Page 125 - Steam Turbines Design, Applications, and Rerating

106 Chapter Five

Figure 5.21 Finite-element mesh for determining isothermal fields and oper-
ating stresses of a welded rotor. (Asea Brown-Boveri, Baden, Switzerland)

Designing a rotor employing the latest technologies requires an
appropriate computer study. Finite-element analysis makes it possible
to calculate accurately the operating stresses in all parts of the rotor.
Figure 5.21 shows the mesh used for the intermediate-pressure rotor of
a 500-MW utility steam turbine to determine its isothermal field and
operating stresses. For the same rotor the combined stresses due to the
rotor speed and temperature (comparative stress) are shown under
full-load conditions in Fig. 5.22.All points on any line in the figure have
the same comparative stress level.
Extensive information is available on the stresses in steam turbine
rotors during start-up.
Experienced manufacturers can make the claim that their welded
steam turbine rotors are designed, manufactured, and inspected in
such a way that a maximum of safety during operation can be guaran-
teed. The welded rotor has the following positive characteristics:

■ Exceptionally large flexural rigidity, which favors smooth running
operation
■ Low stress levels since the solid disks have no central bore

Figure 5.22 Stresses in the intermediate pressure rotor of a 500-MW utility
steam turbine during steady-state operation. (Asea Brown-Boveri, Baden,
Switzerland)