Fluid aeration and cavitation can cause numerous problems in a hydraulic and lubrication oil system:
  • High fluid temperature.
  • Severe fluid degradation.
  • Cavitation damage.
  • Abnormal noise.
  • Poor component response due to the spongy behavior of aerated fluids.
Molykote, a business unit of Midland, Mich.-based Dow Corning Corp., offers the following explanation for how aeration and cavitation occur, how to prevent them, and how to minimize the mechanical failure and costly expense associated with them.

Control Temperatures. Avoid operating a hydraulic system above 180°F (83°C) because it can damage seals and accelerate hydraulic fluid degradation. Hydraulic systems dissipate heat through the fluid reservoir; therefore, the reservoir fluid level should be monitored and maintained at the correct level. To prevent damage caused by high temperatures, a fluid-temperature alarm should be installed. All high temperature indications should be investigated and rectified immediately.

Avoid Aerating the Fluid. Aeration occurs when air contaminates the hydraulic fluid. Symptoms include a banging or knocking noise when it compresses and decompresses as it circulates through the system; foaming of the fluid; and erratic actuator movements.

Air usually enters the hydraulic system through the pump’s inlet. Ensure that the pump inlet lines are in good condition and all clamps and fittings are tight. Flexible intake hoses can age and become porous, so they must be replaced from time to time. Aeration accelerates hydraulic fluid degradation, which in turn can cause the seals to overheat and burn. Regularly check the condition of the pump-shaft seal and if leaking, replace it.

Prevent Cavitation. Cavitation occurs when the pressure acting in a fluid is below the saturation pressure of a dissolved gas in the fluid. This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid, which results in the formation of vapor cavities within the fluid. When these cavities encounter a region of higher pressure, they will collapse. Depending on the load pressure of the hydraulic pump, the collapse can cause broad high frequency vibrations, noise, material damage and oil degradation - all leading to mechanical failure of the system components.

Most often, cavitation occurs at the hydraulic pump. A clogged inlet strainer or restricted intake line can cause the fluid to vaporize. Check the inlet strainer filter on a regular basis to be sure it is not clogged.