Page 38 - Steam Turbines Design, Applications, and Rerating

Introduction 19

ond parameter requires the use of a second set of valves, and so on.
This is illustrated in Fig. 1.15.
Modern control systems use electronic speed and pressure sensors,
digital processing and logic, and hydraulically operated valves. Figure
1.15 shows direct mechanical devices to help visualization of princi-
ples, not to represent the actual implementation. Thus, speed control is
modeled by a flyball governor. A speed increase in response to load
decrease causes the flyballs to move outward, acting to close the asso-
ciated steam valves. Pressure control is modeled by a mechanical bel-
lows. An increase in the controlled pressure causes the bellows to
expand, closing inlet valves or opening extraction valves, as appropri-
ate to the application.
Figure 1.15a represents a unit with a simple speed governor. The
exhaust pressure is established independently of the turbine, either by
a condenser or other steam sources to an exhaust header. This arrange-
ment typifies a straight condensing unit. The valves control speed and
load only, and the turbine can operate in an isolated application or syn-
chronized in parallel with other generating units or electrical systems.
Figure 1.15b shows a unit maintaining exhaust pressure through
the action of inlet valves. (An associated speed governor takes over con-
trol when speed limits are exceeded. Pressure control is lost in the pro-
cess.) This would be an impractical arrangement for an isolated load
requiring accurate speed regulation. It works well when the turbine
drives an electrical generator that operates in parallel with one or
more other generators, which provide system speed control. Under nor-
mal operation one valve regulates one parameter: exhaust pressure.
This arrangement typifies the straight noncondensing turbine. A drop
in pressure at the turbine exhaust indicates an increased demand for
exhaust steam. The pressure governor acts to increase inlet steam flow
to match the changing exhaust requirements. The generator output
varies with use of process steam.
Figure 1.15c represents a single automatic extraction unit. The
speed governor responds to speed/load changes. The pressure governor
regulates extraction pressure as process needs change. Simply chang-
ing the position of the extraction valve at a constant inlet valve posi-
tion would also change the speed/load as well. Therefore, the two
systems interact to maintain constant load. For example, a reduction in
process steam requirements leads to an extraction reduction and an
extraction pressure rise. The pressure governor opens the extraction
valve, increasing low-pressure section flow and power. At the same
time, it acts to proportionally close the inlet valve to reduce inlet flow,
holding constant speed/load.
The two valves of Fig. 1.15c control two parameters: speed/load and
extraction pressure. This unit is incapable of independently controlling
its exhaust pressure as configured. If it were to drive an electrical gen-