Axial Thrust Balancing in High-Temperature Cylinders of Steam Turbines during Transients in Combined-Cycle Units

Abstract

The effect of magnitude and direction of the axial thrust during steam turbine transients on the starting technology is investigated. The behavior of the axial thrust during start-up of a steam turbine with a combined high/intermediate pressure cylinder operating in a single-bypass or two-bypass thermal cycle is presented. At different stages of start-up, varying steam pressure at the turbine inlet and in its flow path can result in a change in the temperature of thrust bearing pads and the magnitude and direction of the axial thrust acting on them. During start-ups with a two-bypass thermal scheme for steam admission to the turbine and its rotor acceleration, connection of the steam turbine generator to a grid and its initial loading are performed with steam supply through intermediate pressure-control valves. In this case, the high-pressure cylinder (HPC) is under “negative pressure” or “countercurrent” conditions passing steam from the exhaust via the bypasses of the check valves in the “cold” reheat steam lines and removing this steam to the condenser via the drains of high-pressure crossover pipelines and from the high-pressure cylinder casing. Connection of the turbine HPC for normal steam supply and change-over to the once-through scheme of steam admission to the turbine are carried out after connection of the turbine generator to the grid and initial loading of the turbine by partial opening of the HP control valve and full opening of the intermediate pressure (IP) control valves. At the same time, the valves of quick-acting pressure-reducing and cooling units (QAPRCU) BROU-1 and BROU-2 are closed to maintain the specified high and intermediate pressure of steam upstream from them. These process operations change the axial thrust acting on the thrust bearing pads with a corresponding change in the pad temperature. The effect of variable axial thrust on the steam turbine maneuverability is examined. Methods are proposed for balancing the axial thrust during start-ups of steam turbines in combined-cycle units having different thermal schemes.