interior walls to minimize head loss. Transparent polyvinyl chloride pipe was
used at the pump discharge heads and immediately upstream of the flow sensors
as observation windows to determine if air or sediment is being pumped through
the piping system. This may be monitored because the pumps are designed to
withstand only small quantities of sand passing though the system and because
the flow sensors, discussed in the next section, are somewhat sensitive to air and
sediment entrainment.
Flow measurement system
Midway along each pump-and-piping system, the main pipeline branches out
into two smaller pipelines (total of 40) running parallel for approximately 4.0 m
before converging back into the main pipelines. These smaller pipelines are the
in-line flow measurement sections and each contains a transparent section of
polyvinyl pipe used for monitoring of air and sediment entrainment, a true union
ball valve, which is either completely closed or completely open, and an
impeller-type flow sensor. Systems No. 1, 2, and 3 have 38- and 75-mm-diam
flow measurement sections, whereas System No. 4 through 20 have 50- and
100-mm-diam flow measurement sections. Figure B-7, Appendix B, shows one
of the parallel flow measurement systems prior to installation into the longshore
current recirculation system. There are a total of 20 of these systems, one for
each pump-and-piping system. For high-flow conditions in a given system, the
valve in the smaller pipe is closed and the flow rate is measured using the flow
sensor in the larger of the two parallel pipes. In contrast, for low flow conditions,
the ball valve in the larger pipe is closed and the flow rate is measured using the
flow sensor in the smaller pipe. None of the valves are used to throttle the flow
rate; they are either completely closed or fully open. The reasons for using two
parallel pipelines with individual flow sensors were two fold: (a) to increase the
water velocity at the flow sensor during low-flow conditions, to minimize
measurement error; and (b) to increase the range of discharge rates over which
the vertical turbine pumps will operate (Section entitled "Hydraulic
Characteristic Curves" in Chapter 3).
Each of the flow sensors has a six-blade impeller with a nonmagnetic sensing
mechanism. Figure B-8, Appendix B, shows the components of a 50- and
100-mm-diam flow sensor prior to assembly. The forward-swept design coupled
with the absence of magnetic drag provides improved accuracy and repeatability
at lower flow rates. The manufacturer states the following specifications:
calibration range 0.3 m/sec to 9.1 m/sec; accuracy +1 percent of full scale;
repeatability +0.5 percent; and linearity +0.5 percent. The frequency of the
output signal, a low impedance 8-volt d-c square wave, is proportional to the
magnitude of the flow rate through the pipe and is transformed into a digital
signal and transmitted to a personal computer using RS-485 protocol. During
operation of the facility, the measured flow rate can be adjusted to match the
target flow rate by changing the speed of the vertical turbine pump. For further
information on the flow sensors, see Data Industrial (1994).
29
Chapter 3
Longshore Current Recirculation System
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