Separation of Water Film From Last Stage Guide Blades of 1000 MW Steam Turbine

Abstract

The paper deals with experimental research of water and steam flow through the grooves in hollow stator blades of the steam turbine last stages with the support of CFD calculations. Also the amount of water sucked by the circumferential groove in the upper limiting wall between the last stage rotor and stator blade was experimentally measured. Measuring took place on a steam turbine with nominal output 1000 MW. With gradual increase of the turbine output it was possible to measure parameters of hollow blades suction for outputs 205, 460, 730, 870 and also 1006 MW. Before starting turbine a complex measuring system was installed consisting of cyclone separator, set of measuring tanks, orifice and pressure sensors and transducers. This measuring system was connected to one hollow stator blade near the horizontal joint. After the measurement the extraction of steam water mixture from this blade was transferred to the condenser via the diffuser chamber in the same way as other non-measured blades. Based on measured data, i.e. the pressure in the hollow stator blade and the flow rate of water captured by the hollow stator blade, it is possible to define the efficiency of suction tract from the viewpoint of total wetness in the inter-stage channel and from the viewpoint of rough liquid phase. The rough liquid phase means water films that flow near the draining grooves and sucked inside to the grooves. The main part of the submitted paper is an analysis of the measured data. Among the analysis results are, besides the flows of rough water phase along the blade surface, the above mentioned efficiency of total wetness suction and of water film suction. For the needs of the analysis there are certain input data, e.g. the value of static pressure and wetness on the blade surface close to the slots that must be defined theoretically using flow path calculations or using CFD methods. In this case, in order to obtain input data, CFD simulations were used when the whole last stage was calculated with the diffuser and exhaust hood. Boundary conditions for CFD were taken from experimental measurements that took place simultaneously with measurement of separated water phase. Numerical simulations were not running for all outputs, but only for three of them — 460, 730 and 1006 MW. For this reason there are no sufficient data for CFD calculations for all outputs and input data of other cases had to be extracted and, based on experience, extrapolated. On the circumferential groove only a part of 30 mm of length was measured, again near the horizontal joint. Due to a short measured groove length it was not possible to obtain the water flow data which would describe suction properties for the whole circumference. The results of experimental measurements provide very important information about the whole suction tract behaviour and its ability to remove liquid water films from the stator blades surface. As very good qualities of the suction tract were confirmed by the measurement, it could be stated that hollow stator blades combined with the circumferential groove on the upper limiting wall is still a suitable technical solution for lowering erosion loading of the last stages. When using a properly dimensioned and correctly working suction system, erosion loading of LSB leading edges on the tip can be lowered by almost a half.