By Rich Wendel, P.E., Plant Mechanical Engineer, Kansas City Board of Public Utilities;
Jason Eichenberger, Project Engineer, Burns & McDonnell;
Terry Larson, P.E., Project Manager, Burns & McDonnell
Low river levels are causing reliability problems with once-through circulating water pump systems at several coal-fired power generating plants sited along the Missouri and Mississippi rivers. River bottom degradation and U.S. Army Corps of Engineers river management practices make low river levels a likely ongoing problem, especially during drought years in the Midwest.
This article outlines circulating water pumping problems that have occurred at the Kansas City, Kansas Board of Public Utilities’ (KCKBPU) Quindaro Power Station due to historic low water levels in the Missouri River as well as the steps taken to resolve these problems. Remedial design solutions that were either considered or implemented included installing supplemental pumps, vacuum lift systems, suction scoops, a new river water intake and converting to a closed-loop cooling tower system. This article includes a discussion of the supplemental pump system placed into service in the fall of 2006 and currently in operation on the Quindaro river intake.
The new supplemental pump system consists of four submersible axial flow pumps mounted on the front of the existing intake. The total flow capacity of the four supplemental pumps is around 200,000 gallons per minute (gpm). This flow is discharged into the existing intake to maintain an acceptable wet well water level for the operation of the existing circulating water pumps. Other system components include an overhead monorail system to facilitate annual supplemental pump installation/removal and a floating ice deflector system. The supplemental pumps are installed and operated during winter months when river levels are low and removed during the river navigation season when the corps maintains higher river flows.
KCKBPU owns and operates the Quindaro Power Station located along the Missouri River in Kansas City, Kan. Quindaro has two coal-fired generation units. Unit 1 has a rated capacity of 75 MW while Unit 2 is rated for 135 MW. Degradation (scouring) of the Missouri River channel has caused reduced river stage elevations at the Quindaro Station intake structures.
Once-through cooling is used for both units and cooling water is obtained through a common water intake structure that houses four 50,000 gpm vertical pumps. The existing cooling water pumps require 8.5 feet of submergence. The bottom of the intake structure is at elevation 709.75 feet, so without any modifications, the pumps were only able to operate at an approximate elevation of 718.25 feet.
The existing intake was fitted with a partial supplemental pump system in 1991. The existing supplemental system had only been sized to provide reliable raw water supply to the adjacent city water plant (no longer in service) and to allow one unit to be operated at reduced load. The existing system included diesel- driven hydraulic pumps and concerns existed about potential oil spills adjacent to the river. For these reasons, KCKBPU did not want to rely on this existing system, which had only been tested, never used. A monorail system was installed with the earlier supplemental pumps but was not oversized. A center rotating crane was installed when the intake was built to facilitate maintenance of the cooling water pumps and traveling screens.
The current configuration of the Quindaro intake and pumping systems does not provide reliable cooling water supply for Units 1 and 2 at low river stages during the winter months when the corps may reduce discharge flows from Gavins Point Dam to 12,000 cubic feet per second (cfs). Assuming the addition of lower than average tributary contributions due to draught conditions, the flows at Quindaro could be expected to reduce to 14,000 gpm during the winter months. Historic mean annual discharge is around 31,000 cfs, with a minimum discharge of 26,600 cfs required to maintain river navigation during the navigation season, April 1 through Nov. 30. The river level was projected to drop to elevation 716.5 feet during extreme low winter flow conditions. The level could drop to 714 feet in the event of an upstream ice jam during extreme low flow conditions. These projections estimated the river level could fall two to four feet below the submergence required for the existing circulating water pumps, affecting the power station’s reliability.
KCKBPU hired Burns & McDonnell to conduct a study to identify and evaluate alternatives to improve the operational reliability of the Unit 1 and 2 cooling water systems during low river flow periods. Effects of current and projected future river degradation were considered in identifying and evaluating viable alternatives.
Alternatives considered in the technology selection study included the following:
- Construct new intake structure
- Convert plant to closed-loop cooling tower system
- Install supplemental pumps on front of the existing intake
- Install vacuum lift system
- Install vortex suppression system
The results of the technology selection study indicated that installing new supplemental pumps was the most economical alternative when considering life-cycle costs and total system reliability.
The study concluded that cooling towers were not economically viable for a number of reasons. First, site constraints would force the cooling towers to be sited farther from the plant, resulting in long and costly circulating water piping runs and associated pumping costs. Second, the same tight site constraints made it difficult to identify suitable piping corridors and tie-in locations, making pipe installation expensive because of required sheet piling and decreased installation efficiency costs.
Third, outage requirements associated with such difficult tie-in would be long and expensive. Fourth, high electrical installation costs would be required to upgrade to the plant’s auxiliary electrical system to accommodate the cooling tower fans and larger circulating water pump motors. Fifth, increased operating and maintenance costs were associated with the water treatment system, tower fans, larger pumps and the towers themselves. And finally, constructing a new intake would be costly, difficult to permit and difficult to construct due to tight site constraints caused by various factors including an adjacent Army Corps of Engineers flood control levee. With Clean Water Act Section 316b compliance requirements still not finalized, investing in a new intake structure did not make sense.
Potential intake modifications considered for improving intake reliability during low river levels included installing a vortex suppression system consisting of a vortex suppression grating or plate, an upstream curtain wall and sidewall fillets to improve flow characteristics to the existing pumps. However, the vortex suppression system would not be sufficient to allow the pumps to operate during the extreme low river levels anticipated in the winter months.
