Possibilities of application of water lubricated thrust bearings in vertical shaft hydro-generators

Michał Wasilczuk,  Michał Wodtke

Faculty of Mechanical Engineering, Gdansk University of Technology, Gdansk, Poland

INTRODUCTION: Increasing requirements for environmental protection make it necessary to introduce new materials and designs. Hydroelectric power plants operating in direct contact with water reservoirs and rivers are potentially endangering water cleanliness, hence they should also be modernized in the way minimizing the environmental hazards. A lot of progress in this field has been achieved in last decades, but still there is much work to be done, since in some cases the modifications are not straightforward and require know-how and/or new research. In case of high speed bearings in which full fluid film lubrication is possible, modifications are focused on substituting mineral based lubricants (mineral oils) with eco-friendly synthetic oils or, preferably, with water. In case of synthetic lubrication there are only minor technical problems, while water lubrication creates numerous problems including material issues and low carrying capacity of water lubricated bearings due to low viscosity of water. On the other hand water lubrication provides no environmental hazard at all, which is not the case with synthetic lubricants. In addition, lower friction losses, as compared to oil lubrication are the other benefit. In vertical shaft machines the problem more pronounced in thrust bearings, which carry substantial load and low load carrying capacity has not allowed for wider application of water lubricated bearings up till now, while in radial bearings in which load is relatively low, introducing water lubrication is generally easier and such bearings are more frequently applied. A research aimed at introduction of water lubricated thrust bearings has been recently started at the laboratory of Faculty of Mechanical Engineering.

METHODS: There are very few literature reports on the research on water lubricated thrust bearings (eg. 1-3) and the authors have not encountered any reports of the application of water lubricated thrust bearings of large diameter. Up till now the study of feasibility of water lubricated bearings was carried with the use of theoretical calculations, at the first stage calculations with the use of fluid structure interaction (FSI), combining finite element (FEM) method calculations for bearing structure, with computational fluid dynamics (CFD) for lubrications film and the fluid domain, in general (4) have been performed. In case of a potentially feasible hydrostatic water lubricated bearing the results were obtained with the use of analytical calculations based on fluid dynamics. In next steps small scale experiments on a specialized thrust bearing test rig are planned.

RESULTS: Preliminary results show that there are possibilities of substantial decrease of power loss. The first case is a hydrostatic thrust bearing of 1.3 m outer diameter operating in a vertical shaft pump turbine with the axial load of 2250 kN and rotational speed of 600 rpm. The data refer to a thrust bearing described in detail in (5), where the measured losses of a hydrodynamic bearings are approximately equal to 250 kW. Alternative hydrostatic water lubricated bearing would have power losses amounting to 34 kW (approximately seven times lower than in the existing bearing) including both components of the power loss: film losses of 14 kW and pumping power of 20 kW. The calculations assumed  film thickness of 20 μm. The

important element of such a hydrostatic bearing would be a high pressure lubricating system, requiring a water pump capable of delivering approximately 150 L/min at a pressure of 10 MPa. The reliability of such a pump would be of a vital importance to the reliability of the whole system (6).

The other example shown in the full paper will refer to the calculations of hydrodynamic tilting pad thrust bearing of similar size and operating conditions. In such a bearing one can observe a decrease of film losses by the factor of five. According to the calculations the bearing will operate in fluid film regime, but the minimum film thickness would drop below 10 μm, as compared to approximately 40 μm in oil lubricated bearing. 

DISCUSSION: According to the theoretical results it seems that water lubricated bearings can be potentially installed even at most demanding applications in large tilting pad thrust bearings. PROS of such applications are mainly of ecological character  including both – environmentally friendly lubricant and substantially decreased power losses. Numerous CONS comprise the need of considerable changes in bearing design,  including polymer lining substituting metal based bearing alloys, efficient design of load equalization system. The redesign will also comprise a stainless lubricating system and a bearing. Novel bearings would certainly require further research, also experimental and field testing. In case of a hydrostatic thrust bearings a reliable, large output, high pressure lubricating system is also a key issue

ACKNOWLEDGEMENTS: This work was a part of research grant no. 2016/23/B/ST8/03104 entitled “Research on water lubricated sliding couples in conditions of improper lubrication conditions” financed by Polish National Science Centre.

REFERENCES: (1) Inoue I. et al. (2012): Development of the water-lubricated thrust bearing of the hydraulic turbine generator. Bearing with PEEK lining. Proc.  26th  IAHR Symposium  on  Hydraulic  Machinery  and  Systems.

(2) Hennsler D. et al. (2015): Qualification and optimization of solid polymer tilting pad bearings for subsea pump applications. Bearing pads made of PEEK, 44^th Turbomachinery Symposium Houston, Texas

(3) Cheng De. et al. (2015): Study on the Water Lubricated Large-scale Tilting Pad Thrust Bearing by Finite Element Method. Proceedings of the World Congress on Engineering and Computer Science 2015, San Francisco, USA

(4) M. Wodtke, A. Olszewski and M. Wasilczuk: Application of the fluid–structure interaction technique for the analysis of hydrodynamic lubrication problems. Proc  IMechE  Part  J: J  Eng. Tribology, 2012, DOI: 10.1177/1350650113481147

(5) M. Wasilczuk, M. Wodtke and L. Dąbrowski Field Tests on Hydrodynamic and Hybrid Operation of a Bidirectional Thrust Bearing of a Pump-Turbine. Lubricants 2017, 5(4), 48; doi:10.3390/lubricants5040048

(6) M. Wasilczuk Friction and Lubrication of Large Tilting-Pad Thrust Bearings. Lubricants 2015, 3, 164-180; doi:10.3390/lubricants3020164