Page 66 - Introduction to Marine Engineering

Chapter 3

Steam turbines and gearing

The steam turbine has until recently been the first choice for very large
power main propulsion units. Its advantages of little or no vibration, low
weight, minimal space requirements and low maintenance costs are
considerable. Furthermore a turbine can be provided for any power
rating likely to be required for marine propulsion. However, the higher
specific fuel consumption when compared with a diesel engine offsets
these advantages, although refinements such as reheat have narrowed
the gap.
The steam turbine is a device for obtaining mechanical work from the
energy stored in steam. Steam enters the turbine with a high energy
content and leaves after giving up most of it. The high-pressure steam
from the boiler is expanded in nozzles to create a high-velocity jet of
steam. The nozzle acts to convert heat energy in the steam into kinetic
energy. This jet is directed into blades mounted on the periphery of a
wheel or disc (Figure 3.1). The steam does not 'blow the wheel around'.
The shaping of the blades causes a change in direction and hence
velocity of the steam jet. Now a change in velocity for a given mass flow
of steam will produce a force which acts to turn the turbine wheel, i.e.
2
mass flow of steam (kg/s) x change in velocity (m/s) = force (kgm/s ).

Force rotating wheel

Nozzle
plate *"
Change in direction
(velocity) of steam
Energy conversion
in nozzle
pressure to kinetic
Steam
entry Blades mounted
around wheel
Figure 3.1 Energy conversion in a steam turbine