Hydro Cyclones

Section 600 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Hydrocyclones

January 1998 Page 1 of 16

Hydrocyclones 1 Introduction .........................................................................................................................2

2 Principle of Operation.........................................................................................................2

3 Performance Parameters....................................................................................................4 3.1 Cone Diameter ..............................................................................................................5 3.2 Plastic Viscosity.............................................................................................................6 3.3 Feed Head.....................................................................................................................6 3.4 Underflow Diameter .......................................................................................................8 3.4.1 Spray Discharge...................................................................................................8 3.4.2 Rope Discharge ...................................................................................................8

4 Desanders.......................................................................................................................... 10 4.1 Recommended Desanders .......................................................................................... 10

5 Desilters............................................................................................................................. 10 5.1.1 Recommended Desilters.................................................................................... 12

6 Sizing Hydrocyclone Manifolds........................................................................................ 12

7 Operating Guidelines........................................................................................................ 13

8 Troubleshooting................................................................................................................ 15

9 Summary............................................................................................................................ 16 FIGURES Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5.

Hydrocyclone operating principles................................................................................3 Cone efficiency. ...........................................................................................................4 Sensitivity to plastic viscosity. ......................................................................................6 Sensitivity to feed head. ...............................................................................................7 Rope flow operation characteristics..............................................................................9

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Section 600 January 1998

SOLIDS CONTROL HANDBOOK

Hydrocyclones

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Schlumberger Dowell

Fig. 6. “Amoco” near optimum core efficiency. ......................................................................11 Fig. 7. Estimated discard rates..............................................................................................11 Fig. 8. Typical hydrocyclone manifold. ..................................................................................13 TABLES Table 1 Effect of Variables on Hydrocyclone Performance......................................................5 Table 2 Cone Capacity............................................................................................................5

1 Introduction Although the shale shaker is considered the primary solids removal device on the rig, hydrocyclones are a cost-effective method of removing many of the fine solids missed by the shaker in unweighted muds. In some formations, the solids are too fine for the shakers to remove; hydrocyclones must be relied upon to remove the majority of the solids. In these instances, the shaker protects the hydrocyclones from oversize particles which may cause plugging. Because the hydrocyclone has no moving parts, it can be a very reliable piece of solids removal equipment when correctly operated and maintained.

2 Principle of Operation Think of a tornado inside a bottle and you have a rudimentary idea of how a hydrocyclone operates. Fig. 1 illustrates the basic concepts of hydrocyclone operating principles. Mud enters the feed chamber tangentially at a high velocity provided by pump pressure. As the mud spirals downward through the conical section, centrifugal force and inertia cause the solids to gravitate towards the wall. The solids settle according to their mass, a function of both density and volume. Since the density range of drilled solids is normally quite narrow, size has the largest influence on settling. The largest particles will settle preferentially. As the cone narrows, the innermost layers of fluid turn back toward the overflow creating a low pressure vortex in the center of the cone. This low pressure area causes air to be pulled in from the underflow outlet. Correctlyoperating cones should exhibit a slight vacuum at the cone underflow. The air and cleaned fluid then report to the overflow through the vortex finder. The purpose of the vortex finder is to prevent some of the feed mud from “short-circuiting” directly into the overflow. Solids with sufficient mass cannot make the turn back towards the overflow because of their momentum and continue out the underflow. Maximum cone wear usually occurs at or near the underflow exit, where velocities are the highest. In cones having a “balanced design‚” whole mud losses out the underflow are slight. Only the solids and bound liquid will report to the underflow. If the solids are too fine to be removed by the cyclone, no liquid should be discharged. “Unbalanced” hydrocyclones will discharge mud without the presence of solids in the mud.

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Hydrocyclones


Fig. 1. Hydrocyclone operating principles. Note: The dark ribbon indicates the path taken by the mud and solids entering the cone. The smaller light ribbon shows the exit path of the cleaned fluid and fine solids.

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Schlumberger Dowell

Because fine solids have more specific area (surface area per unit volume) than large particles, the amount of liquid removed per pound of solids is higher with fine solids than with coarse solids. Therefore, the difference between the feed and underflow density is not a reliable indicator of hydrocyclone performance. Fig. 2 shows the relationship between underflow density and cone efficiency for an unweighted mud. Observe how overall cone efficiency decreases as underflow density increases.

Fig. 2. Cone efficiency. Note: Decreasing underflow diameter to improve dryness impairs cone efficiency.

