Water in oil-base fluids can be just as destructive as particle contaminants. And water exclusion is very difficult to accomplish. Because of its affinity for other liquids, water is present in some concentration in most hydraulic systems. The hygroscopic nature of liquids causes them to pick up a certain amount of water simply from contact with humid air. When condensation occurs in a reservoir, with subsequent mixing into the base liquid, more water can be added to the system. Water can even enter with new oil. An oil barrel stored outdoors in a vertical position is likely to have rainwater collect around its bung. With changes in ambient temperature, some of this moisture can be sucked into the barrel. Eventually, this water enters the system when the reservoir is filled from that barrel.

Besides these natural phenomena, there are several system- and maintenance-related sources of water. In machine tool applications, a good deal of water-base coolant can enter hydraulic systems through breather caps and imperfect seals. Worn and damaged heat exchangers can allow cooling water to leak through seals and ruptured lines into the oil system - and vice versa.

Each fluid has its own saturation level for water. Below the saturation level, water will be completely dissolved in the other fluid. For oil-base hydraulic fluids, the saturation level is likely to be in the range of 100 to 1,000 parts per million (0.01% to 0.1%) at room temperature. At higher temperatures, the saturation level is greater.

Above the saturation level, water becomes entrained, meaning that it takes the form of relatively large droplets. This also is called free water. Sometimes these droplets combine and precipitate to the bottom of the reservoir. At other times, due to churning or other mixing action, undissolved water is emulsified so that it exists as very fine droplets suspended in the oil.

Water's mechanical effects

When water concentration in hydraulic oil reaches 1% or 2%, the response of a hydraulic system may be affected. If water alters the hydraulic fluid's viscosity, the operating characteristics of the hydraulic system change. When the rate of water influx is swift, poor system response could be the first indication that water is present.

Cavitation is another symptom of water in the fluid. Because the vapor pressure of water is higher than hydrocarbon liquids, even small amounts of water in solution can cause cavitation in pumps and other components. This occurs when water vaporizes in the low pressure areas of components, such as the suction side of a pump. Vaporization is followed by the subsequent violent collapse of vapor bubbles against metal surfaces in these areas. The loud characteristic sounds of cavitation may be noticeable when this happens. The result is cavitation damage on the interior surfaces of hydraulic components because the metal has fatigued.

Emulsified water in oil

Tiny water droplets may be emulsified or suspended in oil-base fluids. Evidence of this is when the fluid appears cloudy or milky. Sometimes an oil/water emulsion is so tight that it is very difficult to separate the two fluids - even with the addition of a coalescing agent formulated to do this. While this is desirable in emulsion-type hydraulic fluids, it is highly undesirable for ordinary oil-base fluids. Some fluid additives encourage emulsification, while others (demulsifiers) discourage it. The viscosity of a water emulsion can be much different from its original base liquid. As noted earlier, lower viscosity reduces the thickness of lubricating films, leading to increased wear of surfaces in dynamic contact.

If free water is present in hydraulic fluid and the system operates at temperatures below 32° F, ice crystals may form. These crystals can plug component orifices and clearance spaces. In hydraulic systems, this will cause slow or erratic response.

Without fluid analysis to warn of water's presence and appropriate control measures, water content probably will increase to the point where these and other symptoms appear. Other symptoms include evidence of chemical-reaction products and eventually, component failures.

Chemical reactions due to water

Water reacts with almost everything in a hydraulic system. Water promotes corrosion through galvanic action by acting as an electrolyte to conduct electricity between dissimilar materials. The most obvious sign is rust and other oxidation that appears on metal surfaces. The inside top surface of the reservoir often is the first place rust becomes visible, but such rust still may go undetected unless the reservoir is drained and opened for servicing. Also, the time it takes for rust to form depends partly on the original surface treatment used to protect the metal used to build the reservoir.

Unfortunately, before rust is noticed in the reservoir, water probably has damaged other system components. An inspection of failed bearings and other components may point to corrosion damage. Corroded aluminum and zinc alloys could have a whitish oxide film. Steel bearing and gear surfaces may show signs of rust and pitting.

Water's reaction with oxidation inhibitors produces acids and precipitates. These water-reaction products also increase wear and interference. At high operating temperature (above 140° F), water reacts with and actually can destroy zinc-type antiwear additives. For example, zinc dithiophosphate (ZDTP) is a popular boundary lubricant added to hydraulic fluid to reduce wear in high-pressure pumps, gears, and bearings. When this type of additive is depleted by its reaction with water, abrasive wear will accelerate rapidly. The depletion shows up as premature component failure, resulting from metal fatigue and other wear mechanisms.

Water frequently can act as an adhesive that causes smaller contaminant particles to clump together in a larger mass. These gluey masses may slow down a valve spool, or cause it to stick in one position. Or these clumps could plug component orifices. In any case, the result is erratic operation or a complete system failure.

Microbial growth

Over time, water contamination can lead to the growth of microbes - minute life forms such bacteria, algae, yeasts, and fungi - in the hydraulic system. And the presence of air exacerbates the problem. Microbes range in size from approximately 0.2 to 2.0 µm for single cells, and up to 200 µm for cell colonies. Left unchecked, microbes can destroy hydraulic systems just as they destroy living organisms. Under favorable conditions, bacteria can reproduce (doubling themselves) as rapidly as every 20 minutes. Such exponential growth can form an interwoven mat-like structure that requires significant shear force to break up. This resistance quickly renders a fluid system inoperable. Besides their mass volume, bacteria produce acids and other waste products which attack most metals. When this happens, fluid system performance is degraded, and components fail more rapidly.

Evidence of microbial contamination

The first indication of microbial contamination may be the foul odor that comes from waste and decomposition products of the microbes. Fluid viscosity may increase due to the mass of material produced by these organisms. At the same time, the fluid may have a brown mayonnaise-like appearance, or slimy green look.

Unfortunately, by the time these symptoms appear, system components and the fluid itself may be severely damaged. This could require a major overhaul or replacement of the system.

Properly selected filters will remove microbes. But without adding biocides (substances capable of destroying these living organisms) to the fluid, fast-growing microbes can place a heavy load on system filters. Combined with wear debris and chemical reaction products, microbial contamination can result in rapid plugging of filter elements, requiring frequent replacement.

Water and air are essential for microbe growth. Eliminating water and air from a fluid minimizes microbial problems. But some systems use water as the base fluid, and air is very difficult to exclude from fluids in operating hydraulic systems. With water and air present, microbes can usually find some fluid component to feed their growth. When water can not be controlled by exclusion or removal, biocides should be added to the fluid. A biocide combined with properly selected water-absorption filters can help minimize chemical-reaction byproducts and microbial contamination.