When dealing with a construction site that is underwater or where the water table is just above sea level, contractors must ensure that the excavation stays dry and safe for workers. This can be even more challenging in locations with conditions affected by water depth and complications like silt and sand. Additionally, these difficult conditions may vary during seasonal weather or with the tides. Contractors need to investigate historical data and monitor fluctuations in water levels and speeds of flow to decrease the risks for workers and ensure the success of the project. The contractor must decide what pumping equipment to use based on the hydraulic conditions. Jobsite conditions—such as flow required to mitigate water seepage, depth of a coffer dam and solids content—influence this decision. The contractor usually chooses between self-priming centrifugal pumps or submersible pumps.
Reconstruction and infrastructure upgrades require pumps made from durable materials.
08/12/2014
Dewatering pumps keep construction sites dry in high water table areas. (Article images courtesy of BJM Pumps.)
The self-priming centrifugal pump, a specialized end-suction design, creates a vacuum at the impeller eye to continuously remove air from the suction line. The pump cannot compress the air during the priming phase, so the air must be allowed to escape through the discharge. As air is removed, atmospheric pressure forces water through the suction piping to the pump, allowing the pump to operate.
This self-priming process occurs automatically once the pump is started with the initial quantity of liquid. Sand or solids in the pumped liquid can complicate the priming process. The practical suction lift limit for self-priming pumps is about 8 meters (26 feet) of liquid under ideal conditions.
Self-priming centrifugal pumps have disadvantages. Any small vacuum leak—such as sealing areas around connectors in the suction line or pump seals—can prevent the unit from priming.
The pump will continuously pull air from the leak instead of the air in the suction line, which keeps the priming cycle from being completed. These leaks are a common cause of priming failure. They can be very small or invisible to the naked eye but still prevent priming.
The diameter and length of the suction hose or pipe can also affect the priming cycle’s length of time because of the volume of air that must be evacuated during pump priming. This extended priming time can also add heat to the liquid, which further extends prime time.
Self-priming pumps need to be as close as possible to the water source. A location directly above the coffer dam with few restrictions reduces friction. This can present problems, depending on the excavation location.
Once the pump is primed and moving fluid, it will require adequate net positive suction head (NPSH) to continue pumping without suction limitations.
Without sufficient NPSH, a self-priming centrifugal pump can experience cavitation and lose prime. Lifts through long suction lines, especially lines containing obstructions, can be plagued by cavitation problems that cause unacceptable noise levels and possible equipment damage.
If a self-priming pump must lift water 15 feet or more from the source to the pump’s suction, the pump capacity could be decreased significantly.
Most self-priming centrifugal pumps in remote applications have engine drives. These drives can present their own operational issues—such as maintaining fuel levels for long run times, the engine oil condition and other prime mover maintenance issues.
A storm surge attacks a seawall that protects a coastal city.
Boyer Construction is a 20 year-old construction and engineering company specializing in civil, electrical, mechanical and inland marine construction projects. Based in northwest Houston, more than 250 engineers, master electricians and plumbers, equipment specialists, and skilled fabricators, have worked on rehabilitation and infrastructure replacement projects throughout southeast Texas.
Many of those projects, including those in Galveston, required dewatering pumps on coffer dams and large excavation jobs. These demanding services require continual operation, pumping water that is often laden with silt and sand. The combination of continual cycling with salt and brackish water minimizes the life of most dewatering pumps.
Looking for help in keeping their dewatering pumps from wearing and failing, Boyer Construction contacted a pump specialist. The expert recommended a series of hard metal dewatering pumps.
The chosen pump offered the flow range and high lifts required for a broad spectrum of applications. From 2 horsepower with flows up to 180 gallons per minute (gpm) and heads up to 55 feet to 10 horsepower with flows up to 475 gpm and heads up to 117 feet, the series of pumps is designed and constructed for harsh services. The impeller and wear plate are made of abrasion resistant chrome iron, while the agitator and volute are constructed of hardened ductile iron, making the pump appropriate for the salt/sand slurry and solids-laden water Boyer Construction would encounter. The integral agitator is designed to mix settled solids with pump water to maintain a steady solids concentration and discharge volume.