O&M, Water Treatment

Water Intake System at New Illinois Power Plant Designed for Variable Flows

Issue 2 and Volume 106.

The intake system that will draw cooling makeup water from the Kaskaskia River for Constellation Energy Group’s $250 million Holland Energy plant in Shelby County, Ill., 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 650 MW gas-fired, 2×1 combined-cycle plant, which will enter service in 2002.

In selecting a location for a power plant, candidate sites rarely offer the three essentials-fuel, distribution grid and water-in equally close proximity. “They normally want fuel and distribution together and will build a line to transfer water to the plant,” said Joel Caves, Ph.D., PE, hydraulic engineering consultant with Parsons Energy & Chemicals Group, part of the project team. “In this case, the distance between the Kaskaskia River and the power plant site required construction of a 29,000-foot long HDPE water line.”

Constellation Energy Group’s combined-cycle Holland Energy plant is expected to become operational in Spring 2002. Click here to enlarge image

The essential fuel and distribution links existed much closer to each other. One 36-inch pipeline and one of two 30-inch natural gas pipelines owned by Kinder Morgan will fuel the plant. The plant’s output will tie into Ameren Corporation’s 345 kVA overhead distribution system and then wheel to various wholesale customers.

River Hurdles

Although plant developers logically desire standardization in the supporting infrastructure for their facilities, the uniqueness of every facility, particularly at the intake structures, has thus far scuttled any ‘cookie cutter’ design solution. A number of factors influenced the water intake structure’s design at Holland Energy.

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 designed to comply with Regulation 316 (B) of the Clean Water Act relative to protecting aquatic species.

Although the Kaskaskia River is a generally reliable source, the U.S. 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 ft3/s minimum release to the state, but that flow can be deferred during extended drought or other special conditions. The new Holland plant must shut down if the flow drops below 10 ft3/s. 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.”

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). Constellation Energy Group’s Tom Schwaller, Site Manager, oversees the project.

Design to 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,” said Caves.

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.

Workers ready one of three model CP3231 ITT Flygt 335-hp pumps for placement into the first of the three pump bays. Drawn makeup water from the Kaskaskia River will be pumped 5.6 miles to the Holland Energy cooling towers. Click here to enlarge image

The requirements translated into an easily serviced, three-bay intake structure recessed 60 feet back into the riverbank and 150 feet upstream from a weir remaining from a former mill. Setting it back within a cut 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-foot wide structure is a nominal 0.5 ft3/s with 40 percent blockage. The water reaches the plant through a 24-inch diameter HDPE line.

“The structure’s deck is at Elevation 526, or 1.6 feet above the 100-year flood level,” Caves said. “Vertical curtain walls, located in the intake bays and extending nearly down to the (7Q10) level, further shield against floating ice and debris.”

A variable-speed 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 requirements. Parsons and the owner preferred submersible pumps to long-shaft vertical pumps for several reasons. First, they reduced the structural framework height 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 submersible 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. The pumps easily pull free from the discharge during removal and then reconnect tightly when lowered back down.

The hoist travels along a 21-foot 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 6.5-foot high coarse and fine screens can be cleaned landside.

Overview of the three-bay intake structure under construction. Bays are covered with plywood for worker safety. First pump to be installed in foreground, with Kaskaskia Rver intake structure visible directly behind. Click here to enlarge image

The structural frame for the hoist is 20.5 feet high in order to provide the 15-foot 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-foot 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 and 316(B) 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-inch openings, versus the still common 1/4-inch 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 may provide a good starting point on the drawing boards.