The former Bellwood granite quarry northwest of Atlanta, Georgia, will become one of the biggest water storage facilities in the United States. This is part of a major infrastructure development program to increase drinking water supplies for more than 1 million residents of the city and its surrounding area. As the capital and the most populated city in the state, Atlanta has intensified its efforts to alter the city demographics, politics and culture to become a pioneer in enhancing safe and stable supplies of potable water for its citizens.
Atlanta has been striving to modernize its infrastructure and revitalize its neighborhoods since the 1996 Olympic Games. One of the most important projects is set to dramatically increase the city’s drinking water supplies.
Bellwood quarry is going to become one of the biggest water storage facilities in the country, storing 2.4 billion gallons of water as part of this program. This expansion of the city’s raw storage capacity will provide Atlanta with a reliable supply of drinking water for the next 100 years and increase the emergency raw water reserve from just three days to 30 days. The Department of Watershed Management, which is in charge of the water supply for approximately 1.2 million people in Atlanta and its surrounding area, is investing about $300 million in this project.
Converting a 300-foot-deep quarry into a water storage facility and recreation area includes blasting two circular pump station shafts near the quarry, one 35 feet in diameter and 200 feet deep, and the second 20 feet in diameter and 300 feet deep. Five additional 6.3-foot diameter pump shafts will be constructed, along with a new power substation and various improvements to existing infrastructure. Central to this project are the 136 million gallons per day (mgd) Hemphill Tunnel Pump Station with four vertical turbine pumps and the 200 mgd Quarry Pump Station with four vertical turbine pumps and three submersible turbine pumps.
Image 1. Double-suction submersible motor pumps guarantee a sustainable water supply for Atlanta. (Image courtesy of ANDRITZ)The quarry will be filled through a 5-mile-long tunnel that will connect it with the Hemphill and Chattahoochee Water Treatment Plants and the raw water intake and pump station at the Chattahoochee River, the city’s primary water source. The work also involves building this 10-foot diameter tunnel, as well as the mechanical, electrical and supervisory control and data acquisition (SCADA) systems associated with the pump stations.
Drilling is being done by a 400-foot tunnel-boring machine, which arrived in Georgia in July 2016 on 70 trucks and was assembled on-site. After a public naming contest, the tunnel-boring machine was named “Driller Mike” as a tribute to Atlanta resident Michael Render, aka “Killer Mike,” a famous rapper, actor and activist.
After completion of the project, the city plans to develop a 300-acre recreation area at the site—the Westside Park. It will be the largest park in Atlanta and will be designed with public input in accordance with the Department of Watershed Management requirements.
As water is withdrawn and the water level drops to very low levels, the system requires that each submersible motor pump has a constant capacity rated at 30 mgd, even under fluctuating water surface levels ranging up to 120 feet between minimum and maximum. To achieve this system requirement while operating under this submergence differential, the submersible motors are driven by a variable speed drive, which enables the motors to run at varying speeds between 885 rotations per minute (rpm) and 1,081 rpm. Variable speeds change the axial forces on the rotating unit, which results in varying shaft elongation and ultimately in undefined dynamic loads on the axial thrust bearings.
One of the major benefits of the proposed solution using double-suction submersible motor pumps is the full compensation of the axial thrust independent of the rotational speed. This characteristic neutralizes the load on the pump, the motor and its thrust bearings. With this technology, two contra-rotating submersible motor pumps are arranged on top of each other and driven by a continuous pump shaft. Each of the two pumps transports half of the capacity to the middle of the pump at full pressure. This significantly reduces wear and tear, increasing service life up to 20 years and provides the maximum operational reliability.
The division of work between the two pumps not only achieves the complete compensation of axial thrust, but also halves the suction velocity at the pump inlets. This protects the well walls around the intake openings and minimizes the intake of abrasive solids and silt.
The completion of the project is scheduled for 2019 and will address Atlanta’s goal of achieving a sustainable water infrastructure for future generations and flexibility in systems operation. In the event of a crisis or loss of water service, it provides redundant water storage and can save the city millions of dollars per day.