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.
Self-Priming centrifugal pumps
One weakness of end-suction centrifugal pumps is their poor performance when the liquid is below the pump centerline. Liquid must be delivered to the pump so the process can begin. Air on the suction side of the pump (the piping and pump casing) needs to be completely evacuated. If any air remains in a centrifugal pump, the pump becomes air bound and incapable of functioning.
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.
Submersible pumps offer many operational and application advantages. These pumps are submerged directly into the water for immediate use and unit cooling. This eliminates priming challenges and extended prime times.
No worker intervention is required. The pump is fed without the need for a suction line.
Submerged pumps are quiet. Cavitation is rare and occurs primarily when the sump is too small for the installed submersible pump’s size. They are also lighter weight and portable.
The versatility and low maintenance of submersible pumps make them an ideal option for dewatering service. No regular maintenance is necessary.
Submersible pumps usually need to be fully submerged. The water around a submersible pump actually cools the motor.