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How to Calculate the Settling Velocity in a Pipe

Settling velocity tells you how slowly a fluid or gas must be moving for particles of a given size to drop out of the flow and settle to the bottom of a pipe. Use Stoke's law, which mathematically describes how particles settle due to gravitational and drag forces, to calculate the settling velocity in a pipe. Stoke's law applies to both liquids and gases. You must know the diameter and density of the particles in question to apply Stoke's law.

Instructions

- 1
  Subtract the density of the fluid or gas from the density of the particles. The density of water is 1 gram per cubic centimeter; the density of air is 0.0013 grams per cubic centimeter. Use a table to look up the density if you have another material. For particles with a density of 2.5 grams per cubic centimeter in water, for example, 2.5 g/cm^3 - 1 g/cm^3 = 1.5 g/cm^3.
- 2
  Multiply the result times the acceleration due to gravity, 980 centimeters per square second, and the square of the particle diameters. If the particles have a diameter of 0.003 cm, for example, 1.5 g/cm^3 x 980 cm/s^2 x (0.003 cm)^2 = 0.013 g/s^2.
- 3
  Multiply the viscosity of the fluid or gas by 18. The viscosity of water is 0.01 grams per centimeter-seconds; the viscosity of air is 0.00018 grams per centimeter-seconds. Look up viscosities for other materials in a table. For water, 18 x 0.01 g/cm-s = 0.18 g/cm-s.
- 4
  Divide the result of step 2 by the result of step 3 to find the settling velocity. For example, (0.013 g/s^2)/(0.18 g/cm-s) = 0.702 cm/s.

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