LP Rotor Research Articles

Composite failure analysis of an aero-engine inter-shaft bearing inner ring

The inter-shaft bearing is a critical component in dual rotors aero-engine. Due to the complex and severe loads caused by the interactive excitation of HP and LP rotors during operation, the inter-shaft bearing is prone to damage accumulation and failure. Focusing on a typical failure of inter-shaft bearing inner ring fragmentation and cannot be spliced together completely in high thrust-weight ratio turbofan engine, this paper conducted morphological investigation, fractographic analysis, vibration signal processing, finite element numerical simulation, and fully studied the failure mechanism of this failure.The results indicated that: there exists an uncoordinated resonance speed of the HP rotor in close proximity to the operating speed. This puts the rotor in non-synchronous procession state, accompanied by the uncoordinated angular displacement of the inner and outer rings, which resulting in a lager rolling dynamic load in bearing inner ring. This further leads to high tangential impact load at the root fillet of the anti-rotation pin, causing significant stress concentration, and eventually leads to the generation of initial cracks. These cracks persist and propagate into fatigue oblique fracture over time. After oblique fracture, the modal frequencies of the inner ring become more dispersed. Under the rolling dynamic load (traveling wave load), nodal diameter vibration occurs, causing high stress vibration fatigue damage, and ultimately leading to multiple-point simultaneous fragmentation.The investigation suggests that reducing the stress levels at the root fillet of the anti-rotation pin of the inner ring is an effective approach to prevent bearing failure. This can be achieved through two methods: one is optimizing the rotor structure to decrease the rolling dynamic load on the inner ring, the other is eliminating the anti-rotation pin and appropriately reducing the tightening torque of the compression nut and the interference between the bearing inner ring and the HP turbine rear journal.

Analysis of Nonlinear Vibration Characteristics and Whirl Behavior of Dual-Rotor Systems with Inter-Shaft Rub Impact

Previous studies on rub-impact faults have mainly focused on the rub-impact between rotors and stators, with less research on inter-rotor rub impact. The impact of inter-rotor rub impact on rotor nonlinear vibration is particularly significant. This study investigates the effects of inter-shaft rub impact on the vibration characteristics and whirl behavior of dual-rotor systems. Initially, a dual-rotor model with inter-shaft bearings is established using the finite element method, and inter-shaft rub-impact forces are derived based on contact mechanics. Next, the system response is solved using the Newmark method. Vibration characteristics are analyzed through Campbell diagrams, 3D waterfall plots, time-frequency domain plots, and steady-state rub-impact force plots. Finally, the influence of inter-shaft rub impact on the whirl behavior of the dual-rotor system is studied based on the theory of full-spectrum analysis. The study concludes that inter-shaft rub-impact faults shift the system’s resonance points backward, increase harmonic and combination frequency components, and significantly affect the system response under dual-rotor co-rotation. Excessive friction can lead to self-excited vibrations and sudden amplitude increases, particularly in the LP rotor frequency. Additionally, inter-shaft rub impact primarily affects the whirl behavior of the LP-compressor disk1, showing multiple cycles of forward and backward whirl alternation during acceleration due to combined unbalanced and rub-impact excitations.

Nonlinear Modeling and Vibration Response Analysis of a Dual-Rotor System with an Inter-Shaft Graphite Seal

The inter-shaft graphite seal is a common structure in the dual-rotor system, and may generate nonlinear lateral excitation when there is large relative angular motion at the seal position. This paper represents the first attempt to study the nonlinear behaviors of a dual-rotor system with inter-shaft graphite seal excitations. First, an excitation model of the inter-shaft graphite seal is proposed by the analytical method. This model is then introduced into the beam finite element model of a dual rotor system, and the dynamic equations of the whole system are finally obtained. The vibration responses under the effect of the inter-shaft seal are clarified by analyzing 3D waterfall plots, rotor orbits, and time and frequency domain waveforms. The results show that the inter-shaft seal can lead to vibration coupling between HP and LP rotors. The axial spring stiffness and contact end face friction coefficient of the graphite seal have a significant effect on rotor vibration. When those two parameters are small, only coupling vibration phenomena can be observed, i.e., the rotation frequency of one rotor can be observed in another rotor vibration. With the increase in axial spring stiffness or contact end face friction coefficient, multiple modes of LP and HP rotors are excited, which dominates the vibration behaviors of the dual rotor system.

