RCM Reliability Centered Maintenance

The RCM methodology has its origin in the aviation industry, but is nowadays a well-recognised methodology for preventive maintenance planning.

In the following some basic steps of the RCM methodology will be presented. The RCM decision logic is often considered to be the heart of RCM and is shown below.

WHAT IS RCM?

RCM is a systematic consideration of system functions, the way functions can fail, and a priority-based consideration of safety and economics that identifies applicable and effective PM tasks.

The main objective of RCM is to reduce the maintenance cost, by focusing on the most important functions of the systems, and avoiding or removing maintenance actions that are not strictly necessary.

RCM is a technique for developing a PM program. It is based on the assumption that the inherent reliability of the equipment is a function of the design and the build in quality. An effective PM program will ensure that the inherent reliability is realised.

There is no common approach to an RCM analysis, but according to SINTEF’s experience the following steps have been found to cover the main elements of an RCM analysis:

1. Study preparation
2. System selection and definition
3. Functional failure analyse (FFA)
4. Critical item selection
5. Data collection and analysis
6. FMECA
7. Selection of maintenance actions
8. Determination of maintenance intervals
9. Preventive maintenance comparison analysis
10. Treatment of non-critical items
11. Implementation
12. In-service data collection and updating

In the following the steps are briefly described.

Step 1: Study preparation

An RCM project group is established, and should include at least one person from the maintenance function and one from the operations function, in addition to an RCM specialist. The RCM project group should define and clarify the objectives and the scope of the analysis. Requirements, policies, and acceptance criteria with respect to safety and environmental protection should be made visible as boundary conditions for the RCM analysis.

For the system(s) selected in Step 2, one should:

1. identify and describe the system’s required functions and performance criteria,
2. describe input interfaces required for the system to operate, and
3. identify the ways in which the system might fail to function.

A simplified FMECA approach is recommended for this analysis.

The objective of this step is to identify the analysis items that are potentially critical with respect to the functional failure identified in Step 3. These analysis items are denoted functional significant items (FSI). In addition to the FSIs, one should also identify items with high failure rate, high repair costs, low maintainability, long lead-time for spare parts, or items requiring external maintenance personnel. These analysis items are denoted maintenance cost significant items (MCSI). The combination of the functional significant items and the maintenance cost significant items are denoted maintenance significant items (MSI).

In a complex system we may need a formal approach to identify the FSIs. Depending on the complexity of the system, importance ranking based on techniques like fault tree analysis, reliability block diagram or Monte Carlo simulation may be suitable.

The objective of this step is to identify the dominant failure modes of the MSIs identified in Step 4. A variety of different FMECA forms are used in the main RCM references.

In this step the RCM decision logic is used to guide the analyst through a question-and-answer process. The input to the RCM decision logic is the dominant failure modes from the FMECA in Step 6. The main idea is for each dominant failure mode to decide whether a PM task is applicable and effective, or if it will be best to let the item deliberately run to failure and afterwards carry out a corrective maintenance task.

To determine the optimal maintenance interval some crucial information is needed. We need information about the failure rate function (hazard rate), the likely consequences and cost of the failures the PM tasks are supposed to prevent, the cost and risk of the PM tasks, etc.

Two overriding criteria for selecting maintenance tasks are used in RCM. Each task selected must meet two requirements:

1. It must be applicable
2. It must be effective

Applicability: meaning that the task is applicable in relation to our reliability knowledge and in relation to the consequences of failure. If a task is found based on the preceding analysis, it should satisfy the Applicability criterion. A PM task will be applicable if it can eliminate a failure, or at least reduce the probability of occurrence to an acceptable level - or reduce the impact of failures!

Cost-effectiveness: meaning that the task does not cost more than the failure(s) it is going to prevent. The PM task's effectiveness is a measure of how well it accomplishes that purpose and if it is worth doing. Clearly, when evaluating the effectiveness of a task, we are balancing the "cost" of performing the maintenance with the "cost" of not performing it.

In Step 4 critical items (MSIs) were selected for further analysis. A remaining question is what to do with the items that are not analysed. For plants already having a maintenance program it is reasonable to continue this program for the non-MSIs. If a program is not in effect, maintenance should be carried out according to vendor specification if they exist, else no maintenance should be performed.

