Accurate modelling of bearings and seals for rotating machines for power generation

This paper reviews the most recent achievements obtained in the modelling of oil-film bearings and of labyrinth seals at Politecnico di Milano. These two topics are receiving increasing interest in nowadays rotor dynamics, in particular by OEM, owing to the increasing demand of high-performances/efficiencies and to the remarkable effect that they have on the stability of rotating machinery. The precise evaluation of dynamic coefficients, for both oil-film tilting-pad journal bearings and labyrinth seals, allows reliable evaluation of machine stability in the design phase. At this aim, several models are available in the literature. On the contrary, there are not so many cases, in which the models are validated by means of experimental tests. The bearing and labyrinth seal models presented here are supported by experimental validation

Design and Analysis of CFD Experiments for the Development of Bulk-Flow Model for Staggered Labyrinth Seal

Nowadays, bulk-flow models are the most time-efficient approaches to estimate the rotor dynamic coefficients of labyrinth seals. Dealing with the one-control volume bulk-flow model developed by Iwatsubo and improved by Childs, the “leakage correlation” allows the leakage mass-flow rate to be estimated, which directly affects the calculation of the rotor dynamic coefficients. This paper aims at filling the lack of the numerical modelling for staggered labyrinth seals: a one-control volume bulk-flow model has been developed and, furthermore, a new leakage correlation has been defined using CFD analysis. Design and analysis of computer experiments have been performed to investigate the leakage mass-flow rate, static pressure, circumferential velocity, and temperature distribution along the seal cavities. Four design factors have been chosen, which are the geometry, pressure drop, inlet preswirl, and rotor peripheral speed. Finally, dynamic forces, estimated by the bulk-flow model, are compared with experimental measurements available in the literature

ROTORDYNAMIC PERFORMANCE OF A FULLY-PARTITIONED DAMPER SEAL: EXPERIMENTAL AND NUMERICAL RESULTS

LectureBalance piston seals restrict leakage at the discharge side of single shaft centrifugal compressors. There are multiple types of annular seals including labyrinth seals, textured surface seals such as honeycomb seals and hole-pattern seals, conventional pocket damper seals (PDS), and fully-partitioned damper seals (FPDS). These seals face high pressure differentials with large changes in gas density to develop large radial forces impacting the compressor rotordynamic performance. There is an extensive experimental database available quantifying the dynamic forced performance of labyrinth seals and textured surface seals. On the other hand, an experimental database for PDS and FPDS is limited and prevents a fair and direct comparison of their dynamic force performance against textured surface seals. This lecture presents experimentally derived rotordynamic force coefficients for a FPDS obtained at relevant operating conditions in shaft speed and pressure ratio (exit/inlet) along with a direct comparison to published test data available for a honeycomb seal, similar in size and in operating conditions. The dynamic load tests with the FPDS, 4.5 inch in diameter × 3.37 inch in length, include operation at three shaft speeds (10, 15 and 20 krpm: max surface speed of 120 m/s), a supply pressure of 70 bar(a), three exit/inlet pressure ratios (25%, 50%, 65%), and with circumferential inlet flow pre-swirl conditions ranging from 10% to 160% of shaft surface speed.Of interest to a damper seal performance is the cross-over frequency, which determines the transition from a negative to a positive effective damping coefficient. The FPDS produces effective damping magnitudes near 10 kN.s/m above the cross-over frequency, which ranges from 28-62Hz and above 70 Hz for the lowest (below 0.1) and highest inlet pre-swirl (above 0.6) conditions, respectively. On the other hand, the FPDS generates small direct stiffness coefficients, varying with frequency though contained within +/- 2 MN/m band. Therefore, the FPDS can significantly improve damping ratio of a compressor rotor-bearing system without shifting the natural frequencies. Comparisons of the force coefficients for the FPDS versus a honeycomb (HC) seal demonstrate that both seals generate comparable effective damping coefficients and similar cross-over frequencies with some differences highlighted in the discussion of the results. As expected from a textured seal, the HC produces much larger direct stiffness coefficients when compared to the FPDS. In terms of leakage performance, the FPDS leaks roughly 20%-25% more than the HC. Experimental results are also compared against predictions obtained from CFD and bulk flow models. The CFD and bulk flow predictions yield a good to moderate correlation to the experimental data depending on the operating conditions of rotor speed and pressure ratio. Both methods represent a viable approach for designing FPDSs and predicting their rotordynamic performance

