Going with the Flow
It is common for many water utilities to add 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, regulating agencies may require water utilities to report on the amount of fluoride added to the treated water. As a result of such regulations, the Appomattox River Water Authority (ARWA) in Petersburg, Va., recently installed an electromagnetic flowmeter in their fluoride feed line to measure instantaneous and total flow.
After a short period of time, the ARWA began to experience two major problems with the magmeters.
First, due to the low feed rates (170 to 390 ml/min), the magmeter required a small 0.118 in. orifice to obtain a sufficient flow velocity. This small opening was prone to clogging. As the magmeter began to clog, the meter’s accuracy decreased. Electrode fouling and eventual total clogging of the meter then caused the meter to stop operating. Consequently, maintenance time and meter downtimes were deemed excessive.
The second, and more critical problem, was that the magmeter’s electrodes would fail due to the corrosive nature of fluoride. This situation created two difficulties. The failed electrodes required replacement, increasing maintenance cost and downtime. Even worse, the flowmeter’s analog output would drive downscale to 4 mA DC.
Because the meter was utilized as the primary element in a feedback control loop, the 4 mA DC output signal indicated that the fluoride flowrate was below the established set point.
As a result, the controller’s output to the fluoride feed pump increased the pump’s stroke length. This led to an increase in the amount of fluoride being added. Due to the electrode failure, the magmeter did not respond to the increased fluoride flow and ultimately caused the controller to drive the pump to full scale creating a dangerous overfeed situation. When this situation occurred, the utility was forced to dump its four million gallon clearwell.
The utility decided that the magmeter had to be replaced to minimize downtime and, more importantly, to avoid the possibility of further over-fluoride conditions.
The ARWA began a research project to determine the best possible solution considering the difficult combination of very low flow rates and a highly corrosive liquid. The utility considered a number of different flowmeter designs in the evaluation process.
Their search led them to evaluate a unique flowmeter manufactured by Controlotron, a firm primarily known for clamp-on transit-time and Doppler flowmeters. The company provided a System 1020FT inline CPVC transit-time flow tube flowmeter for evaluation. The 1020FT was designed specifically for chemical feed applications both in chemical compatibility and the ability to handle flow rates as low as 1 gph.
Installation produces results
Upon installation of the 1020FT flowmeter, the ARWA began to measure the meter’s accuracy performance based on batch sample runs. The initial evaluation indicated an accuracy of approximately 1% of rate over the entire flow range.
Additional performance testing was performed over time to evaluate the meter’s ability to avoid the two primary problems associated with the original magmeter, clogging and corrosion.
This lengthy evaluation indicated that the Controlotron 1020FT did not suffer the negative effects that so adversely limited the performance of the magmeter.
The 1020FT flowmeter’s ability to handle the difficult application conditions was the result of two specific design features.
First, the 3/8 in. bore diameter of the flow tube avoided the small restriction required by the magmeter and was therefore able to avoid the conditions that caused the clogging. Even with the large 3/8 in. bore, the 1020FT meter was able to provide the required accuracy at the low flow velocities.
Secondly, the transducers used to make the flow measurement were non-wetted and embedded in the flow tube walls. The wetted surfaces were entirely of CPVC that provided excellent corrosion resistance to the fluoride at operating temperature.
After completion of the evaluation process, the ARWA decided to replace the existing magmeter with the Controlotron System 1020FT flowmeter. Since the time of installation, the 1020FT system has maintained consistent performance within the required accuracy and without the downtime and over-fluoride conditions the ARWA experienced with the originally installed magmeter.
As a result, the ARWA has installed additional 1020FT systems on polymer and alum feed lines with comparable performance.