Page 156 - Steam Turbines Design, Applications, and Rerating

Chapter
8

Governors and Control Systems

8.1 General
Mechanical-hydraulic governor systems as illustrated in Figs. 8.1 and
8.2 are suitable for most turbine-driven pumps, fans, compressors,
small generators, etc. Other systems with NEMA D and C performance
are available for installations requiring better steady-state speed reg-
ulation or remote adjustment features. The separation of governor and
trip systems is evident from Fig. 8.2.
Motion of the flyweights depicted in Fig. 8.2 causes the pilot valve
plunger to change the power piston oil pressures, producing movement
of the terminal shaft that is connected to the balanced, double-seated
valve through linkage and a single lever.
The overspeed trip pin, lever, and trip valve interaction can also be
visualized from Fig. 8.2. A spring-opposed pin in the turbine shaft is
thrown outward by excessive speed and strikes a nonsparking alu-
minum silicon bronze-tipped trip plunger. Plunger movement rotates
the latch to disengage the knife edges. The double-seated trip valve is
snapped shut by a closing spring. A torsion spring normally keeps the
knife edges engaged.
Although the foregoing explanation demonstrates simplicity, steam
turbine control systems can, at first, appear complex and mysterious.
In reality, however, they can be described as a combination of simple
elements. For any system, each element must be considered and ana-
lyzed independently. Basically these elements are sensing, transmit-
ting, and correcting.
Consider first the sensing element. Generally, this is speed. The pri-
mary function of a speed governor is to maintain the speed of a turbine
within set limits for the total load range of the unit. Speed sensing
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