Measurement and Prediction of the Static Operating Conditions of a Large Turbine Tilting-Pad Bearing Under High Circumferential Speeds and Heavy Loads

Abstract

The identification results for the static performance characteristics of a large tilting-pad bearing in load between pad configuration are presented for specific bearing loads between 1.0 and 2.5 MPa and for circumferential speeds up to 79 m/s. The bearing is lubricated by spray-bars and can be described by the following specifications: Five pads, 0.23 nominal preload, 60% offset, 56° pad arc angle, 500 mm inner diameter, 350 mm pad length and 1.28 per mille relative bearing clearance. The axial oil flow is reduced by a fixed seal on both bearing edges which has a nominal radial clearance of 1 mm. The film pressure and the gap width are determined in the whole area of the sliding surface by an axial shift of the shaft. The bearing temperatures are measured by means of 100 thermocouples located 5 mm behind the sliding surface. The experimental results indicate that significant pad deformation occurs in circumferential and in axial direction. Also the effective supply temperatures are much higher than the nominal ones. According to the lubricant flow the sensor temperatures close to the spray-bars at the sliding surface rise about 20 K for 7 l/s and 40 K for 3.5 l/s at 3000 rpm. The temperatures are nearly constant between all pads and depend only on speed and not on load. The theoretical analyses of the bearing performance was accomplished with the bearing calculation software COMBROS. This code models the transition between laminar and turbulent flow and solves an extended Reynolds equation, the three dimensional energy equation of the film and the heat conduction equations of the shaft and the pads considering various boundary conditions due to the judgment of the user. Concerning the minimum film thickness, the maximum temperature on the sliding surface and the maximum film pressure only poor agreement was reached if the influence of the axial pad deformation was neglected. In advanced analyses a co-simulation between COMBROS and a structural mechanics software shows that an improvement of the prediction was achieved. The comparison of the measurement data and the advanced simulation shows very good agreement for the characteristic bearing parameters as well as for the local distributions of film pressure, temperature and film thickness in the whole operating range of the bearing. Further, the applied inlet mixing model for the lubricant supply process proves to be very suitable.