Aeration

Canonical Definition

The entrainment of free air or non-condensable gases into hydraulic or lubrication fluid, resulting in a compressible, foamy fluid mixture. Aeration occurs when fluid is exposed to air above its saturation point — typically through turbulent flow, vortex formation at suction inlets, low reservoir fluid levels, or leaking suction line connections. Aerated fluid compresses under pressure, causing spongy actuator response, increased noise (cavitation-like), elevated fluid temperatures due to adiabatic compression of air bubbles, and accelerated oil oxidation. Aeration is distinct from dissolved air — dissolved air remains in solution and does not compress under operating pressures.

Engineering Context

Aeration is controlled through hydraulic reservoir design: adequate fluid volume (minimum 3–5× pump flow rate per minute), return line submerged below the fluid surface, adequate settling time and baffles to allow entrained air to escape before fluid is recirculated through the pump. Foam-inhibiting oil additives reduce surface tension but do not address the root cause of air entrainment.

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