Velocity Compounding or Curtis Stages

In this compounding, whole pressure drop takes place in nozzle (only one row) and remains constant in fixed and moving blades. Velocity of steam remains constant in fixed blades and decreases in moving blades.

Figure 5.12a shows a two rows Curtis or velocity staging having two rows of moving blades and one row of fixed blade in between them. In three rows of Curtis stage, two row stages is followed by a second row of guide blade and third row of moving blade. Steam of high kinetic energy existing from nozzle impinges on first row of moving blades and gets deflected by the first row of fixed blades of guide blades. The steam after deflection from first row of fixed blades impinges on second row of moving blades and again gets deflected by second row of guide blades. Finally, steam impinges on third row of moving blades and thus, does work on turbine shaft.

Figure 5.12 Velocity Compounding of Impulse Turbine

The velocity diagrams for first stage and second stage of moving blades are shown in Figure 5.12c, d. The blade friction factor K_b may be assumed the same for both moving and fixed blades.

• In two-row curtis stage, three-fourths of the total work is done by steam jets on the first row of moving blades and one fourth of the total work is done on the second row of moving blades.
• In three-rows curtis stage, it can be shown that