Page 122 - Shale Shakers Drilling Fluid Systems

SHALE SHAKER DESIGN 105
high vibration speeds, the peak force, in pounds
(from the peak acceleration), can be calculated
from the equation:
Therefore, the force available on the screen sur-
face is a function of the unbalanced weight (w),
the eccentricity (e), and the rotation speed (N).
Stroke length for a given design depends on the
amount of eccentric weight and its distance from
the center of rotation. Increasing the weight
eccentricity and/or the rpm increases the "G"-
factor. The "G"-factor is an indication of only the
acceleration of the vibrating basket and not nec-
essarily its performnce. Every shaker design has
a practical "G"-factor limit. Most shaker baskets
are vibrated with a five horsepower or smaller
motor and produce 2 to 7 G's of thrust to the vi-
brating basket.
Conventional shale shakers usually provide
a "G"-factor of less than 3; fine screen shale
shakers usually provide a "G"-factor between 4
and 6. Some shale shakers can provide as much
as 8 G's. The higher the "G"-factor, the greater
the solids separation possible and, generally, the
shorter the screen life. The higher the solids ca-
pacity, the less tendency there is for the screens
to blind.
In unbalanced elliptical and circular vibration
motion designs, only a portion of the energy trans-
ports the cuttings in the proper direction. The re-
mainder is lost, due to the peculiar shape of the
screen bed orbit, and is manifested as solids becom- the stroke, rpm, or both. It can also be decreased
ing nondirectional or traveling the wrong direction
on the screen surface. Linear motion designs pro-
vide positive conveyance of solids throughout the
vibratory cycle because the motion is straight-line
rather than elliptical or circular.
Generally, the acceleration forces perpendicular
to the screen surface are responsible for the liq-
uid and solids passing through the screen, or the
liquid capacity. The acceleration forces parallel to
the screen surface are responsible for the solids
transport, or the solids capacity.
On a linear motion shaker, the motion is gen-
erally at an angle to the screen. Usually, the two
TABLE 3-1
5 G's @ 0.44" stroke at 900 rpm
5 G's @ 0.24" stroke at 1200 rpm
5 G's @ 0.16" stroke at 1500 rpm
5 G's @ 0.11" stroke at 1800 rpm
rotary weights are aligned so that the acceleration
is 45° to the screen surface. The higher liquid ca-
pacity of linear motion shale shakers for the same
mesh screens on unbalanced elliptical or circular
motion shakers is primarily related to the fact that
a pool of drilling fluid is created at the entry end
of the shale shaker. The linear motion moves the
solids out of the pool, across the screen, and off the
end of the screen.
On a linear motion shaker with a 0.13-in. stroke
at 1500 rpm, the maximum acceleration is at an
angle of 45° to the shale shaker deck. The "G"-fac-
tor would be 4.15. The acceleration is measured
in the direction of the stroke. If the shale shaker
deck is tilted at an upward angle of 5° from the
horizontal deck, the stroke remains the same. The
component of the stroke parallel to the screen
transports the solids up the 5° incline.
Relationship of "C"-factor to Stroke
and Speed of Rotation
An unbalanced rotating weight vibrates the screen
deck. The amount of unbalanced weight, com-
bined with the speed of rotation, will yield the "G"-
factor imparted to the screen deck. The stroke is
determined by the amount of unbalanced weight
and its distance from the center of rotation and
the weight of the shale shaker deck. (This assumes
that the vibrator frequency is much larger than the
natural frequency of the shaker deck.) The stroke
is independent of the rotary speed.
The "G"-factor can be increased by increasing
in the same manner. The stroke must be increased
by the inverse square of the rpm reduction to hold
the "G"-factor constant. Examples are given in
Table 3-1 to hold 5 G's constant while varying the
stroke length at different values of rpm. The shaker
manufacturers select the combination that appears
to work best for their machine.
POWER SYSTEMS
The most common power source for shale shak-
ers is the rig electrical power generator system.
4 G's @ 0.35" stroke at 900 rpm
4 G's @ 0.20" stroke at 1200 rpm
4 G's @ 0.13" stroke at 1500 rpm
4 G's @ 0.09" stroke at 1800 rpm