Page 205 - Steam Turbines Design, Applications, and Rerating

184 Chapter Ten

Figure 10.11 Frequency spectrum with subsynchronous component
at 48 percent, probably coinciding with first critical frequency. (Gen-
eral Electric Company, Fitchburg, Mass.)

The second example is a two-stage, double-flow, noncondensing tur-
bine, rated 20,000 hp at 11,655 r/min, which went into service in 1969.
The frequency spectrum plot shown in Fig. 10.11 was made in 1979.
This turbine also has tilting-pad journal bearings. The subsynchronous
component frequency is about 0.48X running speed, which probably
coincides with the first critical frequency.
The third example is an eight-stage extraction turbine, rated 35,640
hp at 6112 r/min, that went into service in 1969. The analysis shown in
Fig. 10.12a was made in 1975. In this case, the subsynchronous fre-
quency is 0.26X running speed, which is lower than the first critical
frequency. This turbine has fixed-arc bearings of the axial groove type.
The fourth example is a six-stage condensing turbine, which is typi-
cal of hundreds of turbines that drive boiler feedwater pumps in cen-
tral power stations. This type of turbine has been put into service in
increasing numbers since 1965. The spectrum analysis shown in Fig.
10.12b was made in 1979. It shows a subsynchronous vibration compo-
nent at a frequency corresponding to 0.35X running speed. These tur-
bines have fixed-arc, elliptical bearings.
The frequency spectra, as shown, confirm the fact that limited ampli-
tude, subsynchronous vibrations exist in these turbines, but there have
been no incidents or evidence of classic instability during their long
service times.
Extensive signature analyses have revealed that many turbines
exhibit limited amplitude subsynchronous vibration behavior. How-
ever, these turbines also continue to operate reliably. To date, there has