Before hydrocyclones and centrifuges became available for drilling applications, shale shakers and dilution were the only means of controlling the solids content of drilling fluids. Consequently, solids too fine to be separated by the shaker screens in use could be controlled only by dilution. During drilling with weighted muds, once the solids content reached the maximum acceptable level, the continuing and unavoidable incorporation of drilled solids made it necessary to add a continuous stream of water to control viscosity, while adding barite to control the mud weight. This was, obviously, a costly procedure that generated large quantities of excess drilling fluid.
The decanter centrifuge, in splitting the processed fluid into two streams—the underflow, or ‘‘cake,’’ containing the coarser solids; and the overflow, centrate, or effluent, containing most of the liquid and the finer particles—provides a means of selectively removing the finest, most damaging, solids from the drilling fluid. The removal of these solids in order to control rheology and filter cake quality is the primary reason for centrifuging weighted drilling fluids. When the finest solids are not removed, the only alternative means of reducing their concentration is
dilution, always an expensive process.
Centrifuging weighted drilling fluids routes the coarser solids (both barite and low gravity) to the underflow, and the finer solids (bentonite, barite, and low gravity) to the overflow. Separation of the overflow, consisting of the finer solids together with most of the processed liquid and the dilution fluid, reduces the concentration of the viscosity-building solids, alleviates solids problems, and reduces the need for dilution.
This application is often described as barite recovery, a term that does not accurately describe the process, leads to confusion, and is frequently the reason for improper centrifuge use. The validity of the term depends on the preliminary acceptance of the idea that the fluid entering the centrifuge would otherwise be discarded and that the barite is recovered by the centrifuge. Few, if any, drilling people think of centrifuging in these terms. Centrifuges, like shale shakers and hydrocyclones, are solids-removal devices. Centrifuging weighted muds while drilling is correctly thought of as an alternative to dilution for the reduction of viscosity; not as a means of recovering barite from discarded fluid. Another objection to the phrase is that it lends support to the idea that barite recovery is the reason for centrifuging. It is not. The objective of centrifuging in this manner is the removal of colloids and ultra-fine solids to improve drilling-fluid quality. A third objection is that the use of the term tends to create the totally erroneous impression that the process separates barite from low-gravity solids and that the recovered material is all barite. A natural consequence of this belief is that the underflow from barite-recovery centrifuges is sometimes stored and used to weight up freshly prepared drilling fluids. Inasmuch as the recovered slurry often contains high concentrations of drilled solids, it can be severely contaminated, and is rarely suitable for reuse.
A much better term for the process of discarding the centrate while returning the overflow to the mud system is traditional centrifuging.
Refer again to Figure 1. The mixture of feed mud and any dilution fluid enters the acceleration chamber, or feed chamber, from which it is ejected through the feed ports by centrifugal force. Centrifugal force then carries the slurry to the pool, or pond, where the increased centrifugal forces produced by the rotation of the bowl cause the larger, heavier particles to settle to the bowl wall. For larger solids, this happens almost immediately, while it takes longer for the smaller solids that are large enough to settle. Solids that reach the wall are scraped toward the beach (drainage deck) and solids-discharge ports by the scroll. The pool is the mud in the bowl at any given time, and the beach is the area between the end of the pool and the solids-discharge ports. During their passage across the beach, most of the free liquid is removed from the solids. The discharged solids will, unavoidably, be wet with adsorbed liquid, but no free liquid should be present. The degree of dryness of the solids in the cake is primarily a function of solids size, the characteristics of the feed fluid, and the operating parameters of the centrifuge. Smaller particles have greater surface area per unit of volume and consequently adsorb more liquid.
The depth of the pool is controlled by the adjustment of the weirs, or effluent discharge ports, at the large liquid-discharge end of the machine. Increasing pool depth increases residence time and separation efficiency while reducing flow capacity. On the other hand, if the other parameters are unchanged, reducing pool depth decreases residence time and the separation of finer particles.
Contour bowls, those with a cylindrical shape for part of their length, as opposed to entirely conical bowls, are able to handle higher feed rates at any given cut point. Adjustment of the level of the solids (or cake)- discharge ports controls the flow capacity. When the pool depth reaches the level of these ports, the floodout point has been reached, and liquid is lost with the separated solids. This is ordinarily undesirable.
The relative motion between the scroll and the bowl, which controls the rate at which cake is removed from the machine, is set by the gearbox. Typically available gearbox ratios include 40:1, 52:1, 80:1, and 125:1. In each case, the scroll makes one less rotation than the bowl at the specified number of bowl rotations. For example, at 80:1, the scroll rotates 79 times each time the bowl rotates 80 times. Solids conveyance is faster at the lower ratios. The relative conveyor rpm can be calculated by dividing the rpm of the bowl by the gearbox ratio. For example, with a 40:1 gearbox, a bowl rotating at 1800 rpm has a differential speed of 45 rpm.
Although the fluid within the bowl is rotated rapidly, it is important to note that there is no shear within the fluid itself once it enters the bowl. Consequently, the low-shear-rate viscosity must be low to allow settling of solids.
The primary variable controlling sedimentation rate is the centrifugal force, which is proportional to bowl diameter and the square of rpm. Centrifuges used in drilling applications usually have diameters of 14–28 inches and bowl lengths of 30–55 inches. Rotational speeds are generally from 1500 to 4000 rpm, with most machines operating toward the lower end of this range. High-g centrifuges can produce more than 3000 g: 3000 times the acceleration of gravity. The g force can be calculated from the following equation:
g = (rpm²)(1.42×10^5)(bowl diameter, in)
For example, for a 14-inch bowl rotated at 2000 rpm:
g = (2000)²(1.42×10^5)(14)
g = 795.
A useful rule of thumb concerning rpm and maintenance costs merits consideration when deciding upon the desirable g force. It states that maintenance requirements are proportional to the cube of the rpm. Doubling the rpm can be expected to increase maintenance cost and downtime by a factor of 8, while an increase of 25% would almost double them. Another consideration is that at higher g forces, more solids are separated, and they tend to become more tightly packed, making them more difficult to transport and requiring more torque. Inasmuch as the available torque is limited, the feed rate may have to be limited to avoid stalling the scroll.