How to remove water from hydraulic oil

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If you have worked with hydraulic equipment for any length of time, it's likely that you've come across a hydraulic system with cloudy oil. Oil becomes cloudy when it is contaminated with water above its saturation level. The saturation level is the amount of water that can dissolve in the oil's molecular chemistry and is typically 200 - 300 ppm at 68°F (20°C) for mineral hydraulic oil. Note that if hydraulic oil is cloudy it indicates that a minimum of 200 - 300 ppm of water is present. I recently audited a hydraulic system with cloudy oil that was found to contain greater than 1% (10,000 ppm) water.

Why is water in hydraulic oil bad?

Water in hydraulic oil:

  • Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
  • Reduces lubricating film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
  • Reduces filterability and clogs filters.
  • Increases air entrainment ability.
  • Increases the likelihood of cavitation occurring.

How much water is too much?

A number of factors need to be considered when selecting water contamination targets. These include the type of hydraulic system and reliability objectives for the equipment. It's always wise to control water contamination at the lowest levels that can reasonably be achieved. And ideally, below the oil's saturation point at operating temperature.

Water removal methods

Methods for removing free (unstable suspension) and emulsified (stable suspension) water include:

  • polymeric filters;
  • vacuum distillation; and
  • headspace dehumidification.
Vacuum distillation and headspace dehumidification also remove dissolved water.

Polymeric filters - These look like conventional particulate filters, however the media is impregnated with a super-absorbent polymer. Water causes the polymer to swell, which traps the water within the media. Polymeric filters are best suited for removing small volumes of water and/or maintaining water contamination within pre-determined limits.

Vacuum distillation - This technique employs a combination of heat and vacuum. At 25 inches Hg, water boils at 133°F (56°C). This enables water to be removed at a temperature that does not damage the oil or its additives.

Headspace dehumidification - This method involves circulating and dehumidifying air from the reservoir headspace. Water in the oil migrates to the dry air in the headspace and is eventually removed by the dehumidifier.

In the case of small systems with high levels of water contamination, changing the oil may be more cost-effective than using any of the above methods of water removal.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is cheaper than removing it from the oil. A major point of water ingression is through the reservoir headspace. Many hydraulic system reservoirs are fitted with breather caps that allow moisture (and particles) to enter the reservoir as the oil volume changes through either thermal expansion and contraction, or the actuation of cylinders.

Replacing the standard breather cap with a hygroscopic breather will eliminate the ingression of moisture and particles through the reservoir's vent. These breathers combine a woven-polyester media that filters particles as small as 3 microns, with silica gel desiccant to remove water vapor from incoming air. The result is relative humidity levels within the reservoir headspace that make condensation unlikely, therefore reducing water contamination of the oil.

Related articles:

How to deal with air contamination of hydraulic oil
Hydraulic filters that do more harm than good
Would you pass this simple hydraulics test?

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