Chapter 4.
Shaker Screens
Shaker screen selection has the largest impact on the overall performance of the
shale shaker. It is therefore important to understand the factors which may impact
screen performance and how to properly select screens. Shaker screen perfor-
mance is measured by:
1. Separation Performance - the size of the solids removed
2. Liquid Throughput Performance - the capability of the screen to transmit fluid
3. Service life
Separation Performance
Grade Efficiency
The separation performance of a shale shaker screen (or any other solids control
device) is commonly represented by its percent-s99eparated, or grade efficiency,
curve. This curve is generated from full-scale experimental measurements and
depicts the percent solids removed as a function of particle size. It reports the
screen's probability of separating any specific particle size with a given shaker
under conditions specific to the test. Grade efficiency is the preferred measure of
separation performance because it is independent of feed particle size distribu-
tion.
An example of a percent-separated curve is shown in Figure 4.1. In this example,
the median size separated by the screen was 145 microns. This means that 50%
of the solids with a diameter of 145 microns were removed. A rough estimate of
the median cut point (d50) can be made in the field by the wet sieve procedure
(see Field Procedure to Estimate Cut Point, p 4.9).
Separation Potential
A method was developed that characterize the relative separation efficiency
potential of shaker screens without the expense and time required for full-scale
testing. The technique links the relative separation performance of screens to a
volume-equivalent distribution of their opening sizes.
The screen's openings are measured using PC-based image analysis technology.
Each opening in the screen is then represented by a spherical diameter corre-
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Solids Control Manual
sponding to an ellipsoidal volume calculated from the image analysis data. The
cumulative volume of these ellipsoids, when plotted as a function of spherical
diameter, yields a curve which correlates well with the standard grade efficiency
curve. This curve represents the “separation potential” of the screen. The word
“potential” is used because the screen's separation performance is not measured
directly, but implied by the size of the screen's apertures.
Note: Grade separation efficiencies as measured on the shaker are subject to
specific shaker and flowline conditions. They may not always agree with separa-
tion potential values. For example, the separation potential value for a screen with
rectangular openings may be pessimistic when drilling clean sand sections pro-
ducing predominantly spherical sand grains. The image analysis method
assumes solids of all shapes and sizes are available to the screen. However, on
average, the separation potential values have been shown to adequately repre-
sent the screen's separation performance.
Figure 4.1 Percent Separated Curve. This curve indicates the percentage of solids removed as
a function of particle size.
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Shaker Screens
Liquid Throughput Performance
The liquid throughput capacity of a screen panel is primarily a function of screen
conductance and usable area. Conductance describes the ease with which fluid
can flow through a unit area of screen cloth. In simplistic terms, it is analogous to
permeability with the length in the direction of flow (screen thickness) taken into
account. Higher conductances will result in higher flow rates through the screen.
Conductance is calculated from the mesh count and wire diameters of the screen
cloth by the equations given in Appendix A, Conductance Calculation. Multilayer
screens can also be handled by the conductance equation. The inverse of con-
ductance for each screen layer is summed to equal the inverse of the net overall
conductance:
1 1 1 1
----- = ------ + ------ + … -------
Ct C1 C 2 Cn
This is valid provided that the screen layers used in the composition are designed
to remain in contact.
Oilfield screens are typically bonded to a perforated metal panel or plastic grid to
provide extra strength and improve service life. This practice eliminates some of
the usable area through which fluid may pass. Some metal backing plate designs
may reduce effective screening area by as much as 40 percent. Because conduc-
tance describes screen flow capacity per unit area, the usable unblocked area
available for screening must also be considered when comparing the mud pro-
cessing capacity of shaker screen panels.
Screen Life
The definition of “acceptable” screen life must be judged within the context of the
total solids removal system economics. Besides screen replacement cost, consid-
eration must be given to the costs of drilling mud dilution and waste disposal costs
when determining whether longer screen life is warranted at the expense of solids
removal efficiency. In weighted mud applications, the economic benefits of
improved solids removal efficiency usually outweigh the additional screen costs.
Effect of Screen Composition
Only very general correlations may be made between screen composition and
service life. Unfortunately, features that lead to improved life are usually detrimen-
tal to flow capacity. Using heavier wires with greater tensile strength or adding
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Solids Control Manual
supporting layers of cloth can both reduce conductance. Increasing support
through additional bonding area (smaller plate openings) eliminates usable
screening area. Also, support techniques and screen tension can have a major
effect on screen life. As a result, screen panels are typically designed to balance
flow capacity performance with screen life.
Screen life is heavily dependent upon flow line conditions. Solids loading rate,
drilled cuttings abrasiveness, and shaker dynamics can easily outweigh composi-
tion effects.
Effect of Vibration Pattern
Linear Motion
The abrupt changes in acceleration during the vibration cycle tends to cause
screens to wear more quickly unless close attention is paid to tensioning and
screen support techniques. Perforated metal backing plates and pretensioned
screen panels have been specifically developed to address this problem. Linear
motion shakers usually operate at less than 4.0 G's (normal to the screen) to bal-
ance screen life with processing capacity. Regardless, the finer screens normally
run on linear motion shakers cannot be expected to outlast the coarser screens
used in the past. For screens finer than 100 mesh, expect an average service life
in excess of 100 hours.
