At Work on Flow Metering Devices

Oct. 21, 2003
City's Large Flows Easily Monitored and Analyzed

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.

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