DE-Lecture09-Basic-Pump-Hydraulics

# DE-Lecture09-Basic-Pump-Hydraulics - Dredging Dredging...

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Dredging Engineering Lecture 09 Basic Pump Hydraulics Luis Prieto-Portar 2010

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The Parts of a Dredge Pump. In Lecture 08, slide 13, we saw that the basic dredging centrifugal pump is a (1) volute, single stage, single suction, with a closed, single-curvature, non-clogging, overhung, radial curved-vane runner, and a horizontal, split or solid casing. (1) The term volute means spiral , and is named after the shape of the casing which is a equiangular spiral where its radius r is given by r = ae θ tan α . This shape was derived by the brilliant French mathematician, Blaise Pascal (1623-1662). The curve possesses the property that the acute angle α between the radius and the tangent to the spiral is a constant. The ensuing equiangular shape produces an equal velocity of the flow around the circumference. At the same time the velocity is converted into pressure, and is gradually reduced as it flows to the discharge end of the pump.
(2) The closed-vane runner has shrouds (sides) that totally enclose the vanes. This set up creates closed channels from the “eye” of the pump (the entrance from the suction side) to the periphery of the runner. (3) A straight or single-curvature runner has its vanes curved backwards, opposite to the direction of rotation. On the other hand, double-curvature runners have their vanes curved in two directions, such as the blades of a ship propeller. (4) Non-clogging runner s have fewer vanes so that there are larger areas for larger solids to get through. The largest number is usually five vanes, which is useful for very fine and soft soils (silts and fine sands). (5) An overhung runner has the drive shaft supported on only one side.

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Pump liners are used to protect the pump casing (shell), pipeline, fittings, runners and cutters from wearing out due to abrasion. These liners and replaceable rings are placed inside the pump casing. They also serve the important function of sealing the high pressure discharge volume from the low pressure suction volume. Pump casings are made from 12% manganese steel alloy because it provides high shock resistance, is non-magnetic, very ductile and is easily welded. Another common alloy is Ni-Hard (International Nickel Company), also provided as ABK Metal (by American Brake Shoe Company), Flintmetal (by Morris Machine Works) and Gasite (by Georgia Iron Works). Stuffing box provides a seal around the runner shaft where it enters the pump casing. Runner thrust is the axial thrust of a single-suction runner, due to the two opposing forces: (1) the change in the momentum of the mixture entering the runner, which tends to force the runner backward, and (2) the opposing force caused by the discharge from the runner. The pump’s discharge is at a much higher pressure than at the suction, thus moving the runner forward.
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