Hydraulic Library User Manual - NUPET

Hydraulic Library 4.2User ManualFigure 1.17: Pressure in pipe with saturation pressure 400 bar.The variation between the runs is now very pronounced. The dynamiccharacteristics of the system is completely transformed. A few words ofexplanation are necessary.Normally the air content of a hydraulic oil is well below 1%, and 0.1% is typical.It is normally considered good practice to keep the value as low as reasonablypossible. However, in a few application, such as lubrication oil in gearboxes, theoil and air are well mixed up and 2.5% is typical and up to about 10% is possible.A reasonable quantity of air, given time, will completely or partially dissolve inthe hydraulic fluid. The lowest pressure at which all the air is dissolved is calledthe saturation pressure. For very slow systems all the air is dissolved above thesaturation pressure and partially dissolved below this pressure. Henry’s law givesa reasonable approximation for the fraction of air that is dissolved in equilibrium.Some systems are slow enough to stay very close to this equilibrium position(Figure 1.16). Often classic fluid power systems behave like this. The originalsaturation pressure is better for the current example.However, it does take time for the air to dissolve and this time will not be availablein fast acting systems. Fuel injection systems are a good example of this. Hencewith such systems it may be appropriate to set the saturation pressure artificiallyhigh to allow for significant quantities of air to be undissolved at all pressures.15