A necessary basis for implementing the result of the RCM analysis is that the organisational and technical maintenance support functions are available. A major issue is therefore to ensure that these support functions are available.

Experience has shown that many accidents occur either during maintenance or because of inadequate maintenance. When implementing a maintenance program it is therefore of vital importance to consider the risk associated with the various maintenance tasks.

Step 12: In-service data collection and updating

The reliability data we have access to at the outset of the analysis may be scare or even second to none. In our opinion, one of the most significant advantages of RCM is that we systematically analyse and document the basis for our initial decisions, and, hence, can better utilise operating experience to adjust that decision as operating experience is collected. The full benefit of RCM is therefore only achieved when operation and maintenance experience is fed back into the analysis process.

Brief History of RCM

United Airlines was sponsored by the US Department of Defense to write a comprehensive document on the relationships between Maintenance, Reliability and Safety. The report was prepared by Stanley Nowlan and Howard Heap, it was called ?Reliability Centred Maintenance'. Outside the aerospace industries, the application of MSG-3 is generally known as RCM. The work of the airlines predated similar problems that spread throughout industry during the 1980?s, consequently industry has been fortunate in being able to use the airlines prior experience.

Why is RCM different?

The development of RCM has allowed us to literally re-define maintenance. It re-focuses our thinking by differing in four very significant ways from all that went before it:

1. The objective of a successful PM program is to prevent or mitigate the consequences of failures, not to prevent the failures themselves. Of the thousands of possible failure modes on any facility or installation, each has a different effect on e.g. safety, operations, environment or cost. It is the failure consequence that determines what, if any resources will be used to prevent their occurrence. This leads to the conclusion that that if the consequence of a failure does not have an adverse effect on safety; operations, environment or cost, then there is no need to carry out scheduled maintenance.

2. The consequences of failure differ depending on where and how items are installed and operated. For example, missing an appointment is a likely outcome of mechanical breakdown of a car usually driven in urban areas, however the same failure in the middle of the Sahara desert will have much more severe consequences. A formal review of failure consequences focuses attention on maintenance tasks that have most effect, and diverts energy away from those which have little or no effect. This helps ensure that whatever is spent on maintenance is spent where it will do the most good.

3. We no longer assume that all failures can be prevented by PM or that even if they could be prevented, that it would desirable to do so. Consider puncture of car tyres, what affects the number of failures rate is the number of nails in the road, not the age of tyre. PM is simply not applicable to this failure mode.

4. We are concerned principally with what we want the equipment to do not what it actually is. Say we need a hand held, portable writing instrument, capable of producing erasable text and lines in the width range 0.2 to 0.5 mm. Both a wooden and a mechanical (propelling) pencil broadly satisfy this simple description. The specification of what we need is independent of the method used to achieve it. By identifying what we actually want means we focus our maintenance on what matters and identify any gaps in the required performance and that which a system is capable of.

RCM builds on these simple ideas to determine applicable and effective maintenance for each failure. The mechanics of the RCM process itself are well described by other authors.

How does the RCM process fit into the Preventive Maintenance cycle?

The power of RCM is not in doubt. There is more than enough hard evidence from manufacturing, extractive, transport and process industries that prove the techniques value in establishing and improving system maintenance. It is however a sharp tool, and is usually best applied in selected areas rather than broadly across a facility. To achieve worthwhile results it must also be carried out by (or at least with) the actual operators and maintainers of the systems in question. RCM is not a "quick fix" solution, time and effort must be invested on training, raising awareness, execution and implementation.

It does however achieve an understanding of how plant works, what it can (or cannot) achieve, and the causes of failure. By doing so it focuses maintenance effort on those areas where it is beneficial. The analysis itself is carried out in groups consisting of experienced supervisors, and specialists (if needed). These groups set up maintenance tasks and an ownership concept is developed. The development of RCM had lead to a radical change of direction in our understanding of maintenance and its performance and has presented us with what were entirely new concepts, Today, maintenance directly influences the core aspects of modern business, safety and environmental integrity, energy efficiency, quality, uptime and costs. RCM forms the core of any effective maintenance policy and should therefore be at the heart of your business.

Integrating RCM with Effective Planning and Scheduling-Part 1

Integrating RCM with Effective Planning and Scheduling-Part 2