MAINTENANCE STRATEGY SELECTION
Indian
industries have transformed from the past & focus has been shifted from manual production system to large scale maschine assisted production system. Our industrial technicians & engineers manage huge asset base which
have strategic importance for the organization. Safety & reliability of the
assets / personnel / environment is of prime importance. Keeping the equipment’s production
ready is main goal. This can be achieved only by adopting effective
& efficient maintenance strategies. Certain factors like fault and failure
mode analysis, asset base criticality ranking and adoption of correct
maintenance strategy as per the nature of asset will not only increase the
performance & availability of asset but also reduce the cost &
maintenance time to a great extent. Using the wrong maintenance technique results
into wastage of time, money and resources, and often has no effect on improving
or maintaining availability.
The following topics
are covered in this blog-
·
Introduction of different maintenance
strategies.
·
Key features, advantages & disadvantage of
different maintenance strategy.
·
Selection & Implementation of right
maintenance strategy for different asset kind.
Our industries have various civil, mechnical, electrical & electronics assets. Efficient maintenance of the
equipment’s improves the availability, life & reduces maintenance cost. Every year a budget is allocated for operation & maintenance. A portion of the budget is
allocated for the maintenance of these assets on annual basis. The efficient
operation & maintenance of these assets can be crucial for the upkeep of
above equipment’s for critical operations. The requirement of effective
& efficient maintenance strategies is not only critical in terms of money
but to a great extent for the reliability of the system. If we
categorize the equipment’s / systems, they can be classified into three
categories-
1. Rotating
assets like motors, pumps, blowers, compressors.
2. Static
Assets like control systems, associated electronics, electrical system etc.
3. Civil Structures.
The basic driving force behind
adoption of type of maintenance strategy is business objectives which in turn drives the technologies and solutions
applied to meet desired goals. As discussed earlier, the top most business
objective of the any industry is reliability / safety of the equipment’s &
personnel, environment & the secondary objective is to reduce
maintenance cost & time.
So in nutshell the effective strategy is to
determine which maintenance activities will be performed on which assets, its
frequency & its scope to meet the preset objectives? Out of all equipment’s
listed above, maintenance of a rotating asset is most costly. It has different components
like motors, turbines, DG sets, pumps, compressors, blowers etc. Most of the assets have been in long operation
& it’s expected to have a proper maintenance and life extension plans till
they are replaced with new superior assets. Adoption of suitable maintenance
strategies results into deployment of adequate technology/solutions & overall
an improved business performance.
The four main maintenance strategies adopted by
the industry are:
·
Reactive
Maintenance (RM)
·
Preventive
Maintenance (PM)
·
Predictive
Maintenance (PdM)
·
Proactive
Centered Maintenance (PCM)
Reactive
Maintenance: Reactive maintenance is a fire
fighting approach where maintenance is performed after a failure of the asset. In this strategy, machines are operated in a
run-to-failure (RTF) mode and the maintenance is carried out only after the
functional failure of the equipment.
Preventive
Maintenance (PM): Preventive maintenance
strategy is a template based strategy where periodic maintenance is carried out
as per OEM recommendation / organizational recommendations. The schedule of the
intervals is based on average statistical/anticipated lifetime to avoid failure.
This includes inspection, service / replacement of the defective parts of the
machine.
Predictive
Maintenance (PdM): This maintenance based on the
actual asset condition gauged by objective evidence of probable problem obtained
from on site, non-invasive tests and operating and condition measurements. This is also known as Condition Based
Maintenance (CBM).
Proactive-Centred Maintenance (PCM): This is program of continuous
maintenance optimization based on feedback from Root Cause Failure Analysis
repairs, quantitative preventive maintenance, predictive maintenance routines,
Condition Monitoring Systems, and operational experience.
There are different techniques /
solutions for achieving the ultimate business objectives-
Condition Monitoring (CM):
The is process of recording measurements of different machine parameters that convey
the condition of machine without disrupting operation (for example vibration,
oil analysis, process variable analysis, acoustics electrical characteristics,
and thermal imaging) and comparing each to the permissible limits for each
asset.
Reliability Centred Maintenance (RCM): This is a systematic and disciplined process to ensure the safety and
mission compliance that defines system boundaries and identifies system
functions, functional failures, and likely failure modes for equipment and
structures in a specific operating context. RCM develops a logical
identification of the causes and effects (consequences) of system and
functional failures to arrive at an efficient and effective asset management
strategy to reduce the probability of failure.
