PEABODY, MA, Dec. 12, 2005 -- Operating a municipal water treatment plant can be quite a headache if you cannot rely on the accuracy of the plant's flowmeters. Such erroneous measurements created a good deal of frustration at the Boothbay Region Water District on the coast of Maine. Precision is vital as the district services 3,000 customers year round as well as a greater population during the summer tourism season.
"We lacked confidence in our venturi flowmeters because our empirical readings did not match our theoretical calculations," says Jon Ziegra, the plant manager of the Boothbay Region Water District. "In addition, the measured volume of water flowing into the plant did not correlate with the measurements of the plant's output."
The inaccuracy of the existing flowmeters was primarily due to fouling caused by the high hardness and the high iron and organic content of the source water, as well as the floc generated during the treatment process. The mechanical components of the venturi flowmeters were prone to fouling, even with frequent maintenance.
Expensive overdosing
Consequently, ascertaining the correct dosing of chemicals was extremely difficult. The plant adds aluminum sulfate to incoming raw water for a coagulation/flocculation process that removes dissolved natural organic matter. Without accurate flowmetering, operators would overdose the alum to ensure that sufficient coagulation would occur to remove 99.99% of the total dissolved solids. A similar problem existed at the other end of the plant, where unreliable flowmetering made it difficult to correctly dose fluoride.
"Without accurate flowmeter measurements, we were injecting more alum and other chemicals than required, which meant that we had to pass on this added expense to our customers," said Ziegra.
Faulty metering also caused problems during the backwash of the sand filters that remove the floc created by the coagulation process. Accurate flowmetering is necessary to pace the flow during backwash to ensure that the filter settles down in the correct position, in order to maintain performance and extend filter life.
Accurate electromagnetic flowmeters
In 2003, the Boothbay Region Water District replaced the poorly-performing meters with Aquaflux electromagnetic flowmeters, a proven technology with over 10,000 installations worldwide, manufactured by Krohne.
"We chose the Aquaflux flowmeter because the accuracy and reliability of electromagnetic technology is well-established. And with no mechanical parts, the system is maintenance-free," said Ziegra. This absence of mechanical parts also means that there is no added pressure drop.
This flowmeter is a rugged, electromagnetic unit ideal for measuring the volumetric flow rate of electrically conductive liquids in water, wastewater and environmental engineering applications. It is an economical flowmeter, with a fully welded housing and a tight footprint. The modular KROHNE systems provide the flexibility to implement an optimal solution to meet the particular requirements of each application.
Providing extreme accuracy and durability in various applications, the Aquaflux meters offer superior linear quality and design, exhibiting high chemical, corrosion and abrasion resistance. Ranging in sizes from 1" to 120" (or DN 25 to DN 3000) with hard rubber liner, they comply with all NSF requirements for NSF/ANSI Certification Standard 61 - Drinking Water System Components - Health Effects, and is thus NSF approved for potable water service. Aquaflux meters are available with the 090 converter, now UL-approved for general purpose. The meter's modular design includes a fully welded housing and a tight footprint.
The Boothbay Region Water District selected the Krohne IFC 010, an economical electromagnetic flow converter, with low power consumption, that features digital signal processing and a high-speed signal conversion system at a favorable price-performance ratio. This signal converter has a measuring error of less than 0.5 % of the measured value and a self-monitoring feature to rule out errors in operation.
The plant also chose to equip the Aquaflux flowmeters with Hastelloy "C" electrodes and hard rubber liners that comply with all NSF requirements for NSF/ANSI Certification Standard 61 - Drinking Water System Components - Health Effects, and is thus NSF approved for potable water service.
The district has installed an 8" diameter Aquaflux flowmeter to measure incoming water and in other locations within the treatment operation a 12" diameter Aquaflux flowmeter to measure the backwash process and to serve as the master meter for the distribution system. Currently, the distribution system uses two 8" flowmeters, and a third will be installed next year.
Easy installation after minor retrofits
"The installation of the new Krohne flowmeters was easy, and the display and menus are user friendly," reports Ziegra. "The meters are extremely accurate and have required no maintenance. To achieve the high accuracy, we initially needed to address a few issues that required minor retrofits to the plants piping system."
Distortion of the readings of the sensitive electromagnetic meter caused by electrical interference resulting from chemical reactions of the chemicals used to adjust pH. "We had been injecting acid and base solutions ahead of the meter, but this electrical interference problem was completely resolved simply by moving the chemical injection port six inches downstream past the meter."
There was a minor issue with one of the two 12" meters used during the backwash operation. There was no problem with the meter positioned after a straight run of piping. But the other meter was installed after several twists and turns, and the readings drifted. The fix was easy and relatively inexpensive: a flow straightener was installed and both meters now perform equally well.
Precise dosing without waste
"Our operators are confident in our new flowmeters. The readings consistently match our theoretical calculations," according to Ziegra. "We are on an peninsula extending into the ocean, and our fresh water supply is quite limited. The state Department of Environmental Protection tightly regulates how much water we can draw from each of our reservoirs. With the new Krohne meters, we are confident that we are drawing no more, and no less, than our full allotment," says Ziegra.
"Even more importantly, we are able to dose chemicals with more precision, without wasting additional chemicals to compensate for unreliable flowmeters. We use six different chemicals at various stages of the process and, therefore, the cost savings are significant."
Not only does the reduced chemical consumption benefit the water district's customers by lowering the price, it also benefits the environment by reducing demand for chemical production and transportation.
With its headquarters in Germany and U.S. offices in Peabody, Mass., Krohne (www.krohne.com) is a worldwide technological leader in the development, manufacture and distribution of accurate, reliable and cost-effective, level and flow measurement instruments, forming partnerships with its customers to provide them with the most reliable and innovative solutions available in the marketplace.
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