Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 42 Torsional coupled vibration of a turbine rotor/blade.
rotor with and without blades. It was important that impact test data be
obtained for all blades with interactions in the frequency range of interest.
For fossil units, this is typically the L-0 stage only; in nuclear units, the last
three rows are typically required. Following a redesign and retrofit of the
L-0 row, field torsional testing was done using a system transient bump test.
The test was used to detect torsional modes around 60 and 120 Hz. The test
was performed by manually synchronizing and tripping the machine and
measuring the resulting noise spikes A single-phase ramp test was also used
to excite the system at double-line frequency [73].
There are considerable differences in torsional stiffness of the various
rotor disk designs, such as monoblock, shrunk-on disk, and welded
construction, and as a result of these differences, in the torsional response
characteristics of each, particularly in the higher torsional modes.
Over- and Underfrequency Operation. Prolonged operation at small
variations from 60 Hz (islanding) can cause increased dynamic loading on
tuned blades. Figure 43 shows a composite of the limits on frequency
operation from five manufacturers. The figure shows that operation in the
range 59.5–60.5 Hz would not affect blade life, while outside this range blade
life will be affected. Note that this figure indicates worst-case allowable
times at particular operating levels; specific turbines may not have such
restrictive values [81, 82].
Self-Excitation
Three primary types of self-excited turbine blade vibration phenomena have
been defined: stall flutter, unstalled flutter, and unsteady condensation
shocks.
Copyright © 2003 Marcel Dekker, Inc.