Evaluation of cavitation erosion characteristics of high bulk modulus oils

Toshiharu Kazama1, Shinji Aoki2, Masahiro Kobessho2

1Muroran Institute of Technology, Hokkaido, Japan.  2Idemitsu Kosan Co., Ltd.

INTRODUCTION: Hydraulic systems are used in construction equipment, aeronautical components, and industrial manufacturing, because they provide great power density and high responsivity. Hydraulics use pressurized liquids to transmit power¹, while the power-transfer liquid is either mineral oil or synthetic oil².

Regarding the liquids, the high pressure conditions cause low responsivity and efficiency because of compressibility of the oils. To overcome the compression loss and response lag of hydraulic systems, a high bulk modulus oil was developed³. The advantages in terms of high rigidity, high performance⁴, and low flow loss⁵ for the oil have been examined and confirmed.

Another important drawback associated with high pressurization is cavitation because cavitation erosion is a serious problem affecting hydraulic machinery such as valves, pumps, and motors⁶. Nevertheless, the phenomenon and behavior of the high bulk modulus oils have not yet been examined to date.

In this report, therefore, four types of high bulk modulus oils and a mineral oil were prepared, and then the experimental study was performed using a jet-cavitation erosion tester.

METHODS: The apparatus and test procedures used for this work were almost similar to those prescribed in ASTM standards⁷, but with partially differing geometry and test conditions. The relative deviation of the tests used here from the standards is described in the previous paper⁸.

The liquids tested in this study are listed in Table 1, wherein the bulk modulus K was that K = 1.26 GPa for the mineral oil and K » 1.8 GPa for the high bulk modulus oils⁹. The specimens had a diameter of 15 mm, which were made of aluminum alloy (A5056 in JIS). By installing an inline cooler and recirculating the test oils, the temperature was maintained at 40±1°C. The upstream absolute pressure p_u was maintained at 10.1 MPa. The cavitation number s (= p_d/p_u, p_d: downstream pressure) was maintained at s = 0.02. The exposure time was up to 8 hours.

RESULTS:  Figure 1 plots the mass loss M of specimens in terms of standoff distance L between the nozzle outlet and the specimen surface for the high bulk modulus oils (BF-4M and RO-2M) and the mineral hydraulic oil (TO-2M). Although the test oils of BF-5M and RO-1M had high polymer, the oils of BF-4M and RO-2M had no high polymer (BF: higher molecular weight, RO: lower molecular weight). The standoff distances where M maximized, L_max, were almost the same (L_max » 22.5 mm), although L_max of RO-2M was slightly longer.

Figure 2 presents the maximum erosion rate ER_max (= DM_max /Dt at time t = 8 h) for all oils tested. The ratio ER_max of BF-4M was largest, followed by ER_max of BF-5M, TO-2M, RO-2M, and RO-1M.

Table 1 - Physical properties of test oils (n: kinematic viscosity [mm²/s], r: density [kg/m³]).

Figure 1 - Mass loss M vs. standoff distance L.

Figure 2 - Comparison of maximum erosion rate ER_max.

DISCUSSION: Since high bulk modulus oils essentially have physical properties of high density, the acoustic impedance of the high bulk modulus oils becomes higher than that of mineral oils, resulting in marked cavitation erosion. However, it should be noted here that ER_max of BF-5M by adding high polymer became lower than ER_max of BF-4M. Therefore, the erosion by high bulk modulus oils can be potentially reduced by high polymer additives.

CONCLUSIONS: Based on the cavitating jet method, cavitation erosion characteristics for high bulk modulus oils were investigated experimentally. Although high bulk modulus oils cause severe cavitation erosion because of high density and high acoustic impedance, the erosion can be controlled appropriately by adding high polymer.

ACKNOWLEDGMENTS: The authors thank Mr. S. Suya, Mr. S. Sasaki, Mr. T. Murakami, and Mr. M. Hoshi, the ex-students of the Muroran Institute of Technology, for the experiments.

REFERENCES:  1. Ishihara, Oil-Hydraulics, Asakura (1968),  2. Okabe, Hydraulic Fluid Handbook, Lub. Tech. (1985), 3. Tsubouchi, Proc. 7 JFPS (2008), 4. Liu, Trans. JFPS (2011), 5. Mettakadapa, Proc. FPMC, (2015), 6. Yamaguchi, ASME J. Fluids Eng. (1987), 7. ASTM, G134.95 (1995), 8. Kazama, Trans. JFPS (2007), 9. Aoki, Proc. STLE (2016).