Cutter Head Spillage When Dredging Sand or Gravel
CEDA Dredging Days 2019
S.A. Miedema and B.J. Nieuwboer
Abstract: One of the main types of equipment is the cutter suction dredge (CSD). The theoretical soil production Qc equals the cross section of the cutter head in the bank cutting, perpendicular to the swing velocity vs times the swing velocity vs. If the theoretical soil production is 100%, usually less than 100% will enter the suction pipe. The real production. The difference between the theoretical production and the real production is the spillage. So, this is the percentage of the theoretical production not entering the suction pipe.
Now in practice it is more difficult to define the spillage, because often a number of swings at different levels is necessary to excavate a bank. The spillage of a previous swing may be cut a second time during the current swing and thus enter the suction pipe in the current swing. So, the spillage of one swing does not have to be spillage overall. In this report however just one swing is considered, assuming a fresh bank, where all the soil that does not enter the suction pipe is considered spillage.
The assumption is made that near the ring mixture can flow to the outside, similar to the working principle of centrifugal pumps, while near the hub water will flow into the cutterhead. Since the radius of the cutterhead near the ring is the largest, the pressure generated by the centrifugal effect is the largest. This high pressure generates a circulation with inflow near the hub. The model derived is based on the Euler equation for centrifugal pumps, including inner and outer radii and blade angles. The outflow near the ring is defined as the spillage. Below certain revolutions there will not be outflow at all, so there is no spillage related to the circulation. However, at very low revolutions not all particles will enter the cutterhead, so there will be spillage because the filling degree is not 100%.
The model is calibrated based on the experimental data mentioned in den Burger (2003). The paper covers the theory and the validation, with many experimental data of Miltenburg (1982), never published before. Since the experiments were carried out with model cutterheads, scale laws/rules are required to scale the spillage from model to prototype. The scale laws are derived from the physics of the modeling and give a good match with the scaling factors as applied by den Burger (2003).
Keywords: Spillage, cutter head, cutting sand & gravel, production