This article, published in  » Engeeniring Failure Analysis » of september 2010, describes the root cause failure analysis of a mechanical vapor recompressor (MVR) impeller used in a dairy processing plant. A detailed investigation was commissioned to determine the source of the oscillatory stresses that must have been present to cause fatigue. A vibration analysis indicated large torsional oscillatory stresses, leading to fatigue, were induced by excitation of the 1st and 2nd torsional vibration modes of the MVR impeller and motor system. The excitation was induced by the control system for the motor variable speed drive (VSD).