air contamination

You’ve reached us here, so chances are you are on top of your hydraulic fluid levels and your regular filter replacements. Congratulations, you have done quite well for yourself. The good news about this particular step in the troubleshooting process is that it’s rarely your fault if it is indeed the cause of your malfunction. So, breathe a sigh of relief and let’s get down to the ugly truth – the reality of air contamination in a hydraulic system.

The Four Kinds of Unwanted Air

This type of contamination can take on four different forms/phases each with varying degrees of possible standard operational threat. The progression goes as such (starting with the least threatening and ending with the most): free air, foam, dissolved air, and entrained air.

Free Air

This is generally a natural part of any hydraulic system. That air you bleed out from the system on a regular basis is known as “free air.” Increased levels of free air caused by a neglect to bleed the systems can result in larger maintenance issues, but this is only the case after long operational hours with no regard to system health whatsoever. Free air is generally harmless.


This is the cosmetic that serves as a great indicator of entrained air (a much more serious contamination). It’s a collection of tightly-packed bubbles that settle on the surface of the fluid. You’ll see much of this during the life-cycle of a hydraulic machine, and it generally doesn’t amount to anything more than workshop floor spills or false lubrication. However, it is indicative of greater system issues – read on to “entrained air” below.

Dissolved Air

This type of air isn’t readily drawn out of a solution. Certain petroleum oils can contain a percentage of dissolved air at operational level – and in this dissolved form, the air is mostly harmless. Problems begin to occur when the system overheats or pressure begins to drop. In that case, dissolved air can change state and form small bubbles on the fluid’s surface. Many of these bubbles that remain under 1mm in diameter sink below the surface and become suspended in the fluid’s body. It is at this phase when the air becomes a serious issue and evolves towards air entrainment.

The Real Issue – Entrained Air

As touched on above, this is the real monster of air contamination problems. Surface foam is a great indicator of entrained air, as bubbles on the surface generally accompany bubbles beneath it. This is not to say that entrained air should be treated similarly to foam – the two often beget separate issues within the system. As a whole, entrainment can cause any of these various problems in your system:

  • Erratic operation or behavior of hydraulic workentrained air
  • Loss of precision control in the system
  • Oil Oxidation
  • Low pressure switch trip (equipment shutdown)
  • Pump cavitation
  • Loss of head in centrifugal pumps
  • etc.

The issues listed above are also great indicators of air contamination problems within the system.

Why this step is important:

Air contamination can be prevented by paying close attention to your hydraulic seals – but often or not, it’ll happen eventually. By working through this step in the troubleshooting process, you rule out the number one enemy of your fluid – the very same air we breathe everyday. High temperature issues, physical damage to the outside of the machinery, and missing connector components are all slippery slopes that lead directly to air contamination problems. If you can be sure it isn’t this, you’re well on your way to discovering a much, much worse problem with your hydraulic machinery.

Hydraulic Troubleshooting Step Four – Faulty Operation

Comments are closed


Layout Style

Header Style

Accent Color