Note: Descriptions are shown in the official language in which they were submitted.
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Patent Application
METHOD AND APPARATUS FOR MIxING SOLIDS AND FLUIDS
Backctround ~ Field of the Invent;"
This invention relates to a method and apparatus for
continuously mixing solid particles with a liquid
composition, and especially for continuously mixing
cement particles with mix water or mix fluid in the
oil-, gas-, or geothermal industries, for the cementing
of drilled wells.
BackcLround : Description of the Prior Art
Methods for the mixing of materials have long been
divided into two general classes. In the first of these,
batch mix methods, the required amounts of components
of the mixture are placed in a vessel. The components
are stirred or circulated in the vessel in order to
produce a specified volume of mixture. According to the
second general class of mixing methods, continuous mix
methods, specified amounts of the required components of
the mixture are metered into a mixing region.
Here they are blended together, and the resulting
mixture withdrawn at a rate equal to the volumetric rate
of the incoming components. The mixing region often
consists of a simple stirred vessel but
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various forms of ejectors, jet mixers and the like, in which
mixing is accomplished by eduction, are also well known.
The need for, and advantages of, continuous mix methods over
batch mix methods in many applications are familiar to the
art. Among the advantages are the ability to continuously
change the specified proportions of the mixture in the
course of :nixing; the elimination of inventory or storage of
mixed material prior to any further steps in a sequential
process; and the ability to apply a large amount of power
into a small mixing volume whereby the components are :pore
efficiently mixed together.
An important disadvantage of continuous mix methods has also
long been known to the art. Conventional methods of
continuous mixing require that the inflow of each component,
the outflow of the resulting mixture, and the proportions of
the respective components must be controlled simultaneously.
For example, if a change in the proportion of two components
is specified, ft is not sufficient to change the flow rate
of just one of these into the mixer in order to effect the
specification. The discharge rate of the mixture must be
changed at the same time. If it were not, the mixing region
would flood or starve and the continuous mixing process stop.
When subsequent steps in a sequential process rsquira that
a mixture be supplied at a specified rate, as is often the
case when continuous mixing methods are advantageous, the
rates of each inflowing component must be altered
simultaneously to obtain both the specified proportions and
the specified discharge rate from the mixer. The
requirement for simultaneous control of multiple variables
leads to complex proportioning control systems in which the
advantages of continuous methods are outweighed by the
disadvantages of high cost and unreliability.
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Zingg and Stoskopf in U.S. Pat. No, 3,256,181 (1966)
disclosed a method by which many of the advantages of
continuous mixing methods are retained, and by which the
disadvantage described above can be overcome. The method
depends on a pressure balance principle. Liquid is supplied
under pressure to a mixing region and swirled so tkiat an
"eye" is opened to the atmosphere at the center of the mixing
region. Rotation of an annular body of fluid establishes a
pressure at the periphery of the body of fluid which
balances the pressure of the supply fluid. Liquid cannot
flow into the eye and flood out from the mixing region. Nor
can atmospheric air cross the rotating annular body of liquid
to reach the mixing region. When a specified amount of
material (generally taken to be mare dense than the liquid)
is metered into the eye, it is propelled by rotation out
into the pressurized liquid, mixed with the liquid, and the
resulting mixture discharged under pressure from the mixing
region.
In typical embodiments of the method described by Zingg and
Stoskopf, the liquid supplied to the mixing chamber is
pressurized by a centrifugal pump impeller. These
embodiments constitute one class of "constant volume" '
continuous mixers. When a change in the proportion of
components is required, it is sufficient to change the
flowrate of the component being introduced into the eye of
the mixing region. A change in flowrate of material into
the eye results in a net change of pressure in the mixing
region. This change in pressure will induce the opposite
(volumetric) change in flow of liquid supplied by the
centrifugal pump impeller in order to maintain pressure-
balance in the mixing region. Consequently, control of the
proportion of components of the mixture is simplified.
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Zingg and Stoskopf (1966) did not recognize that ease of
control could be the principal advantage of one of the
embodiments of their method. Its potential value has only
come to be recognized in the subsequent practice and further
development of their method.
Subsequent practice and further development of the method of
mixing a particulate material and a pumpable liquid '
disclosed by Zingg and Stoskopf have also revealed that it
cannot be usefully implemented under many conditions of
practical interest today. As the volumetric ratio of solid
particles to liquid is increased, implementation of Zingg
and Stoskopf's method produces a progressively less
acceptable mixture or slurry. The product becomes an air-
entrained suspension of agglomerated particles. This
agglomerated mixture is not useable in the form produced by
the method. In addition, air-entrainment causes a
substantial loss in pressure in the mixing region, so that
the efficiency of the implementation of the method is poor.
The potentially poor performance of Zingg and Stoskopf's
method was not recognized at the time it was disclosed.
Their method was originally intended to be implemented for
the production of a slurry of sand or sand-like particles
and gel composition Which is used in treatments intended to
increase the productive efficiency of earth wells. At the
tine the method was disclosed, a typical volumetric ratio of
particles to liquid was 1:10. Ratios as high as 1:4 were
reported. but these represented exceptionally high solids
loading and were intended to test the limits of then-current
practice. A greater understanding of the processes involved
in the treatment of earth wells, and improvements in gel
composition and associated equipment, have led to the use of
slurries with a volumetric ratio exceeding 1:1 in modern
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treatments. At these high volumetric ratios, implementation
of Zingg and Stoskopf's method often produces an air-
entrained slurry unsuitable for use.
