Tennessee Valley
Authority's (TVA) Kingston power plant entered operation in 1955 and is one of
11 coal-fired power plants owned and operated by the TVA. The plant is located
just west of Knoxville, TN, at the Junction of the Tennessee and Clinch Rivers.
At the time it was completed, Kingston was the largest coal-fired power plant in
the world, a distinction that it held for more than a decade. The plant
currently burns low sulfur coal from central Appalachia.
Kingston's nine
units produce approximately 10 billion kilowatt-hours of electricity annually.
The plant burns 14,000 tons of coal daily. In 1977 electrostatic precipitators
were added to all nine units. Due to a combination of space and financial
limitations, the induced draft (ID) fans for units 5-9 were left upstream of the
new electrostatic precipitators, while the ID fans for units 1-4 were moved
downstream of the precipitators. The mechanical collectors and the old, smaller
precipitators, located in front of the ID fans, were decommissioned.
Six months after the
old precipitators and mechanical collectors were taken out of service, plant
maintenance personnel discovered that the ID fans for units 5-9 were being
severely eroded by fly ash. At an average run-time of 12-14 months, the steel
fan blades, supporting hardware and center hub had to be repaired or replaced.
On at least one occasion, erosion progressed to the point where several fan
blades were completely worn through, resulting in a fan failure that required
the fan being removed from its concrete foundation for repairs.
Preventive
Maintenance
As part of a
predictive maintenance program, plant engineers take weekly vibration readings
on the ID fans. Although vibration is a good indicator of erosive wear, the
readings don’t always follow a linear trend. In two incidences, fan vibration
levels were well within their acceptable limits, but erosion had progressed to
the point where the ID fans were completely destroyed.
According to Dan
Cowser, manager of outages and capital projects at the Kingston plant, "Fan
erosion has been one of our highest maintenance cost items, costing the plant
over half-a-million dollars annually in parts and labor alone. More than once,
we have evaluated moving the ID fans in order to reduce or eliminate related fly
ash erosion. However, it would take about 20 years for the company to recoup its
costs. The project would entail a major redesign of the fans and a tremendous
increase in fan motor horsepower in order to maintain the required performance
levels."
EPRI Tests
EPRI conducted two
related tests to study the effects of erosion on components protected by tile
most popular wear resistant solutions used in power plants. The tests were
conducted at TVA's Kingston Power Plant. Both tests compared the same basic
materials: plain carbon steel, tungsten carbide composites, tungsten carbide
thermal spray and chrome carbide weld overlay.
The first lest
conducted by EPRI in 1988 (EPRI CS-6068, Project 1649-4), was designed to study
the effects of fly ash erosion in coal-fired power plants. A field test was
conducted by installing a "rainbow" wheel carrying experimental erosion shields
on a number of fans experiencing erosion. More than ten different forms of wear
protection were studied. At the completion of the test, the Westinghouse
Research and Development Center, who authored the study, reported that the
tungsten carbide material displayed superior erosion resistance.
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After seven months
of operation, the tungsten carbide clad blades exhibited erosion of 0.014 inch
on Unit 8A's ID fan.
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Cowser, who came to the Kingston plant in 1997, noted the ID fans were
getting 12 to 14 months of run time between maintenance repairs. In 1999, the
soot-blowers were replaced and ID fan life shortened to 6-8 months, It was
obvious that erosive wear increased due to transporting more fly ash through the
system, says Cowser.
Although a unit is
not taken off-line for fan repairs, a significant derating is needed each time a
fan requires re-blading. Due to the increasing costs associated with shorter fan
run times, and the need to operate at full capacity during periods of peak
demand, Kingston plant personnel turned to TVA's Energy Research &
Technology Applications group (ER&TA) for assistance.
Steve Halcomb, a
project manger at ER&TA, says that ER&TA supports TVA's plants and
transmission system operators with research and development of new technologies:
"We focus on R&D and let the plant staff focus on operations."
In conjunction with
TVA, EPRI conducted another field test to determine which materials would hold
up to the severe erosion experienced by Kingston's units 5-9 ID fans. Donna
Dearmon, project manager for EPRI's Instrumentation & Control Center,
conducted the search for suppliers of various wear resistant materials and
coordinated the day-to-day activities with TVA's staff for the second test.
Fan Blade
Testing
The project was
initiated in the fall of 2001 and 16 wear protected fan blades from six
commercial suppliers were tested. The materials tested included infiltration
brazed tungsten carbide cladding, tungsten carbide high velocity Oxy-Fuel
(HVOF), tungsten carbide plasma spray and chrome carbide weld overlay. Fan
blades were tested over a 60-day period on Kingston's Unit 9 ID fan. The fan is
a double inlet, single exhaust, 400,000 cfm Westinghouse model 16MVID with
forward curve fan blades.
Each fan has 120
blades and a shaft speed of 593 rpm. The original fan blades each weighed 34 lb.
However, because Conforma Clad's infiltration brazed tungsten carbide material
adds five pounds to the fan blade, a new blade was designed. In order to reduce
overall weight and improve strength, the wear pad was removed and a full
penetration weld was used in place of a fillet weld.
Wear protected
blades were distributed throughout the fan. In order to facilitate balancing,
the heavier tungsten carbide-clad blades were located 180 degrees apart. Because
the wear rates of the material were unknown, test organizers were careful to
distribute test blades in such a way that erosion induced weight change wouldn't
require fan rebalancing.
Al the end of the
60-day test, all but the four blades protected with infiltration brazed tungsten
carbide cladding were removed. The blades protected with tungsten carbide weld
overlay and tungsten carbide HVOF were removed due to complete coaling
wear-through. The blades protected with chrome carbide weld overlay experienced
a material loss of 0.150 inch, had a crack at the center junction plate and
extreme wear at the leading edge.
Measurements taken
from the infiltration brazed tungsten carbide clad blades showed a material loss
of only 0.010 inch at the leading edge. Based on the test results, Kingston
began the process of retrofitting Unit 5- 9 ID fans with new blades from
Conforma Clad.
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Wear of forward
curve blade protected with tungsten carbide oxy-fuel HVOF after 60 days of
operation.
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New Blades
The first blades
were installed in mid-October 2002 on fan Unit 8B. After seven months of
runtime, the tungsten carbide clad blades showed material loss of 0.014 inch or
less, and mainly at the leading edge. Based on the applied tungsten carbide
cladding thickness, these blades are expected to last more than 24 months. This
is four times the life of unprotected fan blades.
The test results and
field performance data show that the densely packed tungsten carbide cladding
wears at a uniform and predictable rate. Bond strengths are estimated to be
in excess of 70,000 psi, resulting in a protective barrier that's highly
resistant to chipping, cracking or flaking.
The results of the
collaborative testing look promising. The negligible amount of material war on
Kingston's fan Unit 8B after seven months of run time further substantiates the
test results. Nevertheless, both TVA and EPRI personnel admit that more
monitoring lime is necessary before they can finalize their conclusions. Robert
Frank, director of EPRI’s Instrumentation & ControlCenter, indicates that
the report is in its draft stage and should be published later this year.
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