Pump cavitation and aeration: why a hydraulic pump gets noisy

In simple terms

A pump doesn’t really suck oil. It makes a space, and the air pressure pushing down on the oil in the tank pushes oil into that space. That push is only about 1 bar, and everything between the tank and the pump uses some of it up: lifting the oil if the pump sits above the tank, dragging it through a long or thin pipe, forcing it through a strainer, and speeding it up into the pump.

Cavitation happens when too little of that push is left. The pressure at the pump inlet drops so low that bubbles form in the oil, from dissolved air and oil vapour. A moment later the pump carries those bubbles into the high-pressure side, where they are crushed and collapse. Each collapse is a tiny hammer blow against the metal. Thousands of them a second make the gravel rattle and pit the pump’s surfaces.

Common causes: a blocked strainer, a suction line that is too thin or too long, cold thick oil, a pump running too fast, or a pump mounted above the oil.

Aeration is air getting into the oil from outside. The suction line is below atmospheric pressure, so a loose fitting or a cracked hose there doesn’t leak oil, it sucks air in. You won’t see a drip. Air also gets in when a return line ends above the oil surface and splashes. The bubbles go round the circuit, the oil turns milky and foams, the pump whines, and cylinders move in jerks because air squashes.