Most hydrocyclones are of a balanced design. A properly adjusted,
balanced hydrocyclone has a spray discharge at the underflow outlet and exhibits a central air suction core. Many balanced hydrocyclones can be adjusted so that when water is fed under pressure, nothing discharges
at the apex. Conversely, when coarse solids are added to the feed slurry,
wet solids are discharged at the apex. Even with this adjustment, there
still should be a large opening in the bottom of the cyclone. This will
confirm that the cyclone is hydraulically balanced and discharges at the
bottom (apex) only when solids, which the cyclone can separate, are in
the feed slurry (drilling fluid).
A balanced cyclone should be operated with spray discharge. In this
process, coarser solids separate to the outside in the downward spiral
and pass over the lip of the apex as an annular ring. The apex is actually
a weir, or dam, not a choke or valve.
The high-velocity return stream spinning upward near the center of
the cone into the vortex finder generates a column of lower pressure,
which sucks air inward through the center of the apex opening.
To set a cone to balance, slowly open the apex discharge while
circulating water through the cone. When a small amount of water is
discharged and the center air core is almost the same diameter as the
opening, the cone is said to be balanced (see Figure 1). spray may indicate that the apex orifice is too large. When the apex orifice is larger than required, an excess amount of liquid will exit,
carrying with it finer feed solids, thereby reducing sharpness of
separation and underflow density. However, if more of the fine solids
are discarded through the apex, then the density of the mud returned
to the active system will be reduced. This is not necessarily a bad thing.
A larger opening minimizes plugging of the apex. It depends on what is
most important for your drilling operation. Is it solids removal or liquids
conservation that is most important to you? This is a question that
should be asked for each and every drilling operation so that the
equipment can be chosen and set up accordingly.
It should be mentioned that there are some balanced hydrocyclones
that will not balance in the way described above. They are instead
balanced by operating with a central air core but always having a spray
discharge, even if only pumping water. A cone operation like this is due
to its feed-chamber design and/or the ratio of the apex-opening diameter
to the vortex finder opening diameter. Which type of cone is best
depends on your drilling situation, needs, and parameters.
Several conditions restrict separations and exiting of solids that have
spiraled along the cone wall. These include:
. Excessive solids concentration
. Excessive volumetric feed rate per cone (going past the balance point
on a balanced cone)
. Excessive fluid viscosity
. Excessive vacuum (caused by a long siphon leg)
. Restricted (too small) apex
. Inadequate feed pressure
A greater number of larger solids are entrained within the central vortex
stream to exit with the overflow. The discharge pattern changes from
spray to rope discharge, which is characterized by a cylindrical, or rope like, appearance. With the rope discharge, no air core occurs through the center of the cone. In this case, the apex acts as a choke that restricts
flow, rather than as a weir.
Rope discharge is a process in which material pours from the cone
apex as a slow-moving cylinder (or rope). A hydrocyclone operating like
this is performing an inefficient solids/liquid separation. The apex velocity
in rope discharge is far less than that in spray discharge; therefore,
separation is less efficient because fewer solids are discarded (Figure 2).
A rope discharge can create a false sense of success, as the heavier rope
stream appears to contain more solids and does in fact have a higher
density than a cone operating in spray discharge. In reality, however, a
rope discharge indicates that not all solids that have been separated
inside the cone can exit through the apex opening. Solids become
crowded at the apex and cannot exit the cone freely. The exit rate is
slowed significantly, and some solids that would otherwise be separated
become caught in the inner spiral and are carried to the overflow. Dry
discharge can also produce cone plugging.
With rope discharge, the exiting solids stream is heavier than that
under spray-discharge conditions. All discharged solids will have a surface
film of bound liquid. Since finer solids have a greater ratio of surface
area to volume (size or mass), finer solids’ streams involve greater
volumes of bound water. More bound water causes a less dense underflow
stream (the finer the particle separation, the wetter the apex stream).
This explains why spray discharge stream densities are less than rope
discharge stream densities.
The amount of fluid lost in cone underflow is important. A hydrocyclone
operating with spray discharge gives solids a free path to flow
(to exit the cone). Rope discharge is a dry discharge. Therefore, spray
discharge removes significantly more solids than rope discharge. More
fluid may be lost in spray discharge, but the greater solids-separation
efficiency makes the additional fluid loss insignificant. If fluid loss is a
concern, the underflow can be screened (see Chapter 12 on Mud Cleaners)
or centrifuged for liquid recovery.
Rope discharge should be immediately corrected to reestablish the
higher volumetric flow and greater solids separation of spray discharge.
A rope discharge indicates that equipment is overloaded and additional
hydrocyclones may be necessary. If the desilter cones are roping and the
desander cones are not being run, then turn on the desanders to remove
some of the load from the desilters. If possible, install finer screens on the
shakers to take some of the load off of the hydrocyclones, which may
reestablish spray discharge. If the apex weirs are pinched down, open
them up fully to allow more solids to exit the cones.