3 Performance Parameters Oilfield hydrocyclones are available in cone diameters ranging from 1 in. to 12 in. Hydrocyclones were first used to reduce the API sand content (solids larger than 74 microns). Hence the term “desander.” By convention, hydrocyclones with diameters of 6 in. or larger are labeled as desanders. As the benefits of smaller, more efficient hydrocyclones became apparent, the term “desilter” was coined to reflect the smaller “silt-sized” particles these

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Hydrocyclones

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smaller cones could remove. Hydrocyclones with diameters of less than 5 in. are usually called desilters. However, these terms are not based on any particular performance standard. Separation efficiency varies widely among hydrocyclones classified as desilters. Amoco Production Research has investigated the operational and geometric design factors affecting hydrocyclone performance. Over 500 tests were conducted using bentonite and ground silica slurries. The effect of these variables on cone performance are summarized in Table 1. Selected variables are discussed below.

Table 1 Effect of Variables on Hydrocyclone Performance Major Effect

Minor Effect

Cone Diameter

Feed Solids Concentration (at constant PV)

Feed Solids Distribution

Yield Point

Plastic Viscosity

Inlet Type

Feed Head

Cylinder Length

Cone Angle

Vortex Finder Length

Underflow Diameter

3.1 Cone Diameter Cone diameter is the main factor in determining processing capacity, provided the basic design is sound. Larger cone diameters have higher throughput capacity and generally display inferior separation performance. Individual cone capacity guidelines are listed in Table 2.

Table 2 Cone Capacity Cone Size, inches

Cone Capacity, gpm @ 75 ft head

2

20

3 (Amoco)

50

4

50

5

75

6

100

8

125

10

500

12

500

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Section 600 SOLIDS CONTROL HANDBOOK

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Hydrocyclones

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Schlumberger Dowell

3.2 Plastic Viscosity Hydrocyclone performance is extremely sensitive to the plastic viscosity of the feed mud. Fig. 3 shows the effect of plastic viscosity on median separation size (d50) for a constant underflow solids concentration using a 3-in. hydrocyclone. Note how the median separation size increases rapidly with plastic viscosity from an initial 20 micron cut at PV=6 cp to 50 microns at PV=24 cp.

Fig. 3. Sensitivity to plastic viscosity. Note: Hydrocyclone performance declines with increasing plastic viscosity.

3.3 Feed Head Feed head, or feed pressure, affects hydrocyclone performance as shown in Fig. 4. Insufficient head reduces fluid velocity within the cone and adversely affects separation efficiency. Excessive head will cause premature wear and increased maintenance cost.

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Head is related to pressure and fluid density by the hydrostatic pressure equation: P = 0.052 x H x rmud where P is the feed pressure in psi, 0.052 is a gravitational constant, H is the head in ft, and rmud is the fluid density in lb/gal. Since most hydrocyclones require 75 ft of head, the required pressure for a given mud density can be approximated by: P = 4 x rmud Specific head requirements for most hydrocyclones are provided in Appendix F, Equipment Specifications. A centrifugal pump is used to feed the hydrocyclones because it provides a relatively constant head at a given flow rate. However, correct sizing of the pump is critical to ensure that sufficient head is available at the desired flow rate. Refer to the section on centrifugal pumps for a more detailed discussion on sizing and selecting centrifugal pumps for this application.

Fig. 4. Sensitivity to feed head. Note: This example, for a 3-in. cone, illustrates the importance of maintaining sufficient feed head. CONFIDENTIAL



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3.4 Underflow Diameter As underflow diameter is reduced, fewer solids will have sufficient mass (and momentum) to be discharged. The discharge will be dryer at the expense of separation efficiency. The appearance of the discharge gives a good indication of cone performance.

3.4.1 Spray Discharge A normally-operating cone should have an umbrella-shaped discharge of liquid and solids. The solids spiraling downward and out the cone bottom with their associated liquid are said to be in “spray discharge.” The inside stream moving up toward the overflow at high velocity will pull air with it in the vortex. This causes a slight vacuum to occur in the very center of the cone. The air is replaced by air drawn up through the center of the underflow opening as shown in Fig. 1. Therefore, the presence of spray discharge and a slight vacuum in the center of the underflow opening is a good indication of a properly operating hydrocyclone.

3.4.2 Rope Discharge If the solids concentration is high, there may not be room for all of the downward moving solids to exit the underflow. This causes an undesirable condition known as “rope discharge‚” so-called because of the shape of the underflow stream (Fig. 5). In rope flow, the solids back up near the exit and decelerate. The underflow density is very high, since the liquid volume is severely reduced and only the largest particles will exit the cone. Exit velocities are low; the solids will appear to be falling out of the underflow nozzle. Many of the solids will not be able to exit the cone and will return with the liquid in the overflow. High cone wear will occur in the lower region of the cone. Corrective action consists of opening up the underflow and making sure the opening is clear. If the problem still occurs, this is an indication that the solids loading needs to be reduced by adding more hydrocyclones. If the problem is with the desilter, ensure that the desander is operating and that the shakers are running the finest screens possible.

