POWER August 2011 - 38

2011 MARMADUKE AWARD
The final project specifications required installation
of a SSS clutch size 160FT into the
intermediate block of the BB11-L, between
the compressor and generator. This location
allows complete mechanical separation between
the compressor and generator when
synchronous condensing duty is required.
The project also included modernization of
the unit's control system with digital controls
and replacement of its lubrication oil pumps,
for reasons discussed below (Figure 2).
By October 2010, installation and testing of
the completely new digital control system was
completed and was closely followed by installation
of the new clutch (see sidebar)-one of
the final installation tasks. The plant was successfully
restarted and operated in both power
generation and synchronous condensing modes
on Dec. 21, 2010 (see the table for key project
dates). Since that time, the newly reconfigured
Unit 2 has been available for dispatching by either
CFE's generation department for power, or
by CFE's transmission and distribution organization
for grid-stabilizing reactive power.
Unique Turbine Design
SSS may have determined the project was
feasible, but that doesn't mean the conversion
of a unique 40-year-old combustion
turbine was straightforward or without many
technical challenges. Unlike modern turbomachines
of this rating that arrive on the job
site prepackaged on a single skid that can be
dropped onto foundation bolts, the BB11-L
was a field-assembled set of independent,
free-standing modules that form a single
driveline (Figure 3): turbine, compressor,
turning gear and main lube oil pump, and
generator/exciter.
Normal operation of the BB11-L was also
unique: The source of power for startup was the
exciter, switched to run as a DC motor. When
the power train reached about 2,000 rpm, the
DC starting motor electrically switched to direct
current generator (or rotating exciter) for
the main generator. Conventionally, the automatic
voltage regulator (AVR) determined the
voltage at the generator terminals. When the
generator was subsequently synchronized to
the grid, the generator could supply real and/
or reactive power by adjusting the power factor
controller, within the +15 MVAR to -10
MVAR range set by the manufacturer.
Challenges Overcome
It's often a long leap between concept and reality
when performing retrofit projects on older
equipment such as the BB11-L. The conceptual
design was to remove the existing 700-mm shaft
that connects the compressor and generator and
replace that shaft with a clutch assembly that
would function as a flexible coupling able to
absorb a certain amount of misalignment during
power generation but able to disconnect
during synchronous condenser operation. Aside
from the clutch itself, the plan was to install a
new gear-toothed coupling input hub (on the
compressor output shaft) and a new output hub
(on the generator input shaft). This approach
required the removal and relocation of the driveline
turning gear and removal and replacement
of the motor-operated lube oil pump (Figure 4).
The driveline was originally designed to
grow in the direction of the generator when
3. Modular turbine. The 1970-vintage 14-MW Brown Boveri BB11-L was a collection of individual
modules that were field-assembled into a single combustion turbine driveline. The dark
squares represent the location of driveline bearings, with a thrust bearing located at the connection
of the turbine and compressor shafts. Conceptually, the 700-mm shaft was removed and a
clutch would take its place. In addition to incorporating a clutch, a new location for the turning
gear was selected, the gear-drive main lube oil pump was replaced by a DC and AC pump, and
the mechanical governor was replaced with an electronic governor. The starting motor/exciter
are bolted to the outboard end of the generator shaft. Source: Industria Sigrama
Combustor
Shaft to be replaced
700 mm
TUR
Bearing
(8 places)
Turbine
compressor
coupling
Thrust
bearing
COM
Coupling
hub
Turning gear
Coupling
hub
AVR
GEN
EXC
SM
Tachometer
in power production mode, up to about a
quarter of an inch. However, in synchronous
condensing mode, the combustion turbine is
not operating, so the expected shaft displacement
does not occur. It fell to SSS to design
a clutch assembly that could also accommodate
this driveline displacement. In addition,
the clutch weighed more than the 700-mm
spool piece it was replacing.
Final Driveline Design. The first order of
business on this project was to perform a rotordynamic
study of the proposed driveline with
the new clutch and without the existing turning
gear and gear-operated main lube oil pump. The
reason this analysis was so important relates to
the clutch configuration. The clutch is a semirigid
clutch, which when disengaged leaves two
halves: one half supported by the compressor
and the other half supported by the generator
shaft. SSS engineers were convinced that the
overhung moments (the additional weight of the
clutch halves in relation to the location of the
existing bearings) would be excessive.
On the other hand, by using a spacer-type
clutch, the teeth act like a double-engagement
coupling, and the hub could be reversed
on the compressor shaft. This hub design has
the effect of reducing the overhung moment,
countering the effect of the increased weight
to be supported. A rotordynamic study confirmed
this approach was possible.
However, CFE preferred to retain the existing
hubs (the generator, particularly) because
of their unique tapered bore and three keyway
design and because CFE engineers were concerned
that their removal would be a problem,
as the hubs had not been removed in 40 years
(Figure 5). This approach to securing the clutch
required SSS to manufacture a large, heavy
adaptor, because the clutch outside diameter
was considerably larger than the existing hub. A
4. Shaft in a half-shell. The top shell
(shown in Figure 2) was removed from this assembly
located between the compressor on the
left and the generator on the right. An idler gear
(center) connects the main lube oil pump (foreground)
to the main shaft. This shot was taken
just before the new SSS clutch replaced the 700mm
extension shaft that connects the compressor
and generator. Courtesy: Industria Sigrama
Gear-driven lube oil pump Governor
Notes: AVR = automatic voltage regulator, COM = compressor, EXC = exciter, GEN = generator, SM = starting motor,
TUR = turbine.
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