Flow Measurement at Pipes Leading to Combined Sewers

Flow Measurement at Pipes Leading to Combined Sewers

The wastewater treatment plant on the northeast side of Cleveland between Lakeshore Boulevard and Lake Erie provides wastewater treatment services for more than 300,000 residents and businesses. It also receives and treats storm water from combined sewers. An average 94 million gallons of wastewater per day are treated here. Because of the number of combined sewers in the service area, the plant needed to add capacity and overflow storage. The plant's management decided to also address a need that was going to grow in importance in the near future - accurate and reliable flow metering.

A portion of the enlargement program includes two storage tunnels 24 feet in diameter, two miles long, and 200 feet underground. Further, the plant’s secondary treatment capacity shall increase to 400 million gallons a day. When large storms occur, portions of water and sewage will be diverted to the storage tunnels and held until after the rain stops. When the rain stops, the stored combined sewage will be pumped to the plant for full treatment.

The existing 48” piping in the plant featured venturi (Dall) flow meters, but these had led to ongoing problems as they are intrusive and the differential pressure lines are directly exposed to the wastewater. Due to the solids in the wastewater, the meters would frequently become clogged. Even clean water purges didn’t keep the impulse lines clean so they had to be cleaned manually. 
Back then metering wasn’t critical to plant function. But now, as the plant ramp's up its efficiency, it has to use accurate metering technology to control flow, which is critical to the efficiency.

The plant demanded meters that would be easy to install and maintain. Several flow meter manufacturers where asked for demonstration onsite. Most did not perform very well. One couldn’t even get a signal. Fortunately, there was one meter that that had a strong signal and a good signal-to-noise ratio - a non-invasive FLUXUS ultrasonic flow meter from FLEXIM. 
One of the problems at the measurement points were the absence of long straight pipe runs which most flow meters need for accuracy. But the FLEXIM engineers had a solution for that. They put a dual channel flow meter with two pairs of sensors on the same line and the meter averages the channels. The multiple beam approach maintains a high accuracy with limited straight pipe. It resulted in a much more consistent, repeatable measurement than the old venturi meters.

Unlike the venturi meters, the FLUXUS ultrasonic clamp-on flow meters are not exposed to the flow. This has the added benefit that the plant operators did not have to shut down the line for the meter installation and there is no maintenance required. The twelve installed FLUXUS flow meters have been functioning trouble free for three years now and there are plans in the works to install an additional 50 ultrasonic meters.