Water contamination in hydrocarbon fluids can cause many problems, especially in hydraulic and lubricating oils. This condition frequently results in undesirable mechanical effects such as change in viscosity and loss of lubrication, cavitation in pumps, and chemical reactions with anti-wear additives and oxidation inhibitors to generate solid precipitates. Ideally, water should be removed completely before recirculation of the oil. Free water is defined as large bodies of water in oil. In static reservoirs this water is free to settle to the bottom of the tank, and separate from the oil to produce free water.
Emulsified water represents tiny water droplets in oil, typically dispersed by high shear mixing.
Technically emulsified water belongs to free water. However, emulsified water can be very stable and may not settle out from the oil, especially in the presence of surfactants. Dissolved water is chemically absorbed in solution with the oil. Vacuum dehydration and absorption are only two techniques to remove free, emulsified and dissolved water. The advantage and disadvantage of these technologies will be discussed as follows.