Portland cement slurry is a second example of a liquid-
s particle system in Which implementation of the method fails
to produce an acceptable product. Pumpable slurries of
Portland cement are introduced into earth wells in order to
secure pipe or casing to the rock face of the well bore.
These slurries often have volumetric ratios of particles to
liquid exceeding 1:1. Implementation of Zingg and
Stoskopf's method produces a highly agglomerated, air
entrained slurry of very poor quality. Other examples of
systems which require high volumetric rations of particles
to liquid will be obvious to those familiar with the art.
Zingg and Stoskopf's method is flawed because it
incorporates no means to regulate the proportion of the
inflowing materials at their point of contact. While the
over-all ratio of particles to liquid ca:a be controlled,
their ratio when they are initially mixed cannot. Zingg and
Stoskopf's method calls for the introduction of particles
into the liquid at an uncontrolled volumetric ratio that is
always much higher than.that specified for the product
mixture.. The result.is.an air-entrained paste or mass of
agglomerates which is not readily dispersed into a uniform
slurry of acceptable quality. The reason why this result is
a necessary consequence of implementation of their method,
and the reason why it is a insurmountable flaw of that method
can be best explained by consideration of the various forms
of apparatus which have been applied to implement their
method.
The blender apparatus disclosed by Zingg and Stoskopf in U.S.
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Pat. No. 3,326,536 (1967) has been replaced in current use
bY the apparatus first described by Althouse in U.S. Pat.
No. 4,453.829 (1984). Both of these are continuous process
mixers in which liquid and solid materials are fed at a
relatively high rate through a relatively small mixing
volume. The mixing volume is held almost constant by
hydrodynamic gradients induced by the devices. That is,
according to the method described by Zingg and Stoskop~
(1966), one rotating element acts as a centrifugal-pump
impeller and induces a flow of liquid and slurry through a
casing. A second rotating element, usually termed a
"slinger," is used to open an atmospheric eye at the top
of the mixer where solids may be introduced directly. These
two rotating elements establish a hydraulic balance between
them such that any change in the flow of solids through the
slinger is dynamically eompensated by a change in the flow
of liquid induced by the impeller. Consequently, the mixing
volume, although small with respect to the flowrate of
materials through the mixer, remains almost constant.
Extraneous means of volume- or liquid-flow-eontrol are not
used.
Significant disadvantages of machines like those described
by Althouse (1984), and by Zingg and Stoskopf (1967) have
been discussed in the literature. Improved versions of
configurations based on the slinger-impeller balance
principle axe described by MacTntire in U.S. Pat. Nos.
4,614.435 (1986) and 4,671,665 (1987). Maclntire discloses
therein a means of allowing air to vent itself from the
casing of machines of this type. His improvement was
justified by the observation that machines of this type have
a limited solids flow capacity. when the solids flow rate
reaches a certain value, which appears to be a function of
the size of the slinger, its impeller looses prime and
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ceases to operate as an effective centrifugal pump. The
casing floods with solids, and the mixing process must be
stopped. hn a typical application of a continuous oilfield
mixer, an unanticipated shut-down can result in costly
remediation work and often presents' a serious safety hazard.
MaeTntire (1986, 1987) attributes the capacity limitation to
air entrained in the inflowing solid stream which is carried
out into the casing by centrifugal forces. This entrained
air can find its way to the impeller suction, resulting in a
loss-of-prime condition. The impeller can no longer supply
pressured fluid to the mixing region, and the process must
be stopped. He discloses a means of allowing this air to
vent back to the atmosphere before it reaches the impeller
suction region.
As embodied, the Maclntire device incorporates no means to
assure a flow of air to the vent other than the radial
pressure gradient established in its casing. When the
entrained air is sufficiently finely dispersed, and the
mixture in the casing sufficiently viscous, air can be
carried to the impeller suction in spite of a pravisian for
allowing it to vent. These conditions are common in
practice and are aggravated by an increase in the solids-
liquid ratio of the mixture
Various means of encouraging the air to travel to the vent
instead of the impeller suction might be imagined' by those
familiar with the art. A simple solution would be to place
The centrifugal pump impeller in a separate casing as
described in Zingg and Stoskopf's (1967) preferred
embodiment of their apparatus. However, none of thesa
means overcome the further difficulty that entrained air may
equally well be discharged from the mixer'. Mixers of this
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type are typically used to feed pressurized slurry to
plunger pumps. An air-entrained slurry is relatively
elastic, and its compressibilty results in a gross
degradation of the performance of plunger pumps.
Additionally, the flow of solids into the mixer is typically
controlled by feedback from an instrument or "densitometer"
used to measure the density of the slurry at the outlet of
the mixer The density of an air-entrained slurry cannot be
related to a set-paint or desired density in any
convenient manner. A control system.of this type will
always be more-or -less inaccurate.
The problem of air -entrainment at high solids flow rate is
the result of a flaw in the conception of the machines based
the method disclosed by Zingg and Stoskopf (1966).
