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. 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 sizes and type of blades are changing accordingly. 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 blades has to pass many critical speeds at different operating and dynamic conditions hence these blades are designed such that they can be safely operated beyond or passing through many critical speeds. The blade natural frequency analysis is of great importance for the reliability of steam turbine. Review has revealed that the analytically dynamics of blade can be defined 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 is very difficult through analytical 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 conditions. During the complete 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 in Magnetic Particle inspection (MPI) in 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 become more relevant for blades with larger size of cracks. To imitate the actual rotating condition of blade during their operation, 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 in their root, a parametric study has been done. In this study, 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 idea that the small size cracks does not contribute much in the reduction of frequency. However, larger sized cracks produce iv significant effect. This study of crack with Campbell diagram also shows that there is no substantial shift in critical speed of rotor is observed for small and medium sized cracked. It is only at the very large cracks at which the critical speed dropped significantly and comes below operating range. Investigation of natural frequency and mode shapes for different blade root fixity conditions has been also carried out. Root fixity affects the natural frequency and mode shapes in larger way. During these study thirteen different cases of blade root and groove interaction has been studied. During this study it has 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 laboratory at BHEL, Haridwar. Adopting the basics of modal analysis mechanisms, the study of dynamic behaviour response of normal and cracked blade has been carried out using FFT analyzer. The comparison of vibration analysis results of FEM model and Experimental results has been carried out. It has been observed that the FEM model is very accurate for the first two modes even its accuracy is within 0.1% for the first mode. However, in case of large cracked blade it is in the range of 5%.With this comparative analysis effort has been made to validate the FE model. In situ, vibration analysis of blade has been also studied Using BVMS system. Data obtained from this study is 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 in natural frequency of blade, natural frequency further 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 torsion mode. It has been observed that 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 effect 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.