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Tuesday, 26 March 2013

The dangers of excessive air in oil.

Are you aware that excessive air in oil may be destroying your equipment and costing you money?

We all know that excessive air in oil can’t be good, but just what are the dangers of air trapped in oil? Although air is always present in lubricating oil, it is often justifiable to adopt measures to minimize its presence.

How do you know if your equipment’s lube is suffering because of the air in the oil.

Air can exist in oil in three different states:


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 (often causing cavitation), 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.


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 in Oil.

There are many 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. That is why oils with good dimulsibility qualities are important. 
Many contaminants, such as solvents, numerous chemical contaminants and even oil oxidation by-products have a similar effect. The latter is the primary reason oil foams more as it ages.
  • The loss of antifoam additives, suction leaks, poor reservoir design, using the wrong viscosity or using too much antifoam additive can all result in excessive air in oil.

Effects of excessive Air in oil.

  • Excessive air in the lubricant can damage the oil by increasing the rate of oxidation and thermal degradation, depleting additives, reducing its heat transfer coefficient and reducing its film strength. 

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.

Excessive air in the oil can damage the equipment in several ways.

  • 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 damage 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 Excessive Air in Oil.

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
  • 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.

And finally, don’t forget that the oil itself needs to be stable and of good quality. It’s important to use a quality lubricant, such as those produced by Habot Synthetic Lubricants if you want to safeguard your equipment against the dire consequences of excessive air in oil.