PROJECT: Expand stormwater pumping capacity citywide
OWNER:8City of Dallas Trinity Watershed Management Department
CONTRACTOR:8BAR Constructors Inc., Lancaster, Texas
CONSTRUCTION:8 August 2010 — November 2012
Much of Dallas is protected by a 22-mile levee system that runs along both sides of the Trinity River. These levees form the Dallas Floodway, which receives runoff from an upstream area of approximately 9,000 square miles. The system includes six stormwater pump stations, three behind each levee and each with multiple pumping units, that work with a series of sumps to control flooding. A seventh has been recommended for construction.
Dallas County Levee Control District No. 1 built the facilities in the 1930s in conjunction with the levee system. They were expanded in the 1950s when the U. S. Army Corps of Engineers (USACE) improved the system and again in the mid-1970s to add capacity, making the levees and pump stations a federal program.
Because of their age and inadequacy in a 100-year flood event, the city initiated a program to upgrade the pump stations. The number of pumping units will increase from 29 to 50. This represents a significant increase in operations and maintenance, so selecting equipment best suited for such an application was critical.
The city and its consulting team developed a detailed process to evaluate pump types and options. Their subsequent 15-month investigation included visiting four manufacturers:
- Flowserve Corp.
- Clyde Union Ltd. in the United Kingdom
- Alstom-Bergeron in France
- Kirloskar Brothers Ltd. in India.
|INCREASE THROUGHPUT WITHIN THE SAME FOOTPRINT|
Stormwater managers increase the total capacity of six pump stations 145%.
The City of Dallas Trinity Watershed Management Department is introducing North America to concrete volute pumps for stormwater applications.
The volute is the component in which the velocity imparted to the water by the impeller is converted to pressure by carefully increasing the size of the spiral-shaped flow passage surrounding the impeller assembly. A small unit processes about 80,000 gpm; a very large one, about 400,000 gpm.
Department managers specified the design to increase capacity at six pumping stations and to build a seventh, called Trinity-Portland. Except for Old Baker, which has horizontal pumps obtained from the City of New Orleans in 1930, all are equipped with vertical mixed-flow pumps.
The first table lists the facilities and the second shows how much stormwater each will process as a result. The stations on the East Levee have two separate structures, which is why there are “old” and “new” and “small” and “large.”
At time of publication, Baker No. 3 and New Able were in the design phase. Funding for the other stations is scheduled to be provided through a future bond program.
The team narrowed their options to conventional vertical pumps or concrete volute pumps before settling on the latter.
Although concrete volute pumps are new to the United States, more than 400 units are in service in North America and Europe and have performed well for more than 30 years. They're excellent for high-volume and low to moderate head applications and have an outstanding reputation for reliability, being commonly used in the nuclear power industry without standby units.
Invented by Louis Bergeron in 1917, they're essentially a wet-well/dry-well pump with a vertical configuration that allows for very short drive shafts, eliminating the need for line shaft bearings. A concrete volute enables the use of very large-diameter impellers that wouldn't be practical with cast metal or fabricated volutes because of manufacturing challenges.
Typically, the pump manufacturer provides all components, including the volute. The practical operating range is 80,000 to 450,000 gallons per minute (gpm) with heads ranging from 8 feet to 115 feet. The details vary from station to station, but several facts are relatively typical of the Trinity River installation.
- Pump columns: 45 to 55 feet long
- Required total dynamic head: 55 to 65 feet
- Required flow: at least 100,000 gpm per unit
- Creeks in the area are heavily wooded, so heavy-duty trash screens are necessary to manage tree debris load.
Managers chose the design for several reasons, including:
Wet-well depth. Conventional vertical pumps require a wet well 8 to 12 feet deeper than a concrete volute pump. Controlling factors are the required impeller submergence, submergence required for the suction bell to eliminate possible vortices, and clearance required between the bottom of the suction bell and the floor of the wet well.
Shaft length. Can be as short as 5 feet compared to a conventional vertical pump's 30 to 55 feet. A shorter shaft has fewer line bearings and is thus easier to maintain.
Formed suction inlet (FSI). Developed by the USACE, the design reduces wet-well depth by modifying the inlet geometry and isn't as sensitive to the flow's approach angle as a conventional rectangular intake.
Slow rotating speed and robust impeller. The large-diameter impeller reduces rotational speed to, typically, less than 350 rpm, extending bearing life and reducing concerns about vibration in the pumping unit. The substantial impeller also enhances pump robustness because the unit passes reasonably large debris without damage.
Fewer pumps. Their large size means fewer units are necessary, lowering the overall cost of a facility.
Drivers. Low-speed rotation means the pumps can be driven directly with an electric motor, run on diesel where air quality permitting allows, and incorporate gear reducers where feasible.
Ease of inspection and maintenance. Almost all have manholes in the discharge portion of the volute to permit direct access.
Most of the equipment for a concrete volute pump must be fabricated abroad and assembled in the United States. All related equipment should be witness-tested at the manufacturer's factory before shipping, which may require several overseas trips. There also are likely to be stop points for quality control inspection.
Computational fluid dynamics and physical modeling using ANSI/HI 9.8 protocol, which is available on the Hydraulic Institute's website at www.pumps.org
Installation details also vary from manufacturer to manufacturer, and these differences must be accounted for in construction documents. At a minimum, the design should include generic installation with instructions to the manufacturer to provide required changes, sealed by a licensed professional engineer in the state in which the project is being constructed. The changes should be approved by both the owner and the engineer.
Issues you'll need to investigate and incorporate into contract documents include:
Although not overly complex, building the pump station requires:
For elevation and plan views of a stormwater pumping station with concrete volute pumps, click here.