Moving slurry: velocity, settling and line sizing

Moving a slurry through a pipe is governed by a window. Too slow and the solids settle out, build a bed, and eventually block the line; too fast and the abrasive solids wear the pipe and the pumping cost climbs. Line sizing for slurry is the discipline of staying inside that window, and it separates cleanly into a part that is arithmetic and a part that is not.

The arithmetic this page sizes

The arithmetic part is the part this page sizes. Pipe velocity is volumetric flow over cross-sectional area — v = Q ÷ A, with A = πD²/4 — so for a chosen flow the velocity is set by the pipe diameter, and sizing a line is choosing the diameter that puts the velocity where you want it. Pressure drop along the line follows Darcy-Weisbach, the same relation as for any fluid, with the friction factor from the flow conditions; the slurry’s density and the carrier viscosity enter through that calculation. These are deterministic: given the flow, diameter, density and roughness, the velocity and the pressure drop are calculable, and the two calculators below do exactly that.

The settling-velocity floor

The part that is not arithmetic is the lower bound of the window — the settling velocity, the speed below which solids drop out. This depends on the particle size distribution, the particle density, the solids concentration and the slurry’s rheology, and it is predicted by empirical correlations calibrated to data, not by a clean formula. For a real circuit the deposition or critical-settling velocity comes from established slurry-transport correlations and, for anything demanding, from testwork on the actual material. This page draws a firm boundary there: it treats settling as a concept that sets why a minimum velocity exists, and it does not size a line against a predicted settling velocity. That prediction is correlation-and-testwork territory, and presenting it as arithmetic would misrepresent how the number is actually obtained.

So the working method is: size the line geometry and pressure drop with the velocity and pressure-drop tools here, against a target velocity that a competent slurry-transport assessment has set well above deposition for your material — and treat that target as an input from correlation or testwork, not something this page computes. The arithmetic tells you the velocity a diameter gives and the head a length costs; the settling assessment tells you the velocity you must beat. Keep those two responsibilities separate and the line sizing is honest.

The pipe-velocity calculator returns velocity, flow or diameter from the other two; the pipe-pressure-drop calculator returns the Darcy-Weisbach head loss and pressure drop with the friction factor computed from the flow. Together they cover the calculable half of slurry line sizing; the settling half stays with the correlations and the testwork where it belongs.