Magmeters keep wastewater flowing precisely

Flow calibration is the key to accuracy and reliability of electromagnetic meters.

The Wells Sanitary District wastewater treatment facility depends on electromagnetic meters from Krohne, Inc. for three sludge treatment applications in its newly refurbished plant, located on the southern Maine coast. The magmeters' accuracy and reliability depend on the painstaking calibration during the manufacturing process and recalibration on a regular basis.

The Wells Sanitary District provides treatment for up to two million gallons of wastewater per day. Originally constructed in the late 1970s, the treatment facility underwent a major refurbishment in 2001 to update the facility and to meet all safety standards required by the US Environmental Protection Agency (EPA) and Maine's Department of Environmental Protection.

The district contracted the US consulting engineering firm Wright-Pierce to redesign many of the facility's systems and processes. Project engineers replaced all the electromagnetic flowmeters that measure the flow of waste throughout the facility. According to Dennis Thayer, superintendent of the facility, "The existing meters had seen better days. They were worn out and just needed to be replaced. Many people don't realise that these meters operate 24/7, under demanding conditions, so they need to offer superior performance and reliability." Wright-Pierce recommended electromagnetic meters from Krohne, Inc. of Peabody, Massachusetts, for the newly designed facility.

These magmeters are being used for sludge recirculation, keeping track of total gallons of treated effluent and sludge wasting. In the sludge recirculation area, two of the meters measure the sludge recirculation rates in gallons per minute to measure the number of gallons per day. This is part of the facility's in-line, waste-activated sludge process. Another meter is used in the wasting sludge area to measure the flow of sludge that is removed from the system. A fourth meter is used in the treated effluent area that measures the flow of all the treated wastewater that moves throughout the facility.

"The use of accurate and reliable meters in this environment cannot be overstated," said Thayer. "Flowmeters are essential to the overall process here and at any treatment facility for that matter.

The accuracy and reliability of Krohne's magmeters is no accident. It is, at least in part, the product of painstaking calibration during and following the manufacturing process. Every magnetic-inductive and ultrasound flowmeter from the company has a calibration certificate. The company relies on a calibration rig that is not only the world's largest and most accurate flow calibration rig, it is accurate up to 40,000 m3/h with measurement uncertainty less than 0.013%. Nederlands Meetinstituut B.V. (NMI), the Dutch calibration authority, has certified that the volume serving as the testing standard for this large-scale calibration rig is accurate to within ±0.013% when calibrating volume-flowmeters in the range from 18 to 40,000 m3/h. This flow rate would be sufficient to fill a normal indoor swimming pool in less than one minute.

This test rig permits the precise calibration of flowmeters up to120" in diameter - big enough for a small goods vehicle to pass through. The stainless steel calibration tower is 42 metres high and has a water volume of some 400 m3. High-precision level switches at various heights mark a series of partial volumes; NMI precisely calibrated these partial volumes and total volume.

The test rig itself is certified by the Netherlands Calibration Organization (NCO) under EN 45001, ensuring that it conforms to both national and international standards. The computer automatically produces a calibration report documenting flow values and measurement errors for each calibration. Every flowmeter is wet-calibrated in a direct comparison with the volume calibrated by NMI. The calibration method is 10 times more precise than the flowmeter being tested. This guarantees that the specified error limits of these devices are achieved under reference conditions.

Recalibrating the meters on a regular basis is integral to helping to maintain accuracy. Krohne's MagCheck, a portable testing and verification device for Krohne electromagnetic flowmeters (EMF), enables plant personnel to verify accuracy of the magmeter's flowhead, converter and cables without removal from the pipeline or interruption of the process.

An ISO 9001-compliant portable device that uses available line power at the flow tube, MagCheck can be used with most Krohne flowmeter converters as either an automatic verification instrument or as a manual flow signal calibrator. MagCheck verifies flowmeter performance while creating an archived calibration record accurate within 1% of the original factory calibration. MagCheck is directly traceable to international standards.

Pete Williams of Sullivan Associates, a Krohne distributor, explained that the accuracy and reliability of MagCheck provides customers with a high confidence level.

"MagCheck creates a significant comfort factor," he said. "The calibration figures produced by MagCheck are certifiable and NIST-traceable [National Institute of Testing Standards]. Ultimately the device meets all government standards including the EPA.

Author's Note

Liesel Oliveira, was Krohne's product manager, based in Peabody, Massachusetts, USA, when she wrote this article. She has relocated to Florida as a technical sales representative.