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Description  |
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FIELD OF THE INVENTION
The invention relates to treatment of a multiphase fluid, for example, in a
transport or separator system.
BACKGROUND OF THE INVENTION
The handling of a multiphase fluid, that is, a mixture of at least two
fluids of different phases, presents problems arising for example from the
different physical characteristics of liquids and gases, in particular,
the virtual incompressibility of the former and the ready compressibility
of the latter, and also from variations in the relative amounts of liquids
and gases in the multiphase fluid. For example, in oil production, a well
may produce a mixture of crude oil, crude gas, water and sand or like
particulate material. It is desirable in many instances to place such a
mixture under increased pressure, but this is difficult because pumps with
impellets designed to pump liquid are unsuitable where the liquid contains
a high gas content. Similarly, ordinary gas compressors are unsuitable for
use where liquid is present in the gas in any substantial amount.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided apparatus for treatment
of a multi-phase fluid, comprising an inlet stage leading to a treatment
stage, the inlet stage comprising a cyclonic separator device in which the
multiphase fluid is divided into separate flows consisting at least
substantially of fluid of higher and lower specific gravities
respectively, for at least one of further separation, pumping, and
compression in the treatment stage.
The invention is accordingly concerned in one aspect with the provision of
a pump/compressor unit arranged for efficient pressurising of a multiphase
fluid regardless of variations in the quantities of gas or liquid in the
fluid.
A pump/compressor apparatus in accordance with the invention is thus
arranged for receiving an incoming multiphase fluid and directing the
fluid cyclonically to effect separation of the phases, with a stream of
fluid with the highest specific gravity as a layer at the outer surface of
the cyclone and a stream of fluid with the lowest specific gravity in the
centre of the cyclone. The incoming fluids with the highest specific
gravity are then directed into a helical path at the outer periphery of
the apparatus along which energy is added by means of rotating impeller
guide vane passages increasing the rotational velocity of the fluid, and
thus the pressure. The incoming fluids with the lowest specific gravity
are similarly acted upon by a rotating impeller means, preferably
providing for compression of the fluids which will typically comprise
gaseous material.
The invention thus provides a pump/compressor unit having an inlet for a
multiphase fluid, means for separating the fluid into its components and
for pressurising the components by respective impeller means. Preferably
the two impeller means are parts of a single impeller assembly.
The impeller assembly can thus provide an interior defining a first flow
path along which the gaseous or lower specific gravity fluids are directed
along the impeller assembly axis and then transported radially by blades
or vanes. The cross-sectional area of the flow path preferably reduces
progressively in the flow direction, so as to enhance compression of the
fluid. The compressed fluid of the first stream can then be discharged
from around the impeller assembly periphery.
Radially adjacent of the first flow path, a second flow path is provided
for the higher specific gravity or liquid stream, between the exterior of
the assembly and a housing within which the assembly rotates. The second
path again effects axial re-direction of the stream, into an annular
trough or channel from which the liquid is accelerated by impeller means
to an outlet by way of a fluid pick-up or scoop device.
Such a pump/compressor device would be self-regulating, and also
self-priming because gas would not have to be drained out before pumping
could commence. The device would itself act as a fluid lock, because it
would never empty completely, so preventing gas from blowing back from the
gas outlet in the absence of incoming liquid. Also, gas lock is prevented,
so non-functioning cannot result from intolerance of an essentially
gaseous input.
Alternatively, the invention can be embodied in a centrifugal separator
apparatus for separating the components of a multiphase fluid, the
apparatus having an inlet stage similar to that described above for
providing the separate flows. The fluid flows at the outlet of the helical
path are directed into a rotating separator. The or each fluid flow with
the highest specific gravity is directed into an impeller stage with
passages defined by guide vanes with or without an inner wall. The liquid
layers then proceed axially along the inner surface of the separator
cylinder or drum and are discharged therefrom in any suitable way as by
reception in a discharge chamber into which a discharge scoop extends. The
gaseous component of the multiphase fluid is also brought into rotation by
the guide vanes and proceeds axially through the separator drum. Any
liquid drops remaining will be separated from the gas by centrifugal force
and the dry gas can be withdrawn from the separator without further
pressure increase.
In operation, the incoming fluid is efficiently brought to full rotational
speed, without turbulence in the outlet, and with improved separation. By
selecting appropriate average outlet cross-sectional areas from the
impeller, improved separation efficiency can be obtained because the
average momentum of the fluid in the outlet can be made equal to the
average momentum of fluid in the separator phase.
The invention is further described below, by way of example, with reference
to the accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional side view of a pump/compressor unit
embodying the invention;
FIG. 2 is a perspective view of a cyclonic inlet stage of the unit of FIG.
1;
FIGS. 3 & 4 are perspective, part sectional, views, from different
viewpoints, respectively of a cyclonic inlet stage and of the inlet end of
a rotary stage, of a centrifugal separator apparatus embodying the
invention.
The pump/compressor unit illustrated in FIG. 1 comprises a stationary
casing 10 having axially opposed open ends closed by end plates 11 through
apertures in which respective drive shafts 12 and 14 extend along a common
axis from respective electric drive motors 15 and 16. At the lefthand end
(as shown) an inlet chamber 17 in the form of a volute is provided within
the casing around its axis and into which a multiphase fluid is introduced
in use from outside by means of an inlet fitting 19.