Study on fracture analysis of rotor blade in thermal power plant

The comprehensive failure analysis of the fractured LP rotor blade was carried out by means of appearance morphology analysis, SEM morphology analysis, mechanical property analysis, chemical composition analysis and microstructure detection. The results show that the second and last stage blades at the low-pressure rotor turbine side are damaged by mechanical collision, resulting in damage and deformation at the steam outlet side of the blades and generating stress concentration areas. The blades are prone to generate crack sources in the stress concentration areas of mechanical damage under the working load, and the cracks continue to expand on the condition of alternative stress, eventually leading to fatigue cracking and fracture of the blades. It is recommended to carefully check the blades at other positions of the LP rotor and deal with the mechanical damage and cracking problems in time. Secondly, foreign matters shall be prevented from entering the turbine with the steam to damage the blades, and the fixed parts inside the cylinder shall be ensured to be firm and not fall off, so as to prevent the recurrence of similar blade fracture.

Dynamic characteristics analysis of a dual-rotor system with bolted-disk joint

This article presents a nonlinear vibration characteristics study of a bolted joint dual-rotor system. The motion equations are derived through the lumped mass modeling method and a two-node bolted joint element. Nonlinear time-varying bending stiffness at the joint interface is considered in the numerical integration. Qualitative analysis of the effect of preload on system responses is conducted through changing the value of the transition point of bending stiffness of the bolted joint. Moreover, the transfer path of vibration in the dual-rotor system through inner-shaft bearing was discussed in the present work. The nonlinear differential equations are solved using the Newmark integration method to predict the dynamic characteristics of the dual-rotor system. The results show that the maximum vibration displacement of LP rotor is positively correlated with that of HP rotor as preload changes. Moreover, the maximum amplitude of the time-domain responses of the HP rotor will decrease and the minimum amplitude will increase with the increase of preload. The difference between the maximum and minimum values of the time-domain response will decrease with the increase of preload. This can be explained by the “stiffness hardening” phenomenon of the bolted joint. The research results can help understanding the dynamic properties of the bolted joint dual-rotor system and the vibration transfer path of the rotor system through inner-shaft bearing.

Dynamic characteristics of an aeroengine dual-rotor system with inter-shaft rub-impact

Inter-shaft rub-impact, i.e., the rub-impact between two rotors, may occur in an aeroengine dual-rotor system. This paper represents the first attempt to investigate the nonlinear dynamics of a dual-rotor system with inter-shaft rub-impact. The dynamic model of a complex dual-rotor system is built using the 3D finite solid element method by considering the inter-shaft rubbing forces. An analysis strategy combining the component mode synthesis method and numerical integration method is developed to overcome the problem of solving the large-DOF model. The modes and vibration responses of the dual-rotor system under the effect of inter-shaft rub-impact are clarified. The results show that the inter-shaft rub-impact leads to vibration coupling between LP and HP rotors, leading to the appearance of novel resonance peaks in the vibration response. Three kinds of rubbing types, i.e., continuous rub-impact, intermittent rub-impact and self-excited vibration may be excited when inter-shaft rub-impact occurs in a dual-rotor system. The characteristics and mechanisms of these rubbing motions are examined using the orbit, time and frequency curves as well as the rubbing force and relative velocity curves. By reducing the rubbing stiffness and friction coefficient, the dangerous self-excited rubbing vibrations can be eliminated. Moreover, by comparing the rubbing responses under constant and variable speeds, the key effects of the initial conditions on the inter-shaft rubbing responses are highlighted. Finally, the results of inter-shaft rubbing experiments conducted using a real aeroengine dual-rotor system are discussed and used to verify the correctness of the theoretical results.

Diagnosis and elimination of a typical serious rubbing fault of steam turbine

During the operation of a steam turbine in a power plant, the no.3 bearing of the LP rotor suddenly vibrated strongly, which caused the protection action and the turbine tripped. During the maintenance it was found that serious rubbing occurred between the impeller of the LP rotor and diaphragm gland in the LP cylinder. Through inspection and analysis, the location and cause of the rubbing were found, and corresponding treatment was carried out. During the start-up process after maintenance, the vibration of no.1 bearing of HP/IP rotor increased to trip value. In order to eliminate the rubbing of HP/IP rotor, the appropriate warm-up speed was selected and the speed control method was adopted to make the unit start to the working speed and put into normal operation.

Design of a two spool contra-rotating turbine for a turbo-fan engine

Contra-rotating turbines offer enhanced performance over their conventional co-rotating configurations. In addition, vaneless contra-rotating turbine stages offer lesser stage length along with improved performance. Contra-rotating turbines with a vaned LP stages offer a controlled work-split between the stages over a wider range of operating conditions by maintaining inlet swirl to the second rotor. The objective of the present work is to design an equivalent vaned contra-rotating turbine for an existing co-rotating configuration of a two-spool turbo fan engine. The contra-rotating turbine is designed by retaining the existing flow path and HP turbine, and redesigning the LP turbine for fixed radial distributions of inlet total temperature, pressure and swirl. A comparative study between performance of the co-rotating and contra-rotating turbines is carried out for different speeds. Cascade testing of the LP stator and rotor mean sections was carried out to validate the analysis. The LP stage of contra-rotating turbine exhibits a performance improvement by 2% points at design point, as per flow predictions. The reduced flow deflection in stator row is the primary reason for significant reduction in profile and secondary losses in contra-rotating turbine, which contributed to the performance improvement. A significant reduction of 23% in blade weight and 45% in LP stator vane count is obtained.

