Effect of Stage Reaction and Shaft Labyrinth Seal in a Stage of an Axial Steam Turbine

Abstract

Abstract This paper focuses on the influence of shaft labyrinth seal flow on full stage performance. Experimental data are studied, expected design conditions and experimental results are compared and discussed and a losses breakdown for the design procedure is presented. The experimental investigation was performed in VZLU’s air test turbine which is a part of a closed-loop system equipped with a radial compressor. The test turbine configuration simulated the real drum-stage geometry of an axial steam turbine. The geometry of the turbine represents a typical mid-pressure stage of a steam turbine. The configuration of the test rig was adapted in order to easily change the shaft labyrinth seal geometry. The study covered a wide range of seal clearances from very small to extremely large clearances, reaching a maximum relative mass flow approximately 10% of the stator blade flow. Different types of seal feed were also tested to compare internal feed (the flow obtained from the stator flow by the hub-gap just in front of the stator) and external feed realized by additional piping with external regulation. Three stage reactions were tested in this work — Low Reaction, Mid Reaction and Full Reaction. The stator of the stages was the same in all cases, thus the reaction was changed by implementing three different rotor geometries. The influence of the labyrinth seal clearance was investigated by overall performance measurement and by detailed investigation of the flow field. The turbine stage was loaded by a hydraulic dynamometer used for regulating the rotational speed and a flange torquemeter was used to determine the stage efficiency. The total mass flow was measured using an orifice plate. Each seal geometry configuration was calibrated to compute the seal mass flow. The turbine stage and seal were equipped with a number of static pressure taps, and miniature pressure probes were used for measuring the flow field parameters in detail. The discussion of the results is divided into two areas. Firstly, the influence of the degree of reaction on axial steam turbine stage performance in the configuration without the seal flow is presented. Then, a combination of various degrees of reaction is studied as a function of mass flow through the shaft labyrinth seal. The measured data are evaluated by a breakdown of loss sources. The decomposition of the total loss into row losses, leakage losses and mixing losses is highly advantageous. This total loss analysis is carried out for all three stages and both off-design performance and ratios of the shaft seal flow to nozzle blade flow are measured. The post-processing of measured data through this loss breakdown and the comparison with the design is used to validate the design process.