Failure analysis and retrofit design of low pressure 1st stage blades for a steam turbine

Cracking of blade fingers occurred in a few numbers of low pressure 1st stage blades for a certain type of steam turbines. In order to find out the failure mechanism, one of the cracked blades has been inspected. The inspection results showed that the cracking of the blade finger was caused by high cycle fatigue and surface defect coming from rough machining induced the initiation of crack. Vibratory modes of the blade group have been calculated and measured using a 3D finite element S/W and impact test, respectively. The results showed that resonance of the second type group axial vibration mode with nozzle passing frequency was the source of high cycle fatigue load. To avoid the dangerous resonance, the blade groups have been modified into 10 blades per group without any change of vane and dovetail. The new blade groups have been operated safely more than 2 years since the modification.