Mag meter measures part-full water discharge flows

When the Wybersley water treatment works, near Stockport, needed to meet North West Water’s quality requirements, it had an interesting problem. It needed to measure flow in the works’ sewer discharge pipe – an underground pipe which does not run full. Wybersley provides up to 70Ml/day of potable water, and waste water is discharged to […]

When the Wybersley water treatment works, near Stockport, needed to meet North West Water’s quality requirements, it had an interesting problem. It needed to measure flow in the works’ sewer discharge pipe – an underground pipe which does not run full.

Wybersley provides up to 70Ml/day of potable water, and waste water is discharged to the Hazel Grove waste water treatment works via this pipe.

As the facility was operating close to its limits, the Trade Effluent Officer required that Wybersley engineers install a monitoring system on the pipe, before Consent to Discharge could be given. The system had to provide continuous flow measurement to show that rates were within limits, and to give early warning of increases.

The option of constructing a flume in the pipe, or re-engineering it so that a full-bore measurement could be made was considered. But, it was rejected, because of the physical constraints and cost. There was also concern that such a structure could cause blocking or backflow.

Meanwhile, North West’s engineers favoured the use of electromagnetic flow metering for its accuracy and reliability in applications where water quality and quantity can vary greatly, and where solids may be present.

The solution was a Top Flux electromagnetic flow meter, from KDG Mobrey – in fact the UK’s first installation – project engineered by Bechtel Water Technology. Top Flux uses a magnetic coil installed externally on a flanged pipe section, plus electrodes flush with the inside of the pipe walls, for velocity measurement – making it relatively easy to retrofit.

An ultrasonic MSP 90 level meter on top of the pipe monitors fluid level, and the two measurements combine to give flow.

Peter Thirkettle, from North West’s Wybersley works, said: `Our problem was an unusual one, created by increasing pressure on the waste water treatment works at Hazel Grove, and high standards of quality self-imposed by North West Water.

`Although the flowmeter was relatively expensive, I don’t know of any other acceptable technology, short of re-engineering the discharge pipe’, said Thirkettle.

{{KDG Mobrey,Tel: 01753 534646; Fax: 823589.353 on enquiry card}}