Analisi teorica e sperimentale dell’effetto della direzione del carico in cuscinetti a pattini oscillanti reali

I cuscinetti portanti a pattini oscillanti sono ampiamente utilizzati nelle macchine rotanti per velocità elevate e carichi sia bassi sia elevati. Data l’importanza di questo componente delle macchine rotanti, molti autori hanno pubblicato diversi studi, teorici e sperimentali, per valutare l’effetto dei giochi, della temperatura del lubrificante, della portata dell’olio e di diversi effetti termici sul comportamento di questo tipo di cuscinetto. Tuttavia, le indagini sull’influenza della direzione di carico sulle proprietà dei cuscinetti a pattini oscillanti sono spesso molto limitate e non considerano le geometria reale del cuscinetto. In questo articolo verrà presentata un’analisi teorica e sperimentale degli effetti delle direzione del carico nei cuscinetti reali a pattini oscillanti

Condition monitoring of the rolling stock and infrastructure: results of a pilot project

The application of Prognostics and Health Monitoring (PHM) concepts in rail vehicles and railway infrastructure is a rapidly growing field of research, and extensive efforts are being spent with the aim of improving the reliability and availability of railway systems and of substantially reducing maintenance costs by switching from time-based to event-driven maintenance policies. This paper presents the results of a research project in which concepts were developed and demonstrated for the health monitoring of the rolling stock (traction equipment) and of the railway infrastructure (track and overhead equipment). A prototype monitoring system was installed on a E464 locomotive and results were gathered across a time span of 14 months from December 2014 to January 2016

Numerical Modeling of Thermally-Induced Vibration in Rotor Caused by Light-Rub Against Brush Seal

Clearance is of paramount importance for turbomachinery manufacturers to meet today’s aggressive power output, efficiency, and operational life goals. To minimize leakages, there are various seal types used, and new sealing concepts are in development. Because of their inherent flexibility and compliance, brush seals are capable of significantly reducing the leakage, and allow sufficient geometrical margins to accommodate design and operational variations of turbomachines. Brush seals can be assembled at very tight or zero radial clearance or even with interference on the rotor to minimize the leakage. This means that the risk of contact between the rotor and the seal bristles exists, especially in case of zero clearance or interference. If the contact occurs, a hot-spot develops on the rotor and this may cause the vibration to diverge, resulting in a synchronous instability, the so-called Newkirk effect. The friction forces generated by rotor-to-stator rubs often cause a shaft thermal bow whose main effect on the machine dynamic behavior is a progressive change of the synchronous (1X) vibration. The development of analytical tools able to model this phenomenon is therefore important to assess the rotordynamic stability during the design phase and avoid excessive vibrations which may have severe impact on the operability and on the mechanical integrity of the machine. The objective of this paper is the development of a numerical model to analyze the dynamic behavior of real turbomachines subject to thermally-induced vibration caused by light-rub of the rotor against brush seals. The model developed in the paper is based on the work of Bachschmid et al. [1]: the dynamics is analyzed in the frequency domain using the standard rotordynamic model, whereas the heat transfer analysis, to calculate the temperature distribution and the associated thermal bow, is studied in the time domain. The contact analysis has been deeply revised, aiming at estimating suitable normal and tangential force and the friction heating generated by the contact. The bow determined by the thermal conditions has been reproduced using suitable bending moments, which have been applied to the beam elements in the rotordynamic model

Unconventional Techniques for the Analysis of Experimental Spiral Vibrations

The occurrence of spiral vibrations in rotating machines is a well-known but not very common phenomenon. However, this kind of shaft vibration, usually caused by light rubs between rotating and stationary parts, may give rise to a slow considerable increase of the amplitude of the synchronous (1X) vibration. Owing to the rubs, the normal contact forces cause a friction-induced thermal bow, which in turn determines rather slow changes in amplitude and phase of the 1X vibration vector. The curve described in a polar plot by the tip of the vibration vector is a spiral. The occurrence of expansive vibrations may cause serious damage. Many studies about this malfunction are focused on the stability analysis of spiral vibrations. Simplified or rather rigorous thermal models can be used to evaluate the friction-induced thermal bow of the shaft and the slow continuous migration of the hot spot generated on the external surface of the rotor. However, owing to the complexity of the problem, some basic parameters of the thermal models can be affected by a significant uncertainty. This paper shows some unconventional techniques that can provide useful information for optimizing the rotor-to-stator contact modelling as well as for tuning some critical parameters of the thermal models that affect the velocity with which the hot spot moves around the circumferential surface of the shaft. The effectiveness of these techniques is shown by means of the analysis of the experimental spiral vibrations detected in a steam turbine power unit

