DECANTING CENTRIFUGES

The decanting centrifuge is the only liquid-solids separation device used on drilling fluids that can remove (decant) all free liquid from the separated solids particles, leaving only adsorbed liquid or “bound liquid ,” on the surface area. This adsorbed liquid is not prone to contain solubles, such as chlorides, nor colloidal suspended solids, such as bentonite . The dissolved and suspended solids are associated with the continuous free liquid phase from which the decanting centrifuge separates the inert solids, and are removed with that liquid. The adsorbed liquid can only be removed from the separated solids by evaporation, which has been neither desirable nor practical so far in drilling mud work.

Although centrifuges of other types, notably the nozzle solids discharge type with vertical axis, has been tested periodically in this country since the early thirties for use on weighted water base muds, the decanting centrifuge was first tested and proved adaptable as a practical field tool in 1953. It moved rapidly into the rental field and is now recognized universally as a useful tool, though perhaps not as well understood as it might be.

A more complete description of present decanters would be “continuous conveyor, conical bowl, decanting centrifuges.” A schematic cutaway of the actual separating chamber is shown in Figure 1. The size of the machines is usually given as maximum bowl diameter and length. Decanting centrifuges in drilling fluid use are the Pioneer Centrifuging Company 18″ Dia. x 28 ” Long; Bird Machinery Co. 18″ x 28″ (Baroid and others) ; Sharples 14″ x 20 ” (independent); and the Gilreath approximately 14″ x 22 ” (Dresser-SWACO and others) . The principle of operation is the same for all these . All else equal , the larger machines have higher load capacities at equal separation ability , and finer separation ability at the same loads. The Gilreath machine has departures from conventional design that increase its feed capacity at a he a vy sacrifice of separation ability.

Centrifugal Operating Principles

Referrin g to Figure 1, the separation occurs inside the bowl , which is rotated at speed ranges that vary (accordin g to the bowl size , conveyor design , application , and preventive maintenance program ) from 1500 to 3500 RPM. Inside the bowl is usually a double lead conveyor connected to the bowl through an 80: 1 gear box so the conveyor will lose one revolution for every 80 revolutions of the bowl. Thus there is a relative rotation of 22.5 RPM when the bowl speed is 1800 RPM. This relative rotation is the conveying speed , and it can be varied by using other gear ratios.

 Decanting centrifuge; sectional view of separation
Figure 1 Decanting centrifuge; sectional view of separation

The mud slurry to be treated is metered to the centrifu ge through a feed tube centered in the hollow axle to the feed chamber. From there the feed passes through feed ports into the leads in the separation chamber. The scrolls, or blades (one in the single lead mode ), form the walls of the leads (or lead ). As the slurry is slung outward into the annular rin g of mud called the pool , it also accelerates to the approximate rotational speed of the conveyor and bowl. The depth of the pool is determined by the adjustment of the eight to twelve overflow openings or overflow ports.

The slurry flows through the leads toward the overflow ports under very high centrifugal force and in laminar flow in most machines and applications. Following Stokes Law, the solids settle outward to the outer walls, the most massive particles settling first ; and the lesser ones further along the leads. Those not having committed to the solids cake against the wall by the time the slurry stream is increasing in velocity to exit through the overflow ports, will not be settled. They will pass out the overflow with the liquid. In the normally recommended operating ranges, retention time in the machines will vary from 10 to 80 seconds (less for the Gilreath designed machine).

As the solids are settled outward against the wall, the conveyor con tinuously plows or scrapes them toward the small end of the bowl, out of the pool, and across that portion of the wall between the pool and the solids underflow discharge openings called the “beach.” As the solids cake crosses the beach, all free liquid is removed by both centrifugal squeezing of the cake. The free liquid drains back to the pool , taking with it the colloidal and dissolved solids. As the separated solids discharge out the underflow, they contain only adsorbed, or “bound,” liquid.

Dilution of the feed to a decanter has only one purpose- to reduce the viscosity of the feed to aid in maintaining separation ability. The most convenient rule of dilution is to add only enough dilution water to reduce the effluent to 37 Marsh funnel seconds but do not add any to lower the viscosity to less than 35 seconds . More dilution will decrease settling time
out of proportion to the increase in settling rate.

Dilution is applicable only to water base muds. Continuous oil-phase muds respond better to heat , and stove -oil dilution for centrifuging is not advised. Heating above 90 °F . (32° Celtius) for centrifuging usually is neither economical nor necessary, but separation does benefit at a decreasing rate up to at least 140 °F . (60° Celtius).

Decanter centrifuge Capacities

The solids discharge capacity of a decanting centrifuge is limited by the volume rate of solids cake that can be moved by the conveyor and discharged by the underflow openings. The liquid discharge capacity of a decanter is limited by the capacity of the overflow ports to discharge liquid at whatever pool depth adjustment they may be set. The feed rate capacity of the decanting centrifuge will be determined by the volume of separable solids in the feed or by the free liquid and colloid content of the feed, according to which discharge limit is being approached-liquid or solids. This is to say that a feed mud low in solids has the feed rate limited by liquid discharge capacity; a feed very high in separable solids (a heavily Weighted mud) has the feed limit determined by the solids discharge capacity.

If a very fine separation is desired, it may be necessary to hold down feed volume rate in low solids feeds to stay within laminar flow in the settling chamber and also to maintain retention time for fine separation ability.

Centrifuge Separation Limits

An effective decanting centrifuge for weighted water base mud work should be capable of making a separation approximately at the two micron barite particle size. Due to the settling behavior according to Stokes Law, light solids would be separated down to three microns (1 ½ x 2 microns ) in the same machine under the same conditions. In a continuous oil phase mud, the effective viscosity of the oil is greater than that of water in a water base mud , therefore the separation in the centrifuge is not as fine.

In field operation , the decanting centrifuge is fitted with a housing over the bowl , liquid and solids “hoppers” to take the liquid and the solids discharges of each respectively, a slurry feed pump, dilution water connection, and remix water connection if desired. See Figure 2. The feed is metered with a calibration of one kind or another . The bowl speed usually can be varied , but variation is best left to the centrifuge technician , as it is seldom either necessary or desirable. To reduce damage to expensive gears, there are shear-pins , trips , limit switches , etc. to shut down the equipment automatically in case of a solids overload.

Decanting Centrifuge , Scheme of operations
Figure 2 Decanting Centrifuge , Scheme of operations

Dilution water connections should be equipped with a flow gauge or meter and a flow adjusting valve . Remix connections are usually provided. “Remix water” can be added in the housing to aid in softening the underflow solids for more rapid mixing into the system, or for reducing the underflow solids slurry density to the range of system density , as desired.

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