Customized flow meters go where others cannot tread

Aug. 31, 2007 -- When MWH needed to specify and install flow metering equipment for United Utilities without taking a key water treatment plant offline, customized AquaProbes from ABB provided the answer. As part of a new high rate Rapid Gravity Filter system, the AquaProbes overcame the problem of a lack of straight piping needed for conventional filtrate flow meters.

United Utilities' Lostock Water Treatment Works treats water to remove cryptosporidium and is a major supplier of drinking water to Manchester. Cryptosporidium is a parasite found in drinking water and must be removed as it can cause an unpleasant reaction in humans if ingested.

Cryptosporidium oosysts resist conventional disinfection, so the approach used at Lostock is physical removal by filtration. The flow through the filters decreases as the filters clog. Filters are backwashed periodically to remove the solids they have captured. To optimize the cryptosporidium removal process, accurate flow information is required to ensure correct operation of the filters. A requirement is not to alter the flow through the filters by more than 1.5% per minute, so accurate flow metering is crucial.

ABB was approached by the engineers from MWH to provide a flow metering system that would enable them to measure the flow accurately, yet allow instrument removal without having to shut down.

The chosen measurement points were eight, 450mm pipes in the filtered water channel, discharging into 700mm outlets. ABB AquaProbes were chosen because full bore meters could not be installed and maintained there.

Each probe comprises an electromagnetic sensing head on the end of a support rod. However, the maximum standard length of an AquaProbe is only 1 meter and this application called for 3-meter probes, which ABB built specially. Around 225mm of each probe lies inside each tube, with the remaining section protruding into the surrounding inspection area. The tubes and their support brackets are made from stainless steel 316 to avoid any chance of contamination and the associated transmitters are mounted separately on poles.

In addition, ABB usually recommends straight pipe runs of 50 diameters for most AquaProbe installations and only 12 diameters were available in this case. Even so, the made-to-measure AquaProbes have all been performing within the required tolerance since they were installed.

"We have been very pleased with the results at Lostock and have never looked back," says Fred Ehrlich, the senior process engineer of MWH Ltd. "I am very glad that we pushed for the AquaProbes for the project."

ABB is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact. The ABB Group of companies operates in around 100 countries and employs about 111,000 people.

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