Page 84 - Introduction to Marine Engineering

Steam turbines and gearing 71

Short blasts of steam are now admitted to the turbine through the
main valve to spin the propeller about one revolution. This should be
repeated about every three to five minutes for a period of 15 to 30
minutes. The vacuum can now be raised to its operational value and also
the gland steam pressure. The turbines are now ready for use.
While waiting for the first movements from the bridge, and between
movements, the turbine must be turned ahead once every five minutes
by steam blasts. If there is any delay gland steam and the vacuum should
be reduced.

Manoeuvring

Once warmed through, the turbine rotor must not remain stationary
more than a few minutes at a time because the rotor could sag or distort,
which would lead to failure, if not regularly rotated.
Astern operation involves admitting steam to the astern turbines.
Where any considerable period of astern running occurs turbine
temperatures, noise levels, bearings, etc., must be closely observed. The
turbine manufacturer may set a time limit of about 30 minutes on
continuous running astern.

Emergency astern operation
If, when travelling at full speed ahead, an order for an emergency stop
or astern movement is required then safe operating procedures must be
ignored.
Ahead steam is shut off, probably by the use of an emergency trip, and
the astern steam valve is partly opened to admit a gradually increasing
amount of steam. The turbine can thus be brought quickly to a stopped
condition and if required can then be operated astern.
The stopping of the turbine or its astern operation will occur about 10
to 15 minutes before a similar state will occur for the ship. The use of
emergency procedures can lead to serious damage in the turbine,
gearbox or boilers.

Full away
Manoeuvring revolutions are usually about 80% of the full away or full
speed condition. Once the full away command is received the turbine
can gradually be brought up to full power operation, a process taking
one to two hours. This will also involve bringing into use turbo-
alternators which use steam removed or 'bled' at some stage from the
main turbines.
Checks should be made on expansion arrangements, drains should be