After their start-up in 2008, the two mine exhaust fans at Vale’s Coleman Mine experienced a history of unexplained vibration problems. The fans in question were 2,600 kW (3,500 hp) centrifugal double inlet design and controlled by variable frequency drives (VFDs). The vibration problems culminated in the catastrophic failure of the disc pack spacer coupling on one of the fans. Initial investigations into the root cause of the vibration problems were unsuccessful. A theoretical analysis of the drive train at the design stage had identified torsional natural frequencies in the system; however, these were not deemed problematic since it was believed that they would not be excited during operation by the torque oscillations that are normally associated with VFDs.
Using wireless strain gauge technology in situ, the excitation of a torsional natural frequency above fan running speed was identified as the root cause of the failure. Strain gauge testing demonstrated that very low amplitude inter-harmonic torque oscillations generated by the VFDs were exciting the lowest torsional natural frequency of the drive train during fan speed changes. As a result of the analysis, the fan vendor proposed larger couplings, which have been installed, and modifications to the fan shaft keyway. These changes show promise for resolving the coupling failure problem.
This technical paper discusses the steps that were taken during the investigation into the problem. This includes the site testing, the analysis of the data and the analysis of the remedial options that became identified. The paper concludes with recommendations concerning the approach that should be taken on similar projects to ensure that this type of problem is avoided.