Table 1 lists the primary design specifications of each pump including:
discharge capacity at the best efficiency point; total dynamic discharge head;
discharge pipe diameter; impeller diameter, optimized to the best efficiency point
of each pump; hydraulic efficiency; specific speed; and the electric power
required to drive each pump at maximum discharge. The specific speed of all
four models of pumps ranged from 2,880 to 4,340. Hence, each of the pumps
can be classified as mixed-flow pumps, which generate discharge head through a
combination of centrifugal and axial forces. The electric motors are inverter duty
motors manufactured using special inverter grade insulation so that the motors do
not overheat during low-speed operation. Pump No. 1 is located at the onshore
end and Pump No. 20 is at the offshore end of the recirculation system. For
additional information on the drive motors for the vertical turbine pumps, see
U.S. Motors (1996).
Table 1
Vertical Turbine Pump and Motor Specifications
Total
Discharge
Optimum
Hydraulic
Pump
Discharge
Dynamic
Pipe
Impeller
Efficiency
Required
Location
Capacity
head
Diameter
Diameter
1,800 RPM
Power
Specific
No.
ℓ/sec
m
mm
mm
%
kW
Speed
1-3
37.8
9.8
150
149
79
2,880
4.6
4, 5
50.5
12.2
150
176
81
2,580
7.4
6, 18-20
63.1
8.8
200
149
79
4,340
6.9
7-17
75.7
11.9
200
178
80
3,930
11.0
Variable speed motor controllers
To meet the design requirement that each pump-and-piping system should
have the capability to recirculate between 10 and 100 percent of the maximum
design discharge, variable speed motors were used to drive the pumps. It was
concluded that throttling valves, weirs, or other mechanical control mechanisms
are too labor intensive to operate because there are 20 independent pump-and-
piping systems. Variable speed motors were the most desirable solution for two
primary reasons: (a) there is a linear relationship between flow rate and pump
speed giving accurate control of discharge over a wide range of flow rates; and
(b) pump discharge rates could be remotely controlled and adjusted with a
minimal amount of time and effort. As a result, 20 pulse-width-modulated,
variable-frequency, alternating-current motor controllers were used to control the
speed of the pumps. Eleven of the motor controllers are 11.25-kW systems and
nine are 7.5-kW systems. The output frequency of the motor controllers, and
hence the speed of the pumps, can be manually adjusted using a human interface
key pad, with digital display, mounted on the front panel of each controller
(Figure 17). Electrical power is provided by a 400-ampere, 480-volt, 3-phase,
60-Hz, electrical service.
27
Chapter 3
Longshore Current Recirculation System
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