Hydrocyclones should not be operated in rope discharge because it will drastically reduce the cone separating efficiency. In a rope dis-charge, the solids become crowded at the apex, cannot exit freely from the underflow, and become caught
by the inner spiral reporting to the overflow. Solids which otherwise would be separated are forced into the overflow stream and returned to the mud system. This type of dis-charge can also lead to plugged cones and much higher cyclone wear.
While a spraying cyclone appears to discharge more fluid, the benefits of more efficient solids removal and less cone wear outweigh the addi-tional fluid loss. In cases where a dry discharge is required, the underflow from hydrocyclones can be screened or centrifuged to recover the free liquid.
3.8 DESANDERS
Desanders are hydrocyclones larger than 5” in diameter (6”, 8”, 10” or 12”
ID). Generally, the smaller the cone, the smaller size particles the cone will separate (see Figure 3-24).
Desanders are primarily used to
feed
SPRAY DISCHARGE ROPE DISCHARGE NO CROWDING
AT THE APEX
CROWDING AT THE APEX
Figure 3-23 Spray v. Rope Discharge
remove the high volumes of solids associated with extremely fast drilling of a large diameter hole.
Desanders are installed down-stream from the shale shaker and degasser. The desander removes sand sized particles and larger drilled solids which have passed through the shaker screen and dis-cards them along with some liquid into a waste pit. The partially clean mud is discharged into the next pit downstream.
INSTALLATION
When installing a desander, follow these general recommendations:
• Size the desander to process 110–125% of the total mud cir-culation rate.
• Keep all lines as short and straight as possible with a min-imum of pipe fittings. This will reduce loss of head on the feed line and minimize back-pressure on the overflow discharge line.
• Do not reduce the diameter of the overflow line from that of the overflow discharge mani-fold.
• Direct the overflow line down-ward into the next downstream compartment at an angle of approximately 45°. The over-flow discharge line should not be installed in a vertical posi-tion — doing so may cause excessive vacuum on the dis-charge header and pull solids
Figure 3-24
Particle Removal by Desander Cyclones (200 Mesh Screen Included for Comparison)
through the cyclone overflow, reducing the cyclone’s efficien-cy.
• Keep the end of the discharge line above the surface of the mud to avoid creating a vacu-um in the line.
• Position the underflow trough to easily direct solids to the waste pit.
• Install a low equalizer line to permit backflow into the desander suction. Operating desanders at peak efficiency is a simple matter, since most desanders are relatively uncomplicated devices.
Here are a few fundamental prin-ciples to keep in mind:
• Operate the desander unit at the supplier’s recommended feed head (usually around 75 feet). Too low a feed head decreases efficiency, while excessive feed head shortens the life of cyclone wear parts.
• Check cones regularly to ensure the discharge orifice is not plugged.
• Run the desander continuously while drilling and shortly after beginning a trip for “catch-up”
cleaning.
• Operate the desander with a spray rather than a rope dis-charge to maintain peak efficiency.
MAINTENANCE
Maintenance of desanders normal-ly entails no more than checking all cone parts for excessive wear and flushing out the feed manifold between wells. Large trash may col-lect in feed manifolds which could cause cone plugging during opera-tion. Preventive maintenance minimizes downtime, and repairs are simpler between wells than dur-ing drilldur-ing.
Use of desanders is normally dis-continued when expensive materials such as barite and poly-mers are added to a drilling mud, because a desander will discard a high proportion of these materials along with the drilled solids.
Similarly, desanders are not gener-ally cost effective when an oil-base mud is in use, because the cones also discard a significant amount of the liquid phase.
3.9 DESILTERS
A desilter uses smaller hydro-cyclones (usually 4” or 5” ID) than a desander and therefore generally removes smaller particles. The smaller cones enable a desilter to make the finest particle size separa-tion of any full flow solids control equipment — removing solids in the range of 15 microns and larger (Figure 3-25). This makes it an important device for reducing aver-age particle size and removing
abrasive grit from unweighted mud.
The cyclones in desilter units operate on the same principle as the cyclones used on desanders.
They simply make a finer cut, and the individual cone throughput capacities are less than desander cones. Multiple cones are usually manifolded in a single desilter unit to meet throughput requirements.
Desilters should be sized to process 110–125% of the full rig flow rate.