Failure Modes & Effects Analysis (FMEA): This is a methodology to identify the functions of an asset, the ways it
can fail to perform intended functions, the causes of those failures, and the
methods & techniques for detecting or mitigating those failures.
FMECA: Failure Modes, Effects, and
Criticality Analyses is an integral part of RCM directed to determining type,
probability, cause and consequences of potential failures.
Computerized Maintenance Management System (CMMS): CMMS is a computer maintenance
management system for measuring, managing, and analyzing the maintenance
process. It includes MRO task planning
and scheduling, inventory control and management, labor and material cost
accounting, and asset historical data.
Taxonomy: This is technique of classification and cataloging of items that
shows the relationships between items. A proper taxonomy includes a consistent
method for numbering, describing, naming and classifying items which simplifies
management of the items by facilitating searching, understanding functional
relationships, elimination of duplicate records, etc.
PF curve is a common curve that shows the
behavior of equipment as it approaches failure. The curve shows that as a
failure starts and progresses, the equipment deteriorates to the point at which
it can possibly be detected and is denoted by point ‘P’. If the failure is not detected at that point
& and the fault is not mitigated, it continues until a complete failure
occurs ‘F’. This is known as functional failure i.e. the system is no longer
capable of performing the intended function. For example, a compressor that is designed to
produce 2000 cfm at 7 Kg/cm² is considered to have functionally
failed if it can only produce 900 cfm at 7 Kg/cm². The time
range between P and F, commonly called the P-F interval, is the window of
opportunity during which an inspection can possibly detect the imminent failure
and address it.
Adapted from Moubray, John, Reliability-Centered
Maintenance
Time can be measured in seconds, minutes, days,
months or years. P1-Px indicates detectability intervals by various techniques
or technologies. In this example point ‘P’ is start of potential failure point.
As fault progresses, the point P1 is approached where change in vibration
pattern starts appearing. Further due to
metallic part wear one can detect the worn out metal particles in the oil
filter. Further on the variation in process data starts appearing. Later on the
temperature rise can be detected by IR cameras & finally it can be felt by
touch & audible noise can be heard. This is the point which is very near to
failure & finally the machine reaches point ‘F’ where machine breaks down
completely.
Reactive
Maintenance (RM): Reactive maintenance strategy is just like living
life at the bottom of the P-F curve. In this maintenance strategy, maintenance is
performed after a failure of the asset, or after an obvious & unforeseen
threat of immediate failure. Running machines in run-to-failure mode is an
appropriate strategy for assets where the consequence of failure & also including
the cost of replacement is so low that the expense of maintenance time by doing
preventive maintenance or predictive maintenance cannot be justified. Reactive maintenance in long run can be a very
expensive type of maintenance strategy when applied indiscriminately to all
assets in the plant. This can consume up to 80 percent of the total time and
budget of companies adopting this mode invariably.
Preventive
Maintenance (PM): Preventative Maintenance strategy
was one of the very first strategies adopted by OEM & asset owners and it
is still effective. There are two kinds of PM –
·
Inspection
and observation.
·
Intervention
and replacement.
The first Preventative Maintenance form is the
usual response used for equipment and parts that show signs of age and wear-out
by adopting LLF (look, listen, feel) technique. It involves inspection
and noting down the condition of equipment and servicing it on regular
intervals like changing oil lubricant, greasing, filter cleaning etc. If evidence
of failure is found, the part is changed for new immediately or at the earliest
convenient time before actual breakdown. This strategy is often based on OEM
recommendations with PM template & maintenance is performed at time-based
intervals. The maintenance intervals are generally based on the MTBF (Mean Time
Between Failure) data compiled by the OEM & often very conservative. PM
includes intrusive time-based inspections and requires shutdown of asset for
recommended maintenance activity. Quite often as the asset is already opened
for inspection, wearable parts may be replaced even though they do not show any
sign of wear. As we know, asset failures can happen in between scheduled
maintenance intervals, a time-based maintenance strategy may not be right for
many assets. Intrusive PM employs time based maintenance as per OEM
recommendation where parts would be replaced irrespective of the condition of
the same where as non-intrusive PM for the same machine would comprise of the
inspection, taking measurement of the part & decide not to replace the
part. But non-intrusive maintenance is still time based & they cannot be
performed if the machine is running. Thus PM strategy is better than RTF
strategy. It improves production by reducing unplanned downtime & mitigates
wear and tear on equipment by keeping it clean & lubricated. It reduces
maintenance and repair costs by eliminating emergency repair services.
