Page 252 - Steam Turbines Design, Applications, and Rerating

Reaction vs. Impulse Type Steam Turbines 231

Figure 12.10 Blades of the final stage of a medium-
sized condensing turbine after completing more than
100,000 h of operation. (Asea Brown-Boveri, Baden,
Switzerland)

The causes of erosion are shown in Fig. 12.11. The moving blades col-
lide with the small droplets, and the resulting impact removes blade
material. The water droplets are accelerated to only about 10 to 20 per-
cent of the steam velocity and thus strike the blades at an unfavorable
angle. The most serious erosion occurs in the region b e . The means of
preventing erosion damage include provision of casing internal chan-
nels or troughs (Fig. 12.12) and special diffusion coatings (titanium
nitride) for turbine rotor blades.
The opinion is often expressed that reaction turbines are more sus-
ceptible to erosion. Recent technological advances have made the final
stages of both impulse and reaction condensing turbines almost equiv-
alent in this respect. In stages where there is likelihood of erosion, the
degree of reaction is increased over the length of the blade. Conse-
quently, modern final stages operate more or less as impulse stages on
the inside, with slight or moderate reaction, but very definitely as reac-
tion stages toward the tip.
There is, therefore, no reason to expect a meaningful difference in
the erosion behavior of the two turbine types, impulse vs. reaction. To
this extent, earlier ideas may have to be corrected.