Calculating availability to develop preventative maintenance plan
Our view is that the maintenance organization should be treated as a Profit Center for your company. This view derives from our belief that the overwhelming financial aspects of a maintenance optimization strategy are driven by considerations of downtime elimination (or increased availability), which, when properly accomplished, is a direct contributor to company revenue and profit. Preventive maintenance plays a major role in this scenario.
To further discuss the relationship between availability and preventive maintenance, first, let’s clarify some terminology. We frequently hear discussions about plant productivity that will interchangeably use the terms reliability and availability when referring to technical disciplines that must be invoked. The broadly accepted definition of reliability is as follows:
“Availability is a measure of the percentage (or fraction) of time that a plant is capable of producing its end product at some specified acceptable level.”
Thus, by definition, availability must account for plant outages – both planned (scheduled) and unplanned (forced). Scheduled outages are factored into production commitments as a matter of course, as are forced outages at some small and acceptable level.
To understand how the issue of availability really affects us, consider a hypothetical situation at the ABC Corporation where management has formulated a top-level policy to “maintain output.” This is certainly a sound objective for ABC to pursue, but how do we translate this into meaningful tasks for ABC’s various plants throughout the world?
First, we need to realize that this policy is directly addressing a need to maintain or perhaps even increase availability at its plants. The most direct way to do this is to avoid outages, especially lengthy forced outages. In fact, it is quite common to see specific goals assigned to plants to keep their “forced outage rate” below some specified annual value such as 3.5 percent. If the annual scheduled outage rate is 1.5 percent, then it would be expected that the plant would be capable of producing 95% of the time during the year.
But, down at the plant level, how does the General Manager and his supervisors define specific tasks that will help them to meet or exceed those goals? To answer this question, we need to understand more precisely just what constitutes the availability measure. There are two, and only two, parameters that control this measure:
Mean time between failure, or MTBF, which is a measure of how long, on average, a plant (or an individual item of equipment) will perform as specified before an unplanned failure will occur.
Mean time to restore, or MTTR, which is a measure of how long, on average, it will take to bring the plant or equipment item back to normal serviceability when it does fail.
MTBF, then, is a measure of the plant or equipment reliability (R) and MTTR is a measure of its maintainability (M). Mathematically, we can define availability (A) as follows:
Notice that if MTBF is very large with respect to MTTR – that is, if we have a very high plant reliability – availability will also be high, simply because the MTBF parameter dominates what is physically occurring. Conversely, a very small MTTR can also yield a high availability because, even if the equipment fails frequently, it can be restored to service very quickly. Usually, neither of these two limiting cases exist, and we have to work diligently at retaining or improving both the MTBF and MTTR parameters in order to achieve a high degree of plant availability.
Recognizing that MTBF (or reliability) and MTTR (or maintainability) are the parameters that we must influence will now simplify our job considerably. There are several tasks that can be performed, and usually some investigation and evaluation of plant problems and operating practices will reveal where the resources should be focused. In particular, however, the role that an effective preventive maintenance program (PMP) can play in achieving desired levels of availability is of special note. This is true because the PMP can beneficially impact both reliability and maintainability when it is properly specified and conducted. The right preventive maintenance tasks can, for example, be the primary factor in keeping an item of equipment in top running order – tasks as simple as lubrication and alignment checks performed at specified intervals can be the necessary link to retaining an inherent design reliability. In like manner, the right PM tasks could play a major role in decreasing MTTR simply by the use of periodic on-condition monitoring that would detect failure onset and permit an opportunity for repair or replacement at the timing of your choice, thus avoiding the forced outage.
We can summarize our discussion for the ABC Corporation in a single picture shown in Figure 1. The PMP is particularly potent because, when done properly, it produces a double-barreled effect by impacting both reliability and maintainability within the same execution.
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, Consulting Professional Engineer, G&S Associates Inc. in North Carolina. Copyright 2004. For more information about this title and similar titles, please visit www.books.elsevier.com .