Dissolved air in mineral oil ­dampens cavitation erosion by a factor of 4-5

Cavitation is usually unavoidable in the operation of hydraulic systems and can have serious negative effects, such as noise, premature oil aging, or limitations of the operating range. In addition, cavitation erosion in particular can shorten the service life of hydraulic components such as pumps or valves. Since vapor and gas cavitation usually occur simultaneously, there has been much controversy in the hydraulic literature and in industrial practice as to which type of cavitation is actually responsible for the erosion. In experiments, we have now been able to demonstrate that cavitation erosion in hydraulic components is caused by vapor cavitation and not by gas cavitation. On the contrary, the air released during gas cavitation actually dampens erosion considerably.

Due to the high air solubility of mineral oil, a distinction is made in hydraulics between vapor and gas cavitation. Although both types appear phenomenologically the same, they are based on two fundamentally different physical processes: While vapor cavitation describes the pressure-induced phase transition between liquid and gas analogous to boiling, gas cavitation is a diffusive outgassing process similar to the outgassing of CO2 from mineral water. 

In order to clearly separate the different types of cavitation, a specially designed tank with integrated degassing capability was developed for the test rig. After complete degassing, no dissolved air is present in the oil, so that no gas cavitation can occur either, thus ensuring states of pure vapor cavitation.  A valve model with copper samples in realistic dimensions and under practical operating conditions was used as the flow geometry, thus achieving a very high transferability to real components. A total of four tests were carried out, each lasting over five hours. Subsequently, cavitation erosion was quantified based on the mass loss of the copper samples. Here, experiments with air-free mineral oil showed a mass loss 4.4 to 5.1 times higher than experiments with saturated (normal) oil (5.5 mg mass loss for normal HLP46 and 24.2 mg for air-free HLP46, respectively).

The experimental fact that air-free mineral oil causes significantly more erosion than saturated oil clearly proves that vapor cavitation is the cause of cavitation-induced damage to hydraulic components. This is because if gas cavitation were responsible for erosion, the erosion rate should be lower for air-free oil - however, the exact opposite is observed.

The findings have fundamental implications for the use of air-free mineral oil and, not least, for the further development of hydraulic components and systems in the direction of higher power densities and a longer service life.

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