Flutter of long blades in a steam turbine

Cases of flutter appearing in the last stages of LP steam turbines have recently been reported. This paper presents a numerical analysis of flutter in the last stage of LP steam turbine blades using in-house, non-viscous and viscous codes as well as the ANSYS CFX viscous code. In the case of harmonic blade oscillations, instability regions were calculated using an aeroelastic damping coefficient for various interblade phase angles in five mode shapes. The distribution of this coefficient was also calculated along the blade length. It was found that flutter appears in the first mode shape. 3D viscous and non-viscous calculations showed that flutter appeared only in the first harmonic oscillation mode. In the other four modes, flutter did not appear. A negative aerodamping coefficient for the harmonic blade oscillations is not sufficient evidence of flutter. Conclusive evidence may be obtained using the two-way fluid–structure interaction procedure without assuming harmonic oscillation. In practice, the last stage LP rotor blades are mistuned and the pressure behind them is not uniform. Analysis of such conditions has to be done.

Failure of a Rotating Blade of a 200 MW Steam Turbine LP Rotor and Options for Eliminating other Similar Breakdowns

This contribution deals with the analysis of the failure of a rotating blade of the third wheel (L-1 stage) of the LP rotor of a 200 MW turbine at the power station in Počerady. Material analysis of the blade showed that the cause of the blade fracture was the initiation and growth of a fatigue crack from a corrosion pit. Failures of low-pressure blades have occurred repeatedly on machines of the same design in the power stations of the ČEZ group in recent years and by a similar mechanism. Therefore, based on the knowledge obtained by EPRI (Electric Power Research Institute), ČEZ, a.s. has developed a methodology which can, in real conditions during checks of turbines, reliably detect the parameters of corrosion pits and predict the possibility of development of fatigue damage from these pits. The work summarizes the methodology and the conditions of its use with an emphasis on the fields of its application.

Fatigue Crack Growth Threshold Determination for Welded Joint Constituents of a Steam Turbine LP Rotor

Fatigue Crack Growth Threshold Determination for Welded Joint Constituents of a Steam Turbine LP Rotor

Fatigue Crack Growth Threshold Determination for Welded Joint Constituents of a Steam Turbine LP Rotor

In the present investigation, fatigue crack growth threshold of the base metal, heat affected zone (HAZ), and weld metal of a steam turbine rotor’s welded joint constituents were determined by performing fatigue crack growth rate (FCGR) tests. Two types of test specimens were considered: (1) single-edge notch bend (SENB) and (2) compact tension (CT). Although a CT specimen is more appropriate for an FCGR test according to ASTM E647, in the present investigation the SENB and CT specimens were found to yield an equivalent fatigue crack growth threshold of the welded joint constituent. But, the SENB specimen provided some advantages over the CT specimen in performing the FCGR tests on the welded joints. It was also found that the fatigue crack growth threshold is a function of loading ratio rather than a single material parameter.

Vibration Characteristics and Damage of Turbine Generator Shaft Rubbing

An abnormal journal vibration occurred when a 660MW turbo generator unit was in daily operation. It was identified that rubbing vibration had taken place based on the behavior of the journal vibration and the temperature change of lubricating oil. The rubbing was located inside the bearing box between the HIP rotor and the LP rotor. However, no elimination measures had been taken in time because the damage caused by rubbing was underestimated. Few months later a full inspection revealed that the LP rotor shaft had been worn out with a 70mm wide and 28mm deep groove, which greatly weakened the strength of the LP rotor. The analysis presented here may help in diagnosing and understanding rubbing-vibration-related problems in similar situations.

Transient analysis of LP rotor from NPP 900MW turbine

Thermal stress and the contact stress for centrifuge force field during the start up and shut down is very most important for the safety of the turbine which will affect the design life of the turbine. The stress at startup and shut down is much larger than the stress at other conditions. The stress level and the fatigue life are important for safety and economy of the rotor. In this paper, the mechanical properties of the material varying with the temperature are considered. The vapor pressure and temperature at different position of the rotor and at different history are considered to calculate the film coefficient. The two dimensional thermal-mechanical coupled model is used to calculate the transient temperature field and stress field. The three dimensional contact model is used to calculate the stress field and contact stress under the centrifuge loading conditions.

