Page 244 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

Reaction vs. Impulse Type Steam Turbines 223

12.4 Design
12.4.1 Rotor
The construction and behavior of the rotor are vital in determining
whether a steam turbine performs properly. The differences in the two
kinds of machines can be summarized as follows:

Impulse type

■ Rotors are composed of disks, generally made from the solid.
■ From the vibration standpoint the disks are independent structures,
and their behavior must be accounted for in the design. In service
these disk vibrations are superimposed on the rotor vibration. This
can result in a complex vibration pattern.
■ Material defects may require scrapping the whole rotor.
■ With disk rotors of relatively small diameter, rapid temperature
changes, on starting for example, are accompanied by a greater ten-
dency to distortion.
■ The principal mass of the rotor is at its center. Since the blades are
fitted on the outside of the thin disks, a rotor of this kind, when bal-
anced, is somewhat more sensitive to uniform distribution of the
blade mass than a reaction type rotor.

Reaction type
■ Rotors are executed in the form of a drum machined from the solid or
composed of separate forgings. Since 1930, the rotors of all medium
and large turbines produced by at least one major manufacturer
have consisted of sections welded together. The advantages of this
construction have been reported and are largely fabrication related.
■ The rotor can be uniformly heat treated, hence excellent strength
properties are achievable throughout.
■ The rotor is very easy to test.
■ Sections can be replaced easily and quickly in the event of rejects.
■ The rotor has evenly distributed stresses.
■ If the rotor of a reaction turbine is made from the solid, the same sit-
uation arises in the case of material defects as with an impulse type
rotor.

Critical speed. Just as basic as the question of which is the most suit-
able kind of turbine is the question whether the rotor speed should be
above or below the critical value. Both types have been built in sub-