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If you thought oil analysis was only about monitoring oil properties, you're in for a big surprise. It's a fundamental premise that we don't analyze the oil merely to generate data, but rather we are surveying for answers to machine reliability questions. Many of the answers are imbedded in the body of in-service lubricants, but many others are found in remnants of degraded lubricant and sloughed-off debris. These discarded impurities can be far more revealing to machine reliability than what remains oil-borne.
Too often I've seen lab data show an oil to be relatively clean when, at the same time, filters were plugging prematurely and there was heavy tank bottom sediment. For the most part, low-viscosity industrial lubricants like turbine oils have limited impurity-holding capacity. This simply means that these fluids are quick to release solid and liquid impurities to machine surfaces, filters, separators and system quiescence zones.
We need to figure out:
What is being released and why (dirt, additive floc, sludge, etc.)
Where it is being released (filters, tanks, cool surfaces, hot surfaces, near generating sources, etc.)
The rate of production and release
The resultant damage to the oil
The cleanup and remediation requirement for the machine
Rigid and semisolid contaminants are often quickly removed by filters and centrifuges due to their size, weight and polarity. Some filters can load up with pasty sediment and debris in short order. The source of these oil suspensions can vary but include oxidation, additive precipitation, coolant leakage and thermal degradation.
Used filters should be routinely inspected for abnormal deposits on the media and the adjacent metal surfaces. When abnormally short filter life is observed, this could be a sign of generating to contaminant ingression source. Again, do not rely entirely on oil analysis for confirmation. Analysis of the filter and the impurities it holds is equally important.
Some contaminants and impurities never make it to the filter. Due to their relative weight, they stratify out of the oil in static zones such as tanks, reservoirs, static piping, etc. Many tanks routinely maintain sludge lines while others rarely exhibit heavy phases of solids and liquid impurities. Bottom sediment and water (BS&W) can quickly be observed using special sight glasses that enable easy sampling as well.
Of course, not everything that's an impurity is heavier than the oil. As such, some light or aerated impurities will ride the surface of our sumps and reservoirs. These may be floating patches of waxy, sludgy and/or frothy suspensions. Skimming the surface with a suitable sampling container will help answer questions on their source and formation.
Some impurities are formed due to temperature sensitivities. For instance, splash zones on the walls of tanks just above the typical oil level will often produce a greasy bathtub ring. These cool surfaces cause soft insolubles to be discharged out of the oil. Many types of impurities can cause these rings. They can be sampled easily using an artist's spatula for laboratory analysis.
Hot zones can toast the oil and its additives, forming scabby black surface deposits. Load zones of bearings, heaters, combustion, steam and furnaces can all provide the heat needed to thermally degrade oil. Sampling can sometimes be difficult, but using a knife or a spatula will generally do the trick.
Most soft impurities have polar affinities, meaning they can be stripped from the oil by preferential attraction. For instance, some lubricants going through bulk oil oxidation can cause a goldish varnish to form on all internal, oil-wet machine surfaces. This polar migration of the insoluble impurity takes time and is also temperature-dependent. Evidence of this can show up on filter endcaps, sight glasses and tank walls. Analysis of this can often be performed by doing a varnish insolubles test on the oil using a standard submicron membrane.
What to Test
Muck samples need to be examined for elemental and organic fractions. Solvent extraction and Fourier transform infrared (FTIR) can help isolate the organic components. Microscopic analysis is also a good idea. Compare data to new oil, in-service oil and close-proximity contaminants.
However, don't be surprised if there is not even a trace similarity between the oil samples and the muck samples. This, of course, points to the value of muck sampling as a standard feature of a well-engineered condition monitoring program.