Page 67 - Introduction to Marine Engineering

54 Steam turbines and gearing

This is the operating principle of all steam turbines, although the
arrangements may vary considerably. The steam from the first set of
blades then passes to another set of nozzles and then blades and so on
along the rotor shaft until it is finally exhausted. Each set comprising
nozzle and blades is called a stage.

Turbine types

There are two main types of turbine, the 'impulse' and the 'reaction'.
The names refer to the type of force which acts on the blades to turn the
turbine wheel.

Impulse
The impulse arrangement is made up of a ring of nozzles followed by a
ring of blades. The high-pressure, high-energy steam is expanded in the
nozzle to a lower-pressure, high-velocity jet of steam. This jet of steam is
directed into the impulse blades and leaves in a different direction
(Figure 3.2). The changing direction and therefore velocity produces an
impulsive force which mainly acts in the direction of rotation of the
turbine blades. There is only a very small end thrust on the turbine
shaft.

Rotation

Constant area
steam path

Figure 3,2 Impulse blading

Reaction

The reaction arrangement is made up of a ring of fixed blades attached
to the casing, and a row of similar blades mounted on the rotor, i.e.