Preventive maintenance is not developed based
on FMEA analysis for the assets. No condition monitoring technique is used to
develop the PM strategy. It is only a time-based maintenance practice. PM
includes intrusive time-based inspections and requires taking the asset out of
service and opening it to look for worn parts or incipient failures. It is a
time based, OEM recommended practice to change parts etc. that is adopted. For
quantitative PM the asset needs to be shut down. This may reduce the life cycle
of the assets for which starting & stopping incurs greater wear than steady
state operations. Often we land up doing over maintenance & high
maintenance costs.
Predictive
Maintenance (PdM): Predictive Maintenance (PdM) is a
powerful maintenance strategy that involves monitoring for evidence of changed
conditions within the equipment over a period of time. The amount of
change and the rate of change are tracked and used to predict the time of failure
in future. Typically there is a start point, a gradual worsening, and
eventually a point where the item cannot perform its duty & termed as
functional failure. If it is possible to detect early onset of the failure then
there is often time to manage the equipment carefully and continue operation
until a replacement is actually needed. PdM techniques include thermography,
oil debris analysis, vibration monitoring, process variable analysis,
acoustics, and NDT testing. These methods detect a change, and measure
the rate of change & predictions are on the equipment’s continuing
performance. Predictive Maintenance management strategy enables to detect the
problems immediately and gives sufficient time to act before a failure occurs
that shuts down the operation. It costs less than preventive maintenance or
reactive maintenance. So, without a well-developed PdM Strategy, Condition Monitoring
may not be effective. The only short coming of Condition Monitoring tasks is
that it may not be based on failure analysis & may not be well integrated
with other reliability tools like PM, RCFA, Lean Six Sigma etc. & it’s
difficult to train all maintenance personnel on different technologies.
Proactive-Centred
Maintenance (PCM): A one-size-fits-all approach
utilizing Reactive Maintenance has already been discussed as the most expensive
and least effective maintenance strategy when indiscriminately applied to all
assets. However, the same can be said for both PM and PdM. Simply applying any
particular strategy to all assets, independent of the asset’s criticality, is
non-optimal. PCM recognizes this and emphasizes doing the right maintenance on
the right assets at the right time. In
most cases, a PCM approach increases the use of PdM, while continuing to
utilize PM. It also utilizes RM, but correctly limits this approach to assets
with little or no consequences of failure. However, PCM’s purview encompasses
more than just where to apply RM, PM, and PdM. It also concerns itself with
procedures, operating parameters, processes, and designs in order to limit or
prevent recurring failures, thus reducing the total number of asset failures
and extending the mean time between asset failures. A PCM program is
continually being optimized with feedback from Root Cause Failure Analysis
(RCFA) repairs, Quantitative PM’s, PdM routines, CM systems, and operations.
This feedback is used proactively to keep assets in their optimal operating
condition. In the long run, PCM offers the lowest maintenance expenditure.
The
maintenance strategy is unique to each asset, and should address the specific
root-cause failure modes of the asset. A mix of strategies can be applied to
any one asset – for example, PdM may be used for detectable fault types, but PM
or RM may apply for other failure modes. The purpose of this section is to
introduce the main steps in selecting the right strategy:
1. Identify
the Asset Base
2.
Criticality Analysis
3. Develop
the Strategy
4. Assign
Technology & Tools
5. PM Task
Optimization
The equipment data library is a database of
equipment and their attributes that is loaded into the Computerized Maintenance
Management System (CMMS). This is built using logical conventions and taxonomy.
The first step is to develop a complete and accurate equipment data library in
the CMMS, along with corresponding ID tags on equipment in the field. The extra
effort required to develop this equipment data library will pay for itself many
times over. For example, value from the
equipment data library is delivered every time a repair work order is issued, a
piece of equipment needs to be identified in the field, a reliability or cost
history is needed, and when spare parts need to be identified and purchased. The
equipment library starts with development of a logical equipment hierarchy,
tagging convention, description, equipment class and subclass identification,
and nameplate data. This information is
compiled in a database that is uploaded into the CMMS. Corresponding field tags are also physically placed
on the equipment.
