Page 102 - Steam Turbines Design, Applications, and Rerating

Rotors for Impulse Turbines 83

Figure 4.3 Combination solid and built-up rotor. (Dresser-Rand Com-
pany, Wellsville, N.Y.)

mind by remembering that shrunk-on wheels (20 in or 508 mm pitch
diameter) are suitable for a maximum speed of about 8000 r/min (20 ×
8000 = 160,000). This is equivalent to a tip velocity limitation in the
range of 700 to 800 ft/s (213 to 244 m/s), which is lower than the API
612 requirement.

4.3 Steam Temperature
Because the need for solid rotor construction is so readily related to tip
speed (r/min and pitch diameter), special caution is required to ensure
that the last of the four factors listed above, temperature, is not
ignored. The relationship between steam temperature and the effec-
tiveness of shrunk-on wheels is fairly straightforward. The wheels are
held in place by an interference fit, which is achieved when the heated
wheel is placed in position and allowed to cool to the same temperature
as the shaft. As long as the wheel and shaft are at the same tempera-
ture, the desired interference fit or shrink is sustained. However, if
during any transient operating condition a positive temperature dif-
ferential develops between wheel and shaft, the desired shrink is
reduced and may even be entirely lost. If this happens, the wheel is
constrained from turning on the shaft by the key, but it may move axi-
ally with rather disastrous results. It should be evident that the dan-
ger of incurring a large temperature differential between wheel and
shaft becomes more real as the temperature of the steam to which the
wheel is subjected increases. For this reason, an integral wheel is uti-
lized whenever the maximum temperature for the stage exceeds 750°F
(400°C) or stage inlet temperature exceeds 825°F (440°C). Since stage