
check if it is in working order.
Run to failure RTF- Doing nothing for preventive maintenance deliberately
because of-1. We can’t find any PM task that will do any good irrespective of
how much money we spend. 2. The potential PM is costly than repair when it
fails and has no safety impact. 3. The equipment failure is too low on priority to
provide PM as per budget.
5. CD (condition directed)
Versus FF (failure finding)
The equipment goes into the operation at TA and runs
flawless till TB. At TB the failure mechanism initiated.
If the failure continues to develop it can ultimately
reach to TC where equipment may fail. At TD the
equipment will be repaired and recovered to service.
If TC is well beyond the TB we have time to conduct CD.
If TC >> TB , CD possible. But if TB =TC (Electronic
items) no time for CD.
If TC=TD , The operator knows that something is wrong
that is failure mode is evident. but if TD>TC the failure
is hidden.
To summarize between TB and TC we can define CD
task, between TC and TD we can define FF task if failure
mode is hidden
6. PM practices and Myths
Most of time it was observed that its hard to find the origin of PM
activities.
Failure prevention- There is myth that all failures can be prevented
and this motivates us to major/ minor overhauls without any
understanding of failure mechanism. Which wastes a lot of money and
becomes even counter predictive in lots of cases due to arising human
errors?
Experience- Most common answer given to justify the PM is heard
experience, as it has been done from 15 years so it must be good.
Experience is great but has to be used in a logical framework.
Judgment- PM Activities initiated by judgments like, I think, this
might be good, I think but I am not sure. If these judgments gone
wrong they lead to big damage and money waste.
There are some more like recommendation from OEM, regulations etc.
7. Reliability Centered Maintenance
In 1960, 747 aircraft was manufactured in Boeing factory. The licensing
authority recommended the extensive preventive maintenance which was 3
times than earlier 707 as 747 was 3 times bigger. It became economically
unviable to operate by airlines.
United airlines created a group of specialists who conducted a complete
review of why maintenance was done and how it should be accomplished.
This new technique was defined as MSG-1 (maintenance steering group).
These ideas were adopted by Navy for their all aircrafts and they called it
Reliability centered maintenance.
In 1983, Electric power research institute (EPRI) studied these to apply for
nuclear power plant.
8. RCM Features
Preserve system function (and not equipment)- we must preserve the system function
and try to know what is the expected output of it. Preserving the output is primary work.
We must not assume an equal priority to every item of the function.
Identify the failure modes that can defeat the function- functional failures come in
many shape & sizes, it’s not always a yes or no. A pump supplying water at certain pressure
to a boiler system. The system loss of fluid can be 1- minor leak, dripping, 2- A leak which
will drop the pressure beyond a limit to create negative impact on the system, but system
still operating. 3- Total loss of fluid supply. So a single function gives rise to 3 distinct
failures.
Priorities function needs via failures modes- since our aim is to preserve function, all
functions are not created equal. Thus we need to prioritize the importance of failure
modes for assigning the resources. This is done by passing three ties decision tree for each
failure mode.
Select applicable and effective PM task- address all failure modes in their prioritized
order to identify the candidate PM actions. After listing all activities each has to be judged
to be applicable and effective. Applicable means if task is performed it will prevent, detect
or discover the failure. Effective means it is worth to spend the resources for conducting
PM.
9. RCM Methodology-
RCM requires a comprehensive system analysis to
implement the four features.
System analysis is a seven step process developed from
experience as a convenient way to delineate the
required information.
10. 1.System selection and
information collection
A plant or facility has so many systems. For selecting the system 80/20
rule can be applied. Some basic questions can be asked to select the
system. Like..
A) cost of corrective maintenance actions.
B) Number of corrective maintenance actions.
C) Number of hours attributed to plant outages.
Answers of the above questions will guide us to know who are the bad
actors in the system and need the immediate attention. After selecting
the system, information like drawing, design details, block diagrams,
vendor manuals, and equipment history etc must be collected.
11. 2.System boundary definition
There must be precise knowledge of what components has
been included in the system so that an accurate list of
components can be prepared and there is no overlapping of
systems.
Boundaries will be important in determining what comes
in and what goes out of the system.
12. 3.System description and functional
block diagram
System description- key functional parameters, redundancy features,
protection features and key control features are be recorded. At this
level if the system is very big, can also be divided in subsystems.
Functional block diagram- from the knowledge of description a
simple block diagram needs to be developed. it will help in quick
analysis.
IN/OUT interfaces- interfaces are to be defined at the block diagram.
System may have many in/outs so interfaces must be defined.
System work breakdown structure (SWBS)- list of all components of
the system and their interfaces, related drawing are to be recorded.
Equipment history- equipment history available will be of great help
so it must be recorded as well.
14. 4.System function and
functional failures
Sine the RCM emphasizes on preserving the system so
now at this stage a complete list of system functions
must be prepared. After preparing the list it can be
listed out that how these systems can fail or what will
be the failure mode.
15. 5.Failure mode and effect analysis
Failure mode- this is a brief description of what is wrong like jammed,
worn, frayed, cracked, split, open, torn and so forth.
Failure effect- it is the consequence of failure. It can be analyzed on
local, system and plant level.
From the understanding of failure modes and their effect on different
levels a decision has to be taken further. In one system different failure
modes may have same failure effect.
Failure mode must depict a problem that can be realistically addressed
with PM. For example we never do a PM electronic chip.
Redundancy- if a failure mode can be preventing from redundancy the
priority will not be given to that failure mode.
16. Now since we have listed out
all the functions of the system
and failures, we will try relate
the functional failure with the
system components.
We can use the adjacent
matrix for this purpose. Here
will identify the components
which have potential to crate
one or more failure modes.
As per the matrix all
components are related to one
or more failures. If any
equipment is not related to
any failure, that means either
there is mistake in analysis or
we have discovered a
component that plays no
useful role in system.
17. 6.Logic (decision)
tree analysis
The failure modes which have passed
the earlier screening will again be
checked against the logic tree to allot
the resources.
When the LTA is concluded every
failure mode is either classified as
A,B,C,D/A,D/B, or D/C.
The failure modes in A or B either
hidden or evident have priority over
C. Priority can be as below.
1. A or D/A
2. B or D/B
3. C or D/C
The failure modes in bin C are usually
left for RTF or for status only
instruments if such is possible.
18. 7.Task selection
After gathering all the information about the failure modes, effects and
checking them on logic tree we arrive to failures modes for which PM is
to be done.
The PM task should be applicable and effective. Applicable means
a task which can mitigate the failure and effective means cost
effective among all other options
If no applicable task exist, then only option is RTF
If the cost of PM exceeds the cumulative cost of associated failure,
then effective task option will be RTF
A below roadmap can be followed to further decide the PM task
19.
20. Task Interval & age exploration
Selection of correct interval at which to perform the preventive
maintenance task is the most difficult job. For deciding the task
interval we need to have detailed knowledge of failure
mechanism and age reliability relationship.
Based on the available information we always take a conservative
decision regarding the task interval. But after the we need to
continually monitor and try to improve our knowledge about
failure to explore the age-reliability relation.
Every time we go for performing the TD task we need to inspect
the condition, take photograph and document it for further
study for evaluating the decision of task interval.
Task interval can be increased or decreased based in the input.
21. Age exploration in
United airlines
Age exploration of
hydraulic pump is united
airlines. Initially overhaul
started at 6000 hrs and in 4
year through age
exploration the interval
increased to 14000 hrs.
At the same time the
premature removal has
decreased substantially.
