Page 164 - Shale Shakers Drilling Fluid Systems
P. 164
SOLIDS CONTROL EQUIPMENT 147
In this case, the apex acts as a choke that restricts Hydrocyclones are rated from 40 to 80 gpm of
flow rather than a weir. liquid removal. The normal 4-inch cones will re-
Rope discharge is a process where material move 4 gpm of solids, or 5.7 barrels per hour of
pours from the cone apex as a slow moving cylin- solids, per cone. Therefore, the standard 16 cone
der (or rope). The hydrocyclone effects only ineffi- arrangement will accommodate removal of 510
cient solids-liquid separations. The apex velocity in cubic feet of solids per hour. For a 17-j-inch hole,
rope discharge is far less than that in spray dis- this equates to penetration rates averaging 297
charge. Separations are less efficient and, because feet per hour. Clearly, if design and operational
of the lower velocity, fewer solids are discarded. characteristics are adequately maintained, more
A rope discharge can create a false sense of than ample solids separation can be effected.
success as the heavier rope stream appears to The accelerated gravitational forces generated
contain more solids than a spray discharge. In in hydrocyclones are inversely proportional to the
reality, a rope discharge indicates that not all sol- radius of the cyclone cylinder. Thus, the larger the
ids that have been separated inside the cone can diameter, the coarser the separation. In general,
exit through the apex opening. Solids become the larger the hydrocyclone, the coarser its cut
crowded at the apex and and cannot exit the cone point and greater its throughput. The smaller the
freely. The exit rate is slowed significantly and cone, the smaller size particles the cone will sepa-
some solids, which would otherwise be separated, rate. In other words, the median particle size
become caught in the inner spiral and are carried removed decreases with cone diameter. Median par-
to the overflow. Dry discharge can also produce ticle size also increases with increasing fluid viscos-
cone plugging. ity and density, but decreases as particle-specific
With rope discharge, the exiting solids stream gravity increases. Oil-field hydrocyclones range be-
is heavier than under spray discharge conditions. tween 4 and 12 inches, based on the inner diam-
All discharged solids will have a surface film of eter of the intake cylinder A small hydrocyclone di-
"bound" liquid. Since finer solids have a greater ameter is used for ultra-fine separations.
surface area to volume (size or mass) ratio, finer
solids' streams involve greater volumes of bound Hydrocyclone Tanks and Arrangements
water. More bound water causes a less dense
underflow stream (the finer the particle separation, Hydrocyclones are arranged with the larger cone-
the wetter the apex stream). This explains why size unit upstream of the smaller unit. A separate
spray discharge stream densities are less than rope tank is needed for each size unit. Generally, a
dicharge stream densities.
The amount of fluid lost in cone underflow is desander size and a desilter size are available as
part of the rig equipment.
important. A hydrocyclone operating with spray Suction is taken from the tank immediately
discharge gives solids a free path to flow (exit the upstream of the discharge tank. The number of
cone). Rope discharge is a dry discharge. There- cones in use should process 100% of the flow rate
fore, spray discharge removes significantly more of all fluids entering the suction tank for the hy-
solids than rope discharge. More fluid may be lost drocyclone. A backflow between the hydrocyclone
in spray discharge, but the greater solids separa- discharge and suction compartment of at least 100
tion efficiency makes the additional fluid loss in- gpm usually ensures adequate processing. Estima-
significant. If fluid loss is a concern, the underflow tions based on rig circulation rates are usually
can be screened (see Mud Cleaners) or centrifuged inadequate if the plumbing is not arranged prop-
for liquid recovery. erly. For example, if a 500 gpm hydrocyclone over-
Rope discharge should be immediately corrected
to re-establish the higher volumetric flow and flow is returned to the suction compartment and
a 400 gpm rig flow rate enters the suction com-
greater solids separation of spray discharge. A partment, adequate processing is not achieved
rope discharge indicates equipment is overloaded even though more fluid is processed than is pumped
and additional hydrocyclones may be necessary.
downhole. In this case, the flow entering the hydro-
cyclone suction compartment is 900 gpm. The
Hydrocyclone Capacity fraction of drilling fluid processed would be 500
gpm/900 gpm, or 56%.
Since most hydrocyclones are designed to op-
erate at a constant 75 feet of head at the input Desanders
manifold, and flow rate through any cone is con-
stant at constant inlet pressure, flow rate through Desanding units are designed to separate drilled
any cone is predicatable. solids in the 50 to 80 micron range and barite in
