What Causes Hydraulic Fluid Aeration?

05/02/2020 8:04:06 AM

Hydraulic fluid aeration refers to the presence of air in a hydraulic system in any form including the formation of large bubbles, dissolved air or foam. Regardless of the form, aeration has several negative effects. It causes increased pump cavitation, leading to excessive noise. Air has an erosive effect leading to increased wear on hydraulic components such as rings and vanes. Larger bubbles cause a pounding effect upon pump vanes - leading to extreme wear.

Avoiding air leaks is one of the most important ways to ensure the long term health and performance of your equipment. So, it’s a good idea to understand the most common ways that air can make its way into your hydraulic system, so you can keep an eye on these areas.


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Leaking inlet lines

aerated hydraulic oil

Leaking pipes are some of the most common and obvious culprits that allow air to get into the system. Pipes are flexible and can work loose if not properly maintained. In addition, poor quality pipes or improperly installed pipes can easily allow leaks to occur. Threaded fittings can be porous and must be secured using an approved pipe thread sealer. Damaged or scored inlet flange surfaces - where the pipe mates to the pump - can also reduce the seal at the point where the pipe connects.


Leaking system o-rings

Deteriorated o-rings are another source of potential air. As o-rings degrade, they lose elasticity and become hard, brittle, cracked and porous. Thus, they lose their air-tight characteristics and air is eventually able to make its way into the system. On systems which have been operating at excessive high temperatures, the “O” rings can harden. Fluids type is another factor that leads to o-ring degradation. Fluids with a high sulphur content tend to accelerate o-ring hardness. Keep an eye on your o-ring integrity and follow the recommended maintenance and replacement schedule. And take steps to keeping system operating temperatures within prescribed limits.


Pump shaft seal leakage

The pump shaft seal is another point at which air can make its way into the system. These seals are connected to the pump inlet and since pumps produce a vacuum effect, if these seals are not in good condition, air can be pulled directly into the pump’s working components. Excessively high inlet vacuums can also cause air leakage at the shaft seal. In addition, shaft misalignment, improper installation, or mutilation of the shaft seal at installation can increase the probability of air leakage past the shaft seal.


Leaking cylinder rod seals

Fast-moving cylinders can experience extreme differences in positive and negative pressure which can lead to cavitation. In this situation, a powerful vacuum is produced that is sometimes strong enough to suck air (and dirt) under the cylinder rod seal. For this reason, some cylinders feature an anti-cavitation check valve which allows hydraulic fluid from a reservoir to prevent cavitation from occurring when pressures drop too low.


Turbulence or sloshing in the reservoir


Turbulence or sloshing in the reservoir can cause aeration. Moving vehicles operating on rough ground can cause a back and forth sloshing of the fluid in the

reservoir. Reservoirs should be designed deep enough to reduce aeration due to sloshing. In addition, if the return lines are improperly located in the fluid reservoir this can can cause turbulence as the fluid re-enters the reservoir, which increases aeration. Return lines should be located below the fluid level in order to avoid causing bubbles to form in the system.


Vortexing fluid in the reservoir

Sometimes, poorly designed reservoir inlets lead to an air-sucking vortex effect, similar to the whirlpool phenomenon you see when water flows out of a drain. This has the potential to cause aeration of the fluid. To avoid this, good designs make sure the inlet protrudes away from the reservoir wall.


Releasing of air suspended within the fluid

When hydraulic fluid is cold there is often a significant amount of air suspended in it. On startup, the fluid begins to warm and air is released into the system. A drop in fluid pressure also triggers the release of suspended air. For this reason, relief valves should be located below the fluid level of the reservoir as far from the reservoir outlet as possible. This allows time for the air released by the relief valve to be removed before leaving the reservoir. A special baffle made of 60 mesh screen can be installed into the reservoir to help eliminate most of the larger bubbles.



Hydraulic aeration is the enemy of performance and has a deteriorative effect on hydraulic pumps and other components. So, avoid it at all costs. Make sure you are aware of all the points at which air can potentially enter the system and keep an eye on these areas. Follow your prescribed maintenance routine and be alert for the symptoms of aeration such as increased noise, heat or erratic movement.

If you suspect you may have an air leak, contact our helpful on-site maintenance staff. Berendsen’s fleet of fully-equipped mobile maintenance vehicles is on hand to diagnose and fix many common hydraulic maintenance issues.


Need an Urgent Hydraulic Repair? Click Here To Contact Us!



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