Ultrasonic choice for Flowline

A flow measurement project on a Nottinghamshire river was compounded by potential pollution problems at a wildlife sanctuary. Flowline was called in to provide a metering station.

Minimising the risk of river pollution and protecting a site of special scientific interest was one of the key objectives of a river flow measurement project involving flow monitoring specialists Flowline.

The location of the project was on the River Erewash, a tributary of the River Trent at the Attenborough Lakes Nature Reserve in Nottingham, and at a point where the river is around 10m wide and a depth ranging from 0.5-2m.

The site owners, Cemex, had to overcome the problems of various sewage works and other sites discharging into the river and causing contamination. The critical point was where the river passed through the site of a former aggregate quarry shortly before meeting the river Trent, an area designated of special scientific interest and home to a wildlife sanctuary.

A key requirement was to move the discharge point of the river to ensure it would not continue to pollute the nature reserve. After considering a number of flow measurement options, Cemex asked Flowline to provide a metering station which would continuously measure the river flows to provide data for use in calibrating a hydraulic model of the Erewash and Attenborough lake system.

Another important consideration was that the site was open and accessible to the public and incidents of vandalism had occurred, so any equipment installed had to be robust, without being too intrusive.

Following an initial survey and taking all the dynamics of the site into account, Flowline supplied a four-path ultrasonic flowmeter incorporating Ultraflux Time of Flight sound pulse measurement device with GSM telemetry.

The complete supply package included initial site survey, selection of suitable metering point, installation of the system including support columns and cable runs and finally, commissioning the system. The application of the Ultraflux ultrasonic measurement principle at this site involved the use of a number of ultrasonic paths located at about 45º to the river axis. The requirement for real-time monitoring and continuous recording of flows was satisfied by the use of a GSM modem, which was connected directly to the flowmeter control unit. This ensured that flow could be read at any time and also meant that the built-in data logger could be downloaded when required.

This measurement principle and its configuration at the River Erewash site provided accurate flow measurement data, despite the characteristics of the river, for example significant variations in velocity and turbidity.

These factors often render other types of sensing technology ineffective as they would have been unable to provide accurate data such as accurate average velocity at different depths and widths of the river at the critical point.

The installation at the River Erewash site involves ultrasonic sound pulses being sent alternately between probes mounted on opposite sides of the channel / river. The pulses that travel with the flow are speeded up, while the pulses that travel against it are slowed down. The time between each pulse being transmitted and received is measured accurately, and the greater the time difference between transmission and reception, the higher the flow velocity.

Chris Pointer, senior hydrogeologist within the Geological Services Department at Cemex UK, said: "The Flowline system has enabled hydraulic modelling work that ultimately led to the granting of planning permission for an extension to our sand and gravel operations at the Attenborough Quarry.

"In the past, we have used Ultrasonic Doppler flow meters, and these gave very noisy data due to low suspended solids content, so the data was virtually unusable and we also experienced difficulties with a fixed weir gauge.

"Overall, we are pleased with the system, which has been operating successfully, the problems associated with the Doppler flow meter have not occurred with the Flowline Ultraflux system. Also, we particularly appreciate the instantaneous direct flow measurement data without the need to use a rating curve."

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