Page 139 - Steam Turbines Design, Applications, and Rerating

120 Chapter Six

By a systematic investigation of the possible operating conditions of
a turbine, the range of stages can be determined that may be subject to
this change of steam conditions. The upper limit of the design condition
is defined by minimum superheat, the lower limit by maximum steam
content.
Upstream of the transition zone the blading admits superheated
steam only, downstream only wet steam. The first and last drum rotor
stage of the transition zone can be determined by plotting the transi-
tion zone on the design expansion curve. What is special about the
stages situated in the transition zone is that temporarily, during the
dissolving of the salt deposits on the blading, highly concentrated
sodium chloride solutions can act on the blade material.
Fatigue strength tests have proved that the fatigue strength under
vibration stress measured in a concentrated sodium chloride solution
is much lower than that measured in air. Consequently, the material is
likely to fail because of stress corrosion cracking. For blades employed
in the transition zone the allowable bending stress is thus only a frac-
tion of that for blades in the superheated steam zone.
Extensive use is made of computerized calculation and design tech-
niques for both mechanical and thermodynamic stage characteristics.
Standardization is applied as far as practical. Among the design vari-
ables are the distribution of enthalpy drop, blade height, profile chord
length, and stagger angles of the profiles.

6.5 Low-Pressure Final Stage Blading
With increasing turbine ratings there is an obvious technical require-
ment for increasing exhaust area of the last stage of condensing tur-
bines. One way of satisfying this requirement is to arrange two or more
last stages in parallel. Particularly with single-casing turbines this
leads to a large bearing span and, hence, to sensitive rotors.
A second, more obvious means, is to increase the tip to hub ratio of the
last stage, i.e., to make the last-stage blade longer. Slender blading can
still conform to the obvious reliability requirements as long as high
blade stiffness or a high first mode natural frequency is being achieved.
Experienced steam turbine manufacturers are therefore opting for
small blade-to-chord ratios. This design results in low-steam bending
stresses and thus low dynamic blade stresses. The blades are attached
to the rotor by wide straddle roots and taper pins. This type of root can
be made with such close tolerances, even in small sizes, that all the
prongs are loaded uniformly.
The peripheral speed varies considerably between the hub and the
blade tip so that the blade foil profile and the stagger angle along the
blade height must be matched to the always varying flow directions.