Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to centrifugal submersible pumps. More
particularly, the
present invention relates to such a pump useful in removal of methane from a
water
s methane solution in a downhole well located in coal deposits.
2. Description of the Related Art
There are several problems connected with the downhole pumping of water
containing
dissolved methane gas from a source such as a coal field. These problems
generally result
in premature failure of the submerged pump. One problem is vapor lock which
occurs
when the flow of water is too low compared with the amount of gas present.
Another is
the presence of large coal particles which flow through the pump and cause
damage
thereto. Yet another is excessive wear in a water-coal slurry environment.
U.S. Pat. No. 4,708,589, issued Nov. 24, 1987, to Nielson et al., describes a
submersible
pump of the general type as the present invention. This design may be adequate
for
petroleum wells, but would suffer from unacceptable wear in a water pumping
environment where entrained methane and coal particles are present.
U.S. Pat. No. 4,741,668, issued May 3, 1988, to Bearden, describes a
submersible pump
having centrifugal pump stages with abrasion resistant impeller hub. This
design has the
shortcoming of having rubbing parts, reducing the life of the pump,
particularly in the
environment of a water well having coal particles having coal particles
therein.
U.S. Pat. No. 3,975,117, issued Aug. 17, 1976, to Carter describes a
submersible pump
having an inducer and a multistage centrifugal pump section at opposite ends
of the
driving motor in a pump for cryogenic or boiling fluids, particularly in a
tanker ship or
storage tank. This design is not appropriate where relatively large solid
particles such as
coal may be present in the fluid being pumped.
U.S. Pat. No. 3,975,113, issued Aug. 17, 1976, to Ogles describes a
submersible pump
useful for downhole pumping of water. This design is not adapted to pumping
water
containing high levels of methane or other gas and would be subject to damage
by large
particles and loss of prime by large slugs of gas.
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U.S. Pat. No. 3,961,758, issued Jun. 8, 1976, to Morgan describes a
submersible pump
useful for pumping liquids and liquid slurries such as in a sewage collection
tank, and
provides a grinder at the inlet. This design would not be useful in a downhole
water-
methane solution environment as it is subject to vapor lock from gas slugs and
subsequent
loss of prime.
None of the above inventions and patents, taken either singularly or in
combination, is
seen to describe the instant invention as claimed. Thus a centrifugal
submersible pump
solving the aforementioned problems is desired.
SUMMARY OF THE INVENTION
The present invention is a submersible pump specifically designed for downhole
pumping
of methane-saturated water from wells drilled in coal formations. The
centrifugal pump
configuration has an electric motor driving a vertical shaft having
centrifugal impellers
distributed therealong, each impeller being located in a diffuser, stationary
with regard to
the pump wall to form a mufti-stage pump useful in the petroleum industry, but
is
modified in several respects for adaptation to the specified use. Most
notably, a shroud,
concentric with the pump wall and forming an annulus which is sealed relative
to the
lower portion of the pump wall is provided such that all fluid must enter
holes near the
top of the shroud and travel downward through the annulus to a point below the
pump
inlet. A charge impeller is located near the pump inlet and above the driving
motor,
followed by a solids grinder to grind larger coal particles carried within the
pumped fluid
before entering the first centrifugal stage of the pump. The charge impeller
and solids
grinder are mounted on the same rotating shaft as the centrifugal impellers
and turn at the
same rate. The mufti-stage centrifugal pump may be provided with stages of
diminishing
volume as the methane gas and liquid mixture becomes more and more compressed
as it
travels upward through the pump. Another charge impeller may be located at the
upper
end of the shaft at the pump outlet to boost flow upward into a vertical pipe
sealed to the
pump for carrying the compressed fluid to the surface for separation. Pressure
equalization vents are located to allow flow of fluid from the third
centrifugal stage to the
annulus between the shroud and the pump wall to maintain pump prime when
encountering a slug of gas in the intake. Pump stage centrifugal impellers
each have a hub
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extending upward and downward along the length of the driving shaft such as to
rest upon
each other in turn, while avoiding contact with the stationary diffusers.
Accordingly, it is a principal object of the invention to provide a
centrifugal submersible
pump particularly adapted for pumping methane-saturated water from a well in a
coal bed
to the surface for separation and recovery of methane gas.
