Failure analysis of an MVR (mechanical vapor recompressor) Impeller

This article describes the root cause failure analysis of a mechanical vapor recompressor (MVR) impeller used in a dairy processing plant. An impeller blade was thrown during commissioning after approximately 150 h of service. Visual examination of the blade’s fracture surface indicated that the crack growth mechanism was fatigue and this was confirmed by scanning electron microscope (SEM) examinations. 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).