Hydrocyclones

Hydrocyclones are essentially simple devices that convert pressure
generated by a centrifugal pump into centrifugal force, causing suspended
solids in the mud to be separated from the fluid. This separation is
actually accelerated settling due to the increased gravitational force
cause by the centrifugal action inside the cone. The action inside the
hydrocyclone can multiply gravitational force by as much as 200 times. In
drilling operations, hydrocyclones use these centrifugal forces to separate
solids in the 15- to 80-micron range from the drilling fluid. This solidsladen fluid is discharged from the lower apex of the cone, and the cleaned drilling fluid is discharged from the overflow discharge.

Hydrocyclones consist of an upper cylindrical section fitted with
a tangential feed section, and a lower conical section that is open at
its lower apex allowing for solids discharge (Figure 1). The closed,
upper cylindrical section has a downward-protruding vortex finder pipe
extending below the tangential feed location.

Fluid from a centrifugal pump enters the hydrocyclone tangentially,
at high velocity, through a feed nozzle on the side of the top cylinder.
As drilling fluid enters the hydrocyclone, centrifugal force on the swirling
slurry accelerates the solids to the cone wall. The drilling fluid, a mixture
of liquid and solids, rotates rapidly while spiraling downward toward
the apex. The higher-mass solids move toward the cone wall. Movement
progresses to the apex opening at the cone bottom. At the apex opening,
the solids along the cone wall, together with a small amount of
fluid, exit the cone. The discharge is restricted by the size of the apex.
Fluid and smaller-mass particles, which have been concentrated away from the cone wall, are forced to reverse flow direction into an upward-spiraling path at the center of the cone to exit through the vortex finder.

Hydrocyclone flow illustration.
figure 1.Hydrocyclone flow illustration.

The vortex finder is a hollow tube that extends into the center of the
cone. It diverts drilling fluid from flowing directly to the overflow outlet,
causing the drilling fluid to move downward and into the cone. The
swirling liquid is forced inward and, still rotating in the same direction,
reverses the downward flow and moves upward toward the center of the
vortex finder. In a balanced cone, the inner cylinder of swirling fluid
surrounds a cylinder of air that is pulled in through the cone apex. Solids
and a small amount of liquid are discharged from the lower apex of the
cylinder. The apex opening relative to the diameter of the vortex finder
will determine the dryness of the discharged solids.

Many balanced cones are designed to provide maximum separation
efficiency when the inlet head is 75 feet. To be sure what the recommended inlet head is, check with the manufacturer’s technical group. Fluid will always have the same velocity within the cone if the same head is delivered to the hydrocyclone inlet. Pressure can be converted to feet of head with the equation frequently used in well-control calculations but rearranged slightly:

head (in feet)= PSI ⁄ (0.052)• (mud weight in ppg)

The relationship between manifold gauge pressure and drilling-fluid
weight at constant 75-feet feed head is summarized in Table 1.

Table 1 Pressure for 75 Feet of Head for Various Mud Weights

Pressure (psig) Feed Head (ft) Mud Weight (ppg)
32.5 75 8.34
35 75 9.0
37 75 9.5
39 75 10.0
41 75 10.5
43 75 11.0
45 75 11.5
47 75 12
49 75 12.5
51 75 13

Hydrocyclones separate solids according to mass, which is a function
of both density and particle size. However, in unweighted drilling
fluids, the solids density has a comparatively narrow range, and size has the greatest influence on their settling. Centrifugal forces act on the
suspended-solids particles, so those with the largest mass (or largest size)
are the first to move outward toward the wall of the hydrocyclone.
Consequently, large solids with a small amount of liquid will concentrate
at the cone wall, and smaller particles and the majority of liquid will
concentrate in the inner portion.

Larger-size (higher-mass) particles, upon reaching the conical section,
are exposed to the greatest centrifugal force and remain in their downward spiral path. The solids sliding down the wall of the cone, along with the bound liquid, exit through the apex orifice. This creates the underflow of the hydrocyclone.

Smaller particles are concentrated in the middle of the cone with most
of the drilling fluid. As the cone narrows, the reduced cross-sectional
area restricts the downward-spiraling path of the innermost layers.
A second, upward vortex forms within the hydrocyclone, and the center
fluid layers with smaller solids particles turn toward the overflow. At the
point of maximum shear, the shear stress within a 4-inch desilter is on
the order and magnitude of 1,000 reciprocal seconds.