MacTntire's explanation of the origin of the difficulty is
incomplete, and his improvement only addresses a symptom of
the real problem. All machines utilizing the slinger-
impeller balance principle, as originally disclosed by Zingg
and Stoskopf (1966). bring solid particles into contact with
a liquid composition in a sequence which is known to be one
of the least efficient possible.
The physical properties of mixtures of solid particles in
liquids are strongly influenced by the ratio of the two in
the mixture. A rule of thumb teaches that the particulate
matter should always be introduced into the desired mass of
fluid so that the solids concentration is brought up to the
desired level by progressive addition of solids, and never
the other way around. The reasoning behind this rule is
that the apparent.viscosity of a slurry of particles in a
liquid rises slowly with the addition of particles until a
critical value is reached; at 'ahich.point the mixture turns
from a fluid to a paste or mass of partly wetted
_ g _
agglomerates. It requires several orders of magnitude less
energy to disperse flowable particles into a slurry than it
does to disperse a paste into a liquid. The degree of
respective energy demand (at the same solids-liquid ratio)
is a strong function of particle size. Coarse sand at
relatively low concentration does not form stable
agglomerates. Very fine particles, like Portland cement
particles, readily form an intractable paste. Thus, when
one mixes contrary to the rule, the quality of the mixed
product will be a strong function of the physical
properties. and ratio, of the components of the mixture.
Tn mixers based on the method disclosed by Zingg and
Stoskopf (1966), solids are always introduced into a
partially or fully developed slurry to create an abnormally
high-density, air-entrained paste. In 'the course of normal
operation, the mixer is in a steady-state condition. Its
discharge rate is fixed by suitable external control,
usually by fixing the rate of plunger pumps supplied by the
mixer. The density and_consistency of the discharged slurry
is controlled by the rate of.solids in-flow and is likewise
fixed by feedback control from a densitometer. The bulk of
the slurry in the casing is necessarily at the same density
and consistency.as the discharged slurry. Solids are.
continuously introduced into this slurry at the stinger
where a local volume of heavier-than-desired slurry or paste
is formed. Liquid is continuously introduced at the
impeller where a local volume of lighter-than-desired slurry
is formed. These two slurries are respectively impelled
into the recirculating slurry in the casing. re-mixed to the
desired density, and further recirculated. The heavier-
than-desired slurry created at the stinger has properties
that degrade performance of the entire system.
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The agglomerated paste must be dispersed into previously
mixed slurry and make-up liquid in order to form a blend of
the correct density and consistency before its discharge
from the casing. The energy required to disperse it is
several orders of magnitude greater than that required to
disperse solid particles into fresh liquid at the desired
ratio. Since the energy input into the mixer is relatively
constant, product quality degrades rapidly as the solids-
liquid ratio is increased.
At high solids flow rates, dispersion occurs throughout the
mixer (and not just in the stinger region) so that air
entrained in agglomerates can reach the suction of the
impeller. A still higher solids flow rates, the mixer lacks
sufficient power to fully disperse these agglomerates, and
they are pumped out the discharge, resulting in an
inconsistent, air-entrained slurry of very poor quality.
A second important disadvantage of mixers based upon the .
stinger-impeller balance principle is that they flood with
air at high flow capacities. The size of the atmospheric
~0 eye in the stinger is determined by a balance of stinger
hold-back pressure and impeller discharge pressure, as
explained by Althouse (1984) in the patent cited above.
When the capacity of the mixer is increased, the impeller
discharge pressure falls for two reasons. Firstly, a flow
of fluid through a centrifugal impeller results in a net
subtracive fluid velocity with respect to that tangential
fluid velocity which establishes discharge pressure in the
casing. Secondly, as the capacity is increased, the fluid
friction losses in the supply piping to the mixer grow.
These losses result.in a decrease in absolute pressure in
the casing. It is the absolute casing pressure which is
balanced by the stinger and "held back" to form an eye into
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which solids are added. When the mixer capacity is
increased, the atmospheric eye in the slinger becomes
larger.
In an ideal machine the radius of the eye cannot exceed
the radius of the slinger so long as the pressure in the
mixing region is greater than atmospheric pressure. In
the real machines constructed according to Zingg and
Stoskopf (3.966) the. liquid supply pressure is greater
than atmospheric. So in principle air should never reach
l0 the mixing region. Tn practice air can do so. The reason
is as follows.
The causes of eye enlargement described above usually
occur together and interact additively. At high
capacities the eye becomes so large that the annular
body of rotating liquid and inflowing particles becomes
very thin. In addition, the bulk of the stream of
inflowing solids follows paths along the leading edge of
the stinger blades. The '°wall'° which prevents air from
entering the mixing region becomes unstable and the eye
highly irregular. Atmospheric air over-reaches the
perimeter of the stinger and floods the casing. As a
result the mixer catastrophically looses prime and fails
in service.
In a typical application of the mixer, make-up fluid is
supplied from a storage tank. When the level in this
tank falls during the course of a continuous mixing
process, the hydrostatic head available at the liquid
inlet to the impeller decreases.
Thus the atmospheric eye is further enlarged by the
effective loss of absolute casing pressure in
the mixer. Zingg and Stoskopf (1966, 1967)
CA 02037797 2001-O1-31
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disclosed a constant level supply tank to suppress this undesirable behavior,
but their
solution requires an additional piece of equipment and was never widely used.
