By Jack Sine
The Jacksonville JEA water district in Florida had a problem. Its measurements showed that it was distributing significantly more water than it was removing from its wells.
"Clearly it was a metering problem," said Shawn Arnold, manager of the water acquisition and distribution arm of JEA (formerly the Jacksonville Electric Authority). "We did an investigation and discovered that the meters at 35 of our wellheads were under-reporting the amount of water being extracted."
The Jacksonville water district is massive. Not only does it serve the most populous city in Florida, it also provides potable water to the surrounding counties of Clay, Nassau and St. Johns. It is the seventh largest publicly-owned water district in the country with 4,208 miles of water lines. As such, management must know precisely how much water it is removing from the Florida aquifer.
"The problem was the magnetic flow meters," said Shannon Jordan, water operator maintainer for the utility. "The probes of the meter come directly in contact with the water being pumped from the well. Out of our 135 artesian wells, 35 of them pump water that is high in minerals such as calcium, iron and manganese. The problem is that the minerals collect on the probes degrading the signal and reporting lower than actual flows."
The cleaning itself was an expensive problem in terms of labor.
"You have to remove the entire meter, clean it with a light acid solution and reinstall it," said Williams. "It can take up to half a day. And with those 35 meters, we had to clean them several times within the three year period to maintain the required accuracy."
Enter John Van Nostrand, who had worked with JEA on flow control issues for more than 10 years. As a rep for a few meter manufacturers, he had helped them solve some knotty problems. Now he was South East Regional Sales Manager for Flexim Americas, a manufacturer of ultrasonic flow meters.
"When John showed up and suggested ultrasonics, we weren't immediately excited," said Arnold. "We already had a portable clamp on ultrasonic that everyone hated. We needed it to calibrate the magmeters, but it took a long time to install. You had to be very precise, zero it out, and then there's a long list of procedures we would have to do to calibrate the magmeters. Every thing about these meters was time intensive."
Van Norstrand was prepared. He brought in one of his portable meters for them to try.
"We have a test bench and we installed the Flexim meter and tested against mag meters we had just calibrated," said Arnold. "The accuracy was excellent. Then John showed us how to install the meter on a pipe and it was remarkably fast and easy. Being cautious, we only ordered one portable meter to start and used it to check and calibrate the mag meters."
One of the benefits of ultrasonic flow meters is that, unlike traditional meters, they contain no moving parts and do not need frequent calibration and maintenance. Measurements are made using the transit-time difference method. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium. An ultrasonic signal moves slower against the flow direction of the medium and faster when it is in the flow direction.
For the measurement, two ultrasonic pulses are sent through the medium, one in the flow direction and the second against it. The meter's transducers work alternately as emitter and receiver. The transit time of the signal sent in the flow direction is shorter than that of the signal sent against the flow. The meter measures the transit time difference and calculates the average flow velocity. Since the ultrasound signals propagate in solids, the meter can be mounted directly onto the pipe non-invasively.
Flexim ultrasonic flow meters are not affected by density, which make them ideal for multiple applications from slurries to gas measurements. They automatically compensate for variations in viscosity.
"We already knew of the meter's accuracy from our bench tests," said Williams. "Now, using our first one for calibration of the mag meters, we experienced how easy it was to install and use. We made the decision, as mag meters went out of calibration, to replace them with ultrasonic meters.
"The first ones to be replaced were the 35 'trouble' wells with the high mineral content. We calculate that avoiding three cleanings of a mag meter will pay for the Flexim ultrasonic that replaces it," he said. "One of the other great features of the ultrasonic flow meters is that one meter can be used on any of our pipes from 4 inch to 48 inch. All we have to do is change transducers. That's a huge improvement over the mag meters that started at a high price and went higher as the pipe size increased. The ultrasonics are one-size-fits-all and are very affordable. And since they are not directly exposed to the flow, we don't have to worry about accuracy degradation."
About the Author: Jack Sine is a freelance writer specializing in the HVAC marketplace. He can be reached at [email protected] or at 845-831-6578.
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