Decanter Centrifuge G force on Separation

Since G-force increases with the square of bowl RPM, it is an important parameter. G-force also increases linearly with bowl diameter. shows how solids removal efficiency improves with increasing G-force. For a given particle size and fluid properties, there is a minimum G-force necessary to invoke settling. Although high G-force is desirable, the cost is proportional to the cube of the bowl rpm and there are similar economic limitations on bowl diameter as well. Thus, the required G-force must be obtained from a practical combination of speed and diameter. Most oilfield centrifuges have bowl dimensions from 14 to 28 inches in diameter and lengths from 30 to 55 in. Rotational speeds range from 1000 rpm to 4000 rpm, depending on the application.

From Stokes Law, particle settling velocity is inversely proportional to fluid viscosity. The following figure illustrates the beneficial effects of a feed mud with a low yield value. This shows the merit of diluting the centrifuge feed to improve performance. It also helps explain the relatively poor performance of centrifuges when processing polymer fluids with characteristically high viscosities at low shear rates.

Discharge dryness is commonly considered a direct indication of centrifuge performance. However, test results have shown that cake dryness is more correctly a function of particle size, and therefore, is inversely related to separation efficiency. Test points have yielded the driest solids control corresponded to the lowest efficiency and coarsest D50 separation. As shown in Figure 3, Solids dryness occurs at a threshold Gforce level. Subsequent increases in G-force do not remove additional liquid. Length of the dry beach within the centrifuge bowl (a function of pond depth) also has little effect on dryness. Dry beach length refers to the distance from the solids discharge ports to the surface of the fluid pond within the centrifuge bowl. But the small difference in dryness made a significant difference in the appearance of the solids. At 71% by weight, the solids were quite runny and at 75% weight, the solids seemed much more “stackable”.

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