15 August 2002
Design for Vortex Action. The system design should give such hydraulic
characteristics as will preclude vortex action at the suction inlet and at the suction bell.
Pump Discharge. The design of the pump discharge line is less critical
than that of the suction line. The discharge conduit surfaces should be smooth, and
sharp angles that tend to produce eddy currents must be avoided. The conduit should
be such that the streams from individual pumps converge in as near a parallel direction
as practicable. Changes in conduit sectional areas should be gradual and should not
produce fluctuating velocities.
Allowable Velocities in Discharge Works. At discharge of individual
pumps, use 6.1 m/s (20 ft/s) maximum. In cross section of combining discharge culvert,
use 4.27 m/s (14 ft/s) maximum.
. Head loss computations exclude losses through the
pumping unit. These are included in the manufacturer rating. . Total loss should not
exceed 3.05 to 3.66 m (10 to 12 ft.) Suction loss should be not more than 2 to 3 percent
of total loss.
Valves. Where the discharge terminations are submerged, both gate and
check valves are required. Where the discharge is above water, only a low resistance
discharge flap valve is required. Gate valves should be the outside stem and yoke type
suitable for throttling operation, and should be motor driven with push-button control.
Check valves should be the low resistance, horizontal, nonslam, or dashpot control
Pump Capacity. Base the capacity of the main dewatering pumps on the
desired dewatering time, volume to be removed, type of graving dock construction, and
characteristics of the hydraulic system. As the water level in the dock recedes, the
pump discharge volume falls off accordingly. The unit pump rating in gallons per minute
should be taken as the average discharge volume. This average is based on the pump
discharge when the hydrostatic suction head ranges from that of mean high water down
to the head existing when the water level in the dock is 0.6 m (2 ft) above the dock floor.
Pump Efficiency. Rate dewatering pumps in accordance with average
overall efficiency instead of efficiency at a fixed capacity rating. Average overall
efficiency is based on the range of head stated above, and is defined as the ratio of the
total work done to the total power input to the motor. The total work is determined by
multiplying the amount of water pumped in each interval between water level readings,
by the average head in the same interval.
Pumps. In the design and operation of pumping units, provision must be
made for certain relationships between the suction lift and/or head, discharge head,
capacity, and speed. This design factor is necessary to obtain rated capacity and
efficiency, and to avoid outage and high maintenance from vibration and cavitation.