Page 78 - Introduction to Marine Engineering

Steam turbines and gearing 65

expansion is taking place freely. Various indicators are provided to
enable this to be readily checked. Any sliding arrangements should be
kept clean and well lubricated.

Turbine control
The valves which admit steam to the ahead or astern turbines are known
as 'manoeuvring valves'. There are basically three valves, the ahead, the
astern and the guarding or guardian valve. The guardian valve is an astern
steam isolating valve. These valves are hydraulically operated by an
independent system employing a main and standby set of pumps.
Provision is also made for hand operation in the event of remote control
system failure.
Operation of the ahead manoeuvring valve will admit steam to the
main nozzle box. Remotely operated valves are used to open up the
remaining nozzle boxes for steam admission as increased power is
required. A speed-sensitive control device acts on the ahead manoeuv-
ring valve to hold the turbine speed constant at the desired value.
Operation of the astern manoeuvring valve will admit steam to the
guardian valve which is opened in conjunction with the astern valve.
Steam is then admitted to the astern turbines.

Turbine protection
A turbine protection system is provided with all installations to prevent
damage resulting from an internal turbine fault or the malfunction of
some associated equipment. Arrangements are made in the system to
shut the turbine down using an emergency stop and solenoid valve.
Operation of this device cuts off the hydraulic oil supply to the
manoeuvring valve and thus shuts off steam to the turbine. This main
trip relay is operated by a number of main fault conditions which are;

1. Low lubricating oil pressure.
2. Overspeed.
3. Low condenser vacuum.
4. Emergency stop.
5. High condensate level in condenser.
6. High or low boiler water level.
Other fault conditions which must be monitored and form part of a
total protection system are:
1. HP and LP rotor eccentricity or vibration.
2. HP and LP turbine differential expansion, i.e. rotor with respect to
casing.