When we think of contamination in lubricated systems, we often focus on particle and water contamination. The fact of the matter is that there are many other contaminants we should consider and attempt to control. Most contaminants, which include any material not contained in the lubricant's formulation, can potentially damage the oil and the lubricated components. Although air is always present in lubricating oil, it is often justifiable to adopt measures to minimize its presence.
Air can exist in oil in three different states: dissolved, entrained and foam. Air dissolved in oil exists as individual molecules which are similar to CO2 dissolved in soda water. This type of air is invisible and impractical to detect. Entrained air in oil is comprised of tiny air bubbles suspended in the oil. This type of air contamination is arguably the most damaging, and can be identified by the oil having a cloudy appearance. Although there are several common causes for cloudy oil, this can be identified by taking a sample of the oil and observing whether or not it clears up over time.
If the cloudiness was caused by water or another liquid contaminant, an aqueous layer should form as the sample clears. Foam is the other common type of air in oil. Foam typically refers to the stable layer of relatively large bubbles that accumulate at the surface of a reservoir. In some systems, foam at the surface may not cause a lot of damage, but the presence of a foam layer normally indicates extensive air entrainment.
Causes of Excessive Air Contamination
There are actually quite a few conditions that lead to excessive air contamination, and for this reason, there are several common causes that should be considered. The most common of these is water contamination. When a lubricating oil becomes contaminated with water, its surface tension is lowered, thereby allowing bubbles in the oil to separate into smaller particles that are more easily suspended. Many other contaminants have a similar effect such as solvents, numerous chemical contaminants and even oil oxidation by-products. The latter is the primary reason oil foams more as it ages.
Other causes include loss of antifoam additives, suction leaks, poor reservoir design, using the wrong viscosity or using too much antifoam additive. Under certain circumstances, antifoam additives can be abnormally depleted. This often prompts users to add defoamant to a system and creates the opportunity to add too much. Use caution and follow the proper guidelines when attempting to re-additize the oil.
Effects of Air Contamination
Air contamination can have negative effects on the machine and the lubricant. Air can damage a lubricating oil by increasing the rate of oxidation and thermal degradation, depleting additives, reducing its heat transfer coefficient and reducing its film strength. Oil can oxidize when its molecules come into contact with oxygen. It stands to reason that the more oxygen exits from the oil, the faster the oil will oxidize. This problem is exacerbated when the bubbles move into high-pressure environments where the change in volume causes a drastic increase in temperature. The process, sometimes called microdieseling, causes thermal degradation of the oil as well.
Machine wear can be generated by air contamination by several mechanisms. For one thing, air is compressible. In order for the oil to create the appropriate lubricating film thickness, it must be incompressible. When the oil is heavily contaminated with entrained air, its film strength can be reduced to the point where the film breaks down, allowing mechanical friction between interacting surfaces. Depending upon the type of machine, this effect could be rapid.
In machine environments where dramatic pressure changes occur, such as a hydraulic pump, the dramatic and instantaneous volumetric change causes bubbles to implode violently, which leads to erosion of machine surfaces. In hydraulics, entrained air can create other problems as well, such as spongy operations, loss of controls and an increased likelihood of surface deposits in valves.
How to Detect and Control Air Contamination
When a foaming problem suddenly develops in a sump, there are several factors to look for. First, take an oil sample from the drain to check for the presence of free water. If water is in fact the culprit, the foam actually does us a favor by alerting us to the water problem. If gross water contamination is not observed, take an oil sample for analysis to inspect for a chemical contamination, if the wrong oil was added or if the oil is degraded. Another common cause of a sudden problem is a suction leak in a circulating system. This type of problem can often be detected with the old-fashion shaving-cream technique.
If a system has a history of foaming problems, it may be an ongoing problem of contamination or a system design issue. Some common reservoir problems that lead to foam are using a sump that is too small, having oil return lines that terminate above the oil level causing mixing, or having suction and return lines in close proximity. These problems can sometimes be addressed by using diffusers, installing baffles, or using plates or screens for gravity-return systems.
As I mentioned before, it may be impossible to completely eliminate air from lubricated systems, but steps should be taken to reduce it as much as can reasonably be expected. Eliminating excessive air will likely give more life to your oil, improve system performance, and reduce wear and deposits. Remember, contamination control is not just about filtering out dirt. It involves excluding and removing any foreign material from the system.