It is another object of the invention to provide a centrifugal submersible
pump as above
having a concentric shroud located along its lower portion and forming and
annulus
therewith and sealed to the pump housing both below the pump inlet and the
shroud upper
end wherein holes are provided near the upper end for fluid flow into the
annulus and
downward into the pump inlet.
It is a further object of the invention to provide a centrifugal submersible
pump as above
having a centrally rotating shaft extending upward from a motor and having a
flow
inducer located thereon in the vicinity of pump inlet openings.
Still another object of the invention is to provide a centrifugal submersible
pump as above
having a solids grinder located along the shaft above the flow inducer to
reduce the size
of any coal particles entering the pump.
Yet another object of the invention is to provide a centrifugal submersible
pump as above
having multiple stages reducing in volume as the pumped methane-water mixture
becomes compressed due to increasing pressure as it travels upward through the
pump.
Still another object of the invention is to provide a centrifugal submersible
pump as above
having a centrifugal impeller within each stage and wherein each impeller is
keyed for
rotation to the rotating shaft by a hub extending upward and downward along
the shaft so
as to respectively rest upon each other so as to avoid an contact with
surrounding
diffusers, minimizing wear of pump parts.
Yet another object of the invention is to provide a centrifugal submersible
pump as above
having pressure equalizer conduits communicating between the third pump stage
from the
bottom and the shroud-enclosed annulus to maintain pump prime when
encountering
slugs of gas at its intake.
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It is an object of the invention to provide improved elements and arrangements
thereof for .
the purposes described which is inexpensive, dependable and fully effective in
accomplishing its intended purposes.
These and other objects of the present invention will become readily apparent
upon
further review of the following specification and drawings.
According to an aspect of the invention there is provided a centrifugal
submersible pump
comprising a) a generally cylindrical pump casing of such diameter as to fit
within a well
borehole for insertion and removal of the pump, b) a pump intake in the
vicinity of a base
of the pump, c) an axial drive shaft extending substantially the length of the
pump and
adapted to be driven by a submersible motor located below the pump, d) a
plurality of
centrifugal impellers spaced along the axial drive shaft, each of the
centrifugal impellers
having a central hub attached for rotation to the axial drive shaft and having
an opening
adjacent the drive shaft for fluid intake, e) a plurality of diffusers
corresponding, each
corresponding to one of the centrifugal impellers to form a series of pump
stages, f) the
diffusers being supported by inward compression of the pump casing so as to
remain
stationary relative to the centrifugal impellers, and having a central bore of
such diameter
as to allow fluid to travel upward through the annulus between the central
bore and the
axial drive shaft and into the impeller intake, g) a pump outlet located and
attached for
flow to a conduit for receiving pumped fluid in the vicinity of an upper end
of the pump
casing for connection to a conduit for carrying the fluid to the surface, or
into the casing
of another immersible pump, h) a cylindrical coaxial shroud located along the
lower
portion of the pump casing and having a plurality of fluid inlet holes located
near the
upper end of the shroud, -the shroud being sealed at its upper end with the
pump casing
and at its lower end with the pump casing at a location below the pump inlet
such that all
intake fluid must enter the pump through the shroud.
According to another aspect of the invention there is provided a method of
pumping and
separating methane-saturated water from a well located in a coal bed,
comprising ,
pumping the water upward and around an annulus formed by a casing of the well
and a
pump, reversing flow of the water to enter holes in the upper end of an
annulus formed
by the lower portion of a pump casing and a shroud sealingly engaged therewith
at its
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upper and lower end, so as to induce turbulent flow through the holes then
inducing
partial separation of methane from the methane-saturated water which separated
methane
travels upward within the well casing for collection at the surface thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic environmental, perspective view of a centrifugal
submersible
pump according to the present invention.
FIG. 2 is a diagrammatic elevational view of the centrifugal submersible pump
of FIG. 1
with the lower shroud removed.
FIG. 3 is a diagrammatic elevational view of the centrifugal submersible pump
of FIG. 1
with the shroud removed and the casing broken away.
FIG. 4 is a diagrammatic detail view of the pressure equalizer within the
third pump stage
of FIG. 4.
FIG. 5 is a dia ammatic detail view of the solids
grinder of FIG. 4.
FIG. 6 is an exploded view of a group of pump stages as referred to in the
diagrammatical
depictions of the figures above.
