A large California construction company with more than 5,000 employees had a seepage problem. The company is one of the top 10 producers of aggregate and sand/gravel products. It often extracted aggregate material for road construction and other uses from a gravel pit. While the pit was a great source of materials for concrete production, it had one big problem—it frequently filled with water.
While Northern Californian rains contributed to the water retention issue, the main culprit was an underground river. Water seeped into the pit from this river, filling it with dozens of feet of water in a day. When filled with water, extraction had to stop, and the water had to be pumped out to manageable levels.
The company installed some pumps on barges to attack the seepage. These pumps moved the water away from the pit and into a retention/holding area. If all had worked as planned, the barges would keep the water level low enough so that extraction could occur continuously.
Unfortunately, that was not the result with the pump barges. The vertical cantilever pumps were not adequately dewatering the pit. The expected flow rates were not met, and the water level did not fall to a level at which extraction could be easily accomplished. Rocks and debris frequently blocked the pumps, causing maintenance shutdowns. The pumps also experienced frequent vortexing, entraining air and operating inefficiently. As the company extracted more gravel, the bottom of the pit moved closer to the underground river, increasing the seepage.
The dewatering solution required that the pumps work as efficiently as possible. Because of the high cost of electricity in Northern California, even a few percentage points difference added up to a much higher energy bill. The pumps needed to push up to 18,000 gallons per minute (gpm) from the pit, and they needed at least 250 feet of head to move the water to the retention/storage area.
The system also had to be reliable. The company could not stop the pumps and pull in the barge because of mechanical issues. The pumps had to be able to handle solids, such as pieces of wood, from the bottom of the pit. Preferably, the system would not require mechanical priming to mitigate one more possible mechanical system failure concern.
A distributor for a pump provider visited the site to better understand the hydrology of the situation and offer different solutions. Replacing the cantilevered sump pumps was explored, as well as the possibility of mounting pumps on the edge of the pit.
Mounting pumps on the edge seemed impractical because of net positive suction head available (NPSHa) concerns. The pump would require frequent restaging to the proper elevation because of the changes in the pit’s water level.
The vertical cantilevered slurry pumps that were examined required a much deeper minimum submergence than was available because of the water’s varying levels. The pumps were also not very efficient and had a relatively high net positive suction head required (NPSHr). Maintenance and parts availability were also issues.
The successful solution was two centrifugal pumps barge-mounted to the bottom of the pit at 45-degree angles. This allowed four advantages:
- The pumps could operate without drawing in as much debris and particulates, which clogged the vertical pumps. If the pumps pulled in debris, they had a solids-handling capability of 4.75 inches.
- NPSHr was minimized, reducing the chance for vortexing and allowing the pumps to operate in relatively shallow conditions.
- The pumps were able to maintain their efficiencies—operating at an impressive 82.5 percent.
- The system did not require mechanical priming.
The two pumps were dewatering pumps, providing head of up to 360 feet and flow of up to 11,000 gpm each (the system operated at up to 18,000 gpm). The pumps were mounted to the barge and featured a 4-foot by 3-foot strainer basket with a solid steel top to prevent vortex. The flow pattern was mapped by the pump provider’s engineers, and the screen to stop the vortexing conditions was jointly designed by the distributor and the pump provider.
The pumps were angled at 45 degrees in the water, where the impeller sits partially submerged. This eliminated the need for a priming system. The 45-degree angle also lent itself to a direct-drive power arrangement. The pump distributor wanted to avoid belt drives on the barges because of the issues associated with pulling the barges from the pit for repairs.