In practice,
the degradation of product quality and risk of losing prime are augmented by
the
sensitivity of mixers, based on the stinger-impeller balance principle, to the
absolute
pressure available at their inlet.
The device described by MacIntire (1986, 1987), with provision for allowing
air to vent
itself, actually increases the risk of air flooding the casing. There is an
interface of slurry
and atmospheric air at or near the periphery of the vent during the course of
operation of
the mixer. That is to say, the vent serves to open an atmospheric eye much
like that
opened by the stinger. The size of this vent-eye is regulated by the same
rules which
pertain to the stinger. Thus, when casing pressure falls, the atmospheric
interface at the
vent grows outward radially. At high mixer capacity, this interface will grow
close to the
diameter of the impeller, and air will spill over the impeller rim from the
vent. The mixer
is likely to immediately and catastrophically lose prime, flood with solids,
and fail in
service.
Summary of the Invention
The method and apparatus disclosed herein avoid at least some of the
disadvantages
inherent in the principles and practice of those taught by the prior art, but
incorporate the
objects of a simple, continuous, constant-volume mixing system. The method is
based
upon the invention of a means by which solid particles may be introduced into
a stream of
fresh inflowing liquid before that liquid is recirculated into slurry of the
desired density in
a casing. Hydraulic balance is maintained based on a principle different from
that applied
in the prior art. The method and apparatus also have other advantages over
those used in
current practice.
Accordingly, the invention seeks to provide an improved mixing method and
apparatus
which may continuously and rapidly intermix a liquid and particulated solids,
especially at
high solids concentration and especially where the solids consist of fine
particles.
CA 02037797 2001-O1-31
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The invention may also provide an improved mixer which can be operated over a
wide
dynamic flow-range of solids and liquids while minimizing the risk of
unanticipated shut-
downs and undesirable variations in mixture quality.
Further, the invention may provide an improved mixer which is low in self
contained
inventory, and wherein rapid changes may be effected in the volume of the
materials being
mixed while maintaining predetermined proportions of the components.
Also, the invention may provide an improved mixer which develops a positive
flow
pressure of the mixed slurry useful for moving the slurry to other equipment
without
requiring a pump or the like.
Additionally, the invention may provide an improved continuous mixer wherein
the
mechanism may continue to be operated, even though the delivery line from the
mixer has
been closed or otherwise shut off.
The improved mixer of the invention may continuously produce a liquid-solids
mixture
having a predetermined density.
The invention may also provide an improved mixer, especially for mixing cement
particles
and water in the oilfield industry, with no or little air in the cement slurry
allowing
accurate density measurement.
According to one aspect of the present invention, there is provided a method
of mixing a
pumpable liquid and a particulate material, comprising: (a) swirling a body of
liquid
around an axis such that a vortex or eye is established whose interface
between the liquid
and the external atmosphere is substantially coaxial with the axis of rotation
of the body of
liquid, and such that an increasing radial pressure gradient is established
from the interface
to the outer periphery of the rotating body of liquid; (b) providing a supply
of make up
liquid, the pressure of which is adjusted such that it is greater than
atmospheric pressure
and less than the maximum pressure in the body of liquid; (c) introducing make
up liquid
CA 02037797 2001-O1-31
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from the supply into the body of liquid across an annular section whose inner
radius is
greater than the radius of the eye and whose outer radius is less than the
radius of the outer
periphery of the body of liquid; (d) introducing particulate matter into the
eye and through
the interface into the liquid where the two are mixed together; and (e)
withdrawing mixed
liquid and particulate material from a radial distance greater than that of
the body of
liquid.
According to another aspect of the present invention, there is provided a
method of mixing
a pumpable liquid and a particulate material comprising: (a) rotating a
turbine having first
and second faces around an axis within a casing; (b) swirling a body of liquid
around the
axis on a first face of the turbine by applying power to the turbine such that
a vortex is
established whose interface between the liquid and the atmosphere is
substantially coaxial
with the axis of rotation of the turbine, and such that an increasing radial
pressure gradient
is established from the interface to the outer periphery of the body of
liquid; (c) swirling a
body of make up liquid around the axis on the second face of the turbine such
that a
decreasing radial pressure gradient is established from a radial distance
larger than that of
the vortex to a radial distance greater than one tenth of the radius of the
vortex; (d)
introducing swirling make up liquid into the swirling body of liquid across an
annular inlet
section whose inner radius is greater than the radius of the vortex, and whose
outer radius
is less than the radius of the turbine; (e) introducing particulate material
into the vortex
and through the interface into the liquid where the two are mixed together;
(f) withdrawing
the mixture from the casing; and (g) introducing fresh make up liquid to the
swirling body
of make up liquid via a suction inlet in the turbine.
According to yet another aspect of the present invention, there is provided an
apparatus for
mixing liquids and particulate solids, comprising: (a) a casing or enclosed
housing having
a generally circular peripheral wall, a top, a bottom, a mixture outlet means
coupled to the
peripheral wall, a particulate solids inlet passage centrally disposed in the
top, and an
annular liquid inlet centrally disposed in the bottom; (b) a rotatable turbine
disposed
CA 02037797 2001-O1-31
14a -
within the housing and spaced from the peripheral wall, the rotation axis of
the turbine
being coaxial with the longitudinal axis of the housing, the turbine having an
open central
part facing the solids inlet passage and an annular open part facing the
liquid inlet; and (c)
means for rotating the turbine.
