In the past, increased attention was given to the development of an optimal shape for the inlet part of LP turbine casings in SKODA POWER. A double-flow design is typically used for high power output turbines. An optimized shape for the internal diffuser has been found, which transforms the kinetic energy of steam into increased pressure, thus effectively increasing the thermodynamic efficiency of the stage. Some conclusions have been drawn from laboratory experiments, others derived directly from on-site measurements at power plants. The conclusions from the development of double-flow turbines form the basis for the design of the single-flow turbine arrangement. Single-flow design is typically used for lower output turbines. There are still some limitations in applying this arrangement. The designer needs to resolve the bearing position and how to ensure access to them. Reinforcing the ribs and supports are used, therefore, to ensure the rigidity of the entire casing. The optimization of the single-flow diffuser shape is therefore the subject of the study presented below.

This paper presents the experimental research for the flow of the last stage of a turbine for saturated steam with the nominal output 1090 MW. In addition, the flows in 600, 800, and 1070 MW output turbines were also measured. Pneumatic probes were used to determine the distribution of static pressures and absolute angles at the outlets from the penultimate and the last stages of the turbine. Optical probes were used to measure wetness distribution and were placed in positions similar to the pneumatic probes. The courses of static pressures, angles, and wetness for all outputs respectively were compared and discussed. The difference between wetness courses on the left and right side of the turbine as well as before and behind last stage was minimal. Absolute angles of steam behind the last stage are strongly influenced by the vacuum level in the condenser. Big difference between the outlet angles from last stage on the left and right side of the turbine is confirmed. The influence of the tie-boss was evident in both pneumatic and wetness measurements. Differences of the flow field on the left and right sides of the turbine behind the penultimate stage are noted and discussed. These differences lead to a dynamic loading of the penultimate rotor blades and could reduce the service life.

The paper deals with experimental research of the flow and dynamics of the blades in the last stage of a steam turbine with nominal output of 34 MW and a connected axial exhaust hood. The experiments were carried out on a turbine with relatively low inlet steam parameters “- 64 bars and 445 °C. It was possible to change the operating modes of the turbine during the course of measurement so that significant ventilation would be achieved in the last stage up to the point when aerodynamic throttling occurred in the last stage. In other words the turbine output varied from about 2 to 35 MW. The output of 2 MW was for the case of the island mode turbine operation. The experiments were carried out using static pressure taps and measurements of temperatures at the root and tip limiting wall. In addition to static pressure taps and temperature measurement, it was also possible to carry out probing by pneumatic probe with a diameter of 30 mm. Blade vibration monitoring sensors, so called last stage blade tip-timing, were also installed. The blade tip-timing acquisition hardware was used to monitor rotor blades tip amplitude. Due to the obtained experimental data, it was possible to verify the behaviour of the last stage and the connected exhaust hood for four measured variants. The courses of pressures and steam angles along the length of the LSB were determined. Furthermore, basic parameters of the last stage were determined, i.e. reactions of the stage, Mach and Reynolds numbers and values of pressure recovery coefficients. Based on experimental data the boundary conditions for CFD calculations were determined. Comparison of CFD calculations done for ventilation modes and for a nominal mode was also included. Another phenomenon which occurred during the probing of the flow parameters, particularly in ventilation modes, was the inability to determine parameters of steam due to low values of measured dynamic pressure in the vortex area at the root of the blade. The probe was able to detect dynamic pressure at the level of 50 Pa and more. In other words the transition point between backward and forward flows was identified. This limit point was used for further analysis of ventilation character of the steam flow depending on the ventilation coefficient c2x/u. where c2x is the average axial velocity at the LSB outlet, calculated from volumetric mass flow and u is LSB circumferential velocity calculated at LSB middle diameter. Due to the fact that it was also possible to measure vibration amplitudes of blades using the tip-timing method for a variety of modes, the relationships between pressure ratio over the tip and root of the last moving blade and vibration amplitude were also determined. This verified that the highest amplitude of blade tips occurred just when the compression of the medium on the blade tip was maximum, i.e. c2x/u = 0.05.