Hydraulic Fluid Power. Andrea Vacca
2.9. As the reader can notice from the figure, for a mineral oil, there is an increase in viscosity of about 40% from atmospheric conditions to 200 bar.
Figure 2.9 Viscosity as a function of pressure.
2.7.1 Entrained Air
In some circumstances, the hydraulic fluid can entrain some air from the environment that can lead to suction condition issues for the pump(s). This is commonly referred as pseudo‐cavitation.
This usually can occur in the reservoir (Figure 2.10a) or when gas leakages are present in low‐pressure hydraulic lines, such as suction lines (Figure 2.10b). For example, the return line entering the reservoir might present a non‐submerged pipe or a design creating a high amount of turbulence. In both cases the hydraulic fluid can entrap air that is then carried within the flow and can enter the hydraulic system. If the bubbles of air do not have enough time to settle within the reservoir, foam can accumulate on the surface.
The entrained air can lead to cavitation of the pump or to erratic phenomena such as a slow response of some functions. For this reason, it is always recommended to adopt all possible measures to avoid or limit the air entrainment.
2.7.2 Gas Solubility
All liquids, including hydraulic fluids, normally contain dissolved incondensable gases (typically air taken from the environment).
The liquid absorbs the gas from the surroundings until the saturation state is reached. As long as the gas is dissolved, it does not influence the main properties of the fluid, particularly in terms of compressibility or viscosity.
The (maximum) volume of air dissolved in the liquid can be determined by the following equation, derived from the well‐known Henry–Dalton law:
It is important to remark that Vair, d represents the volume of air measured at the reference pressure p0. This law states that the volume of air that the liquid can dissolve proportionally increases with pressure. For mineral oils, the coefficient αa is also known as Bunsen coefficient and can vary from 0.06 to 0.09. This is not much affected by temperature or viscosity [22]. For water, the Bunsen coefficient is 0.04, which means that mineral‐based oils tend to dissolve more air than water. To better understand the meaning of αa and quantify the actual amount of incondensable gas that a hydraulic fluid can dissolve, one can consider the atmospheric pressure as the reference condition, p0. This is the condition of most hydraulic fluids inside a reservoir open to atmosphere. In this situation, considering p = p0 in Eq. (2.22), a volume of air corresponding to 6%, up to 9%, of the volume of the fluid V0 could be held by the fluid. This amount of air is in equilibrium with the liquid, and it is not released unless the pressure of the fluid is reduced.
Figure 2.10 Typical causes of entrained air within the hydraulic fluid: (a) in a reservoir; (b) in a suction line.
The air release phenomenon is similar to what anyone experiences when opening a bottle of carbonated drink. Before opening the bottle, the fluid in the bottle appears as uniform liquid; however, while opening the bottle, bubbles of gas can be observed while the internal pressure decreases. This means that before opening the bottle, the gas was in equilibrium with the liquid, entirely dissolved. As the pressure decreases, a certain amount of gas gets released, and bubbles start appearing within the liquid.
Considering the gas solubility of the fluid, three possible equilibrium states can be identified. These are graphically represented in Figure 2.11: Above the saturation pressure pSAT, all the air is dissolved in the liquid without affecting the main properties of the fluid. However, below the saturation pressure, only a portion of the air remains dissolved as determined by the Henry–Dalton relation of Eq. (2.22). The rest of the air is released from the liquid and is in free state. In this condition, the fluid is a mixture of liquid and air, in the form of bubbles.
Figure 2.11 Equilibrium states for a liquid considering gas solubility.
For hydraulic systems, the condition in which incondensable gases are released is generally referred to as gaseous cavitation or aeration.
The gaseous cavitation should be treated differently from the entrained air aspect described in Section 2.7.1, which is sometimes referred to as pseudo‐cavitation.
Below the vapor pressure, pVAP, the air is completely released, and the hydraulic fluid is in the vapor form. This condition for the hydraulic fluid is usually referred to as vapor cavitation.
Typical hydraulic oils are always a mixture of different components; therefore there is not a defined value of pVAP, but rather an interval [pVAP, L, pVAP, H] of pressure throughout which the vaporization of the fluid occurs. Typical values of pVAP, L and pVAP, H for mineral oils range between 15 000 and 30 000 Pa (absolute pressure) [23].
All the abovementioned cavitation phenomena can be detrimental to the operation of a hydraulic system. More than the formation of the vapor or of the gas bubbles, it is usually the opposite process (that occurs in an abrupt manner), which causes erosion damages of the mechanical parts. For this reason, designers of hydraulic components normally put the highest attention in preventing cavitation phenomena. More specifically, the selection of hose diameters and length should avoid the fluid pressure to fall below vapor pressure and, if possible, below saturation pressure.
However, gaseous cavitation is often unavoidable, particularly in the lines connecting the reservoir (where the fluid is at saturation conditions) to the pump supply port. This is due to the frictional losses that cause the pressure to decrease as the fluid travels into the line. For this reason, the connection between the pump and the reservoir must be designed to limit these pressure losses as much as possible so that the pump can operate under a minimal (sometimes negligible) aeration condition.
Moreover, in certain parts of the hydraulic system, such as in hydraulic control valves or hydraulic pumps, there are sometimes violent flow restrictions where the fluid accelerates to high velocities, causing the static pressure to fall below the saturation pressure (see Bernoulli's equation, Chapter