Similar reference characters denote corresponding features consistently
throughout the
attached drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is a submersible pump specifically designed for downhole
pumping
of methane-saturated water from wells drilled in coal formations for the
production of
methane gas. The centrifugal pump configuration has an electric motor driving
a vertical
shaft having centrifugal impellers distributed along the shaft, each impeller
being located
in a diffuser, stationary with regard to the pump wall to form a mufti-stage
pump useful in
the petroleum industry, but modified in several respects for adaptation to the
specified
use. Most notably, a gas shroud, concentric with the pump wall and forming an
annulus
which is sealed relative to the lower portion of the pump wall is provided
such that all
fluid must enter holes near the top of the shroud and travel downward through
the annulus
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to a point below the pump inlet. This shroud assists in pumping of water
satwated with
methane where the methane tends to come out of solution and form gas phase
bubbles,
threatening the prime of the pump. Also, chunks of coal or other solids are
present,
threatening damage to the pump.
Referring to FIG. 1 there is shown a coal bed methane pump of according to the
present
invention suspended in a standard well casing and having water level as shown.
Pump 10
includes pump wall 12 having pump cap 13 and fluid exit 16 at its upper end.
Shroud 18
surrounds the lower portion of casing 12 and has inlet holes 20 located in the
vicinity of
its upper end. Motor mount 22 is located at the lower end of pump housing 12
and
attaches to a pump motor seal 14, and pump motor 15.
Referring to FIG. 2 there is shown a diagrammatic elevation view of the coal
bed methane
pump 10, without shroud 18, exposing pump casing inlet holes 24 near its base.
Shroud
upper bracket seal 26 and shroud lower bracket seal 28 are located so as to
support and
seal shroud 18 with pump casing I2 at the shroud's upper and lower ends,
respectively.
The seal are held in place relative to the shroud 18 and pump housing 12 by
means of ring
shaped brackets(not shown) which are mounted on the housing by screws. The
shroud is
so located as to extend below intake holes 24 so that any fluid entering the
pump must
flow into shroud intake holes 20 and down the annulus between the shroud and
the pump
casing to enter pump casing inlet holes 24. Each bracket seal is slotted to
allow a
submersible electrical cable(not shown) to pass through the shroud 18 to the
motor,
allowing the cable jacket to act as the sealing device for the shroud tube.
Referring to FIG. 3 there is shown a diagrammatic elevation view of pump 10
having
casing 12 broken away to show mufti-stage compression pump stack 30 driven by
central
shaft 32 powered by the pump motor(not shown) connected to pwnp motor mount
22.
Charge impeller 34 is mounted for rotation near the lower end of central shaft
32 at a
point slightly below pump inlets 24 and is two-bladed open face impeller, the
blades
being set at an angle so as to impart upward axial momentum to the entering
fluid. An
optional charge impeller 36 (represented by the upper diagrammatic pump stage
may be
employed to assist in directing fluid into the pump outlet 16 for travel to
the surface, or
for entrance into another centrifugal pump, and may be of the centrifugal or
two-bladed
open faced type as desired. Solids grinder 38 is located on shaft 32 slightly
above pump
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inlets 24 and consists of lower plate 40 attached to pump housing 12 and upper
plate 42
spaced above plate 40 and mounted for rotation on shaft 32. Lower plate 40 has
apertures(not shown) as required. One configuration would allow flow in the
annulus
between the lower plate 40 and the central shaft 32. Pressure equalizing plugs
are located
in the wall of the third pump stage from the bottom, leading from the interior
of the third
diffuser and through the pump housing wall to allow fluid to flow between the
inside of
the pump and the annulus between pump housing 12 and shroud 18 when pump
pressure
exceeds that in the shroud, thus maintaining pump prime when ingesting a gas
slug.
Referring to FIG. 4, there is shown a detail view of FIG. 3 showing pressure
equalization
vent plugs 46 located in the fourth stage of multistage compression stages 30
on central
shaft 32.
Refernng to FIG. 5, there is shown a detail view of FIG. 3 showing the solids
grinder 38
having solids grinder lower stationary plate 40 attached to the inside of pump
housing 12
and upper rotating plate 42 attached for rotation with central shaft 32.
Grinding teeth 44
1 S are located on the lower surface of upper rotating plate 42 which is
spaced above
stationary plate 40 an appropriate distance such that grinder teeth 44 produce
the desired
sized coal particles from large particles entering the grinder 38.