Circular, Elliptical Motion
The smooth change in acceleration with respect to direction translates into longer
screen life compared to other vibration patterns. However, many circular motion
shakers were designed before the advent of fine mesh screens and may provide
less support for the screens. This will tend to negate much of the screen life bene-
fit associated with circular motion.
Shaker Screen Designations
Mesh Count
Shaker screens have traditionally been assigned mesh count designations by the
manufacturer. Unfortunately, they do not adequately describe screen performance
in terms of separation efficiency or flow capacity.
Mesh count is defined as the number of openings per linear inch of screen cloth.
Mesh count does not establish the size of screen openings unless wire diameter
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Shaker Screens
is known. The opening size, D, is related to the wire diameter, d, and the mesh
count, n, by the following equation:
1
D = --- – n
d
With the wide variety of wire diameters used to construct the same mesh count,
the actual separation efficiencies of screens with the same mesh count designa-
tion are rarely consistent:
1. Manufacturers commonly designate layered screens by a single mesh count
number. Experimental separation efficiency tests have revealed that these
designations are predominantly optimistic.
2. Oblong mesh screens may be identified by a single number which may be the
sum of mesh counts in both the horizontal and vertical direction. For example,
a 60 x 40 mesh screen may be labelled “100 mesh”. This practice is mislead-
ing: The opening sizes of a 60 x 40 mesh screen will pass much larger parti-
cles than a 100 x 100 square mesh screen.
API RP13E Screen Designation
Recently, a new performance-based screen designation system has been devel-
oped. This designation system has been adopted by the API RP13E as a Recom-
mended Practice for Shale Shaker Screen Cloth Designations. The API has
recommended that all screens be labelled with the following information:
Screen Name
Separation Potential (d50, d16, d84)
Flow Capacity (Conductance, Total Non-Blanked Area)
A comprehensive list of screen designations for most shakers is included in
Appendix C, Screen Designations. The screen designations include additional
information not specified by the API to further define screen performance. Each of
the designation components are described in detail below:
Screen Name
This is the “mesh count” designation or part number used by the manufacturer to
identify the screen. Typically, it consists of a mesh count number preceded by a
letter code which may describe the screen's cloth type or layering technique. For
example, MG100 signifies a 100 x 100 mesh “market grade” bolting cloth, a PWP
HP100 signifies a perforated plate, triple-layer screen composed of oblong mesh
screen cloth.
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Solids Control Manual
Equivalent U.S. Sieve Number
This is the U.S. Sieve Number which has the same median opening size, or d50,
as the screen. Table 4.1 lists the opening sizes of the standard U.S. Sieve series.
In cases where no actual U.S. Sieve exists for a given opening size, the equiva-
lent U.S. Sieve Number is a linearly-interpolated value. This value provides a sim-
ple scale by which to quickly rank the separation potential of screens. Caution
should be exercised when using this value to compare screens of different type
since it represents only the median separation potential of the screen.
Table 4.1 U.S. Sieve Series
U.S. Sieve Opening Size U.S. Sieve Opening Size
Number Microns Number Microns
3.5 5660 40 420
4 4760 45 350
5 4000 50 297
6 3360 60 250
7 2830 70 210
8 2380 80 177
10 2000 100 149
12 1680 120 125
14 1410 140 105
16 1190 170 88
18 1000 200 74
20 840 230 62
25 710 270 53
30 590 325 44
35 500 400 37
Separation Potential (d50, d16, d84)
The separation potential of the screen is represented by 3 points on the separa-
tion potential curve, labelled d16, d50 and d84 (Figure 4.1). These points are the
spherical diameters, in microns, corresponding to 16, 50 and 84 percent of the
cumulative ellipsoidal volume distribution of hole sizes present in the screen. It
must be stressed that these values provide a relative measure of a screen's
potential ability to remove solids. They may not necessarily agree with mea-
sured grade efficiency cut points for a given application.
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Shaker Screens
d50
The d50 is the median aperture size of the screen on a volume-equivalent basis. In
experimental grade efficiency terms, it is analogous to the size of solid that has a
50% probability of separation. The d50 is typically used as a single value indicator
of separation efficiency performance. Because of it's importance, the d50 is listed
first.
d16, d84
The d16 and d84 values indicate the range of hole sizes present in the screen. The
d16 and d84 values can be important when the removal of fines from an
unweighted mud is desired, or when the removal of barite is a concern. The devi-
ation from the d50 describes the screen's implied separation characteristics. As
the difference between the d16 and d50 increases, it is more likely that some solids
finer than the d50 will likely be removed. Conversely, a smaller percentage of sol-
ids coarser than the d50 may be removed as the difference between the d84 and
d50 increases. A multilayered screen will generally have a larger spread between
the d16 and d84 values than a single mesh screen with the same d50.
Flow Capacity (Conductance, Nonblanked Area)
The calculated conductance is reported in units of kilodarcies/millimeter for the
total screen composition.
Non-blanked area is the total effective screening area per panel, in units of square
feet.
