Purging an Hydraulic System, Why?

All hydraulic fluids have some dissolved air, usually 10% of the volume. Under pressure, this condition increases, water absorbs much more air. Aeration in a hydraulic circuit means the presence of free air in places where only oil should be. Commonly the air will be in the form of bubbles dispersed through the fluid.

Difficulties with aeration occur more frequently with increasing flow velocity in hydraulic components. Let's analyze some of the causes of the air entering the system and also see some suggestions to reduce the possible causes of this phenomenon.


The most common places where air is introduced into a hydraulic system or for aeration to occur are.

  • The entry line, an entry line with loose joints or broken seals.
  • The return line, loose joints, seals or defective connectors.
  • Cylinder shank, packing or seals, excessive wear, too much tolerance or breakage.
  • Mounting boards, Tees or cracked pipes.
  • Low oil level, air sucked by vortices caused by low level.
  • Air trapped in the filter due to a bad purge.
  • Return fluid discharged above the fluid level in the tank.
  • Air trapped in the system during original filling or when complete.

  • The hydraulic fluid acts as a seal at atmospheric pressure, when the system falls at a pressure lower than atmospheric pressure, air can be introduced where the oil does not leak.

    System failures.

    The conditions listed in the previous paragraphs contribute to introduce air to the hydraulic system, or as we have called it, aeration. Excessive aeration can cause damage to the pump, which results in system failures. The two causes that cause damage to the components due to the phenomenon of aeration, are the lack of lubrication and overheating. Aeration can also cause uneven or misaligned movements in hydraulic pumps or motors, which in combination with the causes mentioned up to now, will cause faults.

    The lack of lubrication in a hydraulic component will eventually result in adhesion failures and subsequently pump failure. Over heating is caused by a thinning of hydraulic fluid as a result of oxidation. The oxidation of the fluid becomes sediments and varnish. When operating a system with aeration, the oil can be completely oxidized, and eventually the sediment and varnish will cause the pump, or the motor, as the case may be, to overheat, and fail.

    How to avoid aeration?

    Inspection and constant maintenance are the best ways to prevent air from entering a hydraulic system. Keeping all connections and joints tight is the best way to prevent air from entering.

    The return fluid that enters the tank will create aeration if it is drained above the main body of the fluid in the tank. To avoid this condition, keep enough fluid in the tank to have the return line submerged.

    The pump entry line should always be below the fluid surface for the same reason.

    When starting a new system or after the system has been completely drained and flushed, the tendency of the fluid to be aerated may exist until all the oil is purged of all the lines and components. To correct this condition, a good purge to the system should be done.

    If air has been trapped in the reservoir filter, a purge device should be installed on top of the filter or an air respirator in the inlet.

    In general, there is much more life service built into the hydraulic equipment than is often believed. The improvement of the life service depends on maintaining the appropriate conditions within the hydraulic system.