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Metso Thomas dredge pump successfully delivering 1,200 tonnes/h of sand and gravel in northern Italy

Well known throughout the industry for their static and mobile crushing plants, Metso Minerals also offer an ever-expanding slurry-handling solutions product line, including numerous vertical and horizontal pumps specifically engineered for the most abrasive solids-handling applications.

Among this product line is the highly successful Thomas dredge pump range, which Metso have been introducing into European markets for the past few years. Following initial successes in Germany, other European dredger manufacturers and operators are now selecting the Thomas dredge pump as their first-choice solution.

Italy, in particular, has proven to be a good market for Metso and the Thomas range, where an international team effort has recently ensured continued success for one particular dredger manufacturer and operator in the north of the country. Supported by Metso engineers from Italy, the UK and America, Metalmeccanica Strobietto s.r.l. have designed and manufactured what is reported to be the largest aggregate producing cutter suction dredger in Italy.

The dredger, known as Nessie, was commissioned in 2011 and incorporates a 550mm Thomas TU46WD underwater suction pump mounted on a modular suction ladder for maximum aggregate production at a variety of depths (fig. 1). Extensively used throughout North America, the TU46WD dredge pump is made from high-chrome white iron with a minimum hardness of 650 BHN, and has a 46in (1,168mm) diameter high-efficiency solids-handling impeller. Designed for continuous underwater applications, the bare shaft pump alone weighs more than 11 tonnes.

As with all Metso pumps, basic selection was made using Metso’s in-house Pumpdim selection programme – one of the most advanced sizing tools currently available for accurate pump selection – which makes appropriate allowances for the effects of solids on head, efficiency and flow, as appropriate. By not only selecting the right pump for the job, but also considering all mechanical and hydraulic limitations of both the pump and system, Pumpdim ensures safe and reliable operation with maximum time between rebuilds. 

At Nessie’s current extraction site, where the solids being extracted can be up to 450mm in diameter, the dredger is producing, on average, around 1,200 tonnes/h of sand and gravel through a 500mm discharge line to shore for draining, prior to despatch to various road construction projects throughout northern Italy (fig. 2).

Designed and manufactured to specific requirements within the Metalmeccanica Strobietto workshops, Nessie took just four months to complete from order to commissioning. It is currently constructed as a basic 24m model (using four 12m modules), but the design allows for excavation to depths of 45m by simply fitting additional modular elements.

The dredger’s submerged Metso TU46WD dredge pump is powered by an inverter-controlled electric motor, with power being transmitted through a speed-reduction gear box and heavy-duty transmission shaft. Electrical power is supplied via a 15kV power line from the local grid, with on-board processing controlled by PLC (figs 3 & 4).

Nessie’s main control cabin allows one person to control the entire dredging operation, with ‘visualization’ via the control panel’s 17in touch-screen, which provides all the necessary checks and continuous GPS positioning.

Primarily for use in quarries, lakes and other groundwater extraction sites where deposits are compacted or the presence of silt and clay make conventional extraction difficult, the modular design of Metalmeccanica Strobietto’s state-of-the-art cutter suction dredgers allow the company to manufacture to order on a minimal time scale, normally around four months.

Thanks to their versatility, the dredgers can be adapted to suit a variety of applications and dredge depths, currently ranging from 15m to a maximum of 60m. Cutter suction dredgers and the lighter water suction versions cover deposit sizes from 8in (200mm) up to 24in (600mm), with slurry flow rates between 900m3/h and 9,000m3/h.

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