5.1.3 Velocity-compounded stage
The first HP stage of a turbine, where it is desired to give a large pressure drop to the first row of nozzles, sometimes incorporates two impulse stages on the same wheel. This is done to protect the casing and rotor from higher pressure and temperature conditions.
As a velocity-compounded stage uses approximately the same heat drop as four impulse stages, it is used to provide a shorter and cheaper turbine, although with some sacrifice in efficiency. It is no longer used in large base-load turbines.
Figure 1.82 shows the arrangement of such a stage. The nozzles, of the convergent-divergent type, produce very high steam kinetic energy, some of which is absorbed by the first row of blades, the remainder being deflected back by the stationary guide blades and used in the second row. Both moving blades and nozzles are machined from the solid and are necessarily very robust.