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Crews need to be alert to torn screens no matter what shaker is used.
This is especially true during slow drilling, when drill pipe connections
are infrequently made. When riser-assist pumps are used, flow should
be periodically directed to different shakers during connections. This
allows screens to be properly inspected and replaced, if needed.
7.5.4 g Factor
The g factor refers to a ratio of an acceleration to Earth’s gravitational
acceleration. Jupiter has a mass of 418.6 1025 lb and Earth has a mass
of 1.317 1025 lb. A person on Earth who weighs 200lb would weigh
320 times as much on Jupiter, or 64,000 lb. A person’s mass remains
the same on Earth or Jupiter, but weight is a force and depends on
the acceleration of gravity. The gravitational acceleration on Jupiter is
320 times the gravitational acceleration on Earth. The g factor would
be 320. (As a point of interest, Mars has a mass of 0.1415 1025 lb, so
the g factor would be 0.107; a 200-lb person would weigh only 21.4 lb
on Mars.)
The term ‘‘ g force’’ is sometimes used incorrectly to describe a g factor.
In the preceding example, the g force on Earth would be 200 lb and the
g force on Jupiter would be 64,000 lb. This is because the acceleration
of gravity on Jupiter would be 320 times the acceleration of gravity onEarth.
Calculation of g Factor
Accelerations are experienced by an object or mass rotating horizontally
at the end of a string. A mass rotating around a point with a constant
speed has a centripetal acceleration (Ca) that can be calculated from
the equationCa ¼ ro2
where r is the radius of rotation and o is the angular velocity in radians
per second.
This equation can be applied to the motion of a rotating weight on
a shale shaker to calculate an acceleration. The centripetal acceleration
of a rotating weight in a circular motion with a diameter (or stroke) of
2r, in inches, rotating at a certain rpm (or o) can be calculated from
the preceding equation,
ðCaÞ ¼ ð1=2ÞðdiameterÞfog2:
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Ca ¼ 1
2ðstroke, in inchesÞ
1 ft
12 inches
RPM 2 radians
revolution
1 minute
60 seconds
2
Combining all of the conversion factors to change the units to ft/sec2:
Caðin ft/sec2Þ stroke, in inches
ðRPMÞ2
Normally this centripetal acceleration is expressed as a ratio of the value
to the acceleration of gravity:
No. of g’s ¼ Ca
32.2 ft/sec2 ¼ ðstroke, in inchesÞ ðrpmÞ
2
70490 :
Shale shakers are vibrated by rotating eccentric masses. A tennis ball
rotating at the end of a 3-ft string and a 20 lb weight rotated at the
same rpm at the end of a 3-ft string will have the same centripetal accel-
eration and the same g factor. Obviously, the centripetal force, or the
tension in the string, will be significantly higher for the 20-lb weight.
The rotating eccentric weight on a shale shaker is used to vibrate the
screen surface. The vibrating screen surface must transport solids across
the surface to discard and allow fluid and solids smaller than the screen
openings to pass through to the mud tanks. If the weights rotated at a
speed or vibration frequency that matched the natural frequency of the
basket holding the screen surface, the amplitude of the basket’s vibration
would continue to increase and the shaker would be destroyed. This will
happen even with a very small rotating eccentric weight. Consider a child
in a swing on a playground: Application of a small force every time the
swing returns to full height (amplitude) soon results in a very largeamplitude. This is a case in which the ‘‘forcing function’’ (the push every
time the swing returns) is applied at the natural frequency of the swing.
When the forcing function is applied at a frequency much larger than
the natural frequency, the vibration amplitude depends on the ratio
of the product of the unbalanced weight (w) and the eccentricity (e) to
the weight of the shaker’s vibrating member (W ); or
vibration amplitude ¼ we
W :
The vibration amplitude is one half of the total stroke length.
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As noted previously (in the Linear Motion Shale Shakers subsection),
only a portion of the energy transports the cuttings in the proper
direction in unbalanced elliptical and circular vibration motion designs.
The remainder of the energy is lost due to the peculiar shape of the
screen bed orbit, as manifested by solids becoming nondirectional ortraveling in the wrong direction on the screen surface. Linear motion
and balanced elliptical designs provide 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 liquid 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 generally at an angle to the
screen. Usually the two rotary weights are aligned so that the accele-
ration is 45 to the screen surface. The higher liquid capacity of linear
motion shale shakers for the same size openings in the screens on
unbalanced elliptical or circular motion shakers seems related primarily
to the fact that a pool of drilling fluid is created at the entry end of the
shale shaker. The pool provides a liquid head to cause a higher flow rate
through the screen. 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-inch stroke at 1500 rpm, the
maximum acceleration is at an angle of 45 to the shale shaker deck.
The g factor 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 from
the horizontal, the stroke remains the same. The component of the
stroke parallel to the screen moves the solids up the 5 incline.
vy1
1 2
1 2
Position A
Position B
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Relationship of g Factor to Stroke and Speed of Rotation
An unbalanced rotating weight vibrates the screen deck. The amount of
unbalanced weight combined with the speed of rotation will give the
g factor imparted to the screen deck (see preceding paragraphs). The
stroke is determined by the amount of unbalanced weight and its dis-
tance 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.) Stroke is independent of the
rotary speed.The g factor can be increased by increasing the stroke or the rpm, or
both, and decreased by decreasing the stroke or rpm, or both. The stroke
must be increased by the inverse square of the rpm reduction to hold the
g factor constant. Examples are given below to hold 5 g’s constant while
varying stroke length at different values of rpm:
5 g’s @ 0.4400 stroke at 900 rpm 4 g’s @ 0.3500 stroke at 900 rpm
5 g’s @ 0.2400 stroke at 1200 rpm 4 g’s @ 0.2000 stroke at 1200 rpm
5 g’s @ 0.1600 stroke at 1500 rpm 4 g’s @ 0.1300 stroke at 1500 rpm
5 g’s @ 0.1100 stroke at 1800 rpm 4 g’s @ 0.0900 stroke at 1800 rpm
7.5.5 Power Systems
The most common power source for shale shakers is the rig electrical
power generator system. The rig power supply should provide constant
voltage and frequency to all electrical components on the rig. Most
drilling rigs generate 460 alternating-current-volt (VAC), 60 Hz, 3-phasepower or 380 VAC, 50 Hz, 3-phase power. Other common voltages are
230VAC, 190 VAC, and 575VAC. Through transformers and other
1 2
Position C
Position D
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