We are all aware of the awesome power of fluids, particularly air and water, when they move at high velocities. Storms, cyclones, tsunamis, floods and similar quirks of nature can uproot trees, destroy buildings, cause landslides, change coastlines and generally produce massive devastation. On a much smaller scale we can use the same forces of nature for the controlled transportation of solids through pipes.
Water flowing around solid particles creates pressure differentials around them and the resulting drag forces move the particles in the general direction of the flow. The velocity of the solids is slower than that of the water. This is called slippage and particles of different sizes and densities have different slippages. Relative to horizontal flow, slippage increases in uphill flow and decreases in downhill flow because gravity slows down and respectively accelerates the flow of solids relative to the liquid. It follows that, in various parts of a convoluted pipeline, local solids concentrations vary, which influences local slurry velocities, pipe wear and friction losses.
In any pipeline such local variations can cause solids to settle and possibly to block the pipe. Flow conditions must therefore be fully investigated and often confirmed by means of some test work. For short pipelines (up to a few 100 m) on pumping duties, with known materials in typical mining pumping circuits this is not required. We base all calculations only on average solids concentrations (both Cw and Cv) as they exist at the end of a pipeline. When the sizing, density and concentration of solids in a slurry are known, we must determine the average Limiting settling velocity VL of the slurry, which will move the solids and not let them settle. Durand and Condolios carried out most of the original investigative work in the 1950s. They worked with water and narrowly graded solids i.e. the d80 particles were less than twice the size of the d20 particles. They produced the graph shown in Fig.7.1 and the following formula: VL = FL√[2gD(S–1)] (7.1) The diagram and formula are still in wide use today. Diagram A1-5 with nomogram, which gives VL without calculations is given in Appendix A1.