The upward-moving vortex creates a low-pressure zone in the center
of the hydrocyclone. In a balanced cone, air will enter the lower apex in
counterflow to the solids and liquid discharged from the hydrocyclone.
In an unbalanced cone, a rope discharge will emerge from the cone,
resulting in excessive quantities of liquid and a wide range of solids in the
discard. An unbalanced cone is little more than a settling pot, similar in
operation to a sand trap.

There are two countercurrent spiraling streams in a balanced hydrocyclone, one spiraling downward along the cone surface and the other spiraling upward along the cone center axis. The countercurrent directions, together with turbulent eddy currents, concomitant with extremely high velocities, result in an inefficient separation of particles. The two streams tend to commingle within the contact regions, and particles are incorporated into the wrong streams. Hydrocyclones, therefore, do not make a sharp separation of solids sizes. The efficiency of a hydrocyclone can be improved by extending the vortex finder farther into the cone, which eliminates some of the commingling. The farther the vortex finder is extended, the better the separation.

Hydrocyclone sizes are designed arbitrarily by the inside cone diameter
at the inlet. By convention, desanders have a cone diameter of 6 inches
and larger; desilters have internal diameters smaller than 6 inches.
Normally, discharges from the apex of these cones are discarded when
used on unweighted drilling fluids. Prolonged use of these cones on
a weighted drilling fluid will result in a significant reduction in drillingfluid density caused by the discard of weighting material. When these cones are used as part of a mud cleaner configuration, the cone underflow is presented to a shaker screen. The shaker screen returns most of the barite and liquid to the drilling-fluid system, rejecting solids larger than the screen mesh. This is a common application of unbalanced hydrocyclones, since the cut point is determined by the shaker screen and not the cone.

Since most hydrocyclones are designed to operate with 75 feet of head
at the input manifold, the flow rate through the cones is constant and
predictable from the diameter of the cone for a typical tangential feed cone inlet of given orifice size (Table 2). Obviously, manufacturers may
select different orifice sizes at the inlet of the cone. The orifice size determines the flow rate through the cone and the internal geometry of the cone.

Table 2
Flow Rates Through Hydrocyclones*

Designation Cone Diameter (in.) Flow Rate Through Each Cone (gpm)
Desilter 2 10~30
Desilter 4 50~65
Desilter 5 75~85
Desilter 6 100~120
Desilter 8 200~240
Desilter 10 400~500
Desilter 12 500~600

*These are general values. Some cones will vary.
Other design variations of the feed chamber, although more difficult
to manufacture, can minimize backpressure and therefore increase the
cone feed rate or capacity while also increasing separation efficiency.
Hydrocyclones with feed chambers that reshape the incoming drilling
fluid from a round-pipe profile to a rectangular one that conforms better
to the feed chamber geometry can increase the amount of drilling fluid
the cone can handle by 35–45% for the same pressure while at the
same time increasing separation efficiency. The increased performance is
due to the fact that the dead spaces in the feed chamber are eliminated.
If a ramp feed is included in the feed inlet, the drilling fluid is forced
down, which allows incoming drilling fluid to enter without the extremeturbulence caused by impingement of the drilling-fluid streams. This reduces backpressure, which increases through put even more. It also
minimizes turbulence inside the cone, which also increases the separation
efficiency.

The D50 cut point of a solids-separation device is usually defined as
the particle size at which one half of the weight of those particles goes to
the underflow and one half of the weight goes to the overflow. The cut
point is related to the inside diameter of the hydrocyclone. For example,
a 12-inch cone is capable of a D50 cut point of around 60 to 80 microns;
a 6-inch cone is capable of around 40 to 60 microns; and a 4-inch cone is
capable of around 20 to 40 microns. These cut points are representative
for a fluid that contains a low solids content. The cut point will vary according to the size and quantity of solids in the feed and the flow properties of the fluid.

When hydrocyclones are mounted above the liquid level in the mud
tanks, a siphon breaker should be installed in the overflow header or
manifold from the cones. Otherwise, a high vacuum will occur and will
actually vacuum up a lot of the solids that would otherwise be discarded;
instead, these solids are reintroduced back into to the active system. In
some extreme cases, no solids will exit the cone apex if the vacuum is
high enough. The siphon breaker installed as illustrated should be one
quarter of the diameter of the overflow header pipe (Figure 2).

Solution to common discharge header problem.
figure 2

 

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