Experiments with sand slurries pumped in 0.1 to 0.5 m diameter pipeline loops

Abstract: The overall objective was to investigate the effectiveness of using high solids concentrations when transporting sand slurries in pipelines with centrifugal pumps. In detail, the aims are to present large-scale experimental pipe friction loss results and how head, efficiency and net suction head performance are influenced by the solid particles in large pumps and to compare friction loss data with a design model. Experimental results and an analysis related to hydrodynamic effects on sand particles in the pipe wall region are given and the capability of a modified centrifugal pump to convey a highly viscous sand-clay mixture is presented. Experimental results on narrowly-graded sands with median particle sizes of 350 to 390 micron, in a 0.5 m diameter pipeline and a centrifugal slurry pump with impeller diameter 1.4 m, showed that the energy consumption per tonne of dry solids was reduced by about 25% when pumped at a solids concentration by volume, C, of 24 instead of 14%. The effect of solids on the pump efficiency was limited to a few percent and reported field results for C-values up to and over 40% for a 2.5 m impeller pump showed that the effect may be negligible in practice in large systems transporting sands. Corresponding reported field results on friction losses in 1 m diameter pipelines showed a reduction of about 30% in energy consumption when increasing C from about 25 to 40%; for sands with median particle sizes from about 300 to 1000 micron. Friction losses were modeled rather well for the 0.5 m pipeline loop results and the 1 m diameter field data with various sands up to C-values of 40%.With the coarsest field sand (1000 micron), modeled losses were overestimated for C over about 25%.Experimentally found friction losses were reduced about 20% after adding 20% of rock flour particles with a median size of 40 micron to a narrowly-graded sand with median diameter 350 micron. The reduction could not be simulated with the model based on simple assumptions about the rheological properties of a homogeneously flowing fine particle-water mixture. In dredging and phosphate mining applications pumps are sometimes susceptible to high sub-atmospheric suction pressures, where cavitation is common. The net positive suction head requirement was investigated with the 1.4 m impeller pump used in the 0.5 m pipeline loop. Field suction side conditions were simulated with an open tank-valve arrangement. It was found that the net positive suction head required in meters of slurry was about 50% higher than the corresponding specified water values for about 300 to 400 micron sands at C-values of 14 and 24% when operating close to the best efficiency region. The detailed frictional characteristics of both fine and medium-sand slurries were evaluated in a 0.1 m diameter pipeline loop. With the medium 400 micron sand at a C-value of 17%, the excess losses over those of water were high at low velocities, decreased with increasing velocity and dropped below the water curve at about 4.2 m/s. This behaviour was related to an inertial hydrodynamic lift force that is effective in driving particles away from the wall. A lift coefficient was defined for the inertial lift force that diminishes in a way that depends on a dimensionless ratio which includes particle radius and shear velocity. Successive addition of fine sand to a non-Newtonian phosphate clay slurry was found to approximately simulate the rheological behaviour of sewage and fibrous sludges when handled at various solids concentrations. Experiments conducted with the simulated sludge confirmed the capability of a modified centrifugal pump, having an auger-like impeller, to convey slurry with yield stresses well over 200 Pa, corresponding to dewatered paste-like sewage sludges.