POWER September 2017 - 46

OPERATIONS & MAINTENANCE
Upgraded Boiler Feedwater
Pump Improves Efficiency
and Adds Flexibility
Times have changed. Not long ago, baseload units came online and went
straight to full load for days on end. Now, those same units are being
asked to cycle operations to accommodate for variable renewable energy
resources. That can take a toll on equipment such as boiler feedwater
pumps. One plant found that upgrading its pumps was the most-effective
way to adapt to new operational realities.
Leif Bothun and Robert Aronen
A
s wind and solar power supplies have
increased, conventional power plants
have been asked to vary output to
accommodate for intermittent renewable
generation. For many units, that has also
resulted in more cycling operation of boiler
feedwater pumps.
At the Fort St. Vrain Generating Station in
Colorado, greater pump cycling raised a few
concerns. The plant's original boiler feedwater
pumps were not designed for frequent
starts and stops, which created a higher risk
of internal metal parts galling and increased
the potential for pump seizure.
The Fort St. Vrain facility has a unique
history. It was originally commissioned as
a nuclear power station with an innovative
high-temperature gas-cooled design. Unfortunately,
the cutting-edge design created excessive
operating costs and the nuclear plant
shut down in 1989.
But that wasn't the end. The plant was recommissioned
in 1996 as a combined cycle
gas-fired power station (Figure 1). The plant
uses the original General Electric D8 nuclear
steam turbine and three General Electric
7F-Class gas turbines feeding three Vogt
heat recovery steam generators (HRSGs).
The plant has dual exhaust stacks for simple
cycle or combined cycle operation, which
allows extraordinary flexibility in output
from approximately 70 MW to more than
1,000 MW. This flexibility allows the plant
to follow load swings that result as solar and
wind power output rise and fall through the
average day.
Motivation for the Retrofit
Each of the three HRSGs uses two 100%
redundant, 10-stage, axially split boiler feed46
1.
Fort St. Vrain Generating Station. The Fort St. Vrain nuclear power plant ceased
operating in 1989 and was decommissioned from 1992 to 1996. The station was repowered as
a natural gas-fired facility in stages between 1995 and 2009. Courtesy: Leif Bothun/Xcel Energy
water pumps (Figure 2) running at a typical
operating temperature of approximately
260F and pressure of 2,700 psig. The pump
internal seals (wear rings, center bushing, and
throttle bushing) were originally constructed
using stainless steel rotating and stationary
parts. These components separate the highpressure
from low-pressure areas within the
pump and directly impact pump efficiency
and rotor stability.
The original design also created a risk for
galling or seizure at the internal seals. Because
these are 10-stage pumps, the shafts
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are relatively long and thin. When the pump
is not running, there can be a significant
shaft deflection (rotor sag) at the center of
the pump. The rotor sag creates potential for
contact and galling at the internal seals, especially
during a pump start. Once the pump
is running, differential pressure across the
internal seals creates a hydraulic force called
the Lomakin Effect, which stabilizes the rotor
and minimizes the contact between the
internal seals.
The plant's versatility is highly valuable
to meet system demand; however, it creates
POWER | September 2017

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POWER September 2017

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