The availability of all operating
machines in an industry is very important to achieve maximum production. All
efforts are to be taken to eliminate break downs.
Various methods of maintenance being
practiced in various industries are as follows:
1. On
failure maintenance or break down maintenance.
2. Design
out maintenance.
3. Fixed
line maintenance.
4. Condition
based maintenance.
5. Opportunity
based maintenance.
Above policies can be used in combination or
individually, depending on suitability to the equipment / plant. The action
that are carried out before failure can be regarded as PREVENTIVE MAINTENANCE
and those that are carried out after failure as CORRECTIVE MAINTENANCE.
Preventive maintenance actions are deterministic in nature, hence can be
scheduled. Because of probabilistic nature of failure, uncertainty surrounds
corrective maintenance hence cannot be programmed. Using experience corrective
maintenance guide lines are formulated for maintenance decision making.
1. ON
FAILURE MAINTENANCE OR BREAK DOWN MAINTENANCE
Replacement or repair after failure is
the default strategy. It requires no pre-care of the plant, all the management
goes into organizing manpower and stores. It is sometimes called corrective
maintenance, a term which is unspecific and can be applied to other strategies
and hence is better avoided.Examples of such equipments which are
normally adopted for breakdown maintenance are as below:
·
Electrical fuses
·
Rupture discs
Simple replaceable equipments e.g.
pumps which do not affect safety etc. Important task is to restore unit in most
economic way to an acceptable condition.
For complex units this task can be performed
in following ways :
Repair in situ : e.g. repairing of
joints, etc. Replacement of modules, units or
subunits
Many factors influence the
repair-replace choice, following factors will affect the decision:
Cost of unavailability
Time of repair compared with that of
replacement
Availability and cost of resources
Type of repair
Unfortunately on-failure maintenance
encourages the “fire-fighting” syndrome - respond fast and work furiously. “A
job done fast is a job done well” even if it has to be repeated tomorrow.
On-failure maintenance is best
described by its disadvantages:
·
No warning of failure - safety risk.
·
Uncontrolled plant outage
·
Production losses or delays
·
Need to provide standby plant
·
Large standby maintenance team
·
Secondary damage - longer repair times
·
Large spares stock requirements
To be fair when the other strategies
all fail, then this strategy is the backup. To improve on maintenance
on-failure we can:
·
Maintain the plant before failure.
·
Make the units more reliable.
·
Make the process less dependent on
reliability.
The first of these approaches can be
met by employing fixed-time maintenance. The other two come under the umbrella
of design-out maintenance.
2. FIXED-TIME
MAINTENANCE
Replacement or repair at a fixed time
interval before failure is the strategy most commonly applied to reduce
on-failure maintenance. The simple planning structure for manpower and spares
makes this strategy reasoning behind its very common name “planned preventive
maintenance”.
Fixed time maintenance can
·
reduce failures
·
Use the workforce cost-effectively with
planned work schedule in day shifts.
·
allow work to be planned well in advance
But:
·
The maintenance activity and hence cost must
increase because the maintenance interval will be shorter than the
mean-time-to-failure.
(In Fig.-2.1 (i) the probability of
failure after a particular life is plotted against that life for a unit which
has a clear Wear Out characteristic. Repair must be undertaken at a shorter
interval (FTM) than the mean time to failure (MTTF) to ensure that the failures
(hatched area) are reduced.
·
Can only be applied effectively. On fixed-time
intervals where the deterioration is age related (predictable)
In the more common situation where a unit
fails in a near random fashion. Fig.-2.1(ii), to achieve an acceptable
reduction in failures the fixed-time maintenance interval becomes uneconomical
short.
·
The maintenance sometimes induces failures.
Unfortunately these- three factors are
a severe restriction on the effective use of fixed-time maintenance which
therefore must be used in conjunction with other strategies. Process plant
rarely fails in the predictable manner shown in FIG-A (i). Complex replaceable
units or components have failure characteristics more like those in Fig.-2.1
(ii). In such situations, fixed-time maintenance is totally useless.
An example of the consequences of
applying fixed-time maintenance to such plant came to light in a food process
plant because machine condition monitoring had been introduced but maintenance
was still on time rather than condition.
Two boilers provided steam to heating
and process purposes. Each was shut down for statutory inspection every second
summer. At the same time the auxiliary plant including the induced draught fan
were “serviced”. On this occasion the vibration monitoring indicated no
deterioration in either of normal modes of failure. Imbalance or bearing
damage. Nevertheless the fan was stripped down, the rotor cleaned! The bearings
replaced and the system reassembled. Within two months of recommissioning the
monitoring detected bearing damage and replacement was necessary. Analysis of
the past experience on this plant combined with an understanding of the failure
characteristics of rolling element bearings leads one to believe that the mean
life of bearings in these fans was at least five years, some bearings capable
of giving in excess of ten years service if left in place. Replacement every
two years is usually successful but extravagant and, as in this example,
occasionally leads to maintenance induced failure.
To overcome this disadvantage posed by
the random life of typical industrial plant it is possible to detect the onset’
of deterioration and maintain on-condition.
3. CONDITION
BASED MAINTENANCE:
In this policy we arrive the proper
time for performing corrective maintenance by monitoring condition or
performance of equipment. When parameters show deviation from a specified level
corrective maintenance is planned. This eliminates probabilistic element in
failure prediction, the item life is maximized and the effect of failure is minimized.
To implement this scheme we have to identify readily monitor-able parameter.
This policy is most suitable for
complex replaceable items like Compressors, Pumps, Turbines etc.
4. OPPORTUNITY
MAINTENANCE
This term is used for maintenance
action taken after failure or during fixed time maintenance or condition based
maintenance, but directed at items other than those that are the primary cause
of repair or shut down.
5. DESIGN-OUT
MAINTENANCE
Redesigning a unit is also part of the
maintenance function. The other strategies discussed involve a degree of
repetition. They are concerned with responding to or anticipating failure in an
effective manner. In contrast redesigning to avoid failure is, or should be a
once-off activity. There is, therefore, considerable scope for plant
improvement by the application of design-out maintenance.
There are three distinct ways in which
redesign can lead to plant improvement.
·
Redesign the critical units to reduce failures
by modifying or replacing components which are not able to withstand the loads
imposed, and by reducing the deterioration rates of components which wear out.
· Modify the use of the critical unit so as to
make less severs the loads applied to the failing components.
· Minimize the effect on production of the
failures in the critical unit; for example, by providing storage between
processes of installing standby units.
