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"Sometimes our oil analysis reports show high particle counts on a hydraulic system (as high as ISO 21/18). If wear metals and silicon are low (less than 4 parts per million), what are these particles?"
To fully understand the composition of the particles, you should perform a spectrographic analysis and a metallurgy assessment of your system components, which will tell you what the particles are and where they are coming from. At 4 parts per million (ppm), the amount of wear metals is insignificant.
Retest the system on a proper frequency and trend the rate of change. A significant change in the overall amount of wear metals is cause for concern. If the value of wear metals hovers around the same ppm over each test, then you are probably looking at a product of normal operation. Keep in mind that spectroanalysis can pick up particles only 7 microns and smaller at the absolute best. The accuracy can also be off by significant values on some metals.
Approach your concern with ferrous density and a patch test after a particle count. Ferrous density should be considered a primary test for all machines where ferrous wear material is expected. Correlating ferrous density data along with other oil analysis test information can give you a wide picture and a solid understanding of the internal condition of your machine.
Be sure to set your target cleanliness for a particle count. If the tested value is greater than the target value, proceed to a ferrous density test. Set a target for this value as well (maybe 15 percent ferrous). If the value of the ferrous density exceeds 15 percent, proceed to analytical ferrography.
Analytical ferrography allows wear particles to be observed by the analyst via microscopic analysis. In this evaluation, active machine wear as well as multiple different modes of wear can be determined. This method has an outstanding sensitivity for larger particles.
If the value of the ferrous density is less than 15 percent, proceed to a patch test (filtergram). Keep an eye out for a high rate of change in any of the tested values and you will be able to catch problems before they occur.
It is also a good idea to assess the condition and effectiveness of your breathers and filters to make sure you are keeping contamination out and removing it effectively when it gets in.