Still further objects and advantages will become apparent from a consideration
of the
ensuing description and drawings.
Drawing Figures
FIGURE 1 is a front elevation view, mostly in section of the mixer apparatus
of this
invention.
FIGURE 2 is a progressive section view, looking down on the blender from
above.
FIGURE 3 is a front elevation view, mostly in section of the turbine used in
an alternate
embodiment of this invention.
FIGURE 4 and FIGURE 5 represent a front elevation view, mostly in section, of
two
mixers according to the invention, preferred for oilfield applications.
According to the broad concept of this invention, an annular body of liquid
is swirled in a casing by a turbine or an impeller element. The rotation of
<,.~ :.. .,r :J
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and pressure gradients in the liquid. At some finite
inner radius the absolute pressure is taken to be a
minimum. At some finite outer radius, the absolute
pressure is that developed by the rotation of the
annular body of liquid between these radiii plus that at
the inner radius, Supply liquid is introduced into the
swirling body of liquid across an annular section whose
inner radius is greater than that of the inner.radius of
the rotating body of liquid and whose outer radius is
less than that of the outer radius of the rotating body
of liquid. The pressure of the supply liquid is
regulated such that it closely matches that of the
rotating annular body of fluid across the section at
which the supply liquid is introduced.
The inner radius of the rotating body of fluid defines
an "eye". The casing is open to the atmosphere over the
circular section of the eye, and thus the pressure at
the inner radius of the rotating annular body of liquid
is fixed at atmospheric. Supply liquid is introduced at
a radius greater than that of the eye and at a pressure
slightly more than atmospheric. Thus the pressure
gradient in the rotating body of fluid is not disturbed,
and the system remains in balance. Supply liquid cannot
flood the eye and flow out of the casing to atmosphere,
nor can atmospheric air reach the source of supply
liquid, nor can it be introduced into the mixture.
Solid particles and the like may be introduced into the
eye where they contact the inflowing supply liquid
arriving across the annular section. Vigorous mixing
takes place in the rotating body of liquid where solids
and inflowing supply liquid are brought into intimate
contact.
CA 02037797 2001-08-30
-16-
The inflowing solid particles and liquid are continuously contacted together
at the
proper proportions or specified ratio of components for the mixture. Solids
are not
recirculated into already-mixed slurry so that the formation of agglomerates
is
precluded.
Liquid or slurry is drawn off from the casing at a pressure established by the
rotation of
the annular body of mixture in the turbine. Preferably, the liquid is supplied
at an
absolute pressure between the vapor pressure of the liquid and an absolute
pressure of
1.5 atmospheres. Thus only one means of rotating the body of liquid is
required to
impel solids into the supply liquid, to mix the components together, and to
pressurize
the resulting slurry for discharge from the casing.
Referring particularly to FIGURE l, except as noted, the mixer apparatus of
this
invention is generally indicated by the letter M.
Above the mixer is a hopper or silo 10. The hopper serves as a container for
solid
particles, and is equipped with a solids-flow regulating means (valve 1) 12
which
controls the flow of solids into a solids inlet cone 16 of the mixer.
A drive shaft 18 is positioned inside the solids inlet cone 16, such that the
bottom of the
drive shaft extends through solids inlet 17 of the mixer and into a casing 20.
The drive
shaft 18 is coupled to a rotary drive means (not shown) which may or may not
be
supported by an element of the mixer as installation requirements dictate. The
mixing-
pressurizing element of the mixer is a turbine 22 which is secured to the
bottom of the
drive shaft 18 by a bolt fastener 24.
The turbine 22 is disposed within the casing 20 coaxially with the
longitudinal axis of
the casing. The turbine has an insert 26 to which a plurality of blades 28 is
attached.
These blades extend in an inward radial direction along the top of the insert
26 to a
CA 02037797 2001-08-30
- 17 -
radius approximately equal to or a little less than that of the radius 30
(FIG. 2) of the
atmospheric eye of the mixer under "nominal conditions" that are defined
below. The
atmospheric eye is a generally cylindrical volume defined by the interface 32
of
atmospheric air with fluid composition in the mixer. The interface is drawn in
FIGURE 2 as a curly line to indicate that it is never perfectly smooth or
cylindrical in
practice. In the preferred embodiment, the blades are not extended fully into
the eye to
avoid interference with the flow of solids into the turbine.
The blades 28 are also extended in an inward radial direction along the bottom
of the
insert 26 to an inner radius which should be determined as follows.
Choose a "nominal eye diameter" with sufficient cross-sectional area to admit
the
maximum flow of solids specified in normal operation.
Choose a turbine diameter and operating speed sufficient to develop a
specified
discharge pressure, taking the pressure at the eye radius to be atmospheric.
Preferably,
liquid in the turbine is discharged at an absolute pressure between the vapor
pressure of
the liquid and an absolute pressure of 1.5 atmospheres. The turbine outer
radius should
normally be approximately twice that of the nominal eye radius.
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The pressure at the periphery of the turbine insert 36 can
never be allowed to be less than atmospheric or air will have
entry to the suction region of the turbine. This adverse
condition is precluded by seting the radius of the inner edge
of the blades in the suction 34 less than that of the radius
of the perphery of the insert 36. To determine tha exact
ratio, one must specify a minimum net positive suction head
available (NPSHA). The pressure developed in the annualar
body of fluid between the radius at 34 and the radius at 36
at the specified rotational speed of the turbine should be
greater than the diference between atmospheric pressure and
the minimum expected NPSHA.
