Page 138 - Steam Turbines Design, Applications, and Rerating

Turbine Blade Design Overview 119

sional moment from each blade to its respective shroud prevents the
formation of gaps as described by effects 1 and 2.
If the prestress in the shroud area is still not sufficient and gaps
form because of extreme changes of the steam temperature in the blad-
ing, the vibration behavior of the circumferential unlinked shroudband
is still substantially different from that of a row of freestanding blades.
All drum rotor blades have manufacturing and assembling toler-
ances, which cause the natural frequencies of the blades of a rotating
row to be spread over a wide range. Therefore it is statistically impos-
sible for all blades to get into resonance simultaneously.
The blade that is exactly in resonance is prevented from developing its
maximum resonance amplitude by the neighboring blade, which is not in
resonance. The shrouds of the neighboring blades act as amplitude lim-
iters, and the vibration energy is transformed into heat by impact forces.
Energy is also dissipated from vibration amplitude by the following
effect: Because of machining tolerances the existing gaps are not of uni-
form width, but wedge-shaped, crowned or another shape (see Fig. 6.12).
This, for instance, causes the energy of vibration about the axis of
minimum inertia to be partly converted into a torsional vibration by
impact against the neighboring shroud. The available vibration energy
is thus distributed over several forms of vibration so that the maxi-
mum possible amplitude is decreased.
With existing gaps the shrouds act as amplitude limiters and vibra-
tion converters. The shrouds add further to the operational safety
because there is a wide radial gap between shrouding and guide blade
carrier. If because of a drop of the steam temperature the rotor or the
casing should suffer distortion, the thin sealing strips are damaged
without generation of excessive friction heat, but the radial clearance
is never taken up so that the rotor cannot touch the casing.
As of 1992, more than 3000 reaction steam turbines had been fitted
with integrally shrouded blades and their overall reliability was out-
standing in every respect.
The operational reliability of turbines, which work in the transition
zone of steam expansion depends decisively on the correct choice of per-
missible bending loads under these operating conditions.
First, a definition of transition zone. On admission of superheated
steam, salts dissolved in the steam are deposited as a coating on the
blade surface. Blading through which low-superheat steam normally
passes can temporarily admit wet steam because of changes of the tur-
bine operating conditions, i.e., an existing salt deposit is dissolved
again by condensing steam.
On the other hand, it is possible for superheated steam to pass
through stages normally operating in the wet steam zone and for salts
to be then deposited.