The Price Is Right

Magnetic flowmeters aid accurate billing for customers of wastewater treatment facilities

The City of Orlando in Florida treats up to 40 million gallons a day (mgd) of wastewater at its Iron Bridge Regional Water Reclamation Facility. The city bills one major upstream source based on the amount of wastewater treated. Both parties want accurate flow measurements to ensure a fair assessment of costs. A few percentage points of error can quickly add up to thousands of dollars in questionable billings.

The major customer is the South Seminole & North Orange County Wastewater Transmission Authority, referred to as the "North Authority." The North Authority serves about 150,000 people, and, on average, accounts for about 25 percent of the wastewater treated at the Iron Bridge plant. Water from the North Authority comes to the plant through one force main.

Determining Billing Rates
The North Authority sends monthly reports of totalized flow rates to the City of Orlando's Wastewater Billing Group. These reports contain subtotals derived from the readings of 25 flowmeters that measure wastewater coming from area subdivisions. The city compares the sum of the subtotals with the monthly totalized flow measured by the "master" meter on the North Authority's input main. By mutual consent, the sum of the many subdivision flows should agree within five percent with the total measured by the master meter at the main.

Two master magnetic flowmeters, installed in parallel, monitor the wastewater input main from the North Authority. The master meters reside in an underground vault, accessible via a trap door. One meter operates while the other stands by. Manually operated valves in the underground vault, upstream from the meters, determine which meter is active.

Each meter sensor connects by cable to its respective transmitter, mounted on the metering station panelboard. A circular chart recorder above each transmitter plots wastewater flow rate on a seven-day chart and continuously totalizes the flow.

Until a radio telemetry system was installed (see Telemetry System Uses Internet Technology), staff members drove to the metering station twice per week, about two and one half miles from the plant, to obtain totalized flow readings. They changed the active meter's seven-day chart once a week. Back at the plant, the totalized flow readings were e-mailed to the North Authority as interim reports.

Installation of the radio telemetry system now provides automatic digital transmission of instantaneous flow and totalized flow readings every 30 seconds to the Iron Bridge facility. The telemetry connects to a computer that logs the information and automatically sends out e-mails to the North Authority. This not only cut travel time in half, but also took over the task of e-mailing the information. What's more, the readings are more consistent because they occur at exactly midnight each day.

Both the Authority and the City of Orlando benefit from automatically receiving this daily data. It gives both parties a more current picture of flow conditions and provides valuable information regarding any unusual patterns of wastewater input flow.

Selecting the Flowmeters
For sustainable high accuracy and performance, the authority chose MagMaster magnetic flowmeters in a 24-inch diameter size from ABB Instrumentation. These meters can measure wastewater flows up to 64.45 mgd. Daily flow rates from the North Authority vary over a wide range -- typically from 1 to 22 mgd. These flowrates are well within the range of the selected magmeters for accurate flow measurement.

A qualified field service engineer annually visits the site to check the calibrated accuracy of the master meters. The service engineer uses portable equipment that enables him to verify and re-certify the factory-calibrated accuracy (1 percent of flow rate). He can do this easily and quickly in situ -- at the metering station while the meters remain installed on the main. Most other meter makes would have to be removed from service and shipped back to the factory for checking and calibration, which would be costly and impractical for meters of this size. The North Authority respects this "third party" checking method as fair and accurate.

Aside from this capability for inline checking of calibration and proper operation, the magmeter offers a several other advantages. It has an unusually wide rangeability, providing high measuring accuracy at low and high flow rates. Unlike other types of meters, it offers no obstructions to the flow stream, resulting in zero head loss. Additionally, its measurements are unaffected by changes in wastewater viscosity or density.

Checking the Accuracy of Meters Inline
The field service engineer uses portable electronics, trade-named the CalMasterTM system, to verify and re-certify accuracy of the two master meters. The system consists of a portable personal computer (PC) and a control box. The PC contains the necessary software to carry out a pre-programmed verification procedure. The service engineer connects the control box to the panel-mounted magmeter transmitter to test the meter. Each transmitter remains cable-connected to its sensor located in the vault.

To conduct the test, the service engineer places the PC and the control box at convenient locations near the panel-mounted transmitters. After making the system connections, he or she starts the test. In a few minutes, the system carries out a complex verification procedure that checks out the entire magmeter system -- transmitter, sensor, and interconnecting cable.

To confirm that test results meet approved standards, the service engineer reviews them on the PC's display. Upon seeing positive results, he or she prints out a certificate of validation for the tested meter.

The system can also serve as a diagnostic tool for preventive maintenance on the master meters. It stores all previous validation data for each tested meter and, on its PC screen, can plot trend curves of data for selected components. If a curve shows gradual deterioration of the component, the affected meter can be repaired or replaced in-line while the stand-by meter takes over active measurement.

Information for this article was supplied by Roy A. Pelletier, assistant division manager, and Andrew J Harbison, instrumentation chief, Environmental Services Wastewater Division, City of Orlando.

This article originally appeared in the 05/01/2005 issue of Environmental Protection.

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