Compressor aids de-silting

Downham Market, UK - A compressor is at the heart of an aeration system that is being used to de-silt the Denver Sluice on the River Great Ouse, near Downham Market in Norfolk.

The system is using compressed air to agitate the water and overcome the build-up of silt against the tidal doors that protect the river's guillotine gates. The sluice controls the water levels over parts of the fens and prevents high tides from flooding low-lying areas.

Over the past couple of years, the Environment Agency has had to dredge up to 4m of silt from in front of the gates and downstream of Denver Lock, because only a limited amount of water had been allowed to flow from the three 'eyes' of the sluice as a result of a prolonged period of dry weather.

The decision to bring the 'eyes', which include the upstream guillotine gates, back into full service was hampered by the inability to move the silted-up tidal doors (mitre gate type) protecting them.

Rob Barker, project manager from the Environment Agency, Anglian Region, sought advice from NEECA framework consultant Royal Haskoning in order to develop his concept for an aeration system. The task fell to David Goodman, principal mechanical engineer with Royal Haskoning, based in the company's Leeds office, who developed the initial concept to outline design stage and produced the specification and tender drawings for the system.

The final details for the design, along with the supply and installation of the equipment, was provided by Atlas Copco's main distributor in the area, Anglian Compressors, based in Peterborough. The complete installation comprised a GA15 VSD 15kW rotary screw compressor together with an LV500 air receiver and a PD 60 oil filter, housed in a purpose-built GRP kiosk located on site, together with associated pipework and control valves/manifolds. The aeration system comprised three concentric loops of galvanised-steel pipework installed in front of each gate.

In addition, 4mm holes were drilled at 100mm intervals in sections of the pipework loops, with each concentration of holes covering a different section of the individual gate/sill. A set of ball valves was configured to allow independent control of each loop for concentrated aeration at higher pressures and also flushing of the system in either direction.

The decision to install a variable-speed drive version of the GA15 compressor - capable of delivering air at the rate of 16-48 litres per second at a pressure of 7bar(e) - was influenced by the ability of the VSD compressor to match output to demand for optimum energy efficiency and to minimise starting currents without energy penalty.

The system is now operated regularly during outgoing tides, which carry the silt downstream.