As a result, we should always ensure that we are performing our analyses on assets that will show a return on the resources, time and money we invested in performing the analysis, implementing the tasks and performing the tasks.

Prior to starting a RCM analysis, one of the upfront tasks that must be completed is estimating the size of the RCM analysis and the time it will take to complete the analysis phase of the project. The size of your RCM analysis and the time it takes to complete the analysis is dependent on four things;

1. The number of components/functions contained in the envelope of the analysis.

2. The number of failure modes associated with these components/functions

3. The quality of information gathered prior to starting your analysis (how up-to-date are your drawings, OEMs and history?)

4. The experience of your RCM team and facilitator

A new RCM team with an experienced RCM facilitator should be able to complete 85 to 100 functions and 120 to 140 failure modes in a one-week session. Completing failure modes is defined as identifying the failure modes, listing failure effects and making RCM task decisions. As the team and facilitator become more experienced, the number of functions and failure modes will increase over time by as much as 30 percent.

To estimate the size and time it will take to complete a RCM Blitz analysis, select a critical asset and, using a P&ID drawing, begin highlighting and listing all the components within this system. Define a component as a modular asset that can be removed or repaired as a stand-alone item. For example, a gearbox is a component, a pump is a component, a limit switch is a component. While each component may contain several parts, the components are counted as a single item. Each will have one or more functions and can have several failure modes. Once you have completed your component list, multiply the number of components by 1.5; this will give you a estimated number of functions. Remember, you want to limit the number of functions in your first RCM to between 85 and 100.

Now that you have completed your component list, this will become the boundary of your RCM analysis. While other components outside this boundary may impact the performance or reliability of your asset, it is important to not list failure modes for components outside the boundary in this RCM analysis. These components and their failure modes may well be analyzed in your next analysis. Using your component list, you can now estimate the number of failure modes for your RCM analysis by using a failure modes estimating tool. The failure modes estimating tool contains a list of common failure modes for component types. As an example, let's say we have a electrical motor as a component on our list. Looking at the estimating tool, we can see that a motor has five common failure modes:


1. Motor bearings fail due to contamination.

2. Motor bearings fail due to lack of lubrication.

3. Motor bearings fail due to over-lubrication.

4. Motor bearings fail due to misalignment of motor to gearbox.

5. Motor windings fail due to insulation breakdown.

For each component on the list, you will now estimate the number of failure modes associated with that type of component. It should be noted that the failure modes contained in the common failure mode list are not a comprehensive list of failure modes. It is a list of what we most frequently see as failure modes. This list should never be used to create a RCM "template".

While these are common failure modes, we should never assume that these are the failures that your asset is experiencing. As you begin your RCM analysis, your team will develop your failure modes in accordance to what your asset is actually experiencing and what is likely to occur. Once you have noted the common failure modes for the components on your list, add up the total number of common failure modes. This will give you a estimate of the number of failure modes inside the boundary of your RCM analysis. Remember, you should limit the number of failure modes in your first RCM to between 120 and 140 failure modes.

Douglas Plucknette

Doug Plucknette is the founder of Reliability Solutions, Inc., and has worked with large industrial companies worldwide, helping them improve their reliability and operational performance. He is the author of the books, “Reliability Centered Maintenance Using RCM Blitz™” and “Clean, Green and Reliable,” and has published over 60 articles. He has been a featured speaker at numerous industry conferences.

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