An Experimental Study of Nonlinear Oil-Film Forces in a Tilting-Pad Journal Bearing

Journal bearings have been widely used in high-speed rotating machinery. The dynamic coefficients of oil-film force affect the machine unbalance response and machine stability. The oil-film force of hydrodynamic bearing is often characterized by a set of linear stiffness and damping coefficients. However the linear oil-film coefficients with respect to an equilibrium position of the journal are inaccurate when the bearing system vibrates with large amplitudes due to a dynamic load. The study on nonlinear oil-film forces is still rare and most papers are confined to theoretical analyses. The purpose of this paper is to derive some new non-linear force models (28-co., 24-co. and 36-co. models) to identify these dynamic coefficients based on experimental data. The fundamental test model is obtained from a Taylor series expansion of bearing reaction force. Tests were performed with a nominal diameter of 100mm and a length–to–diameter ratio of 0.7 using a suitable test rig in which it is possible to apply the static load in any direction. The results show that these three models are feasible to identify the oil-film forces in which the second-order oil-film coefficients received from the 24-co. model are more stable compared to those of other two nonlinear models

BEHAVIOR OF TILTING–PAD JOURNAL BEARINGS WITH LARGE MACHINING ERROR ON PADS

The use of tilting pad journal bearings (TPJBs) has increased in recent years due to their stabilizing effects on the rotor bearing system. Most of the studies addressing steady state and dynamic behaviors of TPJBs have been evaluated by means of thermo-hydrodynamic (THD) models, assuming nominal dimensions for the bearing, (i.e., the physical dimensions of all pads are identical and loads applied along the vertical direction). However machining errors could lead to actual bearing geometry and dimensions different from the nominal ones. In particular, for TPJB the asymmetry of the bearing geometry has not been well-investigated and can lead to an unexpected behavior of the bearing. The asymmetry of the bearing geometry can arise from large machining errors on only one or more pads, as a consequence of a pivot failure or after bad-mounting of the pads during assembly. These conditions can sometimes be detected by high values of the pad temperature, as measured by the temperature probes installed on the bearing pads, or by the abnormal vibration caused by pad-flutter phenomena. In this paper the authors investigate large machining errors on the pad thickness for a five-pad TPJB and analyze their effects on the bearing operating characteristics. Pad thickness errors correspond to a different preload factor or clearance for each pad. A sensitivity analysis was performed for several combinations of pad thickness using a THD model and the behavior of the bearing was analyzed, including dynamic stiffness and damping coefficients, clearance profile, shaft locus, minimum oil-film thickness, power loss, flow rate, and maximum pressure. The experimental case of a five pads TPJB with an intentional large machining error on the thickness of the pads is also described in the paper. The bearing has a nominal diameter of 100 mm, a diameter to length ratio (L/D) equal to 0.7 and can run at up to 3000 rpm. The experimental measurements are compared with the results obtained from the analytical model. The results show that the effects of asymmetry of the bearing geometry are more evident if the direction of the static load applied on the rotor bearing system, which is different from the vertical load, is also considered. For instance, the shape of shaft locus obtained by experimental tests changing the static load direction at a constant speed is an irregular pentagon if it is compared to the case of the nominal bearing. Based on our findings, we concludes that the machining error on the pads has a large influence on the shaft locus, minimum oil-film thickness and maximum pressure on pads, especially at high rotational speed, but has little effect on the flow rate and power loss. In addition, this error significantly affects the dynamic stiffness and damping coefficients, both in terms of rotational speed and load direction

Thermo-elasto bulk-flow model for labyrinth seals in steam turbines

Over the last few decades, the increasing demand on efficiency and performance for steam turbines has resulted in OEMs operating machines near critical conditions of their structural and thermal capabilities. In this paper, a new thermo-elasto bulk-flow model for labyrinth seals has been introduced. The model includes the bulk-flow model for estimating the dynamic coefficients, heat transfer model for evaluating the temperature distribution in the rotating and stationary parts and structural-mechanics model for calculating the radial growth. By considering a staggered labyrinth seal installed in the balancing drum of a steam turbine, different operating conditions, such as the boundary pressure, rotational speed and inlet pre-swirl ratio, as well as the stability of the seal are investigated in this study