The Pipeline Transport of Different Sand Fractions in Dense Slurries

The development of new techniques in the dredging industry has made possible the hydraulic transport of slurries in pipelines at increasingly higher solids concentration. The modern trailing suction hopper dredges are capable of pumping sand-water slurries at densities higher than 1600 kg/m³. The high concentration of solids influences the economy of the transportation, since it changes the operational characteristics of centrifugal pumps and pipelines. However, little is known about the effect of high solids concentration on the operation of a pump and a pipeline. Laboratory experiment designed to provide information that would lead to greater understanding of the mechanisms governing the pipeline flow of settling slurries were conducted in a 150-mm pipeline of the Laboratory of Dredging Technology of the Delft University of Technology. The investigations revealed the effect of particle size and slurry density on the friction process in a pipeline transporting sand-water slurries. The paper analyses the effect of concentration and size of transported solid particles on the economy of the transportation of slurries by relating the specific energy consumption — the parameter that evaluates the efficiency of a slurry pipeline - to the flow parameters (velocity, concentration and solids throughput) in the pipeline. The laboratory findings are compared with the field observations in two large slurry pipelines (650 mm and 700 mm). The observations indicate that it is beneficial to transport fine to medium sands in highly concentrated slurries. To ensure safe and economic operation the sand-water slurry in a horizontal pipeline should be transported at velocities for which the granular bed sliding over a pipeline wall starts to disintegrate and dissolve. At these velocities the specific energy consumption is minimum.