Failure analysis of gas turbine generator cooling fan blades

Since the optimum operation of a generator is highly affected by increasing in temperature, a cooling system is used to control the temperature. Employing a fan as a cooling system for the generator at the end sides of its rotor is a practical method [Montazer Ghaem Gas Turbine Power Plant. Gas turbine generator manual, Iran, 2004]. In some cases, fracture of blades causes short circuit between rotor and stator and consequently generator explosion and huge financial loss.Since fracture in cooling fan blades has been occurred five times in our case study, in this research, the emphasis has been placed on failure analysis and preventing methods from the fracture in this generator’s fan blades.Survey and analysis of the above-mentioned problem have been conducted in different ways, which the main titles results are as follows:
• All of these failures have been happened after the first operation of gas turbine or at the early after operation 100 h after gas turbine repairs [Alsthom Company. Failure report for gas turbine fan blades, 1997].
• Metallurgical and structural analyses on the failed blades have not shown any microstructure degradation.
• Studies on the ruptured surfaces using scanning electron microscope (SEM) have shown that fracture has been happened as a results of high cycle fatigue (hcf).
• Calculation of alternative stresses has indicated that this stress is less than fatigue endurance limit of blade material and none of these stresses and forces could result in fracture.
• Modal analysis of blade has indicated that the blades natural frequency has been limited too close to the blades operational frequency. It means that making changes in blades installation conditions; blade natural frequency approaches the operational frequency and consequently causes the resonance.Analysis results show that the final fracture of blades is due to fatigue conditions in resonance state and growth of existent tiny probable cracks.