Water Intake At New Illinois Power Plant Designed for Variable Flows
The intake system that will draw cooling makeup water from the Kaskaskia River for the $250-million Holland energy plant in Shelby County, Illinois, was designed to balance construction cost imperatives against the river’s variable flow, regulatory requirements and the owner’s operating preferences. The result is a state-of-the-art vital element for the gas-fired, combined cycle plant.
The facility will generate 650 MW from two gas-fired turbines and a
single steam turbine when it enters service in 2002. Holland Energy LLC, the owner/developer, is an affiliate of Constellation Energy Group, a Baltimore-based energy company. It is a leader among the wholesale energy companies that have emerged since deregulation changed the production and distribution hierarchy for electrical energy. The new facility is among a recent generation of plants that are more efficient and environmentally friendly than earlier counterparts.
Candidate sites rarely offer the three essentials — fuel, distribution grid and water — in equally close proximity, noted Joel Caves, Ph.D., PE, hydraulic engineering consultant with Parsons Energy & Chemicals Group, part of the project team. “They normally want fuel and distribution together and will build a line to transfer water to the plant.
“In this case, the distance between the Kaskaskia River and the power plant site required construction of a 29,000-ft. long water line, while the essential fuel and distribution links existed much closer to each other,” Caves said.
The plant will be fueled by two natural gas pipelines owned by Kinder Morgan. The plant’s output will feed to Ameren Corporation’s 345 KVA overhead distribution system and then wheel to various wholesale customers.
The Reading, Pennsylvania, office of Parsons Energy & Chemicals Group developed the final design of a preliminary concept by ENSR International, also the environmental permit consultants. The overall engineering, procurement and construction contract is held by Holland Engineers & Constructors, a joint venture of Parsons and TIC (The Industrial Company), of Steamboat Springs, Colorado. Constellation Energy’s Tom Schwaller heads up the overall site work for the project.
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, translated into structural redundancy. The design also had to compensate for a wide range in river flows, including flood events along the river. Ice presented a seasonal factor, and waterborne debris and sediment were ongoing considerations. Finally, the required intake screen slots and water delivery velocities helped to comply with Regulation 316 (B) of the Clean Water Act. Fine screens were fabricated with 1/8-in. openings, versus the still common 1/4-in. 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 aquatic species away from the pumps.
“We 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.”
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 maintenance will interrupt delivery of essential makeup water that would force a curtailment of the plant’s power generation.
One 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 supplies 2778 gpm, combining in output to meet the plant’s operating requirement. The water reaches the plant through a 24-in. diameter HDPE line.
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.
Guide rails extending downward from the deck to the pump discharge piping align the pumps when they are reset after service, he explains. The pump easily pulls free during removal and then reconnects tightly when lowered back down.
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 hoist travels along a 21-ft. long beam that spans the entire width of the three, 40-in. wide bays defined by 30-in. reinforced-concrete interior and 36-in. exterior walls, even while they are subjected to the 25-ft. hydrostatic head pressure of a 100-year flood event.
The design yielded a three-bay intake structure recessed 60 ft. back into the riverbank. 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. Ice can present a problem during winter in the Midwest. 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 from hampering the water flow. 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.
For the warm water injection source, a 6-in. line recirculates 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.
As more and more closed-cycle plants are proposed, the concepts that Parsons applied along the Kaskaskia River may provide a good starting point on the drawing boards.
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