Yes, it's true what they told you in school chemistry class. Oil and water do not mix. That is, while water is the universal solvent - most substances dissolve with it, but oil will not. This is because the water molecule is polar. By this we mean that one side of the water molecule is positively charged while the other side is negatively charged. The two atoms of oxygen cling to each other with the single hydrogen atom on the other side. Oil on the other hand is made of carbon atoms bonded to hydrogen atoms forming hydrocarbon chains. This makes the oil molecule non-polar, thus in a container with both substances, the oil molecules are attracted to each other more than they are to the water molecules.

You may have noticed that when you buy Italian salad dressing, you have to shake it up before putting it on your salad. This is because the vinegar and oil in the dressing separate. You can see in the bottle that the oil is floating on top of the vinegar. But if you shake the bottle, the two will mix long enough to coat your salad. Vinegar is mostly water with a little ascetic acid. Both substances are polar, thus they will dissolve and form a chemical bond. The olive oil however is non-polar so, as the salad dressing sits in the refrigerator the oil will separate from the vinegar. No amount of shaking will make the two permanently bond. This is not unlike what happens to water in hydraulic oil. All of the moving around in the machine, passing through a pump, extending and retracting cylinders, turning motors, etc. causes the oil and the water to emulsify, or mix for a period of time. Once the emulsified oil and water sits for a while, they will separate again. At least, this would be the case if the system used pure mineral oil and only distilled water could get in.

You will notice that if you taste some of the oil floating on top of the vinegar in your salad dressing bottle, you can faintly taste some of the spices and a little of the vinegar. This is because other ingredients in the dressing act as a surfactant - a substance that will dissolve with both media. This is also the case with hydraulic oil. Some of the additives in the oil as well as some of the impurities in the water act as surfactants causing small amounts of water to be dissolved in the oil. Even this small amount of water can cause trouble with the hydraulic system.

There are three states in which water can exist in oil - dissolved water, emulsified water and free water. Most hydraulic fluids can hold between 200 and 600 parts per million of water in the dissolved state depending on how hot the oil is and how old it is. Old oil can dissolve 3 - 4 times as much water as new oil. Once the oil is saturated, water will suspend in the oil or emulsify. When this happens, the oil will begin to look milky. If water continues to be added to the oil, a layer of free water will form on the bottom of the reservoir. Water has a higher specific gravity than oil, so the oil will float on top of the free water. Emulsified and free water do the most damage in a hydraulic system, but even high levels of dissolved water can be a problem. Water in the oil can affect its viscosity, its lubricity and its power transfer characteristics. Metals from wear debris can act as a catalyst and cause oxidation leading to higher viscosity and more deposits. It can cause hydrolysis which will result in acids and alcohols which will lead to increased corrosion. Finally, water in the oil can cause premature depletion of oil additives. Free and emulsified water in the oil can freeze easily if your machine is subjected to the elements. This happened on a carnival ride. When the water in the oil froze, of course it expanded. Water is the only substance that expands when it becomes solid. This caused the cylinders on the ride to explode!

It's easier and cheaper to keep water from getting into the oil than it is to remove it once it's there. Replacing your breather with a hygroscopic filter will keep airborne water from entering the system. It uses a desiccant to remove water vapor from the air. The desiccant is visible through the clear plastic of the housing and should be replaced when it changes color. A rupture in a water heat exchanger can result in water getting into the oil. Leaks in the system can allow rain and water vapor in. Care should be taken when cleaning the reservoir to avoid the ingress of water.

Once the water is in the oil, it can be time consuming and expensive to get it out. Regular draining of the reservoir can remove free water. The size of the reservoir can make a big difference in how well the emulsified water will separate. A centrifuge can also remove free and emulsified water. Coalescers separate water from the fluid by arresting them near the surface of a filter or screen. The droplets get bigger which causes them to fall to the bottom where they can be extracted. Coalescers work well on emulsified water but cannot separate dissolved water. Absorbent filters such as the ones we use in our GPM-120 and other flushing machine models can also remove free and emulsified water quickly and effectively. Dissolved water can only be removed by vacuum dehydration, exposing the oil to a partial vacuum. There are two types - flash distillation and mass transfer. Flash distillation also heats the oil to further boil off more water and it operates at a higher vacuum. This makes the flash distillation process more efficient than mass transfer. Both are expensive. It's best not to let the water into the oil in the first place!