15 August 2002
BASIC REQUIREMENTS OF DESIGN FACTORS. Three basic design
factors enter into the design of a pumping system: (1) desired pumping time, (2) initial
cost of pumps, motors, pumpwell structure, and appurtenances, and (3) power supply.
Also consider the relatively small percent of time that pumps are in operation.
Pumping Time. The average time for dewatering Navy drydocks of the
main classifications may be used as a design guide:
Submarine or destroyer docks, l hour 30 minutes.
Cruiser docks, 2 hours 20 minutes.
Carrier and Auxiliary docks, 2 hours 45 minutes.
Initial Cost of Installation. For comparative studies, obtain the cost of
various sizes of pumps, motors, controls, and other mechanical accessories from
manufacturers. The cost of pumpwell and discharge structures increases with the sizes
of pumps. This is especially true with regard to pumpwell foundations that are usually
the deepest part of the entire drydock foundation and lead to special construction
Power Considerations. Dewatering pumps of all naval drydocks should
be driven by electric motors. These main pumping units require a considerable amount
of electrical energy for brief periods of time and at infrequent intervals. If electrical
current is purchased for this purpose, it involves a large demand or service charge. If
current is furnished by the station, this results in a relatively heavy station demand,
necessitating installation of additional power capacity for generating equipment.
Power Demands. Keep the maximum power requirements as low as
practicable, consistent with the required capacity. Keep power demands as constant as
possible through the whole pumping head range. Although the initial cost of high-speed
standard motors is relatively low, their operating characteristics are not suitable for
driving dewatering pump units. Therefore, the design of the motor driven, direct
connected pumping unit for drydocks involves an adjustment and compromise of the
conditions of: varying hydraulic head, minimum range of required power, lower unit
speeds, and relatively good efficiency. These conditions often necessitate a sacrifice in
efficiency. High efficiency, however, is not of prime importance in equipment that is
operated so small a percentage of the time.
Components. The main dewatering system of a drydock usually
The suction inlet located within the dock chamber,
The suction passage and/or culvert,