Four general types of maintenance philosophies or strategies can be identified, namely corrective, preventive, predictive, and reliability centered maintenance. Predictive maintenance is the most recent development. In pratice, all these types are used in maintaining engineering systems. The challenge is to optimize the balance between the them types for maximum profitability. In general, corrective maintenance is the least cost effective option when maintenance requirements are high.Corrosion Monitoring and Inspection Planning
Corrective Maintenance
Corrective maintenance refers to action only taken when a system or component failure has occurred. It is thus a retro-active strategy. The task of the maintenance team in this scenario is usually to effect repairs as soon as possible. Costs associated with corrective maintenance include repair costs (replacement components, labor, consumables), lost production and lost sales. To minimize the effects of lost production and speed up repairs, actions such as increasing the size of maintenance teams, the use of back-up systems and implementation of emergency procedures can be considered. Unfortunately, such measures are relatively costly and/or only effective in the short-term. For example, if heat exchanger tubes have leaked due to pitting corrosion and production must procced as a matter of uregency, it may be possible to plug the leaking tubes on a short-term basis.
Preventive Maintenance
In preventive maintenance, equipment is repaired and serviced before failures occur. The frequency of maintenance activities is predetermined by schedules. Preventive maintenance aims to eliminate unnecessary inspection and maintenance tasks, to implement additional maintenance tasks when and where needed and to focus efforts on the most critical items. The higher the failure consequences, the greater the level of preventive maintenance that is justified. This ultimately implies a trade-off between the cost of performing preventive maintenance and the cost to run the equipment to failure. Inspection assumes a crucial role in preventive maintenance strategies. Components are essentially inspected for corrosion and other damage at planned intervals, in order to identify corrective action before failures actually occur. Preventive maintenance performed at regular intervals will usually results in reduced failure rates. As significant costs are involved in performing preventive maintenance, especially in terms of scheduled downtime, good planning is vital.
Predictive Maintenance
Predictive maintenance refers to maintenance based on the actual condition of a component. Maintenance is not performed according to fixed preventive schedules but rather when a certain change in characteristics are noted. Corrosion sensors supplying diagnostic information on the condition of a system or componenet play an important role in this maintenance strategy. A useful analogy can be made with automobile oil changes. Chaningng the oil every 5000 km to prolong engine life, irrespective of whether the oil change is really needed or not, is a preventive maintenance strategy.
Predictive maintenance would entail changing the oil based on changes in its properties, such as the build up of wear debris. when a car is used exclusively for long distance highway travel and driven in a very responsible manner, oil analysis may indicate a longer critical service interval. some of the resources required to perform predictive maintenance will be available from the reduction in breakdown planning and scheduling. Good record keeping is very important to identify repetitive problems, and the problem areas with the highest potential impact.
Reliability Centered Maintenance
Reliability centered maintenance (RCM) involves the establishment or improvement of a maintenance program in the most cost effective or improvement of a maintenance program in the most cost-effective and technically feasible manner. It utilizes a systematic, structured approach that is based on the consequences of failure. As such it represent a shift away from time-based maintenance tasks and emphasizes the functional importance of system components and their failure/maintenance history.
The concept of RCM finds its foots in the early 1960's, wiht RCM strategies for commercial aircraft developed in the late 1960's, when wide body jets were introduced to commercial airline service. A major concern of airlines was that existing time-based preventive maintenance program would threaten the economic viability of larger, more complex aircraft. The experience of airlines with the RCM approach was that maintenance costs remained roughly constant but that the availability and reliability of their planes improved. RCM is now standard practice for most of the world's airlines.
