Fluid aeration and cavitation can cause many problems in a hydraulic and lubrication oil system, including abnormal noise, poor component response due to spongy behavior of aerated fluids, cavitation damage, severe fluid degradation and high fluid temperature. 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.
Operating a hydraulic system above 180°F (83°C) should be avoided because it can damage seals and accelerate degradation of the hydraulic fluid. 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 and all high temperature indications investigated and rectified immediately.
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 conditions 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 degradation of the hydraulic fluid, which in turn can cause overheating and burning of the seals. Regularly check the condition of the pump-shaft seal and, when leaking, replace it.
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 leading to mechanical failure of the system components.
Most often, cavitation commonly 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 that it is not clogged.
Beware of Air, Bubbles in Hydraulic Systems
April 23, 2010