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Concrete volute pumps for flood protection

Concrete volute pumps for flood protection

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    Illustration: Flowserve Corp.

    A concrete volute pump has a “formed suction inlet” instead of the usual rectangular structure. The suction bell is connected directly to the inlet and the volute sits directly above the suction bell. The transition piece is cast into the volute and becomes the connection to the discharge piping.

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    Construction is under way on a second pump station at the Pavaho site. Three 125,000-gpm Flowserve Corp. BCV 170 concrete volute pumps are replacing Flygt 46,000-gpm submersible and 30,000-gpm vertical pumps. Photo: Trinity Watershed Management Department

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    Table 1 — Existing stormwater pump stations

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    Table 2 — Planned stormwater pump station improvements

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    The Dallas Floodway Extension is one of five projects Congress authorized in 1965 as part of a basinwide plan to control the Trinity River. It was never built, so runoff that should flow to the river is trapped on land and pumped to the river during storms. To alleviate chronic flooding, the Trinity Watershed Management Department and U.S. Army Corps of Engineers are replacing vertical pumps with concrete volute pumps at six pumping stations. Map: Trinity Watershed Management Department

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.

Design factors

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, are absolutely essential. Run the dynamics before the modeling to confirm the layouts of the pump station and adjacent sump area contain no serious errors. Each manufacturer has a slightly different inlet design, so re-run the physical model once you've decided on the supplier.

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:

  • The need for redundant pumping equipment. USACE and Federal Emergency Management Agency (FEMA) guidance diverges, so contact both. According to USACE, redundancy is unnecessary if there's time to warn residents. FEMA 44 CFR Section 65.10: Identification and Mapping of Special Hazard Areas/Mapping of Areas Protected by Levee Systems assumes 17% of capacity is off-line during a 100-year flood, so a pump station would have to have 17% redundancy to control the 100-year event.
  • The need for parallel primary services from the local electric utility. Use USACE guidance to frame the discussion. Because the pumps use high-horsepower motors, make the utility a team member as early as possible in case soft starts are necessary.
  • The impeller type significantly impacts the system's overall hydraulic performance. Use actual pump curves when performing final hydraulic modeling.
  • How much and disposal of cooling water for the thrust bearing. If the gear reducer is water-cooled, amount and location are important issues. If water-cooled motors are used, cooling water will become a major issue and complicate the instrumentation and controls design.
  • Maximum temperature inside the pump room. USACE and the Occupational Safety and Health Administration provide excellent guidelines.
  • Team members are likely to change during these projects, which often take five to seven years from concept to commissioning. Keep a decision log!

  • Construction factors

    Although not overly complex, building the pump station requires:

  • Deciding if the volute should be made from prefabricated concrete units or cast-around formwork provided by the pump manufacturer. Different manufacturers favor one method over the other, and the construction method will be a major sales point. If constructed properly, either method yields a quality product. Maximize value by allowing the general contractor to make the decision at bidding.
  • Extremely well-thought-out construction sequencing, especially if existing pumping facilities must remain in service. Include sequencing in contract documents and discuss details in progress meetings.
  • Make sure the manufacturer's construction experts are onsite to confirm the equipment placement is correct before concrete is placed.
  • Stopping manufacturing for quality control inspections slows delivery, but these inspections are essential. Problems at the foundry are particularly troublesome if allowed to cascade down the manufacturing process. The owner and/or designated engineer should inspect every casting.
  • — Petrasek (al.petrasek@hdrinc.com) is the national technical pump station lead for HDR Inc. Bahta (sirak.bahta@dallascityhall.com) is a senior engineer for the City of Dallas.

    WEB EXTRA

    For elevation and plan views of a stormwater pumping station with concrete volute pumps, click here.