"We suspect that we have an air contamination problem in one of our hydraulic systems. What types of problems can air contamination cause?"
Air contamination can cause a great deal of harm to hydraulic systems. Unfortunately, unless the machine is foaming, with oil spewing from the top of the reservoir, we sometimes overlook this potentially devastating contaminant. Here are a few ways in which air contamination can undermine your reliability effort:
Spongy hydraulics - Air is compressible. This loss of control can slow throughput, increase the percent defective rate, and, in some cases, cause injury or death.
Gaseous cavitation - Some references say gaseous cavitation causes wear, others say it just makes noise. Play it safe and keep oils as free from air as possible.
Oxidation - the rate of oxidation is affected by a number of factors, but all else held equal, it is proportional to the amount of oil in boundary contact with air (which, of course provides the oxygen).
Thermal degradation - Did you know that compressing an air bubble from ambient pressure to 3000 psi increases the bubbles temperature to over 2000°F? This is hot enough to thermally degrade oil at the boundary contact between the air and the bubble, which darkens the oil, sometimes giving it a sooty appearance, and causes varnish to build up on component surfaces.
So, watch for signs of air entrainment or foaming, and take occurrences seriously. Also, periodically test your oil’s ability to release air and its tendency to maintain stable foam. If you have recurring problems, evaluate the lubricant specification and the tank design, and check for low oil levels and signs of a suction line leak.