From Filtration Technology

Water absorption filters usually have an element that combines a particle removal media with water absorption material. The water absorption material may be something as simple as cornstarch, or as complex as a polyacrylate material. The latter is the type of material used in babies' diapers to absorb water. Typically, these filters are used to remove small amounts (less than 1% by volume) of free water from hydrocarbon fluids in circulating systems.

Most water absorption media are sensitive to flow rate, and their efficiency improves as flow velocity is lowered. It is common for these filters to be used on off-line circuits where the flow rate can be controlled by pump selection.

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Air contamination can cause a great deal of harm to hydraulic systems. Unfortunately, unless the machine is foaming with oil spewing from the top of the reservoir, we sometimes overlook this potentially devastating contaminant. Here are a few ways in which air contamination can undermine your reliability effort:

Spongy hydraulics – Air is compressible. This loss of control can slow throughput, increase the percent defective rate, and in some cases, cause injury or death.

Gaseous cavitation – Some references say gaseous cavitation causes wear, others say it just makes noise. Play it safe and keep oils as free from air as possible.

Oxidation – The rate of oxidation is affected by a number of factors, but all else held equal, it is proportional to the amount of oil in boundary contact with air (which, of course provides the oxygen).

Thermal degradation - Did you know that compressing an air bubble from ambient pressure to 3,000 psi increases the bubble's temperature to more than 2,000 degrees F? This is hot enough to thermally degrade oil at the boundary contact between the air and the bubble, which darkens the oil, sometimes giving it a sooty appearance, and causes varnish to build up on component surfaces.

So, watch for signs of air entrainment or foaming, and take occurrences seriously. Also, periodically test your oil’s ability to release air and its tendency to maintain stable foam. If you have recurring problems, evaluate the lube specification and the tank design, and check for low levels and signs of a suction line leak.

Microbial contamination is a pro-oxidant and forms acid products in the oil. Additives are also adversely affected. Over time insoluble suspensions can lead to sludge, deposits and viscosity change.

Filters will plug and oilways will become blocked. Usually the best strategy for controlling microorganisms is to control water contamination. Without water the bacteria can't survive. For water-based lubricants there are biocides available as well.