The incoming mixture has a rotational movement imposed on it by the shape
of the inlet chamber 17 and this movement is enhanced in the next stage by
a fixed guide member 20, shown in FIG. 2, received in an annular chamber
communicating with the inlet chamber and into which the fluid moves in the
axial direction. The guide member 20 comprises an inner sleeve 24 with
external fins 25 defining with the inner wall 26 of the casing 10 plural
helical channels for the multiphase fluid. The centrifugal force generated
by the rotary movement of the fluid causes the heavier fluid or fluids,
that is, the liquid component of the mixture, to concentrate into an
annular flow path A against the casing wall 26 whilst the less dense
gaseous component occupies a flow path B at the inner region of the
channels. The multiphase fluid is thus cyclonically separated into
concentric layers of increasing density in the radially outward direction.
Continuing in the axial flow direction, the interior of the casing 10 next
has a radially enlarged portion 30 constituting a pump/compressor stage.
Carried on the free end of the shaft 12 is a first part of an impeller
assembly comprising concentric inner and outer sleeves 31 and 32 providing
between them an annular passage continuing the annular space between the
sleeve 24 and the inner wall 26. Axially adjacent the inner sleeve 31 is a
member 34 which flares radially outwardly in the flow direction, so as to
redirect the primarily gaseous fluid stream adjacent the inner sleeve 31
along a radially outward direction. The impeller assembly part on the
shaft 12 also comprises an annular disc 35, extending generally radially
outwardly from a position near to, but spaced from, the downstream end of
the outer sleeve 32, so as to form therewith an annular passage 36 through
which can flow the outer layer of the fluid, comprising the denser,
liquid, phase. The inner edge of the disc 35 thus separates the inner and
outer layers, typically of gaseous and liquid components respectively,
formed in the multiphase fluid by the centrifugal force generated
upstream.
The free end of the shaft 14 carries a second part of the impeller assembly
comprising an annular disc 40 extending generally radially outwardly to
oppose the disc 35. Each disc carries impeller vanes or blades 41
extending towards the other disc. The shafts 12 and 14 are driven by the
motors 15,16 so as to rotate in opposite directions and the blades 41 are
shaped to urge the gaseous stream directed to them by the member 34 to
flow radially outwardly. The opposed faces of the discs 35 and 40 slightly
converge in the radially outward direction so as to restrict the flow
passage between them. The gaseous stream is thus compressed in its passage
between the discs 35 and 36 and it flows outwardly from between them into
a discharge chamber 45 in the form of a volute provided in the casing 10
around the outer edges of the discs. A discharge fitting 46 communicates
with the chamber 45 to conduct the compressed gaseous flow outwardly of
the unit.
The more dense, primarily liquid, stream flowing radially outwardly through
the passage 36 between the sleeve 32 and the disc 35, at the side of the
disc remote from the disc 40, is received in an annular channel formed by
a member 50 secured to the disc 35 and comprising a concentric sleeve
portion having at its free end an annular rim portion directed inwardly
towards the shaft 12. Within the channel, impeller vanes or blades 51 on
the disc 35 and the rim portion effect acceleration of the liquid. The
liquid is extracted from this channel by a stationary scoop 52 comprising
spaced disc portions extending outwardly into the channel of the member 50
and providing passages for radially inward flow of the liquid from the
channel. This discharge flow continues axially through a support portion
projecting from an adjacent wall portion of the casing 10, and to a
discharge outlet 55 by way of a passage 56 in the wall portion.
The pump/compressor unit described and illustrated thus provides for the
separation, and separate treatment, of the gas and liquid components of
the incoming multiphase fluid, so that each can be pressurised by impeller
means appropriate to the characteristics of the component which it
handles.
The separation of the gas and liquid stream can of course be maintained
downstream of the unit if appropriate, but if the function of the unit is
simply to effect transport of the multiphase fluid, the separate gas and
liquid outputs can be combined for flow for example along a pipeline to
equipment in which the fluid is subsequently treated.
The centrifugal separator apparatus of FIGS. 3 and 4 has a stationary inlet
stage largely corresponding in design and function to that of the
pump/compressor unit of FIGS. 1 and 2. The inlet stage thus includes a
stationary guide member 60 as shown in FIG. 3 which may be closely similar
to the guide member 20 of FIG. 2 and which again serves to cause an
incoming multiphase fluid to form into an axially flowing stream of
material of higher specific gravity, typically one or more liquid layers,
confined by a housing wall 61, and an inner stream of material of lower
specific gravity, typically of a gaseous nature.
From the stationary inlet stage of the apparatus, the concentric fluid
streams enter a rotary impeller/separator stage, of which the inlet end
only is shown in FIG. 4. This part of the apparatus comprises a drum 65
which is rotated in use by a motor (not shown) about its axis 66. The drum
wall at its inlet end has a short portion 69, with a diameter matched to
that of the guide member 60, followed downstream by a frusto-conical
portion 70 leading to a separator drum portion 72 of constant larger
diameter. The inlet and frusto-conical wall portions mount a series of
impeller vanes 75 extending inwardly preferably but not necessarily, to a
concentric inner sleeve 76 of a diameter equal to that of the sleeve of
the guide member 60.
The impeller vanes 75 receive the fluids flowing concentrically in the
helical paths imposed by the guide member 60 and act to increase the
rotational speed of the fluids in the frusto-conical portion 70. The fluid
layers then flow from the passages defined by the drum portion 70, the
vanes 75 and the sleeve 76, to flow along the drum portion 72 where
further separation occurs by conventional centrifugal separator action.
Any liquid in the central gaseous flow joins the outer liquid layer (or
layers where there are two liquids of different specific gravities). The
liquid or liquids can be removed from the drum by conventional means or
the centrifuge can be designed to be self-regulating as described in
Application GB 91 26 415.0, the contents of which are incorporated herein
by reference. The gas can be discharged from the drum through
appropriately located apertures (not shown).
The invention can of course be carried into effect in a variety of ways
other than as specifically described and illustrated.
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