Fault Diagnosis for Civil Aviation Engine Test Based on the Vibration Signal

In this paper, the fault diagnosis for excessive vibration value of LP Rotor in a turbofan engine test is analyzed. The vibration data collected in the test indicates that the amplitude enlarges with the increase in rotation frequency and reaches its maximum value at a certain frequency-doubling point. By various analysis and judgment, the main reason that causes the vibration of engine's high pressure rotor is the blade damage. The conclusion has certain significance for the fault diagnosis for civil aviation engine test.

A comprehensive theoretical, numerical and experimental approach for crack detection in power plant rotating machinery

In order to describe the state-of-the-art on cracked rotor related problems, the current work presents the comprehensive theoretical, numerical and experimental approach adopted by EDF for crack detection in power plant rotating machinery. The work mainly focuses on the theoretical cracked beam model developed in the past years by S. Andrieux and C. Varé and associates both numerical and experimental aspects related to the crack detection problem in either turboset or turbo pump units. The theoretical part consists of the derivation of a lumped cracked beam model from the three-dimensional formulation of the general problem of elasticity with unilateral contact conditions on the crack lips, valid for any shape and number of cracks in the beam section and extended to cracks not located in a cross-section. This leads to the assessment of the cracked beam rigidity as a function of the rotation angle, in case of pure bending load or bending plus shear load. In this way the function can be implemented in a 1D rotordynamics code. An extension of the cracked beam model taking into account the torsion behaviour is also proposed. It is based on the assumption of full adherence between crack lips, when the crack closes, and on an incremental formulation of deformation energy. An experimental validation has been carried out using different cracked samples, both in static and dynamic configurations, considering one or three elliptic cracks in the same cross-section and helix-shaped cracks. Concerning the static configuration, a good agreement between numerical and experimental results is found. It is shown to be equal to 1% maximal gap of the beam deflection. Concerning the dynamical analysis, the main well-known indicator 2× rev. bending vibration component at half critical speed is approximated at maximum by 18% near the crack position. Our experiments also allowed for the observation of the bending and torsion resonance frequency shifts determined by the extra-flexibility induced by the crack in the shaft. The validated crack model is then applied to predict the dynamical behaviour of large industrial rotating machinery and to verify the crack detection capability based on the vibratory response. With respect to 900 MW turboset units, with cracks affecting LP rotors, a map of crack detection capabilities, based on 1× rev. and 2× rev. components as a function of circumferential extension ratio and crack depth, is drawn. If the crack depth is higher than 37% of the rotor diameter, on-line measurements of 2× rev. vibratory level shift allow to detect the crack. On the opposite, 1× rev. monitoring is necessary for cracks with circumferential extension superior to 270°. It is also observed that LP rotor bending mode shift monitoring theoretically allows to detect cracks with depths equal to or greater than 20% of the rotor diameter or with circumferential extension greater than 120°. The difficulties encountered for distinguishing the LP rotor bending mode frequencies, which may also evolve in time, independently from the cracks, limit the industrial application of this latter technique. Therefore new studies will focus on the analysis of torsion dynamic behaviour and on its sensitivity to cracks. With respect to RCP units, when half of the shaft section is cracked, the 2× rev. component remains very small. Whilst the result is simply due to a small excitation, a more accurate estimation of the external forces acting on the shaft could lead to more accurate numerical predictions.

Control of LP turbine rotor blade underloading using stator blade compound lean at root

Due to a large gradient of reaction, LP rotor blades remain underloaded at the root over some range of volumetric flow rates. An interesting design to control the flow through the root passage of the overloaded stator and underloaded moving blade row is compound lean at the root of stator blades. The paper describes results of numerical investigations from a 3D NS solver FlowER conducted for several configurations of stator blade compound lean. The computations are carried out for a wide range of volumetric flow rates, accounting for the nominal operating regime as well as low and high load. It is found that compound lean induces additional blade force, streamwise curvature and redistribution of flow parameters in the stage, including pressure and mass flow rate spanwise that can improve the flow conditions in both the stator and the rotor. The obtained efficiency improvements depend greatly on the flow regime, with the highest gains in the region of low load.

Life assessment of nuclear LP rotor due to SCC

The stress corrosion cracking (SCC) of nuclear LP rotors has been a worldwide problem since the late 1960s. It has been considered the major factor for maintaining the reliability of turbines for nuclear power plants. Since the problem is not exceptional to the local nuclear power plants, some efforts have been undertaken in order to mitigate or resolve the SCC problem. The main activities in this study were focused on the survey and determination of SCC growth rates, critical crack length calculation by finite element methods (FEM), and comparison of life assessment models. Examples of deterministic and probabilistic life assessment for a local steam rotor disc with SCC cracks are presented.