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This series in hot topics considers modern plant condition management
strategy. The limitations of conventional approaches are evaluated
and developments are described which enable a more effective operations
and maintenance approach to be implemented. Low-cost plant health
assessment systems maximise the value derived from existing equipment
by incorporating them in an integrated plant condition appraisal
architecture, which can predict when assets are at risk before
serious damage has been sustained. Guidelines are presented on
optimising the system scope, vendor selection, and quality assurance
criteria. The benefits of the new approach include centralisation
of resources, high level remote support, improved control of engineering
and maintenance activities, and full accountability.
Introduction
The cost of corrosion has been reported from many studies to be
of the order of 4% of the GDP of any particular country - and
many millions of dollars a year Not all of the loss
is targetable, but it has been calculated that cost-effective
reductions in expenditure are achievable in approximately 25%
of cases.
At company level, Shell has reported at the NACE-sponsored Bahrain
Corrosion Conference held in Bahrain that the cost of
corrosion for that company was calculated to be equivalent to
$400million in 1995. BP has reported that the cost of corrosion
is equivalent to 6% of the net asset value of the company and
has launched a substantial initiative to reduce corrosion damage.
In the power industry, the American Electric Power Research Institute
(EPRI) observed that corrosion is responsible for more than 55%
of all unplanned outages, and it adds over 10% to the average
annual household electricity bill (Ref 3). The potential for
savings has been recognized for more than twenty years, but it
is only recently that technology has emerged which is capable
of delivering the savings in a cost-effective manner
Repair-based maintenance of equipment is the most expensive option to maintain the serviceability of plant. The comparative costs of 'corrective', preventive, and predictive maintenance strategies are shown in Figure 1
This histogram shows that an approximate one-third reduction in
O&M costs is achieved by moving from a 'corrective' (more
realistically termed a 'breakdown') repair strategy to a 'preventive'
regime, and this can be achieved by the introduction of planned
maintenance routines and reliability based maintenance philosophy.
However, the data also reveal that these methods yield only approximately
half of the available cost savings. Although difficult to introduce
by modification of traditional methods, 'predictive' maintenance
strategies are capable of yielding a further one-third reduction
in O&M costs.
PLANT MONITORING
Corrosion
'Corrosion' is the manifestation of uncontrolled electrochemical
activity which takes place when the aggressivity of a plant environment
exceeds the limits of suitability of construction materials.
Corrosion is the primary life-limiting plant degradation mechanism.
Detection of electrochemical activity associated with the corrosion
process allows control action to be initiated before significant
damage takes place.
Corrosion control has in the past been considered an off-line
service activity. The condition was wrongly classified as plant
damage, the costs of which are hidden in 'routine maintenance'.
Conventional corrosion instrumentation is also unsophisticated,
and has lacked the integrity required for plant control instrumentation.
However, corrosion degradation does not take place continuously,
it takes place in episodes which occur from time to time during
normal plant operations (Ref 6). On-line corrosion information
can be used to prompt immediate remedial action, thereby reducing
both the duration and the severity of attack.
Vibration
Condition monitoring normally is assumed to relate to
mechanical damage assessment on rotating or reciprocating plant.
Vibration monitoring, which provides 60-70% of this assessment,
has emerged as the key technology through which to optimize maintenance
management, though other techniques such as thermographic imaging,
performance assessment and lubrication oil and cooling water analyses
can also play an important part.
Vibration monitoring is a well developed technique which can allow
incipient mechanical failure to be diagnosed and its use as part
of reliability-based maintenance strategies has increased significantly
during recent years Although a significant improvement
over breakdown maintenance strategies, reliability-based repair
strategies are only a partial improvement in maintenance optimisation.
Stand-alone techniques of condition monitoring are becoming more
common in reliability-based maintenance practices, and are used
to provide feed-back in order to update the reliability models.
The investment in portable vibration data collection systems
indicates a willingness to improve and optimise machine repair
activities. However, in many cases the return is sub-optimal
in that the systems are normally stand-alone and sampling intervals
allow the development of faults to be plotted in a retrospective
but not anticipated manner.
Off-line Systems and On-line Systems
As the benefits of the condition management approach have become
recognised, there has been a tendency for the 'management' term
to be applied equally to on-line and off-line systems. It is
useful perhaps to clarify the features and functionality of each
type.
Conventional monitoring systems merely collect data. Data is
not information and is not in itself particularly useful. The
essential difference between these and a modern corrosion or condition
management system is that the raw data have been analysed to provide
information on the basis of which the health status of plant equipment
can be deduced. Information has considerably greater value than
data
The important criterion is the time before meaningful data are
available from the system. Several excellent off-line corrosion
data management packages have recently come into the market which
offer ease of data entry and evaluation. They are a considerable
improvement over non-computerised methods of data filing and appraisal,
and equipment reliability performance can be improved even in
relatively un-instrumented systems by an improved structure of
data presentation. However, these systems are, first and foremost,
data management systems, and not condition management systems.
In order that effective action is taken in a timely manner in
operating process plant systems, the response of the condition
management system must be meaningful in the context of the plant.
i.e. in continuously operating process systems, feedback on the
health status of key plant components to operations and maintenance
personnel also must be continuous. This necessarily entails real
time data input to the system in order that it can fulfil the
requisite condition 'management' functionality.