The primary function of the once-through cooling water (CW) system is to continuously circulate cooling water — typically from a lake, river, or ocean — through surface condensers and heat exchangers in the plant, thereby rejecting waste heat back to the water source. A once-through CW system for a typical coal-fired plant requires around 400,000 gpm for each 600 MW of installed capacity.
A typical once-through CW system will include an intake structure, pump house, piping, condenser and other process heat exchangers, seal well, and discharge outfall. Bar and traveling screens collect and dispose of debris that enters with the source water though the intake structure, and large CW pumps in the pump house move water through the plant’s cooling circuit. Depending on discharge thermal criteria, either a shoreline discharge, offshore single port, or multiple port diffuser discharges are typically used.
Many Special Requirements
The 1,135-MW Oak Creek Power Plant (OCPP) has four operating units that have used CW from Lake Michigan since 1953 (Figure 1). The original intake channel extends from the shoreline to about 900 feet into the lake (Figure 2). That intake channel is about 250 feet (ft) wide and 20 ft deep under the low-water datum. The channel width narrows to about 200 ft as it approaches the inlet of the existing OCPP pump house intake. The pump house has four pumps designed for approximately 800,000 gpm.
1. The Oak Creek Power Plant. The two-unit Oak Creek Power Plant Expansion Project is now under construction on plant property located to the right of the existing plant. Courtesy: Bechtel Power Corp.
2. Short intake channel. The existing shoreline intake channel was only 900 feet long. The Clean Water Act Section 316(b) upgrade now takes water from Lake Michigan at a distance of 1.5 miles offshore. Courtesy: Bechtel Power Corp.
We Energies, the owner of OCPP, decided to upgrade the CW system to comply with Section 316(b) of the Clean Water Act by constructing a new common system to serve the four existing units plus the two new 615-MW units of the OCPP Expansion Project now under construction.
Bechtel Power Corp. has the contract to engineer, procure material, and construct the OCPP Expansion Project, including the CW system upgrade. The CW system upgrade scope included a new 27-ft-diameter intake tunnel system that projects approximately 1.5 miles offshore to collect cold water from near the bottom of Lake Michigan, a new lift pump station, and modifications to the existing intake channel. With modifications, the existing intake channel remained functional and now serves as an emergency intake. The existing OCPP pump house, CW pumps, piping, condensers, heat exchangers, and discharges remained fundamentally unchanged.
To accommodate the new intake tunnel system, the existing channel intake was modified with new dike walls with multiple gates. These gates open and close to control the source of CW from the intake channel or from the intake tunnel, respectively. Normally, the gates are closed to realize the benefits of receiving cold water from the bottom of Lake Michigan. However, this design required a new lift pump station to compensate for the additional friction head loss expected in the intake tunnel. The new lift pump design and installation was challenging for several reasons:
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The lift pump station footprint had to be constrained to a corner of the existing intake channel close to the existing CW pump house.
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Lift pump flow and head needed sufficient margin to provide ample submergence and net positive suction head to the existing CW pumps.
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During normal operation with the lift pumps running, the approach flow conditions from tunnel outlet to the lift pump suctions and from the lift pump discharge to the CW pump house must be acceptable hydraulically. Also, in the event of a lift pump station shutdown, the approach flow conditions through the modified intake channel must provide acceptable suction conditions to the CW pump house.
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The lift pump station construction could not disrupt ongoing operations at OCPP.
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