Water in hydraulic fluid is a contaminant that can be just as damaging as hard particles and in some cases, more so.

Water in hydraulic fluid:

Depletes some additives and reacts with others to form corrosive by-products which attack some metals.
Reduces lubricant film-strength, which leaves critical surfaces vulnerable to wear and corrosion.
Reduces filter ability and clogs filters.
Reduces the oils ability to release air.
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, including the type of hydraulic system and your reliability objectives for the equipment.

It's always wise to control water contamination at the lowest levels that can reasonably be achieved, but certainly 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.

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 of mercury, water boils at 133F (56C). This enables water to be removed at atemperature that does not damage the oil or its additives.

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

Vacuum distillation and headspace dehumidification also remove dissolved water.

Prevention is better than cure

Like all other forms of contamination, preventing water ingress is ten times cheaper than removing it from the oil.

Craig Cook