Understanding Filter Debris Analysis

"While inspecting one of our hydraulic systems, we noticed some filter-clogging slime. How can we determine the cause of this?"

The first step is to determine what exactly this “slime” is. Is it a foreign contaminant? Is it an oil degradation byproduct? This can be achieved through lab testing. Many of the major oil analysis labs offer filter debris analysis in addition to oil analysis. The tests performed during a filter debris analysis include elemental analysis by inductively coupled plasma (ICP), acid digestion elemental analysis, gravimetric analysis, micropatch analysis and analytical ferrography.

• ICP Elemental Analysis – This is a type of mass spectrometry that is capable of detecting both metal and non-metal materials at very low concentrations.
• Acid Digestion Elemental Analysis – The filter is flushed using acid digestion, and then the fluid is run through ICP elemental analysis.
• Gravimetric Analysis – This is a form of analytical chemistry for the determination of the amount of solid suspended in a liquid.
• Micropatch Analysis – The sample is drawn through a 0.8-micron patch. A microscope is then used to examine the patch and quantify and qualify contamination and wear debris.
• Analytical Ferrography – Solid debris that is suspended in a lubricant is separated and deposited onto a slide where a microscope is used to determine particle size, concentration, composition, morphology and surface condition. 

A cheaper alternative (although not as scientific or valuable) is a sensory inspection of the “slime.” How does it look, feel and smell? Sometimes these senses can give a very good indication as to what the problem is.

Visually inspect the oil for a change in color. A change in color can indicate the wrong or mixed oil, a photo-catalytic reaction, oxidation, thermal degradation, soot loading or chemical contamination. Look for any emulsions or cloudiness. Take note of any free water that settles to the bottom. Does the sample contain any sediment? If so, what color is it? How fast does it settle? The answers to these questions can provide clues as to what is ailing the oil.

How does the lubricant and “slime” feel? Lubricants should be slippery, not clingy (unless specially formulated with tackifying agents). Greases should feel buttery, not stringy or lumpy. If the “slime” is sludge formation it will feel hard, pasty, and sticky.

Lastly, how does the lubricant and “slime” smell? Can a contaminant be identified? Lots of contaminants have distinct smells: solvents, process chemicals, hydrogen sulfide, etc. If the oil smells like rotten eggs, the culprit may be oxidation. If it smells like burnt oil, that’s exactly what it is, thermally degraded oil.

Sometimes the combination of these sensory inspections can yield a diagnosis as to what the problem is. Other times more info is needed and the sample must be sent to a professional lab for analysis. Either way, if the “slime” needs to be identified, it will need to be interrogated.