Installing a vacuum lift system within the existing intake was also considered to reduce vortexing and pump cavitation problems associated with low river flows. The vacuum lift system involves the installation of sealed walls between the traveling screens and the circulating water pump. The walls would extend below the low water level in the sump and all portals would be sealed to create air-tight pump chambers. Vacuum pumps and associated piping would then be installed to apply a vacuum to the pump chambers and lift the water level in the pump chamber approximately seven feet above the river level, enabling the intake to operate at low river levels where adequate pump submergence conditions would not otherwise have existed. The main reason the vacuum lift system was not selected as the preferred alternative is that the vacuum would likely be lost during extreme winter low flow conditions associated with an ice jam or pluggage of the traveling screens and there were concerns with restarting the system if lost during low flow conditions.
Supplemental Pump System Design
The alternative that was selected involves installing supplemental pumps on the front of the existing intake. The new supplemental pump system consists of four axial flow, submersible pumps (see Photo 1) each rated for 50,000 gpm. The submersible pumps are housed within fabricated pump cans. The pump can bottoms are set at elevation 708.5 feet. The supplemental pumps should operate satisfactorily down to a river stage of elevation 716 feet on a continuous basis and could allow operation down to elevation 714.5 for short durations such as during an ice jam. Overall system cost was approximately $2.7 million.
Photo 1. Two of the four submersible pumps installed on the Quindaro intake. Click here to enlarge image
Submersible, axial-flow pumps were selected because they can pump a large volume of water at low-head conditions and do not require much submergence. The submersible pumps can be installed directly in the river without an extensive wet well system, and are relatively inexpensive to buy and operate. Horizontal axial flow pumps mounted on barges were also considered. However, concerns about potential damage to the units by floating ice, getting warm water out to pumps to prevent the screens from freezing, providing access for maintenance, and providing power or fuel supply to the floating units drove the decision to go with the submersible pumps mounted off the intake.
The design of the supplemental pump system had to address the following concerns:
- The space available to install the low head pumps in front of the existing intakes is limited.
- Pumping a large volume of water (200,000 gpm) into the front of the intake may create unacceptable hydraulic conditions for the existing pumps and cause possible cavitation problems.
- Debris and ice floating down the river may damage the supplemental pumps.
- Any construction out in the river may affect river navigation and would require corps approval.
- Maintaining the intake in service during construction of the improvements is challenging.
Since the supplemental pumps are considered only a short-term solution and are most likely to be operated only in the winter months when flows are reduced and ice jams are likely, they were designed to be removable and to be used only in non-navigation months when release rates and corresponding river levels are lowered. This non-navigation season generally occurs from December through March; however, the corps shortened the navigation season the last couple of years due to prolonged drought conditions.
By designing the system to be removable and to generally be used only in the winter months, many of the concerns about floating debris plugging the pumps were alleviated, especially grass and leaves typically associated with spring and fall flows. Floating ice and slush ice became the primary concerns, so a modular, interconnecting barge system was installed in front of the intake to deflect floating ice away from the supplemental pumps (see Photo 2). The protective barge and its lateral support systems to hold the barges during fluctuating river levels were also designed for relatively simple annual installation and removal. The barges also serve as an access platform, if needed, to reach the pumps while in service.
Photo 2. Pump lowered to service elevation and protective barge system. Click here to enlarge image
Using a temporary system also made permitting the system easier because the barges and pumps may be removed during the navigation season, as required by the corps.
Designing the supplemental pumps system presented many other challenges. The design had to be such that no permanent structure components were placed in the water that could catch logs or other debris during the navigation season. Hanging the heavy supplemental pumps off of the existing intake superstructure required innovative structural design work. The pump cans were mounted to slide plates that slid in a spare channel in front of the intake bar screens (see Photo 3). This system allowed the supplemental pumps to be installed more easily and to be completely removed when not in service. The plate also serves as a weir to raise the water level in the intake to maintain adequate submergence for the existing circulating water pumps and to reduce turbulence around the pumps.
Photo 3. Pump support system. Click here to enlarge image
To facilitate pump installation and maintenance, a new monorail system was installed to allow the pump cans (with the pumps inserted) to be lifted and moved to dry land for maintenance and storage when not in use (see Photo 4).
Photo 4. New monorail system. Click here to enlarge image
Construction and Operation Phase
The intake modifications necessary to allow for installation of the supplemental pumps were completed in the fall of 2006 (see Photo 5). An existing concrete mat left over from construction of the original intake had to be lowered to allow the pump cans to extend to the design elevation. Divers removed the top of the concrete mat and dredging was performed between the intake and the main river channel to provide more water depth in front of the intake.
Photo 5. Operating pump system with floating ice conditions. Click here to enlarge image
The supplemental pump system operated during the 2006 and 2007 non-navigation seasons. The river levels dropped to record lows in each of these two years to the point where the existing cooling water pumps would have been lost if the supplemental pumps had not been installed. The supplemental pumps did not have significant impeller damage during operation. This was a major concern during the design phase since there are no fine traveling screen systems to protect the pumps.
The supplemental pump system has worked satisfactorily for two seasons in keeping the Quindaro intake system online during unprecedented low river levels in the winter months. This is in spite of many design and operational challenges unique to river construction that had to be overcome to make the system a reality.
Authors: Terry Larson is a project manager and civil department manager in Burns & McDonnell’s Energy Division. He graduated from the University of Nebraska with a bachelor’s degree in civil engineering.
Rich Wendel is a mechanical engineer working at the Kansas City, Kansas Board of Public Utilities, Quindaro Power Station. He graduated from the University of Kansas with a bachelor’s degree in mechanical engineering.
Jason Eichenberger is a staff civil engineer responsible for permitting, design, and layout of power-related projects. He graduated from Kansas State University with a bachelor’s degree in civil engineering.