Page 251 - Steam Turbines Design, Applications, and Rerating

230 Chapter Twelve

Figure 12.9 Natural and excita-
tion frequencies of reaction blad-
ing: F = frequencies; N = number
of stage; Indexes: 0 = fundamen-
tal; 1 = first harmonic; e = excita-
tion due to preceding row. (Asea
Brown-Boveri, Baden, Switzer-
land)

rotor becomes distorted, the tips of unshrouded blades are sharpened
to a thin edge. Shrouded blades are sealed by means of thin labyrinth
strips so that rubbing is essentially harmless. Strip-to-tip clearances
are sufficient to avoid rubbing contact under moderate conditions of
rotor bow and vibration.

12.5 Erosion
With condensing turbines attention must be paid to the behavior of the
blades in the region of increasing wetness. Under unfavorable condi-
tions of water content, peripheral velocity and geometry, the water
droplets that form can erode the moving blades, i.e., metal is worn
away from the leading edges (Fig. 12.10). Water deposited on the stator
blades is dragged by the steam flow to the trailing edge where it col-
lects to form large drops. When torn away from the trailing edge, these
drops, which have a diameter of about 1 mm, are broken up into
smaller droplets.