Page 249 - Steam Turbines Design, Applications, and Rerating

228 Chapter Twelve

It is an established fact that a sound design must avoid resonance
due to the preceding blade row causing excitation at the fundamental
bending frequency (nozzle excitation). With that in mind, the relative
vibration behavior of impulse and reaction stages has been summa-
rized as follows:
1. Excitation of a vibration is often described by stimulus S. Substan-
tive investigations confirm the orders of magnitude for S as stated by
texts such as Traupel (see Bibliography). For small ratios of moving
blade pitch/fixed blade pitch (Fig. 12.7), S is typically in field G and
somewhere near line 1. For values of this ratio of roughly 1, S is typically
in field U and somewhere between lines 1 and 2. Field G applies pri-
marily to impulse turbines, and field U to reaction machines. If account
is also taken of the fact that the stimulus diminishes with increasing
axial spacing, then it is higher with impulse stages than with reaction
stages (Fig. 12.8).Thus, for the same dynamic stress, i.e., the same factor
of safety, the reaction stage allows higher specific loading.
2. Another advantage often cited for reaction blading is that reso-
nance between the fundamental oscillation and the excitation due to
the preceding row does not occur in practice (Fig. 12.9), whereas in an
impulse turbine close attention must be paid to this phenomenon. Res-
onances between the first harmonic and the preceding row are either
avoided or accompanied by much smaller dynamic stresses.
3. If resonances causing possible dynamic stresses are unavoidable,
steps must be taken to prevent blade failure. Damping wires and lash-
ing wires are shown in Fig. 12.6. They have for a long time been used to

Figure 12.7 Stimulus S in relation to moving blade pitch/fixed blade pitch: b =
moving blade pitch; l = fixed blade pitch; G = impulse blading; U = reaction blad-
ing. (Asea Brown-Boveri, Baden, Switzerland)