A hydrocyclone (often referred to by the shortened form cyclone) is a device to classify, separate or sort particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance.  Technically, 4 to 6 inch cyclone are crucial part  of desilter and 8 to 12 inch cyclones are parts of desander in solids control system.

Table: Commonly single hydrocyclone size and capacity for waterbased drilling mud
4 inch 5 inch 6 inch 8 inch 10 inch 12 inch
15m³/h 30m³/h 50m³/h 90m³/h 120m³/h 150m³/h

Since most hydrocyclones are designed to operate at a constant 75 feet of head at the input manifold, flow rate through any cone is constant at constant inlet pressure for a given fluid viscosity.

This ratio is high for dense (where separation by density is required) and coarse (where separation by size is required) particles, and low for light and fine particles. Hydrocyclones also find application in the separation of liquids of different densities.

The smaller desilter hydrocyclones are rated from 40 to 100 gpm of liquid removal, depending on the cone design. The normal 4-inch cones will remove 4 gpm of solids, or 5.7 barrels per hour of solids, per cone. Therefore, a 16-cone desilter manifold will accommodate removal of 510 cubic feet of solids per hour. For a 17-inch hole, this equates to penetration rates averaging 297 feet per hour. Clearly, if design and operational characteristics are adequately maintained, more than ample solids separation can be effected.

For deepwater drilling applications, higher flow rates are encountered when boosting the riser. Additional cones may be needed to handle the additional flow rates to ensure that the cone manifolds process all of the mud.

The accelerated gravitational forces generated in hydrocyclones are inversely proportional to the radius of the hydrocyclone cylinder. Thus, the larger the diameter of the cone, the coarser the separation. In general, the larger the hydrocyclone, the coarser its cut point and greater its throughput. The smaller the cone, the smaller the size of particles the cone will separate. In other words, the median particle size removed decreases with cone diameter.

Median particle size also increases with increasing fluid viscosity and density, but decreases as particle-specific gravity increases. Oilfield hydrocyclones range between 4 and 12 inches, based on the inner diameter of the intake cylinder. A small hydrocyclone diameter is used for ultra-fine separations.