significant factor stems from the evidence that a vast majority of our industrial plants and facilities have been operating for extended periods, years in many cases, in a reactive maintenance mode.  That is to say that the maintenance resources have been almost totally committed to responding to unexpected equipment failures.  Corrective, not preventive, maintenance is frequently the operational mode of the day, and this tends to blur what is preventive and what is corrective.

In one actual extreme case, a plant developed an entire culture that fostered a feeling of pride in people's ability to fix things rapidly and under pressure when a forced outage occurred.  Plant personnel viewed their actions as preventive in the sense that they were able to "prevent" a long outage because of their highly efficient and effective reactive and corrective actions.  What the plant staff did not consciously recognize (or acknowledge) was that they were the highest cost per unit producer among their peers.

We use the following definition of preventive maintenance (PM):

Preventive maintenance is the performance of inspection and/or servicing tasks that have been preplanned (i.e., scheduled) for accomplishment at specific points in time to retain the functional capabilities of operating equipment or systems.

The word "preplanned" is the key element in developing a proactive maintenance mode and culture.  In fact, this now provides us with a very clear and concise way to define corrective maintenance (CM):

Corrective maintenance is the performance of unplanned (i.e., unexpected) maintenance tasks to restore the functional capabilities of failed or malfunctioning equipment or systems.  As viewed by the authors, the entire world of maintenance activity is fully encompassed in these two definitions.

However, there are two troubling factors that people frequently question which give rise to some of the confusions over the "PM or CM" discussions.  The first of these involves the games that people play with the terminology.  These games can be driven by such diverse nontechnical factors as accounting practices or political (regulatory) pressures.  For example, some plants, in addition to planned outages and forced outages, have a third category known as a maintenance outage (MO).

The MO occurs as a result of an unexpected equipment problem which hasn't quite yet reached the full failure state but will do so very soon.  So the plant management will delay the shutdown until some off-peak period when the plant outage is more tolerable, and hope that the equipment will hold out until then.

Now from an operational point of view, this is a very smart thing to do — but, as a rule, MOs are not counted when it comes to reporting the plant forced outage rate.  Somehow they seem to wind up in the preplanned category ("after all, we planned to fix it next Saturday!").  Make no mistake about it, an MO is a forced outage and should be labeled as such when measurements are made.  You are only kidding yourself to do otherwise.

A second and more dominant area of confusion occurs when a scheduled task reveals unacceptable equipment deterioration (like the problem above in the MO situation, except it was not unexpected since a PM task discovered its presence).  So actions are taken to repair/restore the full functionality before an unexpected operational impact can occur.  Is the repair/restore action preventive or corrective?

If you will recall that the purpose of the PM task is to perform actions that will retain functional capabilities, then the answer is essentially self evident — the repair/restore action is preventive.  Why? Because a proper structuring of the PM task will always include not only the search for equipment condition, but also the requirement to do something about it if the search uncovers a problem.

This search includes PM tasks that require inspection, monitoring parameters that detect failure onset, discovery of hidden failures and even restoration of equipment that was deliberately allowed to run to failure.  Unfortunately, though, many CMMS programs will not allow the user to create or code a new work order to cover the emergent work as PM.  This additional PM work can only be coded as CM.  This inflates the cost of CM, and can lead management to question why CM costs are increasing even when their PM program had been recently improved.

As a general rule, corrective maintenance is more costly than preventive maintenance.  If anyone should doubt this, then just compare two similar plants or systems where one has a proactive maintenance program and the other a reactive maintenance program.  Which one do you think has the lower overall maintenance cost and higher availability?

Why do preventive maintenance?

For the past 15 years, as part of our seminars and client training programs, we frequently ask the question "Why do preventive maintenance?" The answers that we consistently hear reflect the popular belief that PM is done for a rather narrowly defined reason and this, as such, leads to the exclusion of a number of golden opportunities for PM enhancement.

So why do you do preventive maintenance? The overwhelming majority of maintenance and Plant Engineering personnel will respond "To prevent equipment failures." Would that have been your response? If so, you are correct — but not complete in your viewpoint.  Unfortunately, we are not yet smart enough to prevent all equipment failures.  But that does not mean that our ability to perform meaningful preventive maintenance tasks must end there.

In fact, there are three additional and important options to consider.  First, while we may not know how to prevent a failure, frequently we do know how to detect the onset of failure.  And our knowledge of how to do this is increasing every day, and is creating a whole new discipline called predictive maintenance.  Second, even though we may not be able to prevent or detect the onset of failure, we often can check to see if a failure has occurred before equipment is called into service.  Various standby and special purpose equipments (whose operational state is often hidden from the operator's view until it is too late) are candidates for this area.  Thus, discovery of hidden failures is yet another PM option available to us.

There are also situations in a well planned PM program where economics and/or technical limitations can dictate a decision to do nothing — he appropriately labeled Run-To-Failure (RTF) option.  This RTF option is not to be confused with the more general situation of missing potentially useful PM actions due to oversight or lack of attention to PM planning.

To summarize, there are four basic factors behind the decisions to define and choose preventive maintenance actions:

1. Prevent (or mitigate) failure occurrence.
2. Detect onset of failure.
3. Discover a hidden failure.
4. Do nothing, because of valid limitations.

Printed with permission from Butterworth-Heinemann, a division of Elsevier, from RCM - Gateway to World Class Maintenance, by Anthony M. Smith, AMS Associates Inc. in California, and Glenn R. Hinchcliffe, Asset Performance Technologies Inc., Corrales, NM.  Copyright 2004.  For more information about this title and similar titles, please visit www.books.elsevier.com.