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Forget preventive maintenance. Today's uptime requirements call for an entirely different approach
Maintenance has come a long way since the "fix it when it breaks" mentality of the 1940s and the preventive maintenance philosophy of the 1970s and 1980s. Today, the new world of reliability-centered maintenance (RCM) calls for computers, software, and sensors to achieve maximum plant availability and reliability at the most effective cost (Figure 1).
A big surprise is preventive maintenance (PM) actually can be bad for certain systems! Not only is it expensive, but it doesn't work well with modern, high-tech equipment. So instead of PM, we are entering a whole new world of condition monitoring, loop analysis, and predictive maintenance.
Figure 1: Shoot for Uptime
Reliability-centered maintenance aims to find the correct balance between maintenance spending and process operations. According to Honeywell, only 5% of all plants have achieved the correct balance. (Source: Honeywell)
Although RCM often works best with fieldbus architectures and high-level asset management software (because they can more easily obtain and process data) the techniques involved are not beyond the reach of a typical process control user, even those with legacy control systems. This is because it's not how you acquire the data that's important, it's what you do with it.
Modern maintenance technology procedures began years ago in the military. The recent Gulf War II proves beyond a doubt just how effectively these techniques work.
"I used vibration monitoring and maintenance management [15 years ago] in the propulsion power plants of U.S. Navy vessels," says Robert Rosenbaum, an automation consulting engineer in American Canyon, Calif. "The use of portable handheld vibration instrumentation was so successful that the Navy purchased several permanently installed vibration monitoring systems for its fleet."
Rosenbaum also says he's familiar with RCM as once used by United Air Lines to prevent failure in commercial aircraft systems.
Vibration monitoring is just one part of an RCM program. The overall RCM process includes procedures to determine the functions and performance standards of an asset, what causes it to fail, what happens when it fails, and what can be done to prevent failures (Table I).
Table I: First Answer These
Before considering reliability centered maintenance, determine the functions and associated performance standards of the asset in its present operating context:
1. In what ways does it fail to fulfill its functions?
2. What causes each functional failure?
3. What happens when each failure occurs?
4. In what way does each failure matter?
5. What can be done to predict or prevent each failure?
6. What if a suitable proactive task cannot be found?
The commercial airline industry was the first to realize the benefits of a maintenance decision-making process. According to a white paper by Aladon, this led to the development of the MSG3 process in the aviation industry; in manufacturing, it's just called RCM. (For a complete description and multiple articles on RCM, go to www.aladon.co.uk.)
One of the most startling developments to come out of RCM studies involves preventive maintenance. "Many people still believe that the best way to optimize plant availability is to do some kind of proactive maintenance on a routine basis," says Aladon. This assumes one traditional view of failure (Figure 2), where devices fail as they enter a wear-out zone after a certain period of time.
This may have been true 30 years ago, but equipment is much more complex these days. Now, we identify six patterns of failure to deal with (Figure 3). According to Aladon, studies on commercial aircraft showed only 4% of failures conformed to pattern A, which contradicts another widely held belief that most products have a "bathtub" failure curve. Only 2% conformed to B, 5% to C, 7% to D, and 14% to E.
But here's the startling development: a whopping 68% conformed to pattern F--high infant mortality followed by random failures.
"These findings contradict the belief that there is always a connection between reliability and operating age," says Aladon. "Nowadays, this is seldom true. Unless there is a dominant age-related failure mode, age limits do little or nothing to improve the reliability of complex items. In fact, scheduled overhauls increase overall failure rates by introducing infant mortality back into otherwise stable systems."
Commercial aircraft and process control systems both use similar systems: pneumatics, electro-hydraulics, servomotors, networks, control valves, pumps, miles of wire and cables, networks, computers, electronic controls, and flow, temperature, level, and pressure sensors.
Figure 2: Good Old-Fashioned Breakdowns
Thirty years ago, devices were understood to fail when they wore out. (Source: Aladon)
Some process industry data seems to correspond exactly with patterns E and F. "The majority of failures in valves and control loops is not predictable and the probability of failure does not increase with time," says Lane Desborough, manager of loop management services, Honeywell Industry Solutions, Thousand Oaks, Calif. "There is little evidence that valve failure can be predicted reliably based on accumulated stem travel alone."
If failures of process equipment are random, so much for preventive maintenance. What do we do now?
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