Page 157 - Steam Turbines Design, Applications, and Rerating

138 Chapter Eight

Figure 8.1 Type T sealed, mechanical-hydraulic oil relay governor (NEMA Class A).
The overspeed trip device is a separate subassembly on this single-valve, single-stage
turbine. (Elliott Company, Jeannette, Pa.)

devices use the flyball weight to sense changes to centrifugal force as a
function of speed change, an electric generator or pickup whose signal
can be modified and corrected for control purposes, or the pressure
from a positive displacement pump where a flow (and pressure) change
reflects speed.
The transmitting elements can be mechanical linkage, hydraulic or
pneumatic pressures, electric signals, or a combination of these ele-
ments. Some systems require amplifying elements to put strength into
a weak signal. For example, an ounce in force at the speed governor
may be felt as several hundred pounds at a steam admission valve.
Such amplifiers are called prepilots and servomotors. The tail end of
the control system (correcting element) is the steam admission valve.
These valves are found in all manner of sizes and shapes. And,
although they are officially an integral part of the turbine proper, their
function is clearly control system related and merits being mentioned
again at this time. (The reader will recall our overview discussion ear-
lier under the heading Steam Admission Section [Sec. 2.2].)
Most single-valve turbines utilize a double-seated valve. This type of
valve has a characteristically high flow at relatively low lifts with small