By: Keith Gifford and Chris Harley, Conforma Clad Inc.
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.
Steel Fan Blades showing severe erosive wear after 3 months service. Photo courtesy of Conforma Clad Inc. Click here to enlarge image
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.
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 conducted two related tests to study the effects of erosion on components protected by the 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 test 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.
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 sootblowers 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.
After seven months of operation, the tungsten carbide clad blades exhibited erosion of 0.014 inch on Unit 8A’s ID fan. Photo courtesy of Conforma Clad Inc. Click here to enlarge image
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.
At 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 coating 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.
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.
Wear of forward curve blade protected with tungsten carbide oxy-fuel HVOF after 60 days of operation. Photo courtesy of Conforma Clad Inc. Click here to enlarge image
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 wear 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 time is necessary before they can finalize their conclusions. Robert Frank, director of EPRI’s Instrumentation & Control Center, indicates that the report is in its draft stage and should be published later this year.
Keith Gifford is the Direcotr of Business Development for Conforma Clad Inc. He has a BS in marketing from Bowling Green State University and more than 23 years experience in industrial and commerical market development.
Chris Harley is a Senior Applications Engineer for Conforma Clad Inc. He has studied Engineering and metallurgy at both Purdue University and Indiana University Southeast. Chris has authored and presented papers on wear protection at numerous technical conferences.