A complete and accurate equipment library is
required for effective:
·
PM/PdM
work planning and execution
·
Spare
parts inventory management
·
Cost
accounting
·
Reliability
management
·
Lifecycle
cost management
After the equipment data library has been
developed, the relative criticality of all equipment must be ranked in terms of
safety, the environment, production, maintenance costs, and product
quality. Weightings for each ranking
criteria are determined based on previous experience of team members from
facility, and then the ranking of each asset is determined using consistent and
balanced ranking criteria. The
criticality ranking data is then uploaded to the CMMS so that each equipment
record in the CMMS has a corresponding criticality value. A fine Criticality
Ranking of the equipment focuses more attention on the asset that has the
largest financial impact on your overall operation. Criticality is used by
maintenance planners to prioritize the work based on the risk the asset poses
to the business. It is also used to determine the optimum level of spare parts inventory.
A cross-functional team is usually assembled to agree on criteria weighting and
asset evaluation.
The Primary criticality ranking criteria are:
·
Safety
·
Environment
& Regulatory Compliance
·
Production
·
Maintenance
& Operating Costs
·
Product
quality
·
Other
relevant criteria
Develop the Maintenance Strategy:
Point out that as critical equipment is put on
PdM and results are seen, decisions can be made on a Cost-Benefit-Analysis
basis whether to expand the amount of equipment monitored. In a typical
implementation, 10% of equipment will receive RCM 35% are candidates for
Failure Mode Effects Analysis, and about 50% will benefit from well written
PM’s. Possibly 5% of equipment may be left off a maintenance program altogether
and simply repaired or replaced when it fails. This would only be if it meets
certain criteria. A granular Criticality Ranking of your equipment allows you
to focus attention on equipment that has the largest financial impact on your
overall operation. Critically Ranking Equipment allows ‘Work Order
Prioritization’ supporting both planning and scheduling, as well as Emergency
Work Order generation.
The three alternative tools that can be applied
for maintenance Strategy Development are:
·
RCM
– carried by teams on only the most critical systems. An individual RCM
analysis may take 8 to 10 person-weeks to complete.
·
FMEA
– developed by single analyst on critical systems and equipment. The use of
FMEA libraries helps fastrack this process.
·
Templates
– consist of standard maintenance strategies for low criticality assets.
It is important to understand the asset and its
criticality ranking to select the technology and application frequency to
support the Maintenance Strategy. The criticality ranking of equipment in the
system will influence the Maintenance Strategy and the level of Condition
Monitoring that will be applied.
PM Optimization:
The most tangible and immediate benefits of
implementing a condition monitoring and PdM program is that present the
existing time-based PM program can be optimized. Many PM programs are based on OEM
recommendations or generic templates with little consideration of available
predictive technologies or the operating context of the asset. PM Optimization involves a systematic
screening of existing time-based preventative maintenance tasks with respect to
asset criticality, common failure modes, and available predictive technologies.
Typical benefits are significant reduction in the number of PM tasks through
replacement with non-intrusive tasks, condition-based maintenance tasks, and
deletion of ‘no value’ tasks. Many PM’s
are also strengthened through revision of PM intervals and improved work
packages that detail procedures, parts, and tools.
This is a systematic review of routine
Preventive Maintenance (PM) tasks with reference to the ideal maintenance
strategy. Work plans are strengthened with improved procedures, fully developed
Bill-of-Materials, tools and parts kits. By optimizing existing PM tasks, the
cost of the maintenance program is reduced keeping the reliability high. Savings
from the PM Optimization process can fully fund the introduction of a
Predictive Maintenance program. Typical results from PM Optimization:
·
Approx.
30% of PM tasks can be replaced by more cost effective Predictive Maintenance
tasks.
·
Approx.
30% of PM tasks can be deleted (no value).
·
Approx.
30% of PM tasks need strengthening.
The adoption of any maintenance strategy is
based on kind of asset chosen for maintenance. There is not a single solution
for complete asset base. So, the ideal situation is to classify different naval
assets into different categories based on their criticality. Accordingly, right
maintenance strategy should be selected. This will not only result into
improved performance & availability of assets but also reduce the
maintenance cost & time. The resource saved can be utilized for other
critical jobs.
References:
·
Optimizing Your Maintenance Strategy- Michael
Hanifan- GE
·
Moubray, John, Reliability-Centered Maintenance,
Industrial Press, Inc., New York City, NY, 1997.
·
LICENTIATE THESIS PROPOSAL- CONDITION BASED
MAINTENANCE IN TECHNICAL SYSTEMS- Marcus Bengtsson- Department of Innovation,
Design and Product Development
·
Innovation and Design, Product- and Process
Development, Mälardalen University Eskilstuna, Sweden
