Hydraulic Transport

a method of displacing solid materials by a stream of water. Hydraulic transport is used in moving earth or rock by hydromechanized means, in building earth structures (such as dams or dikes), in removing slag and ashes from large boiler rooms, in transporting mineral products, in removing waste products in ore concentration processes, and in transporting various materials (such as wood chips, paper pulp, and raw materials used in the manufacture of sugar and alcohol).

Hydraulic transport systems are either pressurized or non-pressurized. In non-pressurized systems the hydraulic mixture is moved on inclined troughs or chutes and pipelines that are only partially filled. In this case the pressure on the free surface of the hydraulic mixture is equal to the atmospheric pressure. In pressurized systems the hydraulic mixture in the pipelines is under an excess pressure. This excess pressure is provided by pumps (boring pumps, coal suction pumps, and the like). The pressure resulting from the difference in altitudes between the beginning and the end of a pipeline is sometimes sufficient for hydraulic transport—for instance, when transporting rock into a mine shaft in order to fill an excavated space.

Hydraulic transport occurs only if the travel speed of the hydraulic mixture exceeds a certain minimum, known as the critical speed. The magnitude of this critical speed varies from 1.5-2 m/sec to 4-5 m/sec, depending on the density and the size of the particles being transported, on the concentration of the hydraulic mixture, and on the diameter of the pipeline. In this speed range the small and light particles are transported in suspension, medium particles are transported in interrupted suspension, and the largest and heaviest particles drag and roll along the lower wall of the pipeline. A highly concentrated hydraulic mixture of extremely fine particles of clay, chalk, peat, coal, and the like can be transported even at very low speeds. Such hydraulic mixtures, like colloids, possess certain characteristic properties: the particles in such a mixture remain in suspension even when at rest. Pressurized hydraulic transport makes it possible to transport loads over long distances—for instance, in the USA coal has been transported over a distance of 173 km and ore, over 115 km.

Calculations for the design of hydraulic transport systems usually involve computing the diameter of the pipeline (from a figure given for the capacity of the pipeline and from the magnitude of the critical speed), the concentration of solids in the hydraulic mixture, and the hydraulic resistances. Hydraulic resistances and the hydroabrasive wear of the pipeline are lowered drastically when the size of the particles being transported is less than 1-3 mm. Thus hydraulic transport over longer distances is usually limited to particles of this size.

The advantages of hydraulic transport are high capacity, feasibility of transport over long distances, feasibility of automation, low operational costs, and feasibility of combining the transport of material with some other technological process (hydraulic destruction, enrichment, or rinsing). Its disadvantages include a considerable consumption of water and of electric power, wear of pipeline and pumps when transporting abrasive materials, and, in some cases, a pulverization and maceration of the materials being transported, which requires a subsequent dehydration.