Sound judgement

Most of OnSite's clients - the majority of UK water companies - are now using accoustic doppler flow monitors. And the technology is becoming increasingly popular to measure flow volume at WwTW inlets

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Anecdotal evidence suggests that Minworth WwTW is one of the largest in Europe. Quite what is meant by largest is open for debate. Does it mean size of plant, volume of final effluent, size of operation, or could it simply be gauged by the daily volumes at the inlet? If it is the latter, then Severn Trent Water now has this information to hand.

Thompson RPM, acting as consultants for Severn Trent, was investigating inlet flow and loading conditions at Minworth. The aim was to establish peak conditions in advance of significant capital investment planned for the works during AMP4, and to correlate the InfoWorks model already built by Severn Trent Water’s consultants. Severn Trent also wanted to compare the screenings production to the incoming flow. Thompson RPM commissioned OnSite to install two acoustic doppler flow monitors (ADFM) in each of the inlet channels in June 2005.

Conventional sewer flow monitors measure the velocity at only one position, and can be highly inaccurate, especially in turbulent conditions or in large-diameter pipelines. They are generally unsuitable for use near bends, weirs, or other discontinuities, and even then do not take into account boundary layer effects and variations in velocity throughout the cross-section.

The ADFM Velocity Profiler, manufactured by Teledyne MGD from California, overcomes these limitations by profiling the distribution of velocities throughout the entire flow area. It can produce accurate results in almost any location, including in conditions of high turbulence such as near flumes, weirs and overflows, and also in large-diameter pipelines or channels where there is often a significant variation in flow velocities throughout the cross-section. The ADFM Velocity Profiler comprises an array of four velocity transducers, plus an ultrasonic depth-measuring transducer, incorporated within a low-profile, easy-to-install housing.

The ADFM transducers emit a very short pulse along each of four narrow acoustic beams that are at an angle to each other. The return signals are divided into discrete intervals known as bins, and an independent velocity measurement is recorded for each of these bins. This produces a linear distribution of velocity values at different levels along the length of each beam, allowing the velocity profile throughout the entire flow area to be established. Integration of all the velocity and depth data allows the overall flow rate within the pipeline or channel to be determined with a high degree of accuracy.

For operational and health and safety reasons, it was important that there had been a significant period of dry weather before installation commenced at Minworth. Also, installation took place during early morning low flows, to allow an efficient flow diversion. Two penstocks controlled the larger of the two channels (channel one – 2,600mm x 4,080mm).

These were closed with flow being diverted into the smaller channel (channel two – 2,600mm x 2,400mm). Due to the good hydraulic conditions downstream of the inlet, the channel drained quickly (in less than 30 minutes). The first ADFM was then installed in channel one, with a total installation time of less than one hour. Upon completion, the two penstocks were reactivated and opened, with the flow re-established almost immediately. Flow into channel two was controlled by one penstock. Again, this was closed with flow being diverted into channel one to allow the second ADFM to be installed. Installation of the two monitors was quick and easy, with the total installation time (including penstock operation) being less than three hours.

Dominic Brett, the Severn Trent Water engineer charged with overseeing the project said: “ADFM technology was used on this project due to the package being self-contained – it needs no external power supply. Also, it can be retrofitted in the channel with minimum down time, and can measure flow in less than ideal hydraulic conditions.”

Severn Trent Water and Thompson RPM have subsequently commissioned ADFM surveys at Wanlip WwTW and Trent Vale SPS. OnSite has framework contracts with nearly all UK water companies for flow measurement, CCTV surveys, and various other asset data collection activities, nearly all of which have taken advantage of ADFM technology during the past three years.

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