DYNAMIC ANALYSIS OF LARGE STEAM TURBINE BLADES

Abstract

In thermal power station, steam turbine is the prime mover, which generates enough torque to produce power from generator and the blades are the heart of turbine. These are the only components responsible for the conversion of thermal energy of steam to mechanical energy. Based on the different pressure conditions across the steam turbine flow path, size and type of blades are changing accordingly. Low Pressure (LP) turbine blades are designed to extract the residue energy from the steam. These are the largest size blade in any steam turbine power plant. These blades are exposed to severe dynamic conditions attributed to the variable flow regime and centrifugal forces. A steam turbine blade has to pass many critical speeds at different operating and dynamic conditions, hence the blades are designed such that they can be safely operated beyond or passing through many critical speeds. The modal analysis of blade is of great importance for the reliability of steam turbine. Review has revealed that the dynamics of blade can be defined analytically with the analogous approach to cantilever beam with certain assumptions. Taper and twist of blade can be modelled with certain assumptions. Building the section properties and finding the influence coefficients are very difficult through numerical process, which can give an approximate solution of the problem. Criticality of taper twisted free standing blade with fir tree root blade may also very difficult to model accurately. To understand the dynamic behaviour of critical structures like steam turbine blades, effectively and efficiently, it has been decided to follow the FEM model approach duly validated with the experimental studies. In the present study, dynamic behaviour of steam turbine blade has been investigated using finite element method (FEM) for healthy and defective blade conditions. During the numerical study of blade dynamics using FEM, primary six fundamental modes are studied. Cracked blades have been modelled on the basis of sizes of cracks observed using Magnetic Particle Inspection (MPI) in BHEL – India laboratory. Comparative study of cracked blade and normal blades shows that the there is a definite drop in fundamental frequency in cracked blade and this becomes more relevant for the blades with larger size of cracks. To imitate the actual rotating condition of blade during their operations, the effect of rotor velocity has been studied with the consideration of stress stiffening and spin softening (Coriolis forces). To study the change in dynamic behaviour of blade with different sizes of cracks at their root, a parametric study has been done. In this study, a crack of definite area has been created in top flank of fir tree root blade. Modal analysis of variable size of cracked blade gives the clear indication that the small size cracks does not contribute much in the reduction of frequency. However, larger iv sized cracks produce significant effect. This study of crack in fir tree root blade using Campbell diagram shows that there is no substantial shift in critical speed of rotor for small and medium size cracks. It appears in case of large cracks at which the critical speed dropped significantly and runs below operating range. Investigation of natural frequencies and mode shapes for different blade root fixity conditions has also been carried out. Root fixity affects the natural frequency and mode shapes in larger way. During the study, thirteen different cases of blade root and groove interaction have been considered and analysed. It has also been observed that the Critical speed of blade reached in the range of operating condition for the cases, when only lower flank of fir tree root blade remain in contact. To validate the FEM model, an experimental study of the dynamic behaviour of steam turbine blade has been done in NFT (Natural Frequency Test) laboratory at BHEL, Haridwar. Adopting the basics of modal analysis mechanisms, the study of dynamic response of normal and cracked blade has been carried out using FFT analyzer. The FEM results are compared with experimental study. It has been observed that the results obtained by FEM model are quite accurate for the first two modes, even its accuracy is within 0.1% for the first mode. However, in case of large crack in blade, the validation is in the range of 5%. In situ, vibration analysis of blade has been also studied using Blade Vibration Monitoring Systems (BVMS) system. Data obtained from this study are very useful for asynchronous and synchronous vibration study of blades. During this study, it has been observed that cracks in the blades lead to decrease the natural frequency of blade and further it decreases as the crack size increases. A small crack size does not impact much on natural frequency, however, it is significant in larger crack. The effect crack is more visible in torsional mode. It has been observed that the variation in frequency from suction side contact to pressure side contact surfaces depends on the directional flexibility imparted against the corresponding mode shape. It has been noticed during the study that during experimental modal analysis of any structure, special attention and meticulous effort is required to achieve the designed stiffening conditions. In situ, vibration monitoring system provides very important and critical information for both type of synchronous and asynchronous vibration behaviour of blade. Especially, for the asynchronous vibration various parameters and their effects of dynamic response of blade can be achieved those are very difficult to obtain analytically. Now, this study gives a way to comparatively study the dynamic behaviour of structure using FEM and an experimental modal analysis to characterize its dynamic properties.