On Self-Emptying At High Discharge Mixture Densities
Type:
Presented during:
WODCON XX: "The Art of Dredging" - 2013, Brussels, Belgium
Authors:
De Nijs M
Abstract: Laboratory experiments are presented on unloading of saturated sand at high discharge mixture densities (~1800 kgm-3) through the use of relatively small ratios jet discharge to suction discharge. The experiments show that unloading can be achieved without completely fluidizing the hopper cargo. At the start of unloading, flows between horizontal jet cavities and the unloading point near the hopper bottom undermine an area of non-fluidized hopper cargo at the unloading point. The non-fluidized hopper cargo descends into these flows to form sediment mixtures of high density (~1800-2000 kgm-3) in front of the unloading point. Consequently, the mechanism that creates the high discharge mixture densities at the unloading point is different than the classical breaching mechanism. The lengths of the jet cavities must be long enough to enable transport between the cavities and unloading point. These flows prevent the development of high soil mechanical resistances associated with pore water under pressures in the hopper cargo near the unloading point. These resistances have a negative effect on the stability of the unloading productions and unloading time because they can lead to pump cavitation. Consequently, the soil mechanical resistance in the hopper forces the pump-pipeline-drivetrain system and therefore limits the production instead of the classical hydraulic resistance at the unloading point. A more thoroughly fluidized load at the unloading point is the cause of a different soil mechanical state and hence unloading behavior than observed in previous experimental investigations. Consequently, a new concept is presented in which the unloading rates have increased as a result of higher unloading densities by using less jet-water. The above mechanisms are elaborated using approximate analytical models. Van Oord Dredging and Marine Contractors bv uses this knowledge to design and optimize unloading equipment.
Keywords: unloading; self-emptying; pore water pressures; fluidization; jet cavity