Since 1990, Los Angeles's wastewater collection system
flow monitoring program has been contracted to a company that manufactures and
maintains flowmeters and provides flow reporting services, also known as a
turn-key package. Frequent site visits requiring confined space entry for
maintenance due to fouled submerged sensors and increasing monthly fees for
data reporting and proprietary software resulted in the city making a decision
to look at alternative methods for collecting and reporting basin flows.
Marsh-McBirney proposed a new generation of flow monitoring
equipment coupled with easy-to-use reporting software that would allow the
city's Engineering group to analyze the flow data itself. Encouraged by
the idea of a non-contact sensor, Gary Whitney, staff supervisor for the
Wastewater Collection Systems group, felt this could be the solution the city
had been seeking.
"Continuous flowmeter maintenance costs are what
really should be evaluated when selecting a flowmeter," Whitney said.
"Tens of thousands of dollars in maintenance costs can quickly
accumulate, making the initial cost of the meter itself inconsequential. The
use of proprietary software and associated monthly fees offered by flowmeter
manufacturers providing turn-key services also should be closely
examined."
The Flo-Dar open channel flowmeter measures from above the
flow stream, eliminating confined space entry concerns and maintenance caused
by fouled submerged sensors. The meter transmits a digital Doppler radar beam
that interacts with the fluid surface and reflects back signals at a different
frequency than that which was transmitted. The return frequency is compared
with the transmitted frequency and the resulting frequency shift provides an
accurate measure of the velocity and the direction of the flow. Depth is
detected by an ultrasonic pulse echo sensor. Flow then is calculated.
Based on the outstanding performance of the flowmeter in a
comparison study, a unanimous staff decision was made to purchase 75
intrinsically safe Flo-Dar Model 460 (portable-battery powered) flowmeters for
the city's "near-time" flow applications. The near-time
applications required portable, battery powered units that would be installed
in selected basin locations. Staff members would visit the sites on a predetermined
basis to download collected flow data that would in turn be forwarded to the
Engineering group.
The second phase of the city's project was to replace
the 35 original "real time" flowmeters provided by the manufacturer
originally contracted to provide the turn-key flow monitoring
equipment/services package. Marsh-McBirney again was selected to provide 35
Flo-Dar Model 464 flowmeters for this project. The flowmeters would provide
real-time data through a programmable logic controller (PLC).
According to Whitney, the goal of this phase of the project
is to have an interactive graphic in the collection systems facility that will
provide instantaneous flow readouts of each monitoring location with the push
of a button.
