With hidden failures, the equipment has to be tested periodically, in order to
check whether it is in working condition. When the tests are effected, failure-finding
tasks are carried out. If a component is in a failed state, it is rectified by carrying out a
breakdown maintenance. Under certain circumstances, periodic replacement or
repair of the component is warranted, although it might be still in working condition.
Planned maintenance embraces all of the following:
(1) Testing for hidden failures;
(2) Condition monitoring of incipient failures;
(3) Pre-emptive repair or replacement action based on time (running hours, number
of starts, number of cycles in operation, or other equivalents of time).
Table 20.1 Types of possible maintenance
Type
Description
Breakdown
Repair is carried out following equipment functional failure.
It is impossible to schedule the repairs. It is also called
on-failure maintenance.
Corrective
Repair is carried out after failure initiation entailing degraded
performance. Often, condition monitoring or inspections will
disclose such deterioration. Actual repairs may be carried out
before or after functional failure, depending on the estimated
consequences of failure. This differs from breakdown maintenance in that the repairs could have been scheduled.
Scheduled overhaul or
replacement or hard-time
Repair is carried out depending on age (calendar time, number of cycles, number of starts or similar age metrics as
appropriate). This strategy applies when the age at failure is
predictable. Fouling, corrosion, fatigue and wear related failures are typical examples.
On-condition
Repairs depend on the outcome of inspections or conditionmonitoring activities, scheduled on calendar time, to check if
failure has already started. Typical examples include vibration monitoring and on-stream inspections. Some parameters
may be continuously monitored, using dedicated instruments.
All on-condition maintenance is corrective in nature.
Testing or failure-finding
Attempts to find out whether an item will function if required,
on demand. This applies to hidden failures and non-repairable
components, i.e., the element must be withdrawn from service
if it is known that it has failed.
Predictive
Repairs depend on predicted time for functional failure, to
occur, usually by inferring from the outcomes of on-condition
activities or constantly monitored condition results. It is
identical with on-condition maintenance.
Preventive
Repairs or inspections are carried out before functional failure. They are carried out depending on age-in-service and the
expected time of failure. With a pessimistic estimate, repairs
may be carried out even when the equipment is still in perfect
operating condition.
20.3 Infrastructure and its Maintenance Requirements
579
check whether it is in working condition. When the tests are effected, failure-finding
tasks are carried out. If a component is in a failed state, it is rectified by carrying out a
breakdown maintenance. Under certain circumstances, periodic replacement or
repair of the component is warranted, although it might be still in working condition.
Planned maintenance embraces all of the following:
(1) Testing for hidden failures;
(2) Condition monitoring of incipient failures;
(3) Pre-emptive repair or replacement action based on time (running hours, number
of starts, number of cycles in operation, or other equivalents of time).
Table 20.1 Types of possible maintenance
Type
Description
Breakdown
Repair is carried out following equipment functional failure.
It is impossible to schedule the repairs. It is also called
on-failure maintenance.
Corrective
Repair is carried out after failure initiation entailing degraded
performance. Often, condition monitoring or inspections will
disclose such deterioration. Actual repairs may be carried out
before or after functional failure, depending on the estimated
consequences of failure. This differs from breakdown maintenance in that the repairs could have been scheduled.
Scheduled overhaul or
replacement or hard-time
Repair is carried out depending on age (calendar time, number of cycles, number of starts or similar age metrics as
appropriate). This strategy applies when the age at failure is
predictable. Fouling, corrosion, fatigue and wear related failures are typical examples.
On-condition
Repairs depend on the outcome of inspections or conditionmonitoring activities, scheduled on calendar time, to check if
failure has already started. Typical examples include vibration monitoring and on-stream inspections. Some parameters
may be continuously monitored, using dedicated instruments.
All on-condition maintenance is corrective in nature.
Testing or failure-finding
Attempts to find out whether an item will function if required,
on demand. This applies to hidden failures and non-repairable
components, i.e., the element must be withdrawn from service
if it is known that it has failed.
Predictive
Repairs depend on predicted time for functional failure, to
occur, usually by inferring from the outcomes of on-condition
activities or constantly monitored condition results. It is
identical with on-condition maintenance.
Preventive
Repairs or inspections are carried out before functional failure. They are carried out depending on age-in-service and the
expected time of failure. With a pessimistic estimate, repairs
may be carried out even when the equipment is still in perfect
operating condition.
20.3 Infrastructure and its Maintenance Requirements
579