The operation of the last stage of steam turbine at low-flow rate modes

Abstract

At the present time, thermal power plants and combined heat and power plants operate in highly manoeuvrable modes with almost daily deep unloading, shutdowns on weekends and holidays followed by launches from various thermal conditions. During start-ups, the operation at partial modes and shutdowns, the turbine flow path operates at low-flow rate modes and, as a result, varying tear-off phenomena appear — depending on a relative volumetric flow rate of steam Gv₂ in the low-pressure cylinder starting from the last stage. This is especially true for the operation of the low-pressure path for cogeneration turbines. Under low-flow rate modes a change in the flow structure occurs accompanied by the appearance of the bushing tear-off, the rotating vortex in the gap between stator blades and rotor blades, and an increase in pressure in the main flow when switching to the operation at the compressor mode. The formation of the tear-off area is accompanied by a significant increase in the temperature of steam during its overheating due to ventilation losses created by vortex structures in the areas of tear-off. The temperature change along the working blade length is considered, the characteristic points of this change depending on the relative volumetric flow rate of steam are highlighted. The boundaries of transition from the wet steam to the su— perheated steam with decreasing Gv₂ are determined. The power consumption for the operation of the stage with a decrease in the flow rate of steam and a change in the flow structure is considered.