Hydrocyclones are simple, easily maintained mechanical devices without
moving parts. Separation is accomplished by transfer of kinetic input
or feed energy into centrifugal force inside the cone. The centrifugal
force acts on the drilling fluid slurry to rapidly separate drilled solids and
other solid particles in accordance with Stokes’ law.

The solids that are generated by drilling in some formations are too
fine for shale shakers to remove. Hydrocyclones must be relied on to
remove the majority of these solids. Here the shale shaker protects the
hydrocyclones from oversized particles that may cause plugging.

Hydrocyclones should be designed to provide maximum removal of
solids with minimum loss of liquid. Sufficient hydrocyclones should be
arranged in parallel to process all drilling fluid arriving into the additions
compartment of the mud tank system.

Hydrocyclones produce a wet discharge compared with shale shakers
and centrifuges. Underflow density alone is not a good indicator of cone
performance, as finer solids will have more associated liquid and the
resultant slurry density will be lower than for coarser solids.

As the solids content increases, separation efficiency decreases and the
size of particles that can be separated increases.


Hydrocyclones provide:

. Simple design
. No moving parts
. Easy maintenance
. Good separation ability

Hydrocyclones have the following disadvantages:

. Limited separation of ultra-fines
. Inability to handle flocculated materials

It is impractical to desand or desilt a mud containing appreciable
amounts of barite. Silts and barite have about the same size range. The
majority of barite particles are between 2 and 44 microns, some between
44 and 74 microns, and unfortunately some 8–15% are between 0 and 2 microns. A desander median cut (D50 cut point) falls between 25 and
30 microns. A desilter median cut falls between 10 and 15 microns. Since
much of barite falls above these cuts, it would be discarded along with
the silt and sand.

Generally, hydrocyclones are most efficient when solids have a diameter
greater than 10 microns and are spherical in shape. If the solids are
flat, like mica, movement tends to be random and dependent on whether
the flat surface or edge is toward the gravitational force created in the
vortex. Since separation efficiency depends somewhat on the freedom
and velocity of the solid moving through the liquid phase, it is logical to
use a fluid of as low viscosity as possible.

Hydrocyclones have the following advantages:

1. Replacement of pump fluid end parts is reduced, and pumps operate
more efficiently.
2. Less drill string torque and drag equates to less wear on the string
and less key-seating (a major potential for stuck pipe). Casing is run
3. Bit life is extended; again, due to less abrasion.
4. Penetration rates increase.
5. Water dilution to maintain low mud weights is reduced. This is
reflected in smaller waste pits and drilling-fluid volumes to clean up
at the end of drilling activity.
6. Material additions are decreased.
7. Additions of weight material are made with little or no difficulty.
8. Downhole tools set and release with little or no interference from
drill cuttings.
9. Separation needed
10. Volume of feed slurry
11. Concentration and distributions of solids in feed slurry

In sizing, the bases from which measurements are made are:

1. Free liquid: water at 20 ºC, 68 ºF. Viscosity = 1 cP.
2. Solids: sand (spheres), SG 2.65
3. Feed concentration, extremely dilute: less than 1.0% solids by weight,
0.04% by volume

Because fine solids have more surface area per unit volume (specific
area), the amount of liquid discharged per pound of solids is higher with
fine solids than with coarse solids. Therefore, the difference between the
feed and underflow densities is not a reliable indicator of hydrocyclone
Pressure drop is a measure of the energy being expended in the cone,
and thus a higher pressure drop results in a finer separation.
If the D50 cut point is increased to 75 microns, 25% of the 100-micron
particles are retained and only 25% of the 55-micron particles are
The purpose of a hydrocyclone is to discharge maximum abrasive
solids with minimal fluid loss. Larger particles have a greater probability
to discharge through the bottom underflow (apex), while smaller and
lighter particles have greater probability to move through the top, or
overflow opening.
Cone diameter, cone angle, underflow diameter, feed head, and plastic
viscosity have the largest effects on hydrocyclone performance.
Barite particles 3 microns and smaller have a deleterious effect on
drilling-fluid viscosity due to surface charge imbalance resulting from
unsatisfied broken bonds on the ultra-fine’s surface. Therefore, if a
centrifuge is set such that its median (D50) cut in 14 ppg of mud is a
3-micron barite particle, its median cut for drilled solids will be 5 (or 4.9)


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