Maintenance work typically falls into one of the following four categories:
- Condition-based maintenance;
- Scheduled maintenance; and
- Reliability-centered maintenance (RCM).
All but the first of these can be placed in the overall category of preventive or preventative (either spelling is acceptable) maintenance. The sketch illustrates the maintenance planning and execution process.
The most common type of maintenance is the repair of an item which has failed or which is showing imminent signs of failure. This type of maintenance tends also to be the most hazardous because there may not be much time to plan the job, and the repair work may be going on while the rest of the facility is in operation. In addition, the maintenance workers may be under pressure to get the work done quickly in order to avoid a larger system shutdown. Such pressure may lead to shortcuts being taken.
Condition-based maintenance is carried out when an equipment item starts to show signs of failure or when its performance becomes degraded. For example if the discharge pressure of a pump starts to fall the pump may be taken out of service and repaired before it actually fails.
Condition-based maintenance tends to be less hazardous than repair maintenance because it can be properly planned, and it can be carried out without the workers feeling that they are in a rush. Ideally, condition-based maintenance will be carried out while the unit is shut down, thus making conditions even safer for the maintenance workers.
Examples of monitoring activities that could lead to the need for condition-based maintenance include:
- Process performance;
- Vibration analysis;
- Oil analysis; and
Some equipment and instrument items are serviced on a scheduled basis, regardless of the actual performance or condition of those items. A common example of this type of maintenance is the routine replacement of lubricating and seal oils.
The Table provides an example of the maintenance schedule an instrumentation system.
Example of an Instrument Loop Maintenance Schedule
|High Integrity Protection Loops||6|
|Shutdown and alarms systems (SIL2)||6|
|Shutdown and alarms systems (SIL1)||12|
|Control and monitoring systems||24|
Reliability-centered maintenance (RCM) uses a risk-based approach to organizing maintenance activities. As such, it is analogous to Risk-Based Inspection.
This approach is based on the elements of risk. Those items that contribute the most to overall risk receive a higher maintenance priority than those that are considered to be less critical. (In practice, the RCM program may focus more on the consequence term than on overall risk.)
An RCM analysis is typically built around the following steps:
- A component list is developed for the section of the facility that is being analyzed.
- A failure analysis, possibly using the FMEA method, is carried out for each component.
- A criticality is assigned to each failure mode.
- A scheduled maintenance program that focuses on the high criticality items is set up. This program can be condition-based.
The RCM analysis can also help determine the required inventory for spare parts.
The RCM program should include a process for analyzing equipment failures and applying the lessons learned from those analyses such that overall risk can be reduced.