Oil analysis: 5 things you didn't know

Noria Corporation
Tags: condition monitoring, predictive maintenance, maintenance and reliability

I'm sure that most readers of Reliable Plant are well aware of the value brought by oil analysis. Used appropriately, there is little doubt that an effective oil analysis program can help identify lubrication-related failures, often before any significant machine wear has occurred. But as a veteran instructor of oil analysis and lubrication courses, I find all too often that companies miss the boat on oil analysis simply because they don't understand what oil analysis can and can't do. So in the interests of setting the record straight, I present to you what I like to call the "five fallacies" of oil analysis - things that are often overlooked or not understood but vital to the long-term benefits of oil analysis as a conditioning monitoring tool.

Fallacy #1: Reservoir sampling is fine.

Fact: Oil analysis, just like real estate, is all about location, location, location. While certain homogeneous properties such as viscosity are unchanged no matter where in the system you sample from, the concentration of suspended material such as wear debris, particles and moisture can vary by several orders of magnitude depending on where you take the sample. For maximum effectiveness, you should take samples immediately downstream of the component(s) of interest or source of contaminant ingression. In fact, in large circulating systems with significant reservoir capacity, the dilution effects alone can render the identification of active machine wear virtually impossible with reservoir sampling.

Fallacy #2: Routine oil analysis will always find active machine wear.

Fact: In oil analysis, size really does matter. Depending of the wear mode and degree of severity, wear particle sizes are often 5 to 10 microns and larger. So, why does this matter? Size is important because the most commonly used test method to assess active machine wear - elemental spectroscopy - has a limit to the size of particles it can detect. Depending on instrument and methodology, conventional elemental analysis can't detect particles larger than 3 to 8 microns in size, rendering it useless in situations of advanced machine wear, or where the failure mode naturally generates larger particles, such as fatigue or severe sliding wear.

Fallacy #3: Particle counting is proactive.

Fact: Particle contamination accounts for 60 to 80 percent of all lubrication-related failures. Because of this, most oil analysis practitioners recommend the use of ISO particle counting to measure fluid cleanliness, believing that particle counting is a proactive means to prevent many failures. But unless you have taken the time to determine exactly how clean each system needs to be and have a plan to address fluid cleanliness levels that are too high, particle counting will have little to no effect at reducing the overall number of machine failures.

Fallacy #4: Water is water is water.

Fact: Water, in the form of washdown, airborne humidity or from the process itself is a dangerous contaminant. Because of this, all oil analysis labs test for water. However, in many instances, the test methods used by some labs are unable to detect the presence of water until it is five to 10 times higher than recommended for some machines. Like many oil analysis test parameters, labs have a variety of methods they can use to identify water. The diligent oil analysis end-user should insure that the test methods used by their lab meet or exceed the minimum required detection limits for each test parameter.

Fallacy #5: Vibration analysis is better at finding failures than oil analysis.

Fact: While it's true that some failure mechanisms, such as misalignment, are better detected using vibration, most experts - including those that specialize in vibration analysis - recognize that oil analysis will generally detect active machine wear before vibration analysis. The true value of vibration analysis is its inherent ability to localize the problem (inner race, outer race, cage wear, etc.) rather than any ability to find a problem earlier in the failure cycle. In truth, the combination of oil analysis for early detection coupled with the advanced diagnostic capabilities of vibration analysis make the benefits of these two techniques far greater when treated as teammates rather than opponents.

There you have it - the most misunderstood aspects of oil analysis. Get them wrong and you could be living with a false sense of security. Get them right and you should reap the benefits that many companies get from a well-engineered, reliability-focused oil analysis program.