Page 117 - Shale Shakers Drilling Fluid Systems

100 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

finer mesh screens. Linear motion is produced by
a pair of eccentrically weighted, counter-rotating,
parallel vibrators. This motion provides cuttings
conveyance when the screen deck is tilted upward.
Linear motion shakers have overcome most of
the limitations of elliptical and circular motion
designs. Straight-line motion provides superior
cuttings conveyance (except for gumbo) and su-
perior liquid throughput capabilities with finer
mesh screens. These shakers can effectively re-
move gumbo if they are sloped downward toward
the discharge end; however, they generally do not
convey gumbo uphill. The increased physical size
of these units (and an accompanying increase in
deck screen surface area) allows the use of even
finer screens than those used on circular or ellip-
tical motion shakers.
Screening ability is the result of applying the
energy developed by a rotating eccentric mass to
a porous surface or screen. The energy causes the
screen to vibrate in a fixed orbit. This transports
solids across the screen surface, off the discharge
end, and induces liquid to flow through the screen.
In conventional unbalanced elliptical and circu-
lar motion designs, only a portion of the energy
transports the cuttings in the proper direction, FIGURE 3-8. Linear motion system with two eccen-
toward the discharge end. The remainder is wasted, trically weighted, counter-rotating shafts.
due to the peculiar shape of the screen bed orbit,
and is manifested as solids that become non-
directional or traveling the wrong direction on the
screen surface. Linear motion designs provide posi- Some machines have adjustable angle drive
tive conveyance of solids throughout the vibratory systems that can be changed to account for vari-
cycle because the motion is straight-line rather than ous process conditions (Figure 3-9). If the thrust
elliptical or circular. The heart of a linear motion angle is decreased (for example, to 30° to the hori-
machine is its ability to generate this straight-line or zontal), the "X" component of the resultant vi-
linear motion and transmit this energy in an efficient bratory thrust (force) would increase and the "Y"
and effective manner to the vibrating bed. component would decrease. Conversely, building a
As shown in Figure 3-8, a linear motion system greater angle would cause the "X" component to
consists of two eccentrically weighted, counter- decrease and the "Y" component to increase.
rotating shafts. The net effect of each equal eccen- A larger "X" vector component of thrust will
tric mass being rotated in opposite directions is move solids along the deck faster. A larger "Y"
that resultant forces cancel at all positions along component vector increases liquid throughput and
the vibratory trace, except at the very top and increases residence time of material on the screen.
bottom of each stroke, resulting in a thrust (vibra- Most manufacturers choose a fixed angle near 45°
tion) along a "straight-line." Hence, the term that yields near equal values for each vector. This
"linear" or "straight-line" motion. is a logical approach since the shaker must simul-
To achieve the proper relationship between the taneously transmit liquid through the screen as it
rate of solids conveyance and liquid throughput, conveys solids off the screen.
the drive system must be mounted at an angle to The ability to create linear motion vibration al-
the horizontal bed. A thrust angle of 90° relative lows the slope of the bed to vary up to a +6° in-
to the screen surface would simply bounce solids cline (which impacts residence time and, therefore,
straight up and down. Taken to the other extreme, shaker performance) and to create a liquid pool at
a thrust angle of 0° would rapidly move solids but the flow line end of the machine. This allows a
yield inadequate liquid throughput and solids that positive liquid pressure head to develop, which
were discharged very wet. On most units the ap- helps "drive" liquid and solids through the finer
propriate angle is approximately 45° to the hori- wire cloths. The deck on most linear motion shale
zontal (Figure 3-8). shaker designs can be adjusted up to a maximum