In the paper, an unique event of a steam turbine rotor grabbing and locking will be described and evaluated. The turbine power is 21,740 kW and it is used in a pulp and paper production plant. The turbine is a high speed machine, 8,300 rpm, reaction multi-stage type, coupled by a reduction gear to a 4-pole electric generator, with 1,800 rpm. The steam turbine rotor grabbing and locking event evaluation is based on a proposed rational approach, i. e., based on the connection of the failure mechanisms observed and the identification of possible causes of these mechanisms. It is important to notice that the failure mechanisms and their causes are not the same. The failure mechanisms are the damages observed such as wear, fracture, severe deformation, etc. that impair or limit the safe and economical operation of a mechanical component. The causes, on the other hand, are the operational characteristics, the aging factors, the loads, etc. that introduce conditions to the failure mechanisms development. Using the proposed rational approach, the failure mechanisms causes that induce this unique event of steam turbine grabbing and locking are identified. Also, corrective and improvement actions based on the proposed rational approach are addressed to evaluate similar events and to avoid such a type of event in similar components.

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