One must then specify a maximum NPSHA. When the apparatus is
operated under this condition the absolute pressure at the
periphery of the insert 36 is the maximum NPSHA plus the
difference between atmospheric pressure and the minumum
NPSHA. This pressure will be balanced by the pressure
developed in the annular body of fluid between the actual eye
radius and the periphery of the insert 36 plus atmospheric
pressure. Use the nominal eye radius in this relationship to
find the radius of the insert. Then find the inner radius of
the suction blade edges. If these are pitched as shown in
FIGURE 1, use a hydraulic average. One should also note that
if the insert radius is greater than about 75~ of the turbine
~5 it may be necessary to adjust some of the specifications.
Those familiar with the art will also recognize that
appropriate safety factors should be incorporated in all
calculations. They will also note that the calculation of
exact dimensions may be further refined depending on the
particular type or style of turbine chosen for a specific
application.
/ '.;
- 19 -
To provide fox smoothness of flow, a continuation of the
casing 20 and the insert 26 are configured to form an annular
turbine inlet 40 between them. The cross-sectional area of
this inlet should be chosen such that the fluid i:s not
accelerated in the suction in accordance with good hydraulic
practice. The turbine inlet 40 is connected directly and
smoothly to the liquid suction inlet 42 also formed between
the insert 26 and the inner casing wall. Stator blades 44.
which suppress licruid prerotation and make mixer pErformance~
more predictable, should be installed in the liquid suction
by attachment to the casing inner wall. The annular suction
inlet is continued smoothly into circular section at the
liquid inlet to the mixer 46. A manifold or fluid supply
pipe 48 is provided to supply liquid from a liquid reservoir
49.
The turbine blades 28 extend in an outward radial direction
to the periphery of the turbine and are curved in conformity
With good turbomachine design principles. In the preferred
embodiment shown, an upper shroud 50 is installed on the
turbine between the solids inner edge of the blades 38 and
the periphery of the turbine. The shroud 50 serves to define
a plurality of flow passages 52 between the blades and
prevents inflowing solids from eroding the upper edges of the
blades and the inner wall of the casing 20 opposite. The
height of these passages should be constant so that the
outflowing mixture in the turbine is decelerated in the
radial direction. Deceleration serves to minimize eductor
effects which might induce air entrainment. A plurality of
pump-back vanes 54, in accordance with standard practice, is
used to prevent backflow of materials in the gap between the
shroud and the inner wall of the casing, which gap also
serves as a means to exhaust air.
The turbine 22 discharges across its periphery into a
receiver volume 55 defined by a continuation of the casing
20. In the preferred embodiment, the receiver volume of the
casing 55 is "semi-voluted." The cross-sectional area of
this volume uiewed normal to the tangential flow of mixture
in the casing is increased starting from an edge 56 (FIG. 2)
directly ahead of the discharge outlet 58. The law of
increase is taken from good hydraulic practice and should be
arithmetic with distance around the circumference of the
mixer to the discharge outlet. However, the total cross--
sectional area is always made sufficiently large that the
receiver volume of the casing 55 allows for recirculation of
the mixture. This feature serves to damp out any
irregularities in the flow of solids into the mixer,
providing for more precise control of mixture quality. In
general, the cross-sectional area should at no point be less
than the outlet 58 cross-sectional area, which is determined
by standard hydraulic practice,
In the embodiment shown, the casing is voluted along the
longitudinal axis of the mixer. This configuation is
preferred over the standard method of radial voluting for two
reasons. Firstly the the velocity, and consequently the
pressure, opposite the turbine discharge is held relatively
constant. Thus, the eye remains symmetric with the solids
inlet, avoiding the risk a spray of fluid across a segment of
that inlet. Secondly it provides a device of overall smaller
diameter, which is more convenient and economical.
In order to provide for additional damping and better control
where necessary, a certain portion of the discharged mixture
may be recycled or recirculated from the discharge of the
mixer 58 back to the liquid supply pipe of the mixer 48 by
means of a recirculating pipe 60. The degree of
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- 21 -
recirculation is proportional to the size selected far this
pipe and may be determined by rules and principles well known
to those familiar with the art. A valve (valve 2) 62 is
provided so that the mixer can be operated in either the
recirculating or direct mode according to the cireumstances
described herein.
The precise configuration of the turbine 22 depends on the
performance desired of a preferred embodiment. FIGURES 1
and 2 illustrate a turbine of the radial type which is
-particularly suited for the specification of low specific
speed. It would be selected when a relatively high discharge
pressure with respect to capacity is required. At higher
specific speeds where capacity is more important than
discharge~pressure, a Francis configuration would be
.specified. A vortex-type turbine is shown in FIGURE 3, on
which the names and identifying numbers of the parts are
retained. This configuration would be specified, for
example, where extremely abrasive solids are processed, arid
close clearances in the solids or slurry flow-paths were
especially undesirable.
The extension of blades into the annular suction region in
order to regulate the pressure of the supply liquid is
incorporated into the preferred embodiment because of its
simplicity. Those familiar with the art will also reeognize
-that various commonly known means for controlling the
pressure of the inflowing supply fluid could also be used.
