Page 174 - Shale Shakers Drilling Fluid Systems

SOLIDS CONTROL EQUIPMENT 157
FIGURE 7-7. Tank for continuous removal of solid
particles from a process liquid.
FIGURE 7-8
FIGURE 7-9
FIGURE 7-10. Cross section.
The entire assembly is rotated while increasing
the "G"-force on the solids, which accelerates set-
tling. The fluid moves with the outer cylinder of
the centrifuge so there is no shear within the fluid.
This is the reason that dilution fluid is normally
added to the input stream of a decanting centri-
fuge. The low-shear-rate viscosity of most drilling
fluids is increased to aid hole-cleaning and to pro-
vide weighting agent support. This low-shear-rate
viscosity elevation will also inhibit settling within
a centrifuge.
To convey the solids out of the centrifuge, the
conveyor and bowl must rotate at slightly differ-
ent speeds. This is accomplished using a planetary
gearbox for belt drive centrifuges. Typically, the
entire assembly rotates in the same direction, but
the conveyor rotates at a slightly slower speed.
The conveyor moves the solids to the solids dis-
charge end and the liquid, or effluent, emptys out
of the weirs at the liquid discharge end.
To calculate the "G"-factor a centrifuge imparts
to solids, the formula is as follows:
where G is the ratio of the centripetal acceleration
of the bowl compared to the acceleration of grav-
ity. (Note, this is the same equation used to cal-
culate the "G"-factor of shale shaker vibrators.)
A centrifuge provides a method of increasing
the settling force on particles suspended in liquid.
The force depends on the mass of the particle and
not the chemical composition. Particles with the
same mass, whether they are barite, low-gravity
solids, gold, iron, or silver, will settle at the same
rate. Centrifuges are able to separate solids above
and below the 2- to 10-micron size range. In
weighted drilling fluids, centrifuges are capable of
eliminating very small particles that can cause
dramatic increases in both the low- and high-
shear-rate viscosities. In unweighted drilling fluid
they are used as "super desilters."
Types of Centrifuges
Two types of centrifuges are generally used
to process drilling fluids: decanters and rotary
mud separators.
A decanting centrifuge (Figure 7-10 or 7-11) has
a bowl that rotates at a high speed (from about
1200 to 3600 rpm). The solids are thrown to the
wall of the bowl with a force that depends on their
mass. A conveyer transports the solids along the
inner wall of the bowl so that they are discharged