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Fig. 5. Rope flow operation characteristics. Note: This condition should be avoided; try increasing the underflow opening size. CONFIDENTIAL



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Schlumberger Dowell

4 Desanders With the improved fine screening capability of shale shakers, the need for desanders has diminished. The primary role of the desander should be to reduce solids loading to the desilter cones in unweighted water-based muds. Desanders are recommended when the shakers are unable to screen down to 100 microns (140 mesh U.S. Sieve), or when large hole diameters are drilled at 100 ft/hr or faster. Considering that 75 microns is probably the best performance that can be expected from a desander cone, one might conclude they would have an application in weighted muds as well. This is generally not the case. Hydrocyclones separate solids based on their mass and the density difference between the solid particles and the fluid. Since barite's specific gravity is substantially higher than drilled solids, it will tend to be preferentially removed by hydrocyclones. Also, as shown in Fig. 3, the higher plastic viscosities normally associated with weighted muds will greatly reduce the desander's efficiency. Desander underflows are normally quite dry and abrasive and should be discarded directly. When processing expensive muds, the underflow may be routed to a centrifuge to recover the liquid, provided the solids are not abrasive and the underflow is diluted with whole mud before centrifuging. Another option is to screen the desander underflow down to 200 mesh (74 microns) to remove the larger, abrasive solids before processing with the centrifuge.

4.1 Recommended Desanders Ten-inch diameter desander cones are recommended. They provide the best combination of separation and capacity. The larger 12-in. cones usually cannot make a fine enough cut to be economic. Smaller cones are limited in flowrate and may deteriorate more quickly in abrasive conditions.

5 Desilters Desilters should be used on all unweighted, water-based muds. They are not recommended for use on weighted muds since barite will be lost. When using expensive muds, process the desilter underflow with a centrifuge. APR has developed a 3-in. hydrocyclone which is up to 50% more efficient than some existing oilfield desilters. Fig. 6 shows the improvement in performance over a typical 50 gpm, 4-in. cone. The Amoco-designed, 3-in. cone is not a balanced cone; it will discharge fluid even when no solids are present. In many cases, this cone's underflow should be processed by a centrifuge. The economics of centrifuging the underflow should be checked using the SECOP program. Estimated discard rates per cone are plotted as a function of underflow diameter in Fig. 7. Size

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the centrifuge for the calculated underflow rate. Run the cones intermittently on unweighted mud when no centrifuge is available.

Fig. 6. “Amoco” near optimum core efficiency. Note: The 50 gpm Amoco-designed 3-in. cone exhibits greatly improved performance over a typical 4-in. cone at the same flowrate.

Fig. 7. Estimated discard rates. Note: Use this chart to estimate underflow rates from the Amoco-designed 3-in. cone. CONFIDENTIAL



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Since the underflow opening of the 3-in. cone is smaller than a typical 4-in. cone, it is more susceptible to plugging. Ensure that all of the mud is finescreened or run an efficient, properly installed desander ahead of the 3-in. cones. Desilters can also be used in certain weighted mud applications to reduce the barite loading to the centrifuge thereby improving its efficiency in barite recovery mode (see Chapter 8, Centrifuges). Here, the underflow of the desilter cones are returned to the active system and the overflow is fed to the Barite Recovery centrifuge. The upper limit for this application is generally limited to mud densities of 15 ppg or less due to viscosity and solids content limitations or cone performance. Use only enough 3-in. hydrocyclones to match the feed rate to the centrifuge. Blank off the remaining cones. Use the largest underflow nozzle diameter available to prevent plugging or rope flow.

5.1.1 Recommended Desilters MPE 3 in. (15° Cone) MPE 3 in. (10° Cone) These Amoco-designed cones are recommended because of their superior performance. They will provide the separation performance of a 2-in. cone at the flowrate of a typical 4-in. cone.

6 Sizing Hydrocyclone Manifolds For properly routed hydrocyclones, the minimum number required can be estimated by: No. of Cones Required =

Maximum Circulation Rate x 1.1 Single Cone Flow Rate

This equation does not consider solids loading. If penetration rates in excess of 100 ft/hr are anticipated, the number of cones should be increased. Specific head requirements and flow capacities for each cone are listed in Appendix F, Equipment Specifications. Table 2 may be used to estimate the flow capacity of each cone operating at 75 ft of head. Hydrocyclones are normally provided in banks of 8, 10, 12 and 16 cones per manifold (Fig. 8). Increase the required number of cones to one of these standard manifold sizes.

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Fig. 8. Typical hydrocyclone manifold. Note: This is an “inline” manifold. Circular manifolds are also common.

7 Operating Guidelines 1.

Operate enough hydrocyclones to process over 100% of the circulation rate or to handle the maximum solids loading rate.

2.

