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| Plant Maintenance practices have a large impact on the productivity and sustainability of industrial sites |
The general role
of plant maintenance includes:
- The assurance of safety – this should be the no.1 priority in asset-intensive industries, and for industries like mining, has become a major strategic issue;
- The prevention of functional failures – this is a critical issue, and requires an appreciation of the fact that an asset may be operating, yet may have failed, resulting in risks for product quality, the environment, safety, plant flexibility, throughput and other business objectives;
- To correct failures – not all failures can be prevented, or indeed should be prevented, since the cost of prevention may exceed the cost of allowing the failure to occur and then correcting it after the fact. Also, even a good preventive maintenance programme will not have a 100% success rate;
- To optimise equipment life – note that this is different to maximising equipment life, since obsolete equipment may not deliver competitive performance;
- To ensure that equipment delivers performance as per its design – this is related to prevention of failure, but also includes the prevention of injudicious plant modification;
- To achieve the above at minimum cost – cost here refers to total operating and capital costs, not simply the costs of performing maintenance, and strictly speaking should include the cost of externalities which may come about as a result of failure.
There are a
plethora of approaches to the maintenance discipline, and in general,
organisations do not subscribe to a single philosophy. Let’s explore individual
approaches and what they mean in the context of productivity and
sustainability.
Run-to-failure
Maintenance (RTF)
A run-to-failure
philosophy is one in which failures are allowed to occur and are then dealt
with after they have been detected. The philosophy is sometimes referred to as
“breakdown maintenance” but the difference between RTF and breakdown
maintenance is that, as discussed above, a component may have failed without a
breakdown actually occurring i.e. the failure does not stop the equipment from
running.
This is
essentially the base from which all preventive maintenance philosophies were
ultimately developed. This is not a preventive philosophy, but a reactive one.
If adopted as the dominant philosophy, it has many risks and can also be
expensive due to consequential damage that may occur as a result of component
failure, much of which can be far more extensive than that incurred through the
original failure itself. Often, since spares cannot be held for every
component, the approach leads to long periods of downtime as the site waits for
delivery of the required components. Stable operating performance cannot be
achieved with this philosophy.
From a
sustainability perspective, there are obvious economic drawbacks and also risks
to safety and the environment that arise from use of this approach, and it is
seldom adopted as the primary philosophy on an industrial site. There is
however room for limited application of the philosophy within the context of a
wider preventive maintenance programme. This is for the case where the
consequences of failure of an individual component or sub-component are very
low, and the costs of prevention of the failure exceed the costs of the
consequences of failure. A further criterion is that the failure must be
readily detectable once it occurs. In this case the component may be allowed to
fail and then repaired without incurring risks of multiple failures occurring
at the same time, which may have far more serious consequences than the
original failure.
Time/age-based
Preventive Maintenance (PM)
Preventive
maintenance involves taking action before failures occur to prevent the
failures from occurring. With time-based PM, components are replaced or
reworked at defined frequencies, independent of their condition. The initial
philosophy was based on the age of components, and involved replacing
components when they reached a fixed period in service. The approach was
pioneered in arms manufacturing companies during WW2.
For this
approach to be viable, components would have to display distinct wear
characteristics that would allow some prediction as to when they would be
expected to fail. The approach would then be to replace them as they approached
this failure point, which would be determined by their operating hours.
The American airline
industry, with its strong focus on safety, pioneered many of the approaches
that form the basis for much of modern maintenance practice, and research into
component failure conducted in the 1960’s shows that greater than 90% of
components in the industry did not exhibit a predictable probability of failure
with age. Maintenance tasks driven by time of use hence are not generally
effective, and maintenance programmes using this philosophy will therefore not
prevent functional failures. Infant mortality represents a simple example of
why the philosophy is flawed. Even new components have some probability of
early-life failure, and hence replacement of a working used component may in
fact reduce the reliability of a piece of equipment. Maintenance-induced
failures may also occur due to dis-assembly and re-assembly of complex
equipment, particularly by unskilled artisans.
This approach,
while potentially better than a run-to-failure philosophy, cannot guarantee
improved productivity and sustainability, simply because most components do not
wear in a predictable fashion. However, in replacing or reworking components,
inspection of equipment is naturally carried out, and other potential problems
may be detected and corrected before failure occurs. Reliability should
therefore be somewhat improved over an approach comprising RTF.
Reliability
Centred Maintenance (RCM)
RCM originated in
the airline industry, and was a product of a study commissioned to maximise
availability and control costs, with primary consideration for safety. Its
roots lie in a steering committee representing airlines and aircraft
manufacturers which was formed in 1960 to study the effectiveness of PM. The
committee, known as the Maintenance Steering Group (MSG) developed a logic
known as MSG-1, which after successive iterations and improvements developed
into MSG-3 by 1978. This decision logic, applied with detailed knowledge of
failure causes and consequences, is effectively RCM. The technique quickly
found application in other industries, notably nuclear power plants (due to its
safety focus), manufacturing plants and railroads.
RCM uses a logical,
structured framework for optimising the availability and lifespan of equipment
and systems. It ensures that maintenance tasks are effective and economical.
