Patent GB-1511154-A (GB)
Title: Method of and apparatus for removing material by suction from a bed of particulate material
IPC: E02F3/90
Filed: 1975-05-05
Abstract: 1511154 Dredging apparatus INGENJORSFIRMAN N A SANDBERGS INDUSTRIKONSTRUKTIONER AB 5 May 1975 [29 Aug 1974] 18745/75 Heading E1F [Also in Divisions B1 and B2] A suction nozzle 20 used for dredging or for removing settled sludge from a settling tank floor is adjusted vertically automatically in response to variations in the flow rate through the nozzle in such a way as to maintain the flow rate within certain limits. The flow rate may be monitored directly using an electric-output flowmeter connected to the sludge discharge conduit 7 (Fig. 3 not shown) or indirectly by monitoring the power or current consumption of the electric motor M1 driving the nozzle pump 8. The circuit shown monitors the pump motor current and controls the vertical position of nozzle 20 by feeding raise or lower signals to a winch M2 winding up nozzle support cable 12. It could be modified to control also the movements of the nozzle in two orthogonal horizontal directions, the nozzle being movable along a bridge which is itself movable along a settling tank (Fig. 1, not shown). The various movements can also be controlled by timers according to a predetermined programme. In one mode of operation the nozzle moves continuously to cover the whole tank floor in series of horizontal passes, while its vertical position is adjusted in steps according to the thickness and density of sludge encountered (these directly effect the flow rate and hence the pump motor current), and several complete cycles may be needed to clear all the sludge. In another mode, the horizontal movements are controlled in a stepwise fashion and at each stop the nozzle is raised and then lowered to remove all the sludge at that location, so the sludge is cleared in a single (but longer) cycle. The pump could be located on the bridge instead of being submerged, and could be designed to deliver air or water to the nozzle 20 on the air-lift or injection pump principle. Several nozzles with one or more pumps could operate together. In the latter case the power of a single pump is monitored to control all the nozzles. Fig. 2 shows a valve 15 in the sludge discharge circuit which is automatically adjusted if the variation in pump motor current goes beyond the normal limits which can be corrected by raising or lowering nozzle 20. This happens when the nozzle clogs completely or when the sludge bed is completely removed, causing respectively a large drop and a large rise in pump power. Relays R1, R4 respond to these conditions and move switch S to apply control signals to valve motor M3 instead of winch motor M2. In the former case the pump is stopped and the valve is opened wide and then returned to its normal intermediate setting by a timer, this action clearing the clogging, and control is then switched back to motor M2 and the cycle continues. Fig. 1 illustrates a settling tank with an inlet at one end and a clean water overflow (3) at the other. Pump 8 suspended from moving bridge (5) delivers sludge into shoot (9) which can be washed down by clean water from pump (10). Shoot (9) delivers the sludge to a mixing tank (11) or filter (not shown).