Note: Support rails on the shaker deck can reduce the usable area of screens not
mounted on metal backing plates. This area reduction is not included in the calcu-
lation of usable area because it is not a function of screen panel construction and
will vary with the shaker type.
Transmittance
Transmittance represents the net flow capacity of individual screens. It is the
product of conductance and unblocked screening area. Transmittance permits the
comparison of individual screens which differ in usable screening area.
Aspect Ratio
Aspect ratio describes the average shape of the screen openings.
It is the volume-weighted average length-to-width ratio of the screen openings.
Aspect ratio serves as an indicator of screen composition and provides informa-
tion about the screen's potential resistance to blinding.
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Solids Control Manual
Rectangular, or oblong, mesh screens have been customarily employed to reduce
the “blinding” problems exhibited by square mesh screens when drilling sand sec-
tions. The “near-size” sand grains lodge in the square mesh screen apertures and
reduce mud processing capability. The longer slots in the oblong screens are
more likely to be only partially blocked by these spherical particles and thus tend
to resist blinding. Aspect ratios in excess of 1.5 are typical of oblong mesh
screens (both single and multilayered designs) used in the oil field. Single layer
square mesh screens have aspect ratios near unity.
Layered, unbonded, square mesh “sandwich” screens have the capacity to
“actively deblind” (remove particles) by the interactive movement between the lay-
ers. This feature is lost when the layers are bonded together to improve screen
life. Laboratory tests have shown that blinding increases substantially when the
apertures in the metal backing plate or plastic grid have dimensions of less than
4 x 4 in. Figure 4.2 shows how blinding severely restricts the flow capacity of the
shaker when smaller opening dimensions in the screen panel are used.
Figure 4.2 Effect of Plate Opening Size on Screen Blinding. Plate openings with dimen-
sions less than 4 x 4 in. lose their deblinding ability.
Some improvement in blinding resistance over single layer square mesh cloth is
still apparent in bonded, multilayer square mesh screens: Stacking one screen
cloth over a slightly coarser cloth results in a wide range of hole sizes and shapes.
Only the portion of the screen with openings near in size to the sand will tend to
be blinded. Aspect ratios of layered square mesh screen compositions range from
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1.3 to 1.5. The relationship between screen composition and blinding resistance is
summarized in Table 4.2.
Table 4.2 Blinding Resistance of Common Screens
Screen Panel Composition Aspect Ratio Blinding Resistance
Single or double layer, square mesh < 1.2 poor
Triple layer, square mesh, bonded 1.3-1.5 fair
Triple layer, square mesh, unbonded 1.3-1.5 best*
Rectangular mesh, all types > 1.5 better
* provides “active” deblinding through layer interaction
Field Procedure to Estimate Cut Point (D50)
Note: This procedure provides only a rough approximation of the cutpoint. It
assumes that the mass flowrate of the solids discard is negligible compared to the
feed and screen unders. Results may be inaccurate under high solids loading.
Equipment
• U.S. Test Sieves (Enough sizes to bracket expected cut)
• Sample Containers
• Sand Content Tube and Funnel
Procedure
1. Take equal sized samples of both feed and unders. Avoid taking unders sam-
ples at the point where the fluid enters the sand trap. Where possible, take
them from directly under the screen.
2. Wet sieve each sample and measure the volume retained on each sieve
using sand content tube.
3. Calculate the percent separated for each test sieve by the following method:
Feed Vol. – Unders Vol.
%Separated = ----------------------------------------------------------- × 100
Feed Vol.
4. Plot through the midpoint of each sieve range as a function of volume percent
removed.
5. Read the median cut point (d50).
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Solids Control Manual
Summary
• Shaker screens control the separation and liquid throughput performance of
the shale shaker.
• Separation performance may be measured by two methods:
A. Percent-separated or grade efficiency.
Generated from full-scale measurements, a grade efficiency curve repre-
sents the screen’s probability of separating any specific particle size
under the specific conditions of the test. The median separation of the
screen, commonly called the “d50” or “cut point,” represents the particle
size that has a 50% probability of being removed. A field procedure is
provided to estimate the d50 of the shaker screens.
B. Separation potential.
This method uses the range of opening sizes in the screen to indicate the
relative separation performance of the screen. Because the screen is
visually analyzed, separation potential is independent of operating condi-
tions. This method has been adopted by the API as a Recommended
Practice for Shaker Screen Cloth Designations under API RP13E.
• Liquid throughput performance is represented by the screen’s conductance
and usable screening area. Conductance, calculated from the physical
dimensions of the screen composition, is analogous to the screen’s perme-
ability. The conductance equations are included in Appendix A, Conductance
Calculation. Usable screening area is the area in the screen panel available
for fluid flow.
• Mesh count designations do not adequately describe screen performance
because wire diameters and opening sizes are not consistent, and layered
screen compositions are not correctly represented. The API RP13E recom-
mends that all screens be labeled with: screen name, separation potential
(d50, d16, d84), and flow capacity (conductance and total nonblanked area).
Appendix C, Screen Designations, contains screen designations and dis-
cusses the relative merits of specific screen types for most shaker and screen
combinations.
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