The following are core principles of the methodology:
- It seeks to preserve an item’s function – hence any failure which impacts on a component’s ability to function is addressed, not just those which cause plant stoppages;
- It focuses on the entire life (not to be confused with life cycle) of the system or item, making RCM a continuous process;
- It seeks to understand the failure characteristics of the item in question, and uses this to evaluate whether PM is appropriate (i.e. should an item be allowed to fail or not?) , and if so what type of PM ;
- RCM is driven by safety and environmental considerations first, and then by economics;
- RCM acknowledges that an item has “inherent reliability” which cannot be improved through maintenance, but only through re-design.
The primary
difference between RCM and time/age-based maintenance is that RCM stresses the
examination of the condition of an item, with tasks that are then defined based
on this condition. The philosophy also differentiates between evident functions,
whose failure can readily be detected by the operator of the equipment, and
hidden functions, whose failure may not be readily apparent to the operator. In
the case of hidden functions, there is the potential for exposure to multiple
functional failures, and this is what the maintenance effort aims to prevent.
The consequences of failure determine the options to be followed in terms of
preventive maintenance tasks.
The work of F.S.
Nowlan and H.F. Heap is considered to be where RCM began. Their book on the
subject explores the principles of
failure, age-reliability patterns and the step-by-step process that needs to be
followed in developing a preventive maintenance programme using RCM. Still
relevant, I recommend that you read it, and it can be downloaded here.
Total Productive
Maintenance (TPM)
TPM originated
in Japan in the 1970’s, and while focused to a large degree on plant
reliability, is in fact a broader continuous improvement philosophy and culture
encouraging participation at all levels of the implementing organisation. Central to the philosophy is the performance
measure used to judge its success, which is called Overall Equipment
Effectiveness (OEE). The measure takes into account the so-called “six major
losses”: breakdown losses, setup and adjustment losses, speed losses, idling
and minor stoppage losses, quality defect and rework losses and start-up
losses. It therefore addresses
machine-related impacts on throughput and product quality, but importantly also
addresses the relationship of the operator to the equipment being operated.
Hence operator work practices are a critical aspect of TPM implementation. From
a maintenance point of view, TPM also employs operator maintenance tasks
(so-called autonomous maintenance – a misnomer in my view but perhaps something
that I can discuss in a future post). These generally involve simple aspects of
preventive maintenance such as cleaning, lubricating and tightening, which
together are considered by the philosophy, if neglected, to be accountable for
up to 80% of failures. Cleaning is considered a form of inspection, and hence
there are linkages here to RCM, which stresses the performance of tasks on the
basis of condition rather than time.
As an
organisation-wide philosophy, TPM involves everyone from the shop floor through
to senior management. Communication within and across levels is considered
vital. TPM implementation generally involves the use of a large number of
tools, such as visual management, problem solving, quick-changeovers, 5S, standardised
work and others. While many of the tools
of TPM are now used in implementing philosophies such as Lean manufacturing,
Lean and TPM are not the same thing. The goals of TPM are to:
- Maximise overall equipment effectiveness (OEE);
- Establish a comprehensive PM system spanning the life cycle of the equipment;
- Involve all departments that plan, use and maintain equipment;
- Involve all employees from top management to front-line workers;
- Promote autonomous, small-group activities;
- Promote zero breakdowns;
- Promote zero defects.
TPM seeks to
harness the power of the collective. Its success requires good change
management and strong leadership, and has to be underpinned by a focus on
technical skills at all levels in the organisations that implement it. It is
notoriously difficult to sustain, with a large part of the problem being that
it is generally considered as a “programme” with steps that have to be followed
in order to be able to make the claim that “we have implemented TPM”. Of
course, implementing fundamental change is just not that simple, and generally
once those driving the programme reduce their involvement, the process is prone
to break down. There are however significant benefits to be had from successful
implementation, and while I am no change management expert, my advice would be
to choose the individual tools that will add value for you and implement those
rather than a full-on implementation of a bunch of approaches which may just
add bureaucracy to your organisation simply because they are considered part of
TPM. More important is the development of a culture of teamwork and a focus on
performance.
The above are the
primary maintenance philosophies that are practiced. There are further
derivatives such as predictive maintenance, opportunity-based maintenance and
others, but all of these are based on these core philosophies.
Which then would
be the preferred approach for organisations wishing to be sustainable and
productive? Clearly run-to-failure and time/age-based preventive maintenance
have too many drawbacks to be considered viable. RCM represents a rigorous
methodology and is in my view the best approach for determining maintenance
tasks. It does not however address the need for participative engagement
between plant operators and maintenance personnel, or answer questions as to
who should carry out maintenance tasks. TPM attempts to do this, but lacks the
rigour of RCM as regards task development. The correct approach is therefore to
use a mix of RCM and TPM, with the former used for task development (including
the development of autonomous maintenance tasks) and the latter used to build
teamwork between plant operators and maintenance staff. The principle of
assessing condition before executing tasks can be executed in practice through
inspections and the use of condition monitoring technologies.
A maintenance
philosophy alone cannot guarantee reliability. Successful implementation of a
chosen philosophy requires a significant maintenance infrastructure encompassing
comprehensive task development, planning and scheduling, skill development,
spares selection and acquisition, execution, performance management and many
other aspects of maintenance management. I will explore these in future posts,
but all of them, and others, such as development and execution of the business
processes needed to support plant maintenance, begin with the philosophy
chosen. While the philosophy you choose should draw on best practice, it should
ultimately be an approach adapted to your needs and resources.
Great Information! Its looking Nice. Safety is most important. Useful for me to develop my knowledge. Thank you!
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