Optimising assets through reliability-centred maintenance

In technical literature, reliability-centred maintenance (RCM) continues to show up as the prominent future strategic direction in machinery maintenance, and for good reason. RCM is the best method to use when optimising the operational reliability of plant equipment. It is important for reliability professionals to understand RCM and how condition-monitoring tools fit into the RCM picture.

RCM is the systematic process of optimising reliability and associated maintenance tactics with respect to operational requirements. Economic optimisation of machine reliability relative to organisational goals is the primary objective of the RCM process. Simply stated, RCM helps to ensure that if a dollar is spent on improving reliability, then the full dollar will come back - plus some acceptable return on the investment.

As shown in Figure 1, the law of diminishing marginal returns applies to the implementation of reliability improvement measures. Generally, the first dollar invested in reliability improvement tends to yield a higher return on investment than any dollar subsequently invested. The objective is to reach the point of optimisation that the benefits of reliability, expressed as total operating costs, are maximised through cost reduction. RCM is a set of systematic engineering procedures for achieving and maintaining this objective. The origins of RCM

Figure 1: Economic analysis of reliability investments.

RCM’s roots trace back to the 1960s when it was considered advanced to improve the safety and reliability of commercial aircraft. It has since begun to move into the industrial sector as a result of work conducted by several authors[1,2]. Going further back, however, RCM owes its origins to the development of the reliability engineering discipline.

It was here that the fundamental analytical tools were created to estimate the reliability of electrical and mechanical components and systems. Simply stated, RCM is a component of the quality movement focused on improving the safety, reliability and productivity of the equipment that our society depends on for transportation, power and energy, and goods and services.

Why RCM and why now?

In an economy where prices are set globally, manufacturers must profitably produce products with aging equipment operated and maintained by a workforce that is becoming more expensive as time goes by. This means that manufacturing assets must deliver big - as should the maintenance strategies, such as RCM, to maximise profitability.

For the economic optimisation to be realised, RCM guides the reliability investment with improvement measures and techniques. NASA has identified specific guiding principles of RCM. However, the reliability engineer must answer the following questions:

• What is the system or equipment asked to do?
• What functional failures are likely to occur?
• What are likely consequences of these functional failures?
• What can be done to prevent these functional failures?

In the past, attempts to achieve reliability were made with frequent rebuilds. The strategy was founded on the assumption that the failure rate of machines increased as the asset aged. While some items fail in this manner, most complex systems, such as those found in processing and manufacturing plants, do not. In one study, 30 identical deep-groove ball bearings were run to failure on a test stand under highly controlled conditions. The variation in failure times was so great that if you statistically estimated the appropriate replacement time at the 95% confidence level, the machine would never be started!

In the field, the variation in time-to-failure is even greater. Therefore, the time frame for complex equipment to be rebuilt cannot, in many cases, be effectively estimated.

Selecting a strategy

More recently, vibration analysis, lubrication analysis, thermography and other condition monitoring and predictive maintenance tools have been employed in an attempt to identify early-stage failures so corrective action can be scheduled based on condition. Proactive measures have also been applied to monitor and control the root causes of degradation and failure.

These measures that employ advanced maintenance techniques and technologies have proven effective but, if over applied, can be expensive and counterproductive. Moreover, in some cases, they simply don’t provide the required improvement in reliability to get the job done. In these instances, system redesign or the employment of redundancy is required to achieve the goals of the organisation.

Figure 2: Selecting a reliability strategy.

The process to select a reliability strategy according to RCM is systematic and logical. As Figure 2 suggests, assets are audited with respect to their role in overall system reliability and productivity. If acceptable, no changes are required. If unacceptable, questions about the criticality of the asset define the need to identify the most efficient means of attaining the necessary reliability.

If the asset is deemed noncritical, for example, it is simply run to failure then rebuilt or replaced. For mission-critical systems, advanced maintenance techniques are typically the first choice because their use is relatively inexpensive compared to redesign and the employment of redundancy.

In some cases, redesign or employment of redundancies is required to meet the objectives of the organisation. Redesign in the form of proactive measures to control (and monitor) lubricant contamination, alignment, balance, etc, is usually less expensive to deploy than failure-detection strategies. Conversely, more involved system redesign is typically expensive and often produces unpredictable results.

The employment of redundant systems is the most expensive method to improve reliability, but does provide accurate results. Employment of RCM helps avoid the casual application of the latest panacea strategy, avoiding mistakes that waste resources and provide mediocre and unpredictable performance.

Table 1 summarises strategies for achieving reliability and the conditions in which they are selected in the RCM process. In today’s competitive environment, organisations are looking for advanced maintenance strategies, especially condition-based maintenance, to provide the necessary reliability at minimum cost. The cost to rebuild or replace is quite high and yields dubious value.

Table 1: Maintenance strategy.

Purchasing and maintaining redundant systems is reserved for the most critical systems where no other strategy provides satisfactory results. Advancing technology has brought condition-based maintenance to the forefront of the RCM movement. Lubrication management and oil analysis play an integral role as well.

Analytical tools

The reliability engineer employs a number of analytical tools to optimise reliability relative to mission goals. Some of the more common tools include:

Reliability statistics

Reliability statistics differ from conventional experimental statistics. They provide the means to estimate the likelihood that a system will achieve its mission, given a stated duration and operating conditions. It is important to become knowledgeable about the methods of reliability engineering in advance of undertaking an RCM project.

Reliability block diagrams Once subsystem reliability is determined, the system can be effectively modelled from the reliability perspective. Once constructed, the weak links usually become evident and can be addressed with reliability growth measures to eliminate the deficiencies. Figure 3 illustrates block-diagrammed examples of simple serial, parallel and combination systems.

Figure 3: Serial, Parallel and combination systems.

Failure modes effects and criticality analysis (FMECA)

FMECA is the inductive process of identifying primary functional failures, their related failure modes or states, the effect of the failure modes on the operation of the system, and the associated criticality of the failure mode as a function of impact and likelihood. This valuable analytical tool enables the removal or better management of failure modes through applying advanced maintenance techniques, redesign or redundancy.

Table 2: MP-F interval versus RCFA.

Root-cause failure analysis (RCFA)

RCFA assesses a failure after the fact with the intent to determine its root cause for occurrence. Once the root cause is ascertained, the engineer can assess the risk of recurrence, the success with which the root cause might be controlled, and the cost to control it. With this information, a decision can be made to deploy control measures or to let it go.

Conclusion

Techniques used to improve reliability must harmonise and align with the organisational objective of optimised asset utilisation and maximised profit. RCM is the heart and soul of this process and maintenance and reliability personnel play an increasingly vital role in the RCM process.

References

1. Moubray J, RCM II - Reliability-Centered Maintenance, Industrial Press, Inc, New York, 2nd edition, 1997.
2. National Aeronautics and Space Administration, RCM Guide for Facilities and Collateral Equipment, 1996.

by Drew Troyer, Noria Corporation