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By Paul Slatter*, Principal Engineer, Rheology and Slurry Engineering at ATC Williams
Pumped slurry pipeline design requires accommodating at least three complicating factors. These are:
- The heterogeneous/settling characteristics of the slurry
- The homogeneous/non-Newtonian characteristics of the slurry
- The degrading effect of slurry on pump performance
Both the heterogeneous and homogeneous characteristics of the slurry affect the piping system head requirements, but in fundamentally different ways and must be accommodated in the overall design. This also applies to energy losses in valves and fittings.
The heterogeneous characteristics of the slurry require that sufficient turbulence be maintained in the pipeline in order to suspend the solids particles so that they do not settle and form a bed on the pipe invert. If a bed starts to form, the effective diameter becomes reduced, requiring progressively more and more pumping head to maintain throughput, eventually blocking the pipeline.
The homogeneous characteristics of the slurry increase the rheology of the material. These cause the flow features to change in a fundamental way, the most important of which is the increased risk of obtaining laminar flow. In laminar flow, the coarse particles settle, and there is no mechanism for re-suspension, also eventually leading to pipe blockage.
The degrading effect of slurry on pump performance results from the fact that pump performance curves are always published for water only. Both the efficiency and head performance are degraded by the presence of slurry, and the pump must be appropriately derated for slurry service. The head and efficiency derating ratios are functions of both the heterogeneous and homogeneous characteristics of the slurry.
This issue is further complicated by the need to accommodate the two extreme situations of:
- Charging the line, where the line is filled with water only, and the pump is filled with slurry
- Flushing the line, where the line is filled with slurry only, and the pump is filled with water
The slurry characteristics further impact on pipe sizing and material selection, system start-up, shutdown, ramp up and down, pump station and piping layouts and control, slack flow sections and the associated accelerated velocities and wear rates.
All these complicating factors must also be accommodated within the usual NPSH and cavitation performance requirements of both the pump set and the pipeline.
*Paul Slatter will hold a pre-conference masterclass on Slurry Pumping as part of the 3rd annual Slurry Pipelines Conference. The masterclass will be held on Tuesday 12th November 2013. Paul has been actively involved in flow process research and higher education teaching for the past thirty-three years. His main interests and contributions are for viscoplastic fluids in the engineering hydrodynamic contexts of laminar, transitional and turbulent pipe flow, centrifugal pump derating, losses in valves and fittings, open channels, Launders and sheet flows. These design approaches developed by Paul have been referenced in 15 Design Handbooks on Mineral Processing Plant Design.
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