Predictive maintenance is the application of condition-based monitoring technologies, such as ultrasound, infrared, vibration, oil analysis, or motor testing, for the purpose of early detection. In preventive maintenance, job plans are performed on a time- or cycle-based schedule.
Predictive maintenance uses technologies to assess the condition of equipment and foresee failures before they happen. The trick is in tying the various tools and what they’re telling you together. “Why do we do use condition monitoring?” asked Carey Repasz, CMRP, principal consultant at Allied Reliability Group. He led an afternoon workshop at UE Systems’ Reliable Asset World conference in Clearwater Beach, Florida.
Condition-based maintenance can prevent failures, forecast maintenance needs, extend machinery life, and improve reliability of assets. The basis for tracking failures is the P-F curve. “There are two schools of thought on the P-F curve,” he explained. “Point P is the first place you can detect a failure, and Point P is the moment a defect or unacceptable condition enters the system. I tend more toward the latter. I used to call Point F the point where it is the smoldering hunk of ruin. But Point F represents the loss of acceptable function, but it still might be working.”
As equipment failure modes are understood, all of the predictive technologies are great, said Repasz. “We’re going to try to have detection as close to Point P as possible,” he explained. Maintaining is the act of causing to continue, while repairing is restoring to an acceptable condition. A maintenance organization’s primary function is to effectively and efficiently ensure the plant’s production capacity.
“Does this sound more like the definition of maintain or the definition of repair?” asked Repasz. “The two concepts are easily confused because they both require the same people with the same skill sets and the same tools. However, the differences in the philosophies are huge.”
|“The real purpose of condition monitoring is to assist in the identification and elimination of the root cause of machinery defects. The most immediate benefit of PdM to a maintenance organization is time.” — Carey Repasz, Allied Reliability Group|
Repasz first started his career as a vibration analyst. “I went to my Category 1 class and came back,” he explained. “My first time I called a bearing, it was awesome for the first half-second, and then it was terrifying. I found myself pulled into an abyss of fear. It was a bearing in the worst possible place. I wanted the rolling elements to pour out of that bearing when they pulled the bearing. But we had an outage coming up, so I waited.”
One of the greatest challenges to PdM is in wanting to squeeze all of the life out of equipment before it fails, he said. “Get all the good out of each machine before it fails,” said Repasz. “That’s the real fiduciary responsibility. Unplanned work takes an average of 30% more time to complete. Planned work is a nice linear process. With unplanned work, you get a phone call, and then you go with no equipment because you don’t know what it is. Parts and tools need to be found and coordinated and arranged. Emergency work costs an average of 50% more in parts.”
The most economical time to execute maintenance is the moment the defect begins, said Repasz. “That’s not necessarily the moment you find it, so find it as soon as possible,” he explained. “The real purpose of condition monitoring is to assist in the identification and elimination of the root cause of machinery defects. The most immediate benefit of PdM to a maintenance organization is time. Identify defects nearer to Point P on the P-F curve and act on that information in a timely manner.”
Repasz also shared a real-world example of a motor in the final stages of failure. “Everybody knows it’s about to fail,” he explained. “We’re beyond a simple repair. It’s Thursday, it’s a batch process that can’t be shut down, and the motor needs to run until Sunday to finish the batch. The motor will cost $90,000 to completely rebuild. Losing the motor will result in losing a production run. Losing this production run has a $400,000 implication.”
The plant took the chance and ran the motor, hoping it would last until the end of the production cycle. “It didn’t make it,” he said. “The plant lost $490,000. From a maintenance and reliability standpoint, there are four questions that need to be answered to learn from this situation.”
- How long did we know about this problem? They knew for six months.
- How many chances did we have to do something about it? They had three or four chances — windows of downtime where they could have worked on it.
- What would this have cost if we’d done something about it? They found it on an IR route. It was hot, which meant the motor was dirty from contamination. The motor had a thermal signature.
- What changes to our processes and procedures do we need to make so this never happens again?
|Larry Shyne, Novozymes|