The hydrocyclone overflow should be discharged to a compartment downstream from the feed compartment. Use bottom equalization between compartments.

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Hydrocyclones

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3.

Mechanically stir all hydrocyclone removal and discharge compartments to ensure uniform feed. Mud guns should not be used because they can reduce hydrocyclone efficiency by bypassing a portion of the mud.

4.

Do not allow cones to operate with plugged apexes or inlets.

5.

Spray discharge at the cone underflow is desired. Rope flow will cause premature wear and is less efficient. Rope flow indicates that either more hydrocyclones or finer shaker screens are required or that the underflow apex size is too small.

6.

Because 2-in. cones are extremely susceptible to plugging, consider using the 3-in. cone instead. It has twice the capacity and equivalent performance.

7.

Do not bypass the shale shaker or operate with torn screens.

8.

The hydrocyclone manifold should be located above the mud level in the active system to prevent accidental loss of mud by siphoning when the cones are not operating.

9.

Replace flanged-type hydrocyclones with the quick-connect type to improve servicing time.

10. Replace worn, malfunctioning cones immediately. If no spares are available, remove the cone and blank off the feed and outlet lines. 11. Have a working pressure or head gauge on the manifold feed inlet. 12. Install a siphon breaker on the overflow manifold exit. 13. Size suction and discharge piping to provide flow velocities in the range of 5-10 ft/sec. Refer to Chapter 9, Centrifugal Pumps & Piping. 14. Use one centrifugal pump per hydrocyclone manifold.

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8 Troubleshooting Symptoms

Probable Causes

One or more cones are discharging - others OK.

not

Plugged at feed inlet or outlet - remove cone and clean out lines.

Some cones losing whole mud in a stream.

Backflow from overflow manifold, plugged cone inlet.

High mud loss, conical shape in some cones - others normal.

Low inlet velocity due to partially plugged inlet or cone body.

Repeated plugging of apexes.

Too small underflow shaker or torn screens.

High mud loss, all cones, weak stream, conical shape.

Low feed head-check obstruction, pump size and rpm, partially-closed valve, solids settling in feed line, frozen lines.

Cones at discharge end discharge poorly with a dryer stream.

Strong vacuum in manifold discharge line, usually occurs with long drop into pits - install antisiphon tube.

Cone discharge varying feed head.

Air or gas in feed, too small feed lines, air from upstream equipment discharge.

is

unsteady,

opening,

bypassed

Motor protection fuses “blow.”

Required input horsepower is higher than rated horsepower of motor - check for tees bypassing mud, additional equipment, manifolding.

Low impeller life.

Cavitation in the pump - flow rate is too high need larger lines. Suction line blockage - check for obstructions.

Mud percent solids continues to increase.

Solids removal is insufficient, solids may be too fine to remove, insufficient cones to match drilling rate - add cones.

Cones are discharging a heavy, slow-moving stream.

Cones are overloaded - use larger apex size, insufficient cones to match drilling rate - add more cones.

High mud losses.

Cone opening is too large - reduce size or consider centrifuging underflows.

Aerated mud downstream hydrocyclone overflow return.

of

Viscous mud, return line ends above fluid level in tank - route hydrocyclone overflow into trough to allow air to break out.

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Hydrocyclones

Schlumberger Dowell

9 Summary ·

Hydrocyclones separate solids from fluid by using centrifugal force to cause solids to be settled from the fluid. There are no moving parts. Centrifugal force is created by the conversion of centrifugal pump head into a high velocity stream spiraling within the cone. Solids concentrate in proportion to their mass near the wall of the cone and are discharged at the bottom of the cone in the underflow. Clean fluid and fine solids are returned through the top of the cone in the overflow.

·

Cone diameter, cone angle, underflow diameter, feed head, and plastic viscosity have the largest effect on hydrocyclone performance.

·

Hydrocyclones will produce a relatively wet discharge compared to shale shakers and centrifuges. Underflow density is not a good indicator of cone performance. Finer solids will have more associated liquid and the resultant density will be lower than with coarse solids.

·

Provide enough hydrocyclones to process at least 110% of the circulation rate, more if high penetration rates are expected.

·

Use desanders in unweighted mud when the shakers are unable to screen down to 140 mesh (100 microns). The role of the desander is to reduce solids loading to the downstream desilter. Ten inch diameter desander cones are recommended; they provide the best combination of separation and flow capacity.

·

Use desilters on all unweighted, water-based muds. The recommended Amoco-designed 3-in. cone is up to 50% more efficient than typical 4-in. cones. This cone is an unbalanced design and will discharge a very wet underflow. Process the underflow with a centrifuge to recover fluid, if the economics warrant.

·

Installation and operating guidelines, along with a troubleshooting guide are included in this chapter.

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Hydro Cyclones

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