Pumps provide variable flows for new Illinois power plant
July 15, 2002 -- At a state-of-the-art gas-fired, combined cycle power plant in central Illinois, ITT Industries' Flygt Division has provided pumps for the intake system that helps balance construction cost imperatives against the variable flows of the Kaskaskia River.
The Holland Energy project is a $250 million energy facility being constructed in Holland Township, Shelby County, near Effingham, Illinois. The 665-megawatt combined cycle plant will be fueled by natural gas. The plant will include two natural gas turbines and one steam turbine, and is designed to be among the cleanest power plants in Illinois and in the nation. Construction on the Holland Energy project began in August 2000, and the plant is scheduled to begin commercial operation in June 2002.
Holland Energy is a leader among the wholesale energy companies that have emerged since deregulation changed the production and distribution hierarchy for electrical energy. In addition to the construction considerations and the river flow, the intake system for the new energy plant was designed to find a middle ground between regulatory requirements and the owner's operating preferences. The result has turned out to be a vital element for the power plant.
Location, Location, Location
In selecting a location for a power plant, candidate sites rarely offer the three essentials-fuel, distribution grid, and water-in equally close proximity. Joel Caves, Ph.D., PE, hydraulic engineering consultant with Parsons Energy & Chemicals Group, part of the project team, noted that, "They normally want fuel and distribution together and will build a line to transfer water to the plant. That is the case with the new power facility, which required a 29,000-ft long water line between the the Kaskaskia River and the power plant site.
Plant developers logically want more standardization in the supporting infrastructure for their facilities. However, the uniqueness of every facility, particularly at the intake structures, has thus far scuttled any 'cookie cutter' design solution.
Hurdles to Overcome
A number of factors influenced the water intake structure's design, further emphasized Caves. First, the owner's preferences for the intake's configuration, as well as the anticipated maintenance scheme at the intake, translated into structural redundancy. The design also had to compensate for a wide range in river flows, including flood events along the Kaskaskia River. Ice presented a seasonal factor, and waterborne debris and sediment were ongoing considerations. Finally, the intake required screen slots and water delivery velocities that helped to comply with Regulation 316 (B) of the Clean Water relative to protecting aquatic species.
Although the Kaskaskia River is a generally reliable source, the US Army Corps of Engineers influences its flow by controlling the upstream release from Lake Shelbyville. The reservoir was built in 1958 about 20 miles upstream from the new power plant's intake structure. The Corps has guaranteed a 12.4-CFS minimum release to the state but that could be deferred during extended drought or other special conditions. The new energy plant must shut down if the flow drops below 10 CFS. At yet other times, the Kaskaskia River can still reach flood conditions.
"We therefore designed the intake for water levels ranging from Elevation 505.8 at the 7-day, 10-year low (7Q10), to Elevation 511 as the average, up to Elevation 524.4 experienced during a 100-year flood," explained Caves. "A range of 18.6 feet is considerably more than normal."
From Design Criteria Into Concrete
"Although it cost more to build, the owner preferred an intake structure with three independent bays, each capable of handling 50 percent of the design flow. The redundancy and bay separation decrease the risk that emergency or routine maintenance will interrupt delivery of essential makeup water that would force a curtailment of the plant's power generation. The owner also sought manual cleaning of the coarse (bar) and fine screens," he added.
This runs contrary to the more frequent adoption of automated rakes and traveling bars. A 3-ton hoist is used when exchanging screens during cleaning or when setting the closure gates to isolate and permit dewatering of individual bays to periodically clean out accumulated sediment.
The requirements translated into an easily serviced, three-bay intake structure recessed 60 ft. back into the riverbank and 150 ft. upstream from a weir remaining from a former mill. Setting it back within a cut both protects the intake from floating ice and debris while preserving the channel's full width during high-water events. The water velocity approaching the fine screens of the 21-ft. wide structure is a nominal 1/2 CFS with 40 percent blockage. The water reaches the plant through a 24-inch dia., HDPE line.
"The structure's deck is at Elevation 526, or 1.6 ft. above the 100-year flood level. Vertical curtain walls, located in the intake bays and extending nearly down to the (7Q10) level, further shield against floating ice and debris," Caves said.
A 335-HP ITT Flygt submersible pump serves each bay. The three pumps cycle so that two operate at a time, permitting one bay to be isolated for maintenance. Each of the two active pumps operating in a cycle supply 2778 GPM, combining in output to meet the plant's operating requirement. Parsons and the owner preferred submersible pumps to long-shaft vertical pumps for several reasons. First, they reduced the height of the structural framework for the hoist by at least seven feet for initial construction savings. The other alternative to serving the long-shaft pumps would have involved a mobile crane that would have increased long-term maintenance expense.
"The Flygt pumps are also easier to service because of a rather clever lifting eye provided at the top of the units that enables you to connect a cable hook even while the pumps are fully submerged," Caves added.
Guide rails extending downward from the deck to the pump discharge piping align the pumps when they are reset after service, he explains. The pumps easily pull free from the discharge during removal and then reconnect tightly when lowered back down.
The hoist travels along a 21-ft. long traveling beam that spans the entire width of three, 40-inch wide bays defined by 30-inch concrete interior and 36-inch exterior walls. In addition, the beam can travel the structure's full length so that the 6-1/2-ft. high coarse and fine screens can be cleaned landside.
The structural frame for the hoist is 20-1/2-ft. high in order to provide the 15-ft. lift height at hook level. This also places the mechanical apparatus above any potential high water. The reinforced-concrete walls of the intake structure are thicker than normal to permit a bay to be closed, dewatered and serviced even while they are subjected to the 25-ft. hydrostatic head pressure of a 100-year flood event. The hoist was also oversized to have enough force to remove the gates and filters even during ice conditions or debris jams.
Ice can present an inevitable problem during winter in the Midwest. The records in this case were relatively poor so Parsons again adopted conservative measures. The setback offers some protection, as do the vertical, concrete curtain walls that extend deep enough so that the openings remain submerged to prevent a solid freeze over even during the lowest water levels. To minimize the buildup of frazil ice, the type that can coat and block a screen, Parsons specified HDPE bars for the coarse screens and polyethylene panels for the fine screens. These nonconductive components are complemented by a warm water injection system.
To comply with Regulation 316(B) of the Clean Water Act, the fine screens were fabricated with 1/8-ins. openings, versus the still common 1/4-ins. slots. Dual slots cast into the walls of the bay enable a clean screen to be set into place before removing a fouled shield. This keeps the bay operational and fish away from the pumps.
For the warm water injection source, a 6-inch line re-circulates a portion of the pump discharge to electric heaters and to the intake where it mixes with water delivered from the river. The slightly higher temperature combats the otherwise crippling buildup of frazil ice on the intake structure and related components during winter months.
Caves has full confidence in the design of the intake whose pumps and valves can be remotely monitored and controlled from the plant, much like the SCADA systems used to monitor and control lift stations along wastewater distribution systems. As more and more closed-cycle plants are proposed, the concepts that Parsons applied along the Kaskaskia River with the help of Flygt submersible pumps may provide a good starting point on the drawing boards.
For More Information: www.flygt.com
