Pelton Wheel meter helps with Dounreay decommissioning

18 February 2011

Customised Pelton Wheel meters have been supplied by Litre Meter for use in the latest stages of decommissioning at the former nuclear fuel fast reactor at Dounreay in Scotland.

Parts of the site contain intermediate level radioactive material which requires special controls to be in place to ensure safe decommissioning of the facilities. A Pelton wheel flow meter is being used to measure the flow of decontaminated water from one of the facilities. The radioactively contaminated water is drawn from the silo, which stores intermediate level waste. The water removal and clean up is part of the decommissioning process.

Some of the most hazardous radioactive waste from an abandoned fast reactor experiment has lain submerged in water for 50 years in the 65m deep vertical shaft beneath Dounreay.

The first phase of the clean-out involved isolating the shaft from the surrounding groundwater by building a ‘grout curtain’ in the rock up to 80m beneath the surface.

The next stage is the removal, treatment and storage of the waste. Waste retrieval, treatment and storage facilities will be used to decommission the shaft and the nearby wet silo, which succeeded the shaft in 1971 as a storage facility for the site's intermediate-level waste.

Stuart Tod, shaft and silo facility manager at Dounreay, said: “We currently have a Pelton wheel meter fitted on an outlet line located within a shielded cell, which runs from an ultra filtration facility that decontaminates radioactive water from the silo.

“We have recently purchased another meter to the same. The unit purchased is to be held as an 'essential spare' should our current unit fail – although it been extremely reliable so far. As a 'like for like' replacement changing the unit, should we need to, would be a relatively simple task that can be carried out quickly with no engineering modifications.”

Charles Wemyss of Litre Meter said: “There are several reasons why companies opt for customised meters but the most important is the ‘bottom line’ – the amount that can be saved in costs. “There are six common areas of customisation – high pressure, special connections, communication, application environment, low flow and flow range. Customising a meter from a standard range is usually carried out in one or more of these six areas in order to reduce the user’s installation and operational costs.

“In the case of Dounreay the meter required special connections. We are often required to adapt meters to meet very specific connection requirements but the cost to the customer is much less than doing the reverse – adapting equipment to suit a standard meter.”

The Pelton Wheel has a large blade area compared with the flow inlet port size which produces a wide range flowmeter that can measure very low flows. The flow through the inlet of the meter is accurately directed onto the rotor which rotates at a speed in linear proportion to the flow rate.

A small sensing coil detects the ferrites mounted in the rotor blade tips as they pass. No drag is imposed on the rotor which assists in the measurement of extremely low flows. Sapphire bearings provide a low friction mechanically-robust bearing with long life characteristics. Turndown can be as high as 280:1.

Alternatively the Pelton Wheel can be mounted across an orifice plate to produce linear measurement over larger flows. Due to the unique characteristics of the Pelton Wheel there is a linear relationship between the frequency and flow rate over a wide range of up to 65:1.
Litre Meter is currently running a competition to find the oldest Pelton wheel still in operation. One entrant has been in continuous operation for well over 20 years.

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