Page 77 - Introduction to Marine Engineering

64 Steam turbines and gearing

lubricating oil would be necessary. This is usually provided from a
gravity tank, although main engine driven lubricating oil pumps may
also be required.
A lubricating oil system employing both a gravity tank and an engine
driven pump is shown in Figure 3.11. Oil is drawn from the drain tank
through strainers and pumped to the coolers. Leaving the coolers, the
oil passes through another set of filters before being distributed to the
gearbox, the turbine bearings and the gearbox sprayers. Some of the oil
also passes through an orifice plate and into the gravity tank from which
it continuously overflows (this can be observed through the sightglass).
The engine driven pump supplies a proportion of the system
requirements in normal operation.
In the event of a power failure the gearbox sprayers are supplied from
the engine driven pump. The gravity tank provides a low-pressure
supply to the bearings over a considerable period to enable the turbine
to be brought safely to rest.

Expansion arrangements
The variation in temperature for a turbine between stationary and
normal operation is considerable. Arrangements must therefore be
made to permit the rotor and casing to expand.
The turbine casing is usually fixed at the after end to a pedestal
support or brackets from the gearbox. The support foot or palm on the
casing is held securely against fore and aft movement, but because of
elongated bolt holes may move sideways. The forward support palm has
similar elongated holes and may rest on a sliding foot or panting plates.
Panting plates are vertical plates which can flex or move axially as
expansion takes place.
The forward pedestal and the gearcase brackets or after pedestal
supports for the casing are fixed in relation to one another. The use of
large vertical keys and slots on the supports and casing respectively,
ensures that the casing is kept central and in axial alignment.
The rotor is usually fixed at its forward end by the thrust collar, and
any axial movement must therefore be taken up at the gearbox end.
Between the turbine rotor and the gearbox is fitted a flexible coupling.
This flexible coupling is able to take up all axial movement of the rotor
as well as correct for any slight misalignment.
Any pipes connected to the turbine casing must have large radiused
bends or be fitted with bellows pieces to enable the casing to move freely.
Also, any movement of the pipes due to expansion must not affect the
casing. This is usually ensured by the use of flexible or spring supports
on the pipes.
When warming through a turbine it is important to ensure that