Suction dredgers are used on land or on water as oversized wet or dry vacuum cleaners (Fig. 1). Dredgers of this kind are used, for example, to maintain the depth of water at port facilities or to mine gravel and sand for the construction industry. The company Jan De Nul is an operator of suction dredgers and charges for its services based on the mass pumped. To precisely determine the mass pumped, the company uses measuring technology from Endress+Hauser: The electromagnetic Promag 55S flowmeter is used for volume measurement and the radiometric Gammapilot M FMG60 is used for density measurement.
Basically, a suction dredger consists of a ship‘s hull with one or two slewing pipes attached to it. The pipes, which have specially shaped suction heads attached to their ends, have a diameter of up to 1.5 m and reach down to the bottom of the body of water. Powerful pumps are used to pump the sediment loosened by the suction heads from the ground to the surface of the water. From there, the pumped material is either released away from the ship via connected piping or first stored in the hull and later dumped in a suitable place.
Along with the conventional measuring technology used in ship construction, the suction pipes are also fitted with Promag 55S flow meters in order to measure the quantity of the sludge-water mixture extracted and pumped through the suction pipes. The demands placed on the measuring technology are extremely high because the Promag 55S must be capable of correctly measuring the sand and rock-filled seawater at flow rates of up to 10 m/s − and at operating pressures of between 16 and 40 psi. In comparable applications in the mining industry, flowmeters with a 27 mm thick natural rubber lining and brush electrodes have proven to be a very effective solution in the past.
Radiometric measuring technology is used when other measuring principles are not suitable due to extreme process conditions or due to physical, geometric or structural circumstances. The measuring principle is based on the attenuation of gamma rays when material passes through them. On suction dredgers, radiometric measuring systems are used to determine the density of the extracted sludge. If the piping is filled with water, the detector receives a specific quantity of gamma rays. As soon as sand and silt are mixed into the water, the density of the mixture in the pipe increases. Due to the greater density, the level of gamma rays received by the detector reduces. This allows the density of the sludge to be determined. The density value which is obtained is fed into the Promag 55S flowmeter. Based on this density value and the measured flow velocity of the medium, the mass flow rate is calculated. This, in turn, is used by the company to charge for its services. Due to the cramped conditions, a „piggyback“ solution was developed, whereby the density measurement device is attached directly to the Promag 55S. This device arrangement makes it possible to measure both the density and velocity of the medium at the same point (Fig. 2). Further Promag devices (ANSI < 200) are used on the suction pumps. These measure the quantity of seawater which (under a slightly higher pressure than in the suction process) is forced into the bearings of the suction pump in order to prevent the penetration of solids.
Endress+Hauser Messtechnik GmbH+Co. KG,
Weil am Rhein (D), Tel.: +49 7621 975-01,