When using a kidney-loop cart to filter oil, I suggest plumbing the cart's filters in series. As the primary filter catches debris and starts to plug, the pressure drop across the filter increases. This increase in pressure differential tends to push previously captured debris though the filter. The secondary filter ends up having a lower pressure differential because most of the debris is captured by the primary filter.

This lower pressure differential will allow the secondary filter to retain more of the debris that it captures. The secondary filter also acts as a "safety" filter in case the primary filter ruptures. In cost-sensitive applications, the secondary filter can be placed in the primary position after the primary filter plugs, and a new filter can be placed in the secondary position. (Paul Tokach, Butler Machinery Company)

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"I had a Mobil DTE 797 that was left on a 5-micron filter for an extended period of time. The original count was 21/20/18 and the final test was 5/5/5."

While the original particle count seems reasonable for new oil that has not been filtered (particularly if it was delivered in bulk); the final test results do not appear to be genuine. Stating that the particle count is 5/5/5 suggests that there are only 16 to 32 particles greater than 4 microns in every 100 ml (approximately 4 ounces) of fluid.

While prolonged filtration at 5 microns will undoubtedly get the oil fairly clean, outside of a controlled lab environment it is almost impossible to obtain this low level of particle contamination when even breathing into the sample bottle would probably yield a higher count! I suggest that you first re-run the test to obtain a more realistic result.

To answer the other part of your question, however, there is no problem with filtering this fluid which is an ISO VG 32 rust and oxidation-inhibited mineral oil at 5 microns for an extended period of time. Using an even finer filter would also be acceptable, assuming it is required to achieve the desired system cleanliness.

With some higher viscosity oils, particularly those containing polymeric additives such as methyl silicone defoamants or viscosity index improvers, you should be cautious if you plan to filter at 3 microns or lower. If necessary, test for potential problems before hand, but generally speaking, prolonged fine filtration shouldn’t be a concern for most fluids and certainly not for low-viscosity turbine-type oils.

Mark Barnes, Noria Corporation

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