For example, a regulated low°pressure booster pump could be
placed in the line between the liquid reservoir 49 and the
fluid supply pipe 48. This means and others of the like
.f ~'~s, .,. ,. " ::J .;
- 22
sort may also be selected in accordance with the method
of this invention.
The invention may be illustrated by describing a typical
operation in which portend cement powder is mixed with
water to obtain a cementatious slurry suitable for
pumping into a well in order to provide a hydraulic seal
between the casing and rock formations opposite that
casing.
At the start of the operation, a drive means rotates the
drive shaft 18 and turbine 22. Once the turbine is in
motion, water is supplied to the inlet of the mixer 48.
The water flows into the turbine through the liquid
inlet passage defined by the liquid inlet 46 of the
mixer, the liquid suction inlet 42, and the annular
7.5 turbine inlet 40. The water is rotated by the turbine
and develops pressure and velocity at is flows out into
the casing receiver volume 55. Air in the mixer is
discharged through the gap between the turbine upper
shroud 50 and the inner wall of the casing 20 directly
opposite. Thus the mixer can be primed even when it is
convenient to keep its outlet 58 blocked. Once the mixer
has been primed in this fashion and is pumping, it will
remain in a primed state even if the absolute pressure
along the liquid inlet path is allowed to fall below
atmospheric.
After the mixer is primed, cement powder is metered into
the turbine along the solids inlet path defined by the
means of flow regulation 12, the solids inlet cone 16,
the solids inlet 17, and the air-liquid interface 32.
The water and cement particles are brought into contact
~i~.i ~:: ~~.. ,~:i 1?~ ",.
23
at this point. They then pass through the passages in
the turbine 52 where they are mixed and pressurized as a
slurry. Under these conditions, the mixer is operated in
the recirculating mode with valve 62 open. When the
density of the slurry reaches the desired value as
determined by a measurement means, the outlet is opened
and the slurry flows under pressure to a high-pressure
pump which delivers it into a well.
As pumping begins, cement powder continues to flow along
the solids inlet path. Water is drawn into the mixer
through the liquid inlet path based on a volumetric
balance which says that the rate of inflowing water
equals the rate of outflowing slurry less the rate of
inflowing cement powder. Thus the density of the slurry
may be controlled by regulating the flow of cement
powder into the mixer or by regulating the flow of
slurry out of the mixer in combination or singly.
Multiple control actions are not required. Once the
mixer has reached a steady state condition, the
recirculation value may be closed. This action is
desirable when the mixer is required to operate around
the maximum of its design capacity, and flow losses must
be reduced. At lower capacity the valve should be left
open in order to maintain more precise control of the
density of the slurry.
A modified version of the mixer according to the
invention, and especially adapted for preparing
continuously cement slurries for the oil', gas-, or
geothermal industries, namely for the cementing of
drilled wells, is represented on Figures 4 and 5.
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- 24 -
In Fig 1 to 5, same numerical references have the same
meaning.
Referring to Figure ~, the casing 20 of the mixer M
contains a turbine 22 with blades 28. The cement
particles flow from the non-represented hopper 10 into
the solid particles inlet 16, 17. Water, or water-based
fluid with usual oilfield cementing additives, enters
through the inlet 46, 48 from an atmospheric fluid
reservoir, either by gravity or through a feeding pump.
1O A stator 80 prevents vhe incoming fluid to spin,
allowing a constant pressure to establish in the volume
82 immediately below the turbine 22.
The receiver volume 55 is most preferably limited
outwardly by a somewhat cylindrical wall 81, and most
preferably the slurry outlet 58 is placed behind the
said wall as represented on Figures 4 and 5.
An horizontal disk 83 is provided above the turbine 22
so as to partially overlap with blades 28 as shown on
Figure 4. While not essential for the operation of the
mixer, this disk is most preferred since it prevents the
outcoming of solids dust through the air escape vent 84.
The blades 28 may optionnally extend downwardly to form
a snoop 85, the purpose of which is to keep the machine
primed even at low presssure and expecially when the
whole mixer M is built in an inclined configuration, by
helping the incoming fluid to be forced upwards.
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The machines shown on Figure 4 and on Figure 5 present
the advantage of a very stable eye, what is an essential
condition to reach the purposes of the inventions as
recited above. In that respect, the horizontal disk 86
5 at the bottom of the turbine defines the position of the
eye or interface 32 for a given water or fluid pressure
at the inlet 46.
Such machines are especially useful for continuously
mixing cement particles With mix water or mix fluid in
10 the oilfield industry and related industry with a very
accurate control and monitoring of the dentity of the
produced slurry.
Figure 5 represents a further version of the machine
represented on Figure 4, where the water or fluid inlet
15 46, 48 is located at the top of the casing 20. Water or
fluid is flowing, either from an atmospheric tank 49 by
gravity or-through a feeding pump.
Most preferably, the water is introduced in the top
cylindrical chamber 90 of the mixer, as defined by the
20 upper part of the casing 20 and an intermediate
horizontal wall 91. Both the upper part of the casing 20
and the horizontal wall 91 feature a central hole as
represented on Figure 5, aimed at providing
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space for the air vent 84 and the solid particles
inlet 1s.
