Appomattox River Water Authority (ARWA), like many water agencies and districts, adds fluoride to potable water for its cavity prevention benefits. However, excessive amounts of fluoride in drinking water can have serious consequences. Because of fluoride’s toxic nature, ARWA is required to report to the Virginia State Health Department on the amount of fluoride added to its water.
ARWA initially installed an electromagnetic flowmeter on its fluoride feed line to measure instantaneous and total flow. The meter experiences problems with clogging because of the small 0.118 inch orifice needed to measure the feed rates of 170 to 390 ml/min).
The corrosive nature of the fluoride also tended to cause the magmeter’s electrodes to corrode. When the electrodes failed, the flowmeter’s analog output would drive down scale to 4 mA DC. Since the meter was used as the primary element in a feedback control loop, the 4 mA DC output signal indicated that the fluoride flow rate was below the established set point, and led to an overfeed situation. When this situation occurred, it forced ARWA to dump its 4 million gallon clearwell.
ARWA researched other flowmeter designs and selected the Controlotron System 990DFT in-line CPVC transit-time flow tube flowmeter for evaluation. The 990DFT is designed for chemical feed applications both in chemical capability and the ability to handle flow rates as low as 1 gph.
During initial testing, ARWA measured the meter’s accuracy based on batch sample runs. The evaluation indicated an accuracy of approximately 1 percent of rate over the entire flow range. Further testing over time confirmed the meter’s performance. Other testing was performed simply to evaluate the meter’s ability to avoid clogging and corrosion.
The DFT flowmeter’s ability to handle the difficult application was the result of two specific design features. The 3/8-inch bore diameter of the flow tube avoided the small restriction required by the magmeter and was therefore able to avoid clogging. Even with the large bore, the DFT meter was able to provide the required performance at the low flow velocities.
In addition, the transducers used to make the flow were non-wetted and embedded in the flow tube walls. The wetted surfaces were entirely of CPVC, which provided excellent corrosion resistance to the aggressive fluoride at operating temperature
After completing the evaluation, ARWA elected to replace its existing equipment with the Controlotron System 990DFT flowmeter. The installation was completed in early 1996. Since that time, the system has maintained consistent performance within the required accuracy range and without the downtime and over-fluoride conditions ARWA experienced with the originally installed meter.
ARWA has since installed additional DFT systems on polymer and alum feed lines with comparable performance.
