Cut Point

Notice the removal range, or Cut Point, is given as a range of the particle size removed. Mechanical solids control equipment classifies particles based on size, shape, and density. It is typical to refer to particles as being either larger than the cut point of a device (oversize) or smaller than the cut point (under-size).

Figure 3-2 shows a typical cut point curve. The cut point curve represents the amount of solids of a given size that will be classified as either oversize or undersize. Particles to the right of the cut point curve, in the area labeled "A", rep

How Create Cut Point Curve

Particle size, microns (i

Figure 3-2 Typical Cut Point Curve

Particle size, microns (i

Figure 3-2 Typical Cut Point Curve resent the removed, oversize solids. Particles to the left of the curve, in the area labeled "B", represent the undersize solids returned with the whole mud.

Particular interest is given to three points along the cut point curve, the D50, the D 16, and the Ds4. Given these three points, the removal characteristics of screens, hydrocyclones, or other devices can be compared.

The D50, or median cut point, is the point where 50% of a certain size of solids in the feed stream will be classified as oversize and 50% as undersize. The D16 and Ds4 are the points where 16% and 84%, respectively, of the solids in the feed stream will be classified as oversize. These two points are statistically significant because they are one standard deviation from the D50 in a normal distribution. An "ideal" classifier (the dashed line) would show very little difference between the D50, D16 and Ds4.

Separation Efficiency is a measure of the D50 size relative to the number of undersize particles that are removed or oversize particles that are not removed. The higher the separation efficiency, the lower the

Separation Curve

Particle size, microns ¡J

Figure 3-3 Separation Curve

Particle size, microns ¡J

Figure 3-3 Separation Curve false classification. An example will assist in understanding this concept.

Figure 3-3 shows the cut point curves for two screens, each with the same D50. Curve No.1 is almost vertical with a small tail at each end. This results in a very sharp, distinct cut point. Almost all particles larger than the cut point are rejected, with very few undersize solids. Almost all particles smaller than the cut point are recovered, with very few oversize particles included.

Curve No. 2 is an S-shaped curve with a large tail at each end. Even though the D50 is the same as for Curve No.1, the Die and Ds4 are very different. Many solids larger than the D50 are returned with the under-size solids and many solids smaller than the D50 are discarded with the oversize solids.

If curves number 1 and 2 in Figure 3-3 illustrate typical removal gradients for two different types of oilfield shale shakers screens, we can draw conclusions about separation performance. The area between the curves marked "A" represents solids Screen No.1 removes and Screen No. 2 returns. Likewise, the area marked "B" represents solids recovered by Screen No.1, but discarded by Screen No. 2.

This is not to say that Screen No.1

is "better" than Screen No. 2, or vice versa; it simply illustrates that two devices with similar "cut point" (as measured by the D50 alone) may perform very differently. As an example, consider solids removal from a weighted drilling fluid using vibrating screens.

An effective solids control program for weighted mud should remove as many undesirable, sand-sized solids as practical, while retaining most of the desirable, silt-sized barite particles. Referring back to Figure 3-3, Screen No. 2 would return all the sand in area "A" that Screen No.1 would catch, and Screen No. 2 would remove the silt-size material in area "B" (including all weighting material) that Screen No.1 would recover.

Therefore, in a weighted mud,

Screen No. 2 would not perform as well as Screen No.1. Further, if the area to the right of both curves (representing total mass solids removal) were calculated, Screen No.1 could prove superior in terms of mass solids removal.

As shown by this example, it is important to view "cut point" as a continuous curve, rather than a single point. This concept is equally true with screens, hydrocyclones, centrifuges, or any other separation equipment — the relative slope and shape of the cut point curve are more important than a single point on the curve.

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