The operation of mineral processing equipments is affected by natural disturbances coming from changes of the ore feed characteristics and operating practices. A method to quantify the impact of natural disturbances and equipment operating practices on plant performances is described in this paper. The method that is based on process simulation is applied to an industrial hydraulic classifier. For the studied process it was found that most of the variation observed in the production results is due to the operation of the classifier rather than changes in the feed mineral composition and particle size distribution. (C) 2011 Elsevier B.V. All rights reserved.