(1) Deep-well turbine or submersible pumps are generally used to pump large-diameter deep wells and consist of one or more stages of impellers on a vertical shaft (fig. 4-34). Turbine pumps can also be used as sump pumps, but adequate stilling basins and trash racks are required to assure that the pumps do not become clogged. Motors of most large-capacity turbine pumps used in deep wells are mounted at the ground surface. Submersible pumps are usually used for pumping deep, low-capacity wells, particularly if a vacuum is required in the well.
(2) In the design of deep-well pumps, consideration must be given to required capacity, size of well
Table 4-2. Capacity of Various Size Submersibleand Deep – Well Turbine Pumps
U. S. Army Corps of Engineers
screen and riser pipe, total pumping head, and the lowered elevation of the water in the well. The diameter of the pump bowl must be determined before the wells are installed, as the inside diameter of the well casing should be at least 3 to 4 inches larger in diameter than the pump bowl. Approximate capacities of various turbine pumps are presented in table 4-2. The characteristics of a typical three-stage, lo-inch turbine pump are shown in figure 4-35.
(3) Submersible pumps require either electric power from a commercial source or one or more motor generators. If commercial power is used, 75 to 100 percent of (connected) motor generator power, with automatic starters unless operational personnel are on duty at all times, should be provided as standby for the commercial power. Spare submersible pumps, generally 10 to 20 percent of the number of operating pumps, as well as spare starters, switches, heaters, and fuses, should also be kept at the site.
(4) Deep-well turbine pumps can be powered with either electric motors or diesel engines and gear drives. Where electric motors are used, 50 to 100 percent of the pumps should be equipped with combination gear drives connected to diesel (standby) engines. The number of pumps so equipped would depend upon the criticality of the dewatering or pressure relief needs. Motor generators may also be used as standby for commercial power. For some excavations and subsurface conditions, automatic starters may be required for the diesel engines or motor generators being used as backup for commercial power.
c, Turbovacuum pumps. For some wellpoint systems requiring high pumping rates, it may be desirable
(Courtesy of Fairbanks Morse, Inc., Pump Division)
Figure 4-35. Rating curves for a three-stage 10-inch-high capacity deep-well pump.
to connect the header pipe to a 30- or 36-inch collection tank about 20 or 30 feet deep, sealed at the bottom and top, and pump the flow into the tank with a high-capacity deepwell turbine pump using a separate vacuum pump connected to the top of the tank to produce the necessary vacuum in the header pipe for the wells or wellpoints.