Two good sources of velocity information for slurry and sludge pumping can be found in:
Addie,G.R., "Slurry Pump Selection Considerations", Pumps and Systems Magazine, Oct.1993.pp.32-39
P&SM Staff, "Sludge Handling-Guidelines for the Tough Stuff", Pumps & Systems Magazine, June 1994, pp.38-41
The first gives, in Fig. 3, nomograms for velocity at limit of stationary deposition attributed to:
Wilson,K.C., "Effects of Solids Concentration on Deposit Velocity", Journal of Pipelines, 1986, pp.251-257
The second recommends that "sludge velocities should be kept above 4-5 ft/sec to avoid sedimentation in the line and reduce grease buildup". This article provides a comprehensive Sludge Services Applications Chart for various %solids including pump type selection and relative severity of in-service abrasion and plugging.