Thermodynamic Aspects of Designing the New Siemens High Pressure Steam Turbine With Overload Valve for Supercritical Applications
Authors: Deidewig Frank , Wechsung Michael 
Published: October 2008

Huge coal fueled power plants in the 1000MWel class are requiring high efficient steam turbines which can handle supercritical steam conditions up to 300bar and 600°C. Besides these boundary conditions, the capability for stabilising the grid fluctuations is also one key requirement. Siemens is focussing on this topic by using the so-called overload valve(s), which enhance the maximum amount of main steam mass flow entering the high-pressure turbine by use of additional valve(s). Using this technique, a power increase in the range of up to 20% is theoretically achievable. Siemens PG has collected a lot of positive service experiences throughout the past decades with this technique, and therefore this principle is being well established in the field. The connection between the additional steam mass flow passing through the overload valve and the standard blading path is somewhat downstream from the first stage. These connecting points can be varied (for this current turbine design) — if necessary — between the third and fifth stage after the turbine inlet. From an economic point of view, the approach of extending the power range via overload valves is even better than throttling the whole machine during standard operating condition and opening the valves fully at certain peak load requirements. Historically based, Siemens designs and manufactures reaction stages, ‘reaction turbines’, which must be thrust compensated via a separate piston to equalize and reduce the overall axial thrust down to a small number. Increasing the main steam temperatures up to the previously mentioned levels makes the internal cooling device of this thrust equilibrium piston a major key point for the whole turbine. No external cooling pipe-work or special materials are required. In Figure 1, a longitudinal cross-section 3D-view of the newly designed high-pressure turbine is drawn. The outer casing — at the steam inlet regime — is cast steel of 10% chromium content with significantly reduced wall thickness, whereas the outer casing at the hp-exhaust is a 1% chromium steel. The thrust-balancing piston on the shaft can be identified on the right hand side near the steam inlet channel. As noted further on, the steam outlet channels are both connected to the lower part of the turbine, whereas the inlet chambers are located at 3 o’clock and 9 o’clock, respectively. The outer casing has no horizontal splitting line; the turbine is being built as a barrel-design. This paper deals with the described turbine regarding the major design criteria from the thermodynamic point of view. Based on several calculations, the following design topics were covered: • Developing a turbine-internal cooling system for the thrust equilibrium/balancing piston as well as for the inner and outer casing. • Evaluation of staged piston with new internal cooling system adjusted for the impact on heat rate. • Quantification of all related mass flows, temperatures and pressures. • Axial thrust calculation to determine the required diameters of the staged piston. • General remarks concerning efficiency behaviour of hp-turbines with different geometrical designs.