Modifications of Balancing Slots in Drum Rotor Arrangement of Steam Turbine Stages and Their Influence on Efficiency

Abstract An analysis of the industrial steam turbine market shows that many customers require steam turbines with relatively small output and high inlet steam pressures and temperatures, causing small nominal mass flows of steam through the turbine flow parts. As a result, the blades of the first stages tend to be relatively short, which is connected with the high energy losses. This problem can be partially alleviated when the drum rotor arrangement is used. In order to increase efficiency further, the balancing slots, which are created on the rotor blades, can be used. These slots can be of different shapes, dimensions, and positions. Doosan Skoda Power has investigated this issue using measurements on an experimental turbine as well as numerical simulations. Some results of this study have already been presented. However, many details regarding the influence on efficiency have not been presented yet. This paper presents several possible balancing slots shape modifications. These modifications influence both the mass flow through the balancing slots and the torque of these slots. It can lead to an efficiency drop or an increase. A numerical model based on the experimental turbine was created to simulate several balancing slot designs. It represents the turbine flow-through part with two drum stages, including adjacent seals and investigated balancing slots. The numerical simulations were carried out using an Ansys software tools package. The computed flow fields and important integral results are shown. The influence of each design change is discussed. The resulting rules for designing the balancing slots are presented.