New Intake Design Concept
Formed suction intakes (FSI) have been used with large vertical pumps in flood control applications for many years. The well-proven design concept uses a low-profile rectangular-shaped inlet that is generously sized to accept incoming flow approaching in horizontal directions from a suction basin. After the inlet, the FSI passage shape transitions in a prescribed manner to efficiently turn the flow to the vertical direction at the FSI outlet. The FSI outlet is circular and concentric with the centerline of the pump suction. The shape of the FSI is illustrated in several of the following figures.
Model testing and similar installations have proven that pumps with FSIs are more compact and have lower submergence depth requirements than the same pumps with suction bells and forebays. The Hydraulic Institute (HI) standard for pump intake design provides guidelines for designing forebays for wet-pit vertical pumps as well as FSIs for dry-pit vertical pumps. In 1998, the HI adopted the Type 10 FSI standard in its design guideline. The Type 10 FSI offers several advantages, such as improved approach flow hydraulics in high cross-flow conditions, less sensitivity to the skewed velocity distribution at its entrance, and excellent subsurface vortex suppression capabilities.
The limited space available for the new lift pump station made the FSI concept attractive. More-traditional forebay designs required at least two times the transition distance and 1.5 times the width of FSI designs to convey the 800,000-gpm capacity from the vertical tunnel shaft to the lift pump impellers. In fact, traditional forebays would have resulted in the total constriction of flow paths through the remaining intake channel to the CW pump house. The FSI approach also reduces the submergence required at the pump intake because of its insensitivity to cross flows, thereby reducing the excavation depth during construction within the confines of the intake channel.
Unique Design Approach
The upgraded CW system for the OCPP Expansion Project includes the new intake tunnel, suction basin, lift pump station with four lift pumps, and discharge basin. These new elements are integrated with the existing CW pump house and the existing inlet channel, which has been modified with new dike walls.
System control is simple: The number of lift pumps operating equals the number of CW pumps operating. During the summer and peak power demand periods, all four lift pumps will operate. During winter and low power demand periods, two or three lift pumps will meet the cooling water flow requirements of the existing CW pumps. Rated flow of each lift pump is somewhat higher than the flow of each CW pump to ensure margin under all operating regimes. Excess flow from the lift pumps is recirculated from the discharge basin back to the suction basin over a fixed-elevation weir.
At the heart of the upgraded CW system for OCPP is the new lift pump station. The station incorporates four FSIs and houses four large lift pumps with driver sets (Figure 3). Each driver set includes a right-angle, speed-reducing gearbox and a horizontal, six-pole electric motor. Individual discharge pipe subassemblies are attached to each pump, and they exit through the wall of the lift pump station to the common discharge basin.
3. Model pump station. The OCPP lift pump station as viewed from the shoreline (top) and from Lake Michigan (bottom). Note the shape and location of the formed suction intake at the bottom on the structure. Courtesy: Bechtel Power Corp.
No discharge valves are needed for this pool-to-pool pumping arrangement. When a lift pump is shut down, reverse flow siphoning is stopped by opening a 12-inch vacuum release valve mounted at the high point of each discharge pipe subassembly.
The FSIs are engineered to be integral with the civil design of the lift pump station. The FSI design is intentionally oversized, with the outlet diameter made somewhat larger than the impeller eye diameter. This hydraulic adjustment improves flow distribution at the FSI inlet and reduces friction head losses between the FSI inlet and the pump impeller (Figure 4).
4. Some assembly required. The formed suction intakes were prefabricated off-site as steel subassemblies. Each FSI is made of three sections to facilitate shipment to the job site. Lifting eyes and vertical leveling screws are engineered into the design, so that complete subassemblies can be lifted into place and positioned. Courtesy: Bechtel Power Corp.
Once in their correct position, the FSIs have structural strength sufficient to serve as forms for poured concrete in multiple lifts. The design incorporates a curb ring that allows elevation and levelness adjustments at the FSI outlet connection just prior to the final pour of concrete for the lower floor.
A loose-fitting mounting ring design on the pump suction housing provides flexibility for the fit-up of the pump to the fixed FSI. This design flexibility must be carefully engineered and verified during design reviews with the pump supplier.
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