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 shaker 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.
- 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.
- 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
- Bit life is extended; again, due to less abrasion.
- Penetration rates increase.
- 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.
- Material additions are decreased.
- Additions of weight material are made with little or no difficulty.
- Downhole tools set and release with little or no interference from drill cuttings.
- Separation needed.
- Volume of feed slurry.
- Concentration and distributions of solids in feed slurry.
In sizing, the bases from which measurements are made are:
- Free liquid: water at 20 °C, 68°F. Viscosity = 1 cP.
- Solids: sand (spheres), SG 2.65.
- 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 performance.
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 discharged.
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) microns.