Page 40 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

Introduction 21

erator in parallel with others, then the two valves could be used to reg-
ulate extraction and exhaust pressures with a speed override.

1.3.5 Automatic extraction condensing
controls
The turbines shown in Figs. 1.14b through d are automatic extraction
condensing turbines commonly applied in industrial plants and third-
party cogeneration systems. A typical governing system would permit
simultaneous control of extraction pressure (with varying extraction
flow) in one or more steam systems at the same time that the speed
governor is maintaining frequency, even though the requirements
for extraction steam and generated power may be varying. Thus, the
turbine-governing system for either a single automatic, double auto-
matic, or triple automatic extraction condensing turbine can respond to
varying demands for steam in one-, two-, or three-process steam sys-
tems in addition to frequency or load.
Turbine inlet and extraction valve gears have the ability to control
turbine section flows as required to accomplish these many tasks. This
turbine can operate as an isolated unit or in parallel (synchronized)
with other generating units or the utility tie. If the required kilowatt
output for an isolated unit should ever be lower than the power pro-
duced by extraction steam (plus turbine exhaust cooling steam), then
the speed governor would automatically decrease the controlled extrac-
tion flow to maintain system frequency. The deficit in extraction steam
could then be made up by pressure-reducing valves throttling steam to
the process system in parallel with the turbine extraction steam.
Thus, available steam turbine control systems provide a wide variety
of system control functions to simplify plant operation and increase
plant energy system reliability.

1.3.6 Geared and direct-drive types
Geared and direct-drive turbine-generator sets are a cost-effective and
flexible means of generating power. With today’s competitive economic
climate, geared sets, with their lower first cost and competitive efficiency
levels, are finding extensive application in the 15,000- to 20,000-kW
range. A packaged gear-drive turbine is shown in Fig. 1.16. Direct-drive
turbine-generator sets can be designed for very large ratings. Packaged
direct-drive units, as depicted in Fig. 1.17, are available up to approxi-
mately 60,000 kW depending on the shipping limitations associated
with last-stage bucket length and exhaust hood size.
Packaged equipment sets offer the benefits of single-point responsi-
bility for the assembly of the turbine-generator or other driven equip-
ment package, which includes all major equipment as well as the