Page 170 - Steam Turbines Design, Applications, and Rerating

150 Chapter Eight

Since the rotating pilot of the EGR is similar to the standard PG
series, the EGR requires a motor driver and a lube system. This system
is usually offered as a complete package with the only customer con-
nection being an electrical power supply connection to the motor.

8.7 Governor Systems
8.7.1 General
One sensing element for the multivalve turbine control is the Wood-
ward governor. This governor contains its own servo with given power
outputs. For single valve applications this power output is generally
sufficient to move a double-seated valve throughout its design range.
However, for multivalve applications the forces required to operate the
valves can approach 20,000 lb or 9070 kg. The Woodward servo cannot
generate forces of this magnitude. To achieve these force levels, the
Woodward servo is remotely mounted to serve as a prepilot (or slave)
to a master pilot that controls the flow of high pressure oil to a large
piston. This assembly of Woodward servo (prepilot), pilot valve, and
piston is called a servomotor. Such a control system is shown in Fig.
8.8. Servomotors are available in various diameters. For example,
using 75 psig (approximately 5 bar) oil pressure and an 8-in (203-mm)-
diameter servomotor piston, the force that can be generated is 3760 lb
(1700 kg) and with a 10-in (254-mm)-diameter servomotor piston uti-
lizing 125 psig (8.6 bar) oil pressure the force generated is in the neigh-
borhood of 9800 lb (4440 kg). Thus, it is readily seen how a few ounces
in governor force can be multiplied through a hydraulic mechanical
advantage to generate forces required to operate multivalve turbine
governor valves.

8.7.2 Extraction control
Steam at constant pressure for process use can be supplied from a
steam turbine by adding a pressure-regulating system to the speed
control system. This combined speed-pressure control system is called
extraction control.
The simplest manner in which to explain extraction principles and
the action of the turbine is to think of the turbine as having two steam
flows, exhaust and extraction, as indicated in Fig. 8.9. The power gen-
erated by the steam depends on the available energy and the quantity,
in the conventional manner. Thus, if exhaust flows and extraction flows
are equal, the exhaust flow will generate more horsepower than extrac-
tion flow, because it is subjected to a greater pressure drop.
These simple rationalizations are shown in the extraction diagram,
Fig. 8.10, in terms of tangible pounds of steam and effective horse-