Fracture of Inducted Draft Fan Shaft at Commercial Power Station

A large induced draft fan comprising a commercial power station experienced increasing levels of vibration during the initial phases of hot commissioning and transition to full operation. A number of upset conditions associated with the downstream flue gas desulfurization (FGD) process were initially thought to have been the cause of the vibrations. After multiple attempts to identify the apparent source of vibration, the fan was shutdown and the rotor pulled for examination. The examination revealed the presence of a crack at a keyway in the rotor shaft. A metallurgical evaluation of the crack and shaft identified the crack to have been caused by fatigue due to excessive clearance between the keyway and key, as well as the presence of a geometric stress concentration associated with the machining of the keyway during initial manufacture of the fan shaft. The investigation also evaluated various means available to repair the fan shaft as opposed to replacement.