Page 135 - Steam Turbines Design, Applications, and Rerating

116 Chapter Six

Figure 6.10 Fully bladed reaction turbine rotor with integral
shrouds. (Siemens Power Corporation, Milwaukee, Wis. and Er-
langen, Germany)

To keep the lowest natural frequency of the blades principally above
the sixth harmonic frequency of the turbine speed, the aspect ratio, i.e.,
the ratio of blade length to profile chord length, is limited to a value
below 5.
In the transition zone, which is particularly endangered by vibration
failures, this ratio is further reduced. Transition zone means the range
of the turbine blading, which depending on the turbine operating point,
alternately admits superheated steam or wet steam. The operating
point is determined by the power generated by the turbine and the live
steam conditions.
As a general rule the width of the axial gap between guide blades
and moving blades is made at least 20 percent of the profile chord
length. The actual value may be larger and is determined by the
expected relative expansion between guide blades and moving blades.
Manufacturers usually standardize shroud dimensions for each pro-
file chord length. The clearance between moving blade shrouds and
guide blade carrier, as well as between guide blade shrouds and rotor is
several millimeters. Sealing is effected by caulked-in sealing strips a
few tenths of a millimeter thick (Fig. 6.11). The moving blades are held
in the shaft groove by T-roots. Axial root dimensions typically equal the
profile chord length. All sizes of T-roots produced by a given manufac-
turer are geometrically similar. For all the reaction blading only a sin-
gle profile shape and a single root shape is necessary.