Why Your Hydraulic System Should Be Kept Free Of Air

Generally speaking, hydraulic oil pollutants are classed as substances that prevent the fluid from working properly. Air is one such substance, so when it finds its way into the oil, remedial action is needed to keep the fluid and other parts of the hydraulic system protected.

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There are several different forms of air that can exist in oil, as follows:

1) Air that has dissolved – Typically, hydraulic oil has a dissolved air content (v/v percent) of six to twelve percent.
2) Air that is free – This might be an air pocket somewhere inside the hydraulic system.
3) Foam – Bubbles of air with a diameter of over one millimeter, which appear on the oil surface.
4) Air that is entrained – Bubbles of air with a diameter of under one millimeter, which disperse inside the oil.

Entrained air causes more problems than any other form of air. Most of the time, bleeding the hydraulic system when starting it up, and pre-filling system parts, will eradicate free air. Low quantities of foam rarely present any major issues, although larger quantities of foam might lead to an overflowing reservoir. This could indicate a more severe problem with oil degradation and/or air contamination.

How Does Entrained Air Wreak Havoc?

Entrained air has a number of negative impacts, including:

  • Increases heat load
  • Reduces bulk modulus, which damages the system controls and causes unreliable operation
  • Causes fluid to deteriorate, due to dieseling (thermal degradation) and increased oxidation
  • Hinders thermal conductivity
  • Erodes cavities
  • Reduces the viscosity of fluid, which makes vital surfaces more prone to wear
  • Makes the hydraulic system less efficient
  • Increases the level of noise

External Ingestion

Air can become entrained via external ingestion. This usually happens at the inlet pump, due to:


  • Intake lines that have become porous
  • Fittings or clamps on the intake line that have become loose
  • A defective seal on the pump shaft
  • A low level of reservoir oil

Furthermore, air ingestion can be caused by defective load control valves, which might draw air past the double-acting cylinders – sending oil into the reservoir (the drop pipes that extend below the minimum level of oil should be correctly installed).

Cavitation In Gas

In some conditions, dissolved air can leave the solution and become entrained air, in a process called cavitation. Once static pressure reduces, or the temperature of hydraulic oil rises, air solubility declines and the fluid can develop bubbles.

As with external ingestion, there might be a static pressure reduction and dissolved air emission at the pump, due to:

  • Turbulent activity produced by isolation valves on the intake line
  • Blocked suction strainers or inlet filters
  • An intake line that is restricted or collapsed
  • An inlet with a bad design (either too long, not big enough in diameter, or too many bends)
  • A reservoir breather that is undersized or blocked
  • A lift that is too excessive (the distance, measured vertically, between the minimum level of fluid and pump intake)

Additional reasons for static pressure reduction are fluid velocity changes via flow transients, orifices and conductors, and badly adjusted load control or anti-cavitation valves.

Take Precautions To Avoid Air Contamination

As with many other issues relating to hydraulics, efficient design, and the use of high-quality parts, will reduce the risk of air contamination from causing any damage. At Hydrastar, we design and support unique hydraulic systems for clients in a range of industries.

Please get in touch to find out more.

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