Page 148 - Steam Turbines Design, Applications, and Rerating

Turbine Auxiliaries 129

Figure 7.5 Oil actuated trip-throttle valve. This balanced valve is easy to operate and per-
mits hand throttling of the turbine against full-line pressure. The valve acts as a power-
ful, quick-closing valve to shut off all steam to the turbine when actuated by the turbine’s
tripping devices or on loss of oil pressure. (General Electric Company, Fitchburg, Mass.)

are typically put in operation before the turbine is started up and just
after it is shut down. Turning at approximately 20 r/min prevents tem-
porary sag of the rotor at standstill while still hot, or while it is being
heated up. With the hydraulic barring gear the turbine rotor (1) is
intermittently turned by a hydraulically actuated ram (5) through a
sprocket (2). Mechanically driven turning gears operate on a similar
principle. If failure of the auxiliary power supply occurs, the turning
gear can also be operated manually.
Occasionally, steam turbines are executed with provisions to auto-
matically engage the turning gear whenever the rotor has slowed down
to a preset speed. Upon start-up and steam admission, the turning gear
will automatically disengage whenever the rotor again reaches a pre-
set speed of 40 to 60 r/min.

7.3 Trip-Throttle or Main Stop Valves
Trip, or main stop valves, are provided upstream of the steam chest.
These valves are opened either under oil pressure or by manually turn-
ing a hand wheel connected to a valve spindle.
When one of the safety devices trips, the pressure is relieved in the oil
system, causing the valves to close within a fraction of a second under