Flow channel and intake system
As shown previously in Figure 1, flow channels are used at the upstream end
of the facility to guide water from the discharge pipes to the upstream boundary
of the beach. Likewise, on the downstream end of the facility, flow channels
guide the longshore current from the downstream boundary of the beach to the
vertical turbine pumps. Each of the flow channels is 0.75 m wide. The two
sidewalls of the channels consist of fabricated aluminum flow guides. A
continuous neoprene gasket is used to seal the interface between the flow guides
and the reinforced concrete floor and sidewalls of the facility, so that each flow
channel is independent and water tight. To ensure maximum flexibility of the
facility, the recirculation system has been designed with the capability to conduct
experiments with water levels ranging from 0.5 to 1.0 m. As a result, 20 sumps
were constructed to ensure vortex free operation of the vertical turbine pumps at
maximum discharge, and hence maximum drawdown, with a minimum operating
water level of 0.5 m. Each sump is constructed of reinforced concrete and is 0.75
m wide, 1.5 m long, and 1.2 m deep (Figure B-6, Appendix B). Hence, design of
the intake system to each pump consists of a straight open flow channel with a
sump at the downstream end of the channel. Turns and obstructions in the intake
system were minimized to avoid the possibility of eddy currents causing
submerged vortices under high flow conditions.
Hydraulic Characteristic Curves
By the use of variable speed motors, the discharge of each pump can be
varied to suit the recirculation requirements for a given test condition. Figure 18
shows the performance curve for Pump No. 10 operating at speeds of 540, 1,080,
1,440, and 1,800 rpm, which correspond to 30, 60, 80 and 100 percent of the
nominal operating speed of 1,800 rpm. The manufacturer supplied the
performance curve for a pump speed of 1,800 rpm. The performance curves
representing slower pump speeds were calculated using the standard homologous
pump equations.
Figure 18 also shows two characteristic curves for Pump No. 10 piping
system. These two curves represent head loss through the 70-m-long, 200-mm-
diam main pipeline combined with the head loss through the 5-m-long flow
measurement section of 100- and 50-mm-diam (FS = Flow Sensor in legend of
figure). The intersections of the pump performance curves with the two
characteristic pipe curves define the range of operating conditions of the
combined pump-and-pipe system. The following technique is used to meet the
design requirement of a pumping range between 10 and 100 percent of the
maximum discharge of each pump. Point No. 1 in the figure, shows that the
maximum discharge through System No. 10 is approximately 75 ℓ/sec. The pump
speed then can be reduced to 30 percent (540 rpm) of the nominal pump speed of
1,800 rpm depicted by Point No. 2. However, if the pump speed is reduced to
less than approximately 30 percent of the nominal speed, the pump discharge will
plummet to zero because of the inadequate centrifugal and axial forces at the
impeller. Therefore, to obtain lower discharge rates, the valve in the larger pipe
is closed and the flow rate is measured using the flow sensor in the smaller pipe.
However, this increases the head loss in the piping system. To overcome this
additional head loss, the pump speed is increased to approximately 1,680 rpm
30
Chapter 3
Longshore Current Recirculation System
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