Ever since it hosted the Summer Olympic Games in 1996, Atlanta – capital of the US state of Georgia – has been striving to modernize the city’s infrastructure. One of the most prestigious and important projects in this campaign concerns the old Bellwood granite quarry in the northwestern part of the city. This area is to be transformed into one of the largest drinking water reservoirs in the United States, with a capacity of 9.5 billion liters of water (2.5 billion US gallons). In addition to the construction work, the project also includes building two central pumping stations that will transport the water to the population of over one million in and around the city of Atlanta.
One of these two pumping stations is the Quarry Pump Station, which pumps 40,834 m³ of water per hour (253 MGD) every day. This pumping station has an extremely low water level, which in turn creates complex general conditions for transporting the water. In this system, each submersible motor pump must be able to provide a constant capacity of 4,842 m³/h (30 MGD), also under fluctuating water surface levels with up to 36.6 m (120 ft) between minimum and maximum. In order to meet these requirements, the pumps must be driven by submersible motors with a variable speed that allows the motors to operate at different speeds of between 885 and 1,081 r.p.m. In conventional pumping technology, this variable speed change results in fluctuating hydraulic forces that affect the rotating unit and would lead to an undefined dynamic load status of the axial thrust bearing.
Andritz was awarded to supply three submersible motor pumps complete with peripheral equipment for Quarry Pump Station. One of the most important technological advantages of the suggested solution to meet the special technical requirements is the use of submersible motor pumps in double-suction design. For this purpose, two impellers rotating in opposite directions are arranged one above the other and driven continuously by a pump shaft. This compensates in full for the axial thrust, regardless of the rotation speed. The hydraulic thrust forces acting on the pump, the motor, and the axial bearings, are completely neutralized.
The division of work between the two pump halves not only enables full compensation of the axial thrust, but also halves the suction speed at the pump inlets. Each of the two pumps thus transports half of the flow to its center at full pressure. Splitting the flow reduces the suction speed of the flow medium along the inlet channels to the pumps and, ultimately, it also reduces the wear. In addition, the halved flow speed along the walls of the well has a very positive effect on the service life, especially with abrasive solids and sludge. In this way, the maximum possible operating reliability can be guaranteed, which increases the required maintenance intervals to up to 20 years and more.