12.2 Influence on machine design
The need for flexibility has brought with it inherent problems which have needed to be redressed at the design stage of the turbine, with the knowledge of future operational requirements. The main problem is the effect of thermal cycling with respect to thermal fatigue damage.
The flexible mode of operation involves some degree of temperature cycling of components, leading to strain cycling of material in the critical regions. The damaging cumulative effects of thermal cycling (i.e., stress-strain cycling) must be assessed over the turbine life to ensure that sufficient margin exists in the design for the effects of steady state creep.
Figure 2.93 shows a typical stress-strain cycle as experienced by the surface of a large HP rotor. The cycle shows the effects of two-shifting using a typical 'warm' start where the temperature gradients cause thermal strain. On heating with steam at 565°C, the surface attempts to expand but is compressed by restraint from the underlying material. This compression, represented by line A-B in the figure, may cause the surface to go into compressive yield, represented by line B-C, in areas of high stress concentration. Subsequent heating and expansion of the underlying material then causes tensile stressing of the surface
as full power is attained (line C-D). Stress relaxation (line D-E) can then occur at the maximum operating temperature, during which elastic strain is converted to creep strain and may cause microstructural damage. Shutdown causes a drop in temperature at the surface, the thermal contraction being constrained by underlying material, resulting in further tensile stressing. However, this straining takes place outside the creep range of CrMoV steels used in rotor construction. Details of rotor materials and their properties with reference to creep resistance are given in Chapter 1. The stress-strain cycle of Fig 2.93 is similar to that experienced by the inner surface of a high temperature casing under severe starting transients.
The magnitude of thermal stresses produced during two-shifting or cycling, depends on the amount of temperature change imposed in a given time in relation to the sizes of components involved; for example, casing thickness or rotor diameter. Stress concentrations in casings and rotors must be reduced to a practical minimum by the avoidance of rapid changes of section and by proper fillet radii. The need for flexible operation has influenced the design of turbines in several areas.