Reliability-centred maintenance (RCM)

Reliability-centred maintenance (RCM) is a systematic process of preserving a system’s function by selecting and applying effective Preventive Maintenance (PM) tasks.

However it differs from most approaches to PM by focusing on function rather than equipment.

RCM governs the maintenance policy at the level of plant or equipment type. In general the concept of RCM is applicable in large and complex systems such as large passenger aircraft, chemical plant, oil refineries and power stations.

The RCM approach arose in the late 1960s and early 1970s when the increasing complexity of systems and consequent increasing size of the preventive maintenance task forced a rethink of maintenance policies among manufacturers and operators of large passenger aircraft. Pioneering work on the subject was done by United Airlines in the 1970s to support the development and licensing of the Boeing 747.

The principles which define and characterise RCM are:

• a focus on the preservation of system function;
• the identification of specific failure modes to define loss of function or functional failure;
• the prioritisation of the importance of the failure modes, because not all functions or functional failures are equal; and
• the identification of effective and applicable PM tasks for the appropriate failure modes. Applicable means that the task will prevent, mitigate, detect the onset of, or discover, the failure mode. Effective means that among competing candidates the selected PM task is the most cost-effective option.

These principles are implemented in a seven step process:

1. The objectives of maintenance with respect to any particular item/asset are defined by the functions of the asset and its associated desired performance standards.
2. Functional failure (the inability of an item/asset to meet a desired standard of performance) is identified. This can only be identified after the functions and performance standards of the asset have been defined.
3. Failure modes are identified, which are reasonably likely to cause loss of each function.
4. Failure effects are documented, describing what will happen if any of the failure modes occur.
5. Failure consequences are quantified to identify the criticality of failure. RCM not only recognises the importance of the failure consequences but also classifies these into four groups: Hidden failure; Safety and environmental; Operational and Non-operational.
6. Functions, functional failures, failure modes and consequences are criticality analysed to identify opportunities for improving performance and/or safety.
7. Preventive tasks are established. These may be one of three main types i) scheduled on-condition tasks which employ condition-based or predictive maintenance; ii) scheduled restoration; and iii) scheduled discard tasks.

Although one of the prime objectives of RCM is to reduce the total costs associated with system failure and downtime, evaluating the returns from an RCM program solely by measuring its impact on costs hides many other less tangible benefits. Typically these additional benefits fall into the following areas:

• improving system availability
• optimising spare parts inventory
• identifying component failure significance
• identifying hidden failure modes
• discovering significant, and previously unknown, failure scenarios
• providing training opportunities for system engineers and operations personnel
• identifying areas for potential design enhancement
• providing a detailed review, and improvement where necessary, of plant documentation