Failed Compressor Impeller

MSI was contracted by a customer to investigate a failed compressor impeller. An FEA model of the impeller was created to perform a natural frequency modal analysis. The model closely correlated with experimental modal analysis test results.

The model calculated the first 200 natural frequencies of the impeller. Most of these occurred in groups distinguished by the "lobes" in their mode shape, known as the number of "nodal diameters."

The analysis confirmed that the failure was vibration-induced fatigue caused by the synchronization between vane/blade "sum and difference" pressure-pulsation at blade-pass frequency and the natural frequency of the disc three nodal diameter mode shown in the lower right mode shape picture. Based on the worst case loading situation occurring in service, the design was marginal, and the factor-of-safety was very close to 1.0. Several possible modifications were tried using the FEA model, and a final recommendation which was suggested by MSI was able to increase the factor- of-safety by a minimum of 50%, as shown in the Goodman Diagram.

The whole project was compressed into a very small time window due to a pressing overhaul schedule requirement. MSI was able to meet the requirement by constructing and running the complicated model very quickly and recommending a fix that minimized component changes.