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It is often said that a well-engineered oil analysis program has three essential categories. In the past, I’ve referred to this as the iron triangle of oil analysis. The three categories are:
Adequate coverage of these three categories builds a fundamental foundation for success with oil analysis. However, using this same data, an oil analysis report can convey information and prescribe maintenance actions well beyond what was generally intended by the three categories. It has a lot to do with responding to what a maintenance professional really wants to know about an oil’s in-service conditions.
To start, the oil analyst should know the machine’s operational criticality. This can be broken down into two basic elements. The first is mission criticality, which considers the consequences of failure (production losses, safety, etc.) in relation to the machine’s intended mission. The second is functional restoration, which basically asks in the event of failure, what would it cost to replace, repair and rebuild the broken machine?
It is important to note that these two elements of operational criticality don’t always go hand-in-hand. Because of redundancy and standby equipment in some processes, an expensive repair may not always result in costly downtime. Likewise, in other cases, huge production losses may be triggered by small throw-away machine components. It stands to reason that operational criticality is the starting point in defining most machinery maintenance functions. For more information on this, refer to the reliability centered maintenance (RCM) articles on Noria’s Web site.
So, how does this relate to oil analysis? Well, for one it defines such things as how often the oil should be sampled, what tests should be performed and the setting of alarms and limits on oil analysis results. As a practical matter, the ability of an oil analyst to make insightful and useful comments about oil analysis data is rather limited without reference to operational criticality. In my opinion, a statement on operational criticality should be a permanent fixture on every oil analysis report - placed at the top alongside the description of the machine and lubricant.
Operational criticality is best defined by the asset owner, not by outside oil analysts or other nonstakeholders. For instance, consider using a scale from one to five for both mission criticality and functional restoration. A rating of one might mean failure is inconsequential while a rating of five alerts that failure could have devastating consequences. The cost, frequency and quality of oil analysis will likely vary in accordance to how the machine is rated for operational criticality.
Now let’s talk about how to move oil analysis beyond the three categories. To do this, let’s consider the following two oil analysis scenarios:
Sample A. A machine is sampled and the oil’s properties have degraded beyond the point of a comfortable safety margin. Considering the machine’s high degree of operational criticality, an oil change is recommended immediately by the on-site lubricant analysts. For this machine, oil has historically been changed on-condition.
Sample B. In another machine, oil is changed at a fixed interval of 2,000 meter-hours or one year, whichever first occurs. The oil is always sampled at 1,000-hour intervals, which is midway into the drain interval and again just before the drain. In this example, the sample is collected at the second interval - the 2,000-hour point. This machine has high operational criticality as well.
Next, think of each charge of oil in these machines as being a single generation in the evolution of the machine. A generation begins when a full charge of oil is added to its reservoir or compartment. It ends with the drain. The interval between the full charge and drain is the oil’s service life.
I use the term generation to suggest the human analogy. Humans pass knowledge and wisdom down through the generations. The growing repository of human intelligence and technology gives rise to wiser living and enhanced quality of life. Now back to oil.
As I stated, the oil drain marks the end of a generational era. However, let us not fail to learn those valuable pearls of wisdom from the departing generation before it goes to its final resting place. Within the aged and decaying oil (and the oil’s filter) is a life’s journal, often rich in information. To enhance the life expectancy and performance of subsequent oil generations, these stories should be studied for lessons learned.
Now, let’s move this discussion from the abstract to the practical. What do these oil analysis reports need to be telling us about the two oils described above? The following table provides a list of questions that, in a more ideal world, I would want answered about both oil samples:
Think of the data from each oil sample, especially the one taken at the end of the oil’s service life, as the end result of a 2,000-hour field study or research project. Based on the data, what theories were proven or disproved by the experiment? What questions can be answered that could only be pondered before? How does the new information modify or refine the effectiveness of oil analysis and lubrication practices going forward?
Perhaps it’s time to make a closer inspection of your oil analysis report.