A large tilting-pad journal bearing (TPJB) with “PEEK” polymer-lined pads was tested over a range of operating conditions representative of those experienced on turbogenerators used in fossil fuel power plants. The 500 mm diameter test-bearing has four offset-pivot pads, ball and socket pivots, load-between-pivot configuration, directed lubrication and hydrostatic jacking features. The operating conditions explored during the test campaign characterize the static and dynamic behaviour of the bearing over a range of shaft surface speeds between 40 m/s and 95 m/s and maximum specific load of 4.75 MPa. Similar test conditions were previously investigated on the same bearing with whitemetal lined pads, allowing for a direct comparison. A thermo-elasto-hydrodynamic (TEHD) model for TPJBs with polymer-lined pads is introduced in this paper and validated against experimental test data. Experimental data along with numerical results reveal and confirm the superior performance that can be attained using TPJBs with polymer lined pads at high specific loads.

The progressive upgrading of heavy-duty gas turbines, aimed at increased performance, can ultimately introduce more onerous operating conditions, to the point that original design limits can be approached. An increased gas turbine pressure ratio together with compression and expansion line adjustments can directly affect the rotor axial thrust. Other than the individual forces acting on the rotor, a key component to be taken into account is the fluid film thrust bearing, which should assure safe and reliable operation during the worst case operating conditions. Typically, such bearings are designed with large safety margins, yet it is possible that the new and more challenging conditions require a bearing capability upgrade, especially when field retrofit needs pose additional constraints. A succession of performance upgrades have been carried out on Ansaldo Energia’s AE94.2 E-Class GT. An accurate understanding of the thrust-related phenomena proved necessary and drove improvements in the thrust bearing design along with hardware adjustments to lower the rotor thrust. This paper addresses calculations and experimental arrangements for the rotor axial thrust evaluation on the aforementioned GT and considers both the matters related to the secondary air system for the thrust generation and the mechanical/functional matters for the bearing upgrade. It is shown that issues such as uneven load sharing across the thrust bearing, or the variability of rotor thrust from engine to engine within the fleet, strongly affect the maximum thrust assessment and thus the requirements used in the process of selecting a suitable bearing. A predictive calculation method is described considering the main thrust contributions. Field experimental setups and main observations are reported. Measurements have been carried out using thermocouples and load cells placed on many of the thrust bearing pads. Moreover, the engine cavities carrying the highest and/or the most uncertain thrust share have been instrumented and characterized by pressure sensors. The development of an upgraded thrust bearing is finally depicted through the main issues addressed, such as improved thrust pad lining material, increased load sharing efficiency and enlarged thrust bearing active surface area. Waukesha Bearings test results on the upgraded lining material, a high-tin aluminium alloy are reported as well. A multidisciplinary approach is presented as necessary to manage the crucial challenge of improving the thrust balancing system, especially in the case of a formerly designed engine which receives a powerful upgrade.

Flexure pivot® journal bearings (FPJBs) have typically been used in small high-speed applications such as integrally geared compressors and multistage high-speed compressors, where the temperature management and the rotordynamic stability of the machine are the main targets. Nevertheless, the need for high-speed applications may also be applicable to large compressors and for this reason a large 280 mm diameter four-pad FPJB with L/D = 0.7 has been designed, built, and tested by the Authors. The test facility is a novel rig, setup at the University of Pisa, that includes a floating test bearing and a rigid rotor supported by two stiff rolling element bearings. Both static and dynamic loads are applied through hydraulic actuators, capable of 270 kN static and 40 kN overall dynamic load. The instrumentation can measure all the relevant test boundary conditions as well as the static and dynamic quantities that characterize the bearing performance. This paper presents the results from a test campaign conceived to explore not only the design conditions (7000 rpm rotational speed and 0.75 MPa unit load) but also the sensitivity to the unit load (from 0.2 MPa minimum load up to 2.2 MPa maximum load) as well as the oil flow. The results are discussed and compared with predictions from an existing numerical code.

The presence of high subsynchronous vibrations and other rotordynamic instabilities in steam turbines can prevent operation at full speed and/or full load. The destabilizing forces generating subsynchronous vibrations can be derived from bearings, seals, impellers or other aerodynamic sources. The present paper describes the case of an 11 MW steam turbine, driving a syngas centrifugal compressor train, affected by subsynchronous vibrations at full load. After the occurrence of anomalous vibrations at high load and a machine trip due to the high vibrations, the analysis of data collected at the site confirmed instability of the first lateral mode. Further calculations identified that the labyrinth seal at the balance drum was the main source of destabilizing effects, due to the high pre-swirl and the relatively tight seal clearance. The particular layout of the turbine, a passing-through machine with a combined journal/double thrust bearing on the steam admission side, together with the need for a fast and reliable corrective action limited the possible solutions. Based on the analyses performed, adjusting the clearance and preload of the journal bearings could not have ensured stable operation at each operating condition. The use of swirl brakes to reduce the steam pre-swirl at the recovery seal entrance would have required a lengthy overhaul of the unit and significant labor to access and modify the parts. The final choice was a drop-in replacement of only the rear bearing (on the steam exhaust side) with a bearing featuring integral squeeze film damper (ISFD) technology. In addition to being a time efficient solution, the ISFD technology ensured an effective tuning of stiffness and damping, as proven by the field results. The analyses carried out to understand the source of the subsynchronous vibrations and to identify possible corrective actions, as well as the comparison of rotordynamic data before and after the application of the bearing with ISFD technology, are discussed.

Flexure Pivot® Journal Bearings (FPJBs) have typically been used in small high-speed applications such as Integrally Geared Compressors (IGCs) and multistage high-speed compressors, where the temperature management and the rotordynamic stability of the machine are the main targets. Nevertheless, the need for high-speed applications may also be applicable to large compressors and for this reason a large 280mm diameter four-pad FPJB with L/D = 0.7 has been designed, built and tested by the Authors. The test facility is a novel rig, set up at the University of Pisa, that includes a floating test bearing and a rigid rotor supported by two stiff rolling element bearings. Both static and dynamic loads are applied through hydraulic actuators, capable of 270kN static and 40kN overall dynamic load. The instrumentation can measure all the relevant test boundary conditions as well as the static and dynamic quantities that characterize the bearing performance. This paper presents the results from a test campaign conceived to explore not only the design conditions (7000rpm rotational speed and 0.75MPa unit load) but also the sensitivity to the unit load (from 0.2MPa minimum load up to 2.2MPa maximum load) as well as the oil flow. The results are discussed and compared with predictions from an existing numerical code.