FluidFlow gives a saltation/blockage warning if the pipe is horizontal AND the flowing slurry velocity is below the (maximum) deposition velocity. The deposition velocity varies with slurry concentration and FluidFlow uses the maximum deposition velocity and not the deposition velocity at the actual slurry concentration. The reason for using maximum deposition velocity instead of actual deposition velocity in the comparison is that localised concentrations can differ from the average slurry concentration. Also operating concentrations can change over time. Consensus of users is that it better practice to use maximum deposition velocity to cover all localised and time based concentration scenarios.
For sloping pipes and vertical pipes generally, blocking does not occur. FluidFlow never issues a blockage warning in these cases as the deposition velocity calculation equations only hold for horizontal pipes.