Page 245 - Shale Shakers Drilling Fluid Systems
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EQUIPMENT GUIDELINES 227
particles are removed and few drilled solids A centrifuge usually processes only a small
are available to settle. (API defines anything portion of the total rig flow. Backflow should
larger than 74 microns, or 200 mesh, as sand.) not be expected between the centrifuge
With the introduction of linear motion shale suction and discharge compartments. All
shakers (using 200-mesh screens) and the em- other removal equipment (degasser, desander,
phasis on minimizing rig discharges, fewer desilter, and mud cleaner) should process all
rigs are including sand traps in their removal of the fluid entering the suction compartment.
systems. Many rigs place their desander pump This may exceed the rig flow if drilling fluid
suctions in the former sand traps. This requires enters upstream from another process or from
that these compartments be well agitated. mud guns.
6. Use top equalization between the degasser suc- 11. Based on experience a rule-of-thumb for the
tion and discharge compartments. minimum square feet of horizontal area for a
The degasser should process more fluid than compartment is as follows:
is entering its suction compartment. This will .. . . , _ f . . „. Max. Circulating Rate (epm)
&
create a backflow from the discharge com- Horizontal Surface Area (sqjt) = — ^ '
partment into the suction compartment. Only
processed drilling fluid can flow over the top It has been found from experience that this rule
weir. Any drilling fluid still containing gas will of thumb provides fluid velocities low enough to
overflow over the top weir back to the degasser allow entrained air bubbles to rise to the surface
suction compartment. and break out. Note: This rule-of-thumb was de-
veloped by George Ormsby and is included in the
7. Use bottom equalization between the suc- IADC Mud Equipment Manual, Handbook 2:
tion and discharge compartments ofdesanders, "Mud System Arrangements," pp. 2-17.
desilters, mud cleaners, and centrifuges.
This is strictly an empirical guideline. Follow-
Openings in the partitions between the suc- ing this guideline, usually results in most of
tion and discharges are primarily needed to the air leaving the drilling fluid, although many
allow a small backflow. Since the level can types of drilling fluids (polymer, synthetics,
vary in these compartments, an underflow is relaxed fluid-loss oil muds, mineral oil muds,
needed so that no adjustment is necessary etc.) were not in existence when this rule was
while drilling. Also, an overflow tends to en- developed. Since the air break-out is a func-
train air as the fluid cascades into the up- tion of the interfacial tension and the low-
stream tank. shear-rate viscosity of the fluid, this formula
should be used with caution. This is also the
8. Use an adjustable equalizer between the re- reason that all tanks should be adequately
moval and addition sections when cyclones and/ stirred. The entrained air must be brought to
or centrifuges are being used. Run with the high
position on the downstream side. the surface so that it can leave the system.
An adjustable equalizer is usually a curved 12. Mechanical Stirrers are preferred for stirring
pipe that can swivel in the equalizing line. removal compartments.
The upper end is in the downstream compart-
ment. Fluid exits the bottom of the removal All fluid entering a suction compartment should
section and flows through the adjustable equal- be processed. Mud guns suctioning from a
izer. The downstream end of the equalizer is downstream compartment will increase the
elevated so that the liquid level in the removal quantity of fluid that must be processed. Me-
section remains constant. chanical stirrers eliminate this problem. Mud
guns can be used in removal compartments if
9. Use bottom equalization in the Addition Section each centrifugal pump stirs its own suction.
and in the Check/Suction Section.
13. Mechanical stirrers should be properly sized
This will allow the maximum use of the fluid and installed according to manufacturer's
contained in these sections. recommendations.
10. For removal devices processing flow rates greater Mechanical stirrers are available in two stir-
than the rig circulating rate, equalizing flows ring blade shapes. One type is canted to ac-
should always be in the reverse (or upstream) tually pump fluid vertically downward. The
direction. second type has vertical blades, similar to the
