System Contamination

Although oxidation is the greatest contaminator in a hydraulic system, many other contaminants exist. All contaminants can be controlled through the use of chemicals and good maintenance procedures. The best control of course is through the use of good maintenance. Maintenance procedures can always be checked accurately, whereas chemical stability cannot.

 
Water

Small of water get into the hydraulic fluid in the form of water vapor that is carried in with the air that constantly finds its way into the system. When the air is suspended in the fluid, the water vapor also becomes emulsified in the fluid and remains in suspension as a result of the churning conditions that exist in the system. Water-fluid emulsions may be thin and slurry, thick and pasty, or heavy and gummy depending on the amount of water and the fluid viscosity. Emulsions frequently promote the collection of impurities which increases friction and wear in the system. Many fluids use chemicals to prevent emulsions from forming. If a fluid (with or without the aid of chemicals) resists the formation of water-fluid emulsions and promotes the breakdown of emulsions, it is said to have Demulsification.

Demulsification, or the breakdown of emulsions, usually takes place under conditions of low pressure and reduced churning found in return lines and reservoirs. As the emulsions break down the water separates and settles to the bottom of the reservoir. During overnight and week-end shutdowns the reduced pressure in most of the valves, hydraulic lines, accumulators, and cylinders also allows the water to settle out. Because water breaks through the lubricating films, it causes rust to form on the rubbing surfaces and in ports and orifices. If the system drains properly, all free water will eventually accumulate in the reservoir, from which it should be drained periodically. Any free water in the fluid decreases its lubricity and causes increased leakage and erratic pump action.

The water in a hydraulic system can form VAPOR PRESSURE. Vapor pressure is formed when water in the fluid comes in contact with high temperatures and starts to boil. In an open container this vapor would escape to the atmosphere, but in a sealed system it becomes trapped just as air does. Not all vapor pressure is caused by water. Petroleum oils subjected to head may also cause vapor pressure. When this happens, they evaporate.