Page 68 - Introduction to Marine Engineering

Steam turbines and gearing 55

moving blades (Figure 3.3). The blades are mounted and shaped to
produce a narrowing passage which, like a nozzle, increases the steam
velocity. This increase in velocity over the blade produces a reaction
force which has components in the direction of blade rotation and also
along the turbine axis. There is also a change in velocity of the steam as a
result of a change in direction and an impulsive force is also produced
with this type of blading. The more correct term for this blade
arrangement is 'impulse-reaction'.

Rotation

Narrowing
steam
path /JC ^""--* Steam
flow

Figure 3.3 Reaction blading

Compounding
Compounding is the splitting up, into two or more stages, of the steam
pressure or velocity change through a turbine.
Pressure compounding of an impulse turbine is the use of a number
of stages of nozzle and blade to reduce progressively the steam pressure.
This results in lower or more acceptable steam flow speeds and a better
turbine efficiency.
Velocity compounding of an impulse turbine is the use of a single
nozzle with an arrangement of several moving blades on a single disc.
Between the moving blades are fitted guide blades which are connected
to the turbine casing. This arrangement produces a short lightweight
turbine with a poorer efficiency which would be acceptable in, for
example, an astern turbine.
The two arrangements may be combined to give what is called
'pressure-velocity compounding'.
The reaction turbine as a result of its blade arrangement changes the
steam velocity in both fixed and moving blades with consequent gradual
steam pressure reduction. Its basic arrangement therefore provides
compounding.
The term 'cross-compound' is used to describe a steam turbine unit
made up of a high pressure and a low pressure turbine (Figure 3.4). This
is the usual main propulsion turbine arrangement. The alternative is a