Frequent failures of long turbine blades forced an electrical utility to sponsor research work to find out the causes of the failures. One of the techniques applied in this work was finite element analysis.
The paper presents an application of the finite element method for computation of the natural frequencies, steady-state and alternating stresses, deformations due to forces acting on the blades and modal shapes of the turbine long blade groups. Two stages, L-1 and L-0 of the low pressure part of a steam turbine, were analyzed.
It has been postulated that the results of the FEM analysis of the blades groups would be complementary to those obtained from the radio telemetry test (which was carried out during operation of the turbine) for the purpose of blade group failure diagnosis. However, the results of the analysis show that the FEM results were decisive in blade failure identification (L-1 stage moving blades).
The graphical post processor of the FEM code revealed that the first blade in the group was the one most protruding from the stage rotating plane, thus indicating that this blade was the most prone to erosion. This was confirmed in the inspection of the turbine. This finding showed why only the first blade in the group was cracked (erosion induced cracks). The mode shapes were also very helpful in identifying other types of cracks which affected other parts of the blades.
It can be concluded that the finite element method is very useful for identification of very difficult cases of blade faults and indispensable for carrying out modifications to prevent future failures.