INSTALLATION
Installation of desilters is normally downstream from the shale shaker, sand trap, degasser and desander, and should allow ample space for maintenance. Here are some funda-mentals for installing desilters:
• Take the desilter suction from the compartment receiving fluid processed by the desander.
• Do NOT use the same pump to feed both the desander and desilter. If both pieces of equipment are to be operated at the same time, they should be installed in series and each should have its own centrifugal pump.
• Keep all lines as short and straight as possible.
• Install a guard screen with approximately 1/2” openings at the suction to the desilter to prevent large trash from enter-ing the unit and pluggenter-ing the cones.
Figure 3-25
Particle Removal by Desilter Cyclones (200 Mesh Screen Included for Comparison)
• Position the desilter on the pit high enough so the overflow manifold will gravity-feed fluid into the next downstream com-partment at an angle of approximately 45°. Remember
— no vertical overflow dis-charge lines.
• Keep the end of the discharge line above the surface of the mud to avoid creating a vacu-um in the line.
• Install a low equalizer line for backflow to the desilter’s suc-tion compartment.
• Position the underflow trough to easily direct solids to the waste pit.
Running a desander ahead of a desilter takes a big load off the desilter and improves its efficiency.
If the drilling rate is slow and the amount of solids being drilled is only a few hundred pounds per hour, then the desander may be turned off (to save fuel and mainte-nance costs) and the desilter may be used to carry the total desand-ing/desilting load. Appendix C includes a chart to calculate the pounds per hour of solids generat-ed for a range of hole size and rate of penetration.
Operating efficiencies of competi-tive desilters vary widely according to differences in design features.
The same technique described in
Appendix B for comparing two desanders will work to compare the efficiencies of competing desilters operating on the same rig.
GUIDELINES
To operate desilters at maximum efficiency, follow these basic guide-lines:
• Operate the cones with a spray discharge. Never operate the desilter cones with a rope dis-charge since a rope underflow cuts cone efficiency in half or worse, causes cone plugging, and increases wear on cones.
Use enough cones and adjust the cone underflow openings to maintain a spray pattern.
• Operate the desilter unit at the supplier’s recommended feed head. This is generally between 70–80 feet of head.
Too much energy will result in excessive cone wear.
• Check cones regularly for bot-tom plugging or flooding, since a plugged cone allows solids to return to the mud system. If a cone bottom is plugged, unplug it with a welding rod or similar tool. If a cone is flooding, the feed is partially plugged or the bottom of the cone may be worn out.
• Run the desilter continuously while drilling and also for a short while during a trip. The
extra cleaning during the trip can reduce overload conditions during the period of high solids loading immediately after a trip.
MAINTENANCE
A desilter’s smaller cyclones are more likely than desander cones to become plugged with oversized solids, so it is important to inspect them often for wear and plugging.
This may generally be done between wells unless a malfunction occurs while drilling. The feed manifold should be flushed between wells to remove trash.
Keep the shale shaker well main-tained — never bypass the shaker or allow large pieces of material to get into the active system.
A desilter will discard an
appre-ciable amount of barite, because barite particles fall within the silt size range. Desilters are therefore not recommended for use with weighted mud. Similarly, since hydrocyclones discard some absorbed liquid along with the drilled solids, desilters are not nor-mally used with oil-base mud, unless another device (centrifuge or mud cleaner/conditioner) is used to
“deliquor” the cone underflow.
3.10 DECANTING CENTRIFUGE
Centrifuges for oilfield applica-tions were first introduced in the early 1950s. These early units were adapted from existing industrial decanting centrifuges. In the mid-1960s, a perforated rotor type machine was developed which
Figure 3-26 Decanting Centrifuge SCROLL
SCROLL FEED CHAMBER
LIQUID DISCHARGE
SOLIDS DISCHARGE HOLLOW
SHAFT
BOWL
WEIRS
FEED PIPE
DRILLING MUD
does not perform like a pure decanter. Commonly called “barite recovery” centrifuges, these early designs were limited in capacity and application. Today, the cen-trifuge is even more important part of solids control. In addition, the increased use of low-solids mud and environmental dewatering applications require higher process volumes, greater clarification and solids capacity, and additional fine solids removal.
Equipment selection is decided by site specific requirements. Proper system selection is the first step to effective solids control.