Decanting centrifuges are mechanical devices used for the separation of solids from slurries in many industrial processes. In oilwell drilling, centrifuges are used to condition drilling fluids by dividing the fluid into high-density and low-density streams, permitting one to be separated from the other. The division is achieved by accelerated sedimentation. As the drilling fluid is passed through a rapidly rotating bowl, centrifugal force moves the heavier particles to the bowl wall, where they are scraped toward the underflow (heavy slurry) discharge ports by a concentric auger, also called a scroll or conveyor, which rotates at a slightly slower rate than the bowl. The separation of the heavier particles divides the processed fluid into two streams: the heavy phase, also called the underflow or cake; and the lighter phase, which is called the overflow, light slurry, effluent, or centrate. (see picture 1.)
If time were not a factor, sedimentation could be accomplished in any container. To reduce the time required, the geometry could be manipulated to limit the depth of the fluid and, consequently, the distance the settling particles would have to traverse before reaching the bottom of
the container (picture 2, 3, and 4). If this approach were used, a scraping device could remove the settled solids from the bottom of the container, and one end of the container could be sloped to permit the solids to be removed from the liquid by the scraping mechanism.
They could then be left to dry on this ‘‘beach,’’ known as the drainage deck, before being discharged.
Conditions on drilling rigs obviously preclude the use of this approach. The decanter centrifuge, however, utilizes essentially the same process. The inner surface of the rotating bowl receives the settled solids, as the container bottom does in the procedure just described, and the scroll functions as the scraper, conveying the settled solids to, and across, the beach, where they are dried by the removal of free liquid, then to the underflow discharge ports. Essentially, the centrifuge design wraps the surface corresponding to the bottom of a sedimentation container around the scraping device, the conveyor (Pictures 5, 6, and 7).
Basically, a decanting centrifuge is a simple machine: a rotating bowl containing a concentric conveying scroll. The rotation of the bowl forces solids to the wall, where the scroll transports the solids, as previously described. Two different bowl designs are available: conical, in which the entire bowl is cone shaped; and cylindrical/conical, in which the
effluent (or light slurry) end of the bowl is cylindrical. This configuration is preferred for drilling-fluids applications because it offers greater capacity.
The elevated centrifugal forces created by the rotation of the bowl accelerate the sedimentation process so that separation that might take hours or days under the normal gravitational force of 1 g in an undisturbed container is achieved in seconds at the 400–3000 g generated by the centrifuge.
Inasmuch as sedimentation is used to achieve the separation, an
understanding of the factors influencing the process is required for
the proper use of centrifuges. An Irish mathematician and physicist,
Sir George Stokes, who described the basic principals of fluid mechanics
in the mid-nineteenth century, defined sedimentation in Stokes’ law.
k=a constant that is dependent on the units in use
g=the gravitational constant
D=the diameter of the solid particle
ds=the density of the particle
df=the density of the fluid
µ=the viscosity of the fluid
The equation permits the calculation of the maximum sedimentation rate achieved by spherical particles. Note that it confirms what is intuitively clear, that particles settle more rapidly in less viscous fluids, and that heavy particles settle more rapidly than light particles. The utility of Stokes’ law lies in the fact that when used with proper and consistent units, it permits independent evaluation of each of the variables: particle size, the density of the particle and of the fluid, and the viscosity of the fluid. (For additional discussion of this topic, see on Settling Pits/Sand Traps.)
The mass of a particle depends on its size and density. While there is a technical difference, mass is essentially equivalent to weight. Stokes’ law shows that at any given viscosity and fluid density, the sedimentation rate depends directly on the mass, or weight, of the particle.