In a pneumatic conveying system, the air conveying velocity below which the solids being conveyed starts to settle at the bottom of horizontal pipe is the saltation velocity. In order to be able to convey in dilute phase a bulk solids, the minimum air conveying velocity in all part of the line must be HIGHER than the saltation velocity.
The saltation velocity is better defined through trials in a pilot plant. However, in case one tries to make a 1st design assessment or to try troubleshooting an existing line, correlations are available. The one below is due to Rizk, it is not very precise, +/-54% in average, but may be useful :
From recent works, this correlation looks to be best suited for fine powders, below 200 microns particles diameter.
Note : The saltation velocity, which happens in horizontal lines, should not be confused with the choking velocity which happens in vertical lines. However, the saltation velocity is higher than the choking velocity, thus designing the system to run above the saltation velocity will allow to avoid crossing the choking velocity in vertical pipes.Source