Page 71 - Introduction to Marine Engineering

58 Steam turbines and gearing

shorter than the ahead turbine. The turbine rotor is supported by
bearings at either end; one bearing incorporates a thrust collar to resist
any axial loading.
The turbine casing completely surrounds the rotor and provides the
inlet and exhaust passages for the steam. At the inlet point a nozzle box
is provided which by use of a number of nozzle valves admits varying
amounts of steam to the nozzles in order to control the power developed
by the turbine. The first set of nozzles is mounted in a nozzle ring fitted
into the casing. Diaphragms are circular plates fastened to the easing
which are fitted between the turbine wheels. They have a central circular
hole through which the rotor shaft passes. The diaphragms contain the
nozzles for steam expansion and a gland is fitted between the rotor and
the diaphragm.
The construction of a reaction turbine differs somewhat in that there
are no diaphragms fitted and instead Fixed blades are located between
the moving blades.

Rotor
The turbine rotor acts as the shaft which transmits the mechanical power
produced to the propeller shaft via the gearing. It may be a single piece
with the wheels integral with the shaft or built up from a shaft and
separate wheels where the dimensions are large.
The rotor ends adjacent to the turbine wheels have an arrangement of
raised rings which form part of the labyrinth gland sealing system,
described later in this chapter. Journal bearings are fitted at each end of
the rotor. These have rings arranged to stop oil travelling along the shaft
which would mix with the steam. One end of the rotor has a small thrust
collar for correct longitudinal alignment. The other end has an
appropriate flange or fitting arranged for the flexible coupling which
joins the rotor to the gearbox pinion.
The blades are fitted into grooves of various designs cut into the
wheels.

Blades
The shaping and types of turbine blades have already been discussed.
When the turbine rotor is rotating at high speed the blades will be
subjected to considerable centrifugal force and variations in steam
velocity across the blades will result in blade vibration.
Expansion and contraction will also occur during turbine operation,
therefore a means of firmly securing the blades to the wheel is essential.
A number of different designs have been employed (Figure 3.6).
Fitting the blades involves placing the blade root into the wheel