BAKER HUGHES - Drilling Fluids Reference Manual

MECHANICAL SOLIDS CONTROL
Figure 10 - 4 Cross-Sectional View of Hydrocyclone Operation
Desanding hydroclones are usually 6 inches or larger in diameter. As the name implies, desander
hydroclones are designed to handle large volumes of fluid and remove essentially all low-gravity
particles above 74 microns, the sand designation size. The desander is economical and performs
very well in low solids, inexpensive drilling fluids.
Desanders, operated with a spray discharge, will normally make a 30-micron median cut on low-
drilling fluids. Median cut implies that there is a 50-50 chance for
gravity solids in low-density
particles of 30 microns and above to be ejected through the underflow opening, normally called the
apex valve. Median cut is called the D-50 cut point. A 30-micron median
cut on low-gravity solids
would be equivalent to a 20-micron median cut on barite. Since quality barite normally contains
40% of particles greater than 20 microns in size, it would be economically impractical in most
cases to desand a weighted drilling fluid. An exception to this would be in higher viscosity drilling
fluids where the median cut point would be greatly increased. In these cases, it may be practical to
desand a weighted drilling fluid for short intervals providing the barite content of the underflow is
closely monitored.
Hydroclones are normally rated using water which provides the minimum viscosity possible. This
results in reported cut points that are comparable from one unit to the next, but do not give realistic
numbers that can be related to a drilling fluid. Because the cut point is directly proportional to the
viscosity of the fluid, the actual performance on a viscosified fluid will be much higher than the
manufacturer's specifications. Remember, cut points mentioned here are based on optimum
conditions.
Twelve inch diameter desander cyclones are the most common and will process approximately 500
gal/min with 75 ft feed head. This is equal to 35 psi when a 9 lb m /gal is being processed. Sufficient
cyclones should be utilized to process 1.25 to 1.5 times the maximum circulating rate. The back
flow in the fluid pit, caused by this excess of processing capacity from the desander discharge pit to
the suction pit, will ensure that all of the fluid is processed. In each and every case where
hydrocyclone solids processing equipment is being utilized, fluid back flow is important
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 10-12
REFERENCE MANUAL