Entrained gas or gas bubbles in the hydraulic fluid is caused by the sweeping of air out of a free air pocket by the flowing fluid and also when pressure drops below the vapor pressure of the fluid. Vapor pressure is that pressure at which the fluid begins changing into vapor. This vapor pressure increases with increase in temperature. This results in the creation of fluid vapor within the fluid stream and can in turn lead to cavitation problems in pumps and valves. The presence of these entrained gases reduces the effective bulk modulus of the fluid causing unstable operation of the actuators.
The phenomenon of cavitation is in fact the formation and subsequent collapse of the vapor bubbles. This collapse of the vapor bubbles takes place when they are exposed to the high-pressure conditions at the pump outlet, creating very high local fluid velocities, which impact on the internal surfaces of the pump. These high-impact forces cause flaking or pitting on the surface of components such as gear teeth, vanes and pistons leading to premature pump failure. Additionally the tiny metal particles tend to enter and damage other components in the hydraulic system. Cavitation can also result in increased wear on account of the reduced lubrication capacity.
Cavitation is indicated by a loud pump noise and also by a decreased flow rate as a result of which the pressure becomes erratic. Air also tends to get trapped in the pump line due to a leak in the suction or on account of a damaged shaft seal. Additionally it has to be also ensured that air escapes through the breather while the fluid is in the reservoir or otherwise it tends to enter the pump suction line. To counter the phenomenon of cavitation in pumps, the following steps are recommended by manufacturers:
1. Suction velocities to be kept below 1.5 m/s (5 ft/s)
2. Pump inlet lines to be kept as short as possible
3. Pump to be mounted as close to the reservoir as possible
4. Low-pressure drop filters to be used in the suction line
5. Use of a properly designed reservoir that will help remove the trapped air in the fluid
6. Use of hydraulic fluid as recommended by the manufacturer
7. Maintaining the oil temperature within prescribed limits, i.e. around 65 °C or 150 °F.