Lift Pump Design Details
The lift pump design is a mixed-flow vertical type with open line-shaft using HI nomenclature. Each 1,200-bhp pump is single-stage, non-pullout by design and operates at a fixed rotating speed of 225 rpm. Each pump delivers from 205,000 to 260,000 gpm, depending on the relative static heads between the suction and discharge basins. The pumps are engineered to operate with pool-to-pool heads ranging from 17 to 0 feet.
Each lift pump subassembly includes a suction housing, impeller, column pipes, and discharge elbow. The suction head, which attaches to the FSI, transitions the flow from the FSI outlet to the impeller. The impeller has a semi-open design, which facilitates setting and resetting of the running clearances without dismantling the pumps. The impeller discharges into a multivane diffuser, which balances radial forces over the operating range of the pump. The diffuser discharges into a vertical column pipe, which further diffuses flow to the final discharge pipe diameter of 96 inches. The column pipe attaches to the pump discharge elbow (also known as pump discharge head), which redirects the flow from the vertical to the horizontal direction.
Mounting flanges on the pump discharge elbow are designed to rest on soleplates imbedded in the foundation at the top floor of the pump station. The mounting flanges, soleplates, and upper foundations are the primary supports of the pump subassembly and gearbox weights. Horizontal alignment fixtures and jack screws are provided for adjusting pump subassemblies as they are lowered into position above the respective FSIs. Allowable height tolerances between the curb ring elevation below and the soleplate elevation above must be carefully specified and closely verified with the pump supplier to enable constructability at the job site.
Separate discharge pipe subassemblies are attached to each pump at the discharge elbow connection. A discharge pipe subassembly consists of an expansion joint, a pipe spool, a pipe elbow, and a vertical draft pipe. In addition to transferring water to the discharge basin, the pipe subassemblies perform two critical functions:
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The elevation of the horizontal section facilitates a vacuum break to prevent reverse flow and thereby isolates a pump when it is not operating.
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The vertical section of the discharge pipe subassembly acts as a draft tube, which siphons flow forward when the pump is operating.
Supports for discharge pipe subassemblies are mounted on separate foundations from the pump subassemblies to allow the use of nominal civil tolerances and to make equipment installation easier during construction.
The pump subassemblies, driver sets, FSI fabrication, and discharge pipe subassemblies were supplied by ITT Flygt Corp. (ITT) of Pewaukee, Wis.
Testing Ensures Quality Results
A physical model test program was used to ensure that the hydraulics of the actual lift pump station functioned properly. Model testing was conducted at Clemson Engineering Hydraulics (CEH) in Anderson, S.C.
The Oak Creek Power Plant Expansion Project model was built to 1:12 scale of the full-size pump station with all features, including four sets of FSIs and pumps (Figure 5). The model was tested for a variety of cases with varying flow and varying numbers of pumps in operation. It was also used to:
5. Scale-model testing. An accurate 1:12 scale model was used to test and refine the hydraulic performance of the new intake system design. Shown is the lift station hydraulic model as viewed from the suction basin. Courtesy: Bechtel Power Corp.
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Evaluate the suitability of the as-designed suction basin and determine if any approach flow anomalies could adversely affect lift pump performance.
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Determine the impact, if any, of flow from the overflow weir introduced back into the suction basin.
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Measure the head loss associated with the as-designed FSI.
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Qualitatively observe the approach flow into the CW pump house to determine if any anomalies were present as a result of the new lift pump discharge configuration.
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Test and document the approach flow conditions with the final as-tested design with modifications physically added to the model.
Overall, the as-designed suction basin and FSIs performed well. No problems with the approach flow to the CW pump house were identified. However, two hydraulic issues were observed for some testing cases:
To remedy the first problem, three vertical baffles combined with a surface beam were added in the basin upstream of the FSIs to eliminate air-entraining surface vortices. Addressing the second concern required rounding the edges at the FSI inlets to prevent flow separation at sharp corners. The final hydraulic model test documented that the lift station with these modifications functioned properly for all cases.
With the as-tested design validated by physical hydraulic model testing, modifications to the actual full-size lift pump station design were made prior to construction. By utilizing a physical model program of this kind, a hydraulically sound lift pump station design was achieved.
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