To be noted that the horizontal disk 91 ends inwardly
with a certain overlap of the turbine blades 28 while
the upper part of the casing 20 extends inwardly
beyond the limit of the disk 91 so that the eye
(air/slurry interface) can establish and get
stabilized in a position which is intermediate
between the two inwards limits of, respectively, the
upper part of the casing 20 and the disk g1.
In a preferred embodiment a fixed system of
blades, generally represented as 92, is positioned
in the above mentioned central hole, so that
the incoming water in chamber 9o is prevented
from spinning,
The principal objects of the invention are
effected because the proportions of the
components of the mixture are never allowed
to exceed the design or desired proportions
in any part of the apparatus under a wide
variety of typical operating conditions.
The mixing of portland cement slurry was
used to illustrate the operation of the
invention because this slurry is
;,~ '. -.''; ri ' ',. ,
- 27 -
notoriously difficult to mix to specification. In machines
configured according to the prior art, cement powder would be
introduced into slurry which has already been mixed to the
desired density. Hut oil-field cement slurries are mixed to
a desired density such that there is a minimum amount of
"free water" available. That is to say, the proportion of
powder to liquid is specified such that there as a minimum of
excess water beyond what is required to wet the powder. Any
additional volume of cement powder above the "free water
point" results in a viscous paste or agglomerates of partly
wetted and generally air-entrained solids. These are
difficult to reduce to a uniform, pumpable, air-free slurry
by any economical means. The mixer disclosed herein is not
subject to this fault. Cement powder is introduced directly
into water and before the mixture is substantially
pressurized by the turbine. Thus, high-quality cement slurry
can be supplied by the mixer with no further processing steps
required.
Some unexpected advantages accrue to a mixer based on the
method suction pressure balance in~ a single turbine as
opposed to discharge pressure balance between a stinger and
impeller as disclosed in the prior art. The turbine can be
designed according to well-known design principles for turbo-
machinery. The blades may be swept back to define a best
efficiency point or best-efficiency-point range for the
machine. Erosion by abrasive solids is greatly reduced.
Additionally, because all of the fluid flows through the
turbine in the form of a pumpable slurry, its hydraulic
efficiency can be made as high as is consistent with modern
turbo-machine design practice. The impeller element
described in the prior art can be designed according standard
design principles, and its efficiency. may be high. Hut the,
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- 28 -
stinger described in the prior art serves as a "hold-back"
means. It absorbs motor horse--power to little apparent
advantage. With respect to the slurry in the casing, it
operates in a "shut in" condition where its hydraulic
efficiency is very low. The small boost in discharge
pressure noted by ~ingg and Stoskopf (1966) is due to the
fact that the stinger peripheral velocity is necessarily
larger than that of the impeller and this "exce.ss velocity'°
can be recovered as pressure by a suitable diffuser means.
However, the effect is always small, ~and.its contribution to
the overall efficiency of devices described in the prior art
is virtually negligible.. A stinger. demands horsepower which
serves neither to pump nor to mix, but is directly lost to
heat. The mixer disclosed, here does not set an inefficient
stinger against a potentially efficient impeller. The normal
drop-off of discharge pressure with increased capacity
becomes a positive advantage instead of an insurmountable
conceptual flaw. Under comparable operating conditions, the
mixer requires about half the input horsepower of machines
designed according to prior art.
A further advantage of the method is that a turbine
configuration may be selected from a larger group of standard
types than is possible in the design of machines based upon
the teaching of prior art. A turbine configuration may be
chosen from a spectrum whose limits range from "radial flow"
to "mixed flow" configurations. A '°vortex" or "recessed
impeller" configuration. may also be selected in accordance
with this invention.
A further advantage is that the apparatus disclosed herein is
smaller and cheaper than machines designed according to the
prior art.
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A further advantage is that the apparatus described here can
be used in a variety of oilfield services. The mixing of
cement powder is used to illustrate its operation in detail
because cement slurries are difficult to mix. Mixers
designed according to the teaching of the prior art were
intended exclusively as sand blinders. They cannot mix
cement slurry of acceptable quality under many circumstances.
Cement particles readily form pastes and agglomerates which,
because of the small size of the particles, are difficult to
disperse into a consistent slurry. Nor can mixers designed
according to prior art mix acceptable gel or r~lymer
solution. Water soluble polymers are also difficult to
disperse in aqueous media. Many of these are effectively
undispersible in a medium which already 'contains dissolved
polymer. Consequently, a mixer which does not bring polymer
powder in immediate, direct contact with a fresh aqueous
medium will produce low quality product. The mixer disclosed
here calls for the contact of solid particles with fresh
make-up fluid at the proper solids-liquid ratio and maintains
that ratio throughout their passage through the device.
Consequently, it will mix high quality cement, gel, or sand
slurry indifferently.
A furtYa.er advantage is that the mixer disclosed here can
process a large flow of solid particles whose density is less
than that of the liquid also composing the mixture. Machines
designed according to the prior art are able to process small
ratios of low-density solids due to recirculation of mixture
in the chambers defined by the-slinger blades, but they stop-
up at typically higher rates. Since this mixer contacts
inflowing particles with the full volumetric flow of fresh
make-up liquid at the air-liquid interfaee, mixing of high
solids-liquid ratios of low-density particles is not
precluded.
