Page 140 - Steam Turbines Design, Applications, and Rerating

Turbine Blade Design Overview 121

Flow velocities are high in slender last-stage blades because circum-
ferential velocities are high. The steam in the blade passages is accel-
erated from subsonic to supersonic flow and, thus, transonic flow is
obtained.
Because of the large centrifugal forces acting in long last-stage blades,
their profile thickness is reduced from the hub toward the blade tip.
Because the blades are designed for variable speed operation, they
must be capable of running safely at resonance points. The resonance
frequencies should, therefore, be as high as possible, and the blade row
should have sufficient damping.
It is not possible on last-stage freestanding blades to provide integral
shrouding because of the blade taper. The last stage blades are there-
fore interconnected with loosely fitted lacing bars. The blind bores
required for these lacing bars are machined into integrally forged
bosses to minimize weakening and prevent excessive stress concentra-
tion around the bore (Fig. 6.13).
When the blades are rotated, centrifugal force presses the lacing bars
against the upper outer wall of the bores. All blades are thus essentially
coupled to a single vibrational system. This coupling raises the reso-
nance frequencies and makes the resonance criterion more unlikely: in
a coupled system resonance only occurs when the excitation frequency

Figure 6.13 Integrally forged bosses at low-pressure
blading of reaction turbine. (Siemens Power Corpo-
ration, Milwaukee, Wis. and Erlangen, Germany)