Refernng to FIG. 6, there is shown an exploded perspective view of a typical
section of
the multistage compression stages 30. Diffusers 60 comprise cylindrical walls
62 and
radial vanes 64 mounted on diffuser inner plate 66 having central bore 68. The
back side
of diffuser 60 (not shown) is in the form of a shallow cup having the opposite
side of
diffuser inner plate 66 as a base and conforming with the dimensions of
impeller 70 while
leaving adequate mechanical clearance. Impeller 70 comprises curved radial
vanes 72,
similar to diffuser vanes 64, surrounded by disk-shaped shrouds 74 attached to
either
respective sides of vanes 72 to form water passageways which force fluid
outward from
the central shaft 32. There are fluid inlets on each of impellers 70 near the
hub 76 in the
opposing shroud 74 from that shown to allow fluid to flow from diffuser axial
opening
66. Hub 76 is slidingly engaged with and keyed for rotation with central shaft
32. Hub 76
is of such length and diameter that it fits inside stationary diffuser central
bore 68, while
leaving space for upward liquid flow, and engages the hubs of impellers in
adjoining
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stages(not shown). Hubs 76 are self supporting for rotation with central shaft
32 so they
are free of any physical engagement with diffusers 60. Each stack of seven
impellers 70
are separated by a bearing(not shown) capable of supporting the shaft from
lateral
movement. The inner race portion of this bearing is attached to the central
shaft for
rotation and the outer race portion is held by lateral compression by the pump
wall. This
type of bearing is commonly used in the industry in compression pump assembly.
The
assembly of several stages on central shaft 32 forms a pump stage stack 80.
This general
type of pump configuration is standard in the industry as shown by Nielsen et
al. in U.S.
Pat. No. 4,708,589, granted Nov. 24, 1987. The cap structure of Nielsen et al.
may be used in the present invention or, alternatively, a compression plate
(not shown) forming an annulus with central shaft 32 for fluid flow and
which is slidably engaged with the inner side of the pump housing 12. The
compression
plate presses downward against the stack of diffusers, keeping them tightly
engaged. The
compression plate is forced downward by screwing down pump upper cap 13 which
is
threadably engaged with the upper end of pump housing 12. This is also a
commonly
used structure in centrifugal pump design in the industry.
In operation, the water level in coal bed methane pump 10 is initially the
same as water
level in the surrounding well casing. As the pump rotates, methane-saturated
water flows
upward within well casing where it enters shroud 18 through shroud intake
holes 20. The
fluid then travels downward within the annulus between the shroud and the pump
wall to
pump inlets 24. The shroud intake holes 20 are of a size to create a pressure
drop between
the outside well bore pressure and annulus inside the shroud so as to induce
turbulent
flow through the holes 20. This results in some methane gas coming out of
solution to
travel up within the well casing. The shroud length is determined by the
amount of water
that is being produced. The volume of the annular space between the shroud and
the
pump wall must equal the displacement value of the.pump, calculated using a
well bore
pressure, i.e. the pressure at the pump inlets 24, of not less than 32 psi.
The number of
holes in the shroud are determined such that the ratio of collective diameters
of the holes
to the sum of the diameters of the pump inlets 24 exceeds about 3 to 1. The
holes are
sized to avoid plugging due to solids, allow sufficient fluid to fall to keep
the pump from
running out of liquid while pumping, and to allow a pressure drop from the
outside of the
shroud and the inside of the shroud to promote gas separation at the hole
inlets. As liquid
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and gas enter the pump inlets 24 they encounter charge impeller 34, a two
blade, open-
faced impeller, which increases the intake pressure with whatever fluids are
available at
the intake. This device increases lift by about 25 feet or 11 psi to charge
the first stage of
the compression pump with fluid. This first charge impeller will also increase
the volume
of fluid being pushed into the first stage, allowing any solids to be carried
more quickly
through the pump. The open faced, two blade charge impeller will also impact
solids
particles and, through impacting their surface, reducing their size. This
reduction of size
will allow the pump to "digest" the solids more effectively and, with the
increased
velocity created by the impeller, allow the solids to move more quickly to the
pump,
itself. The fluid then encounters the solids grinder 38 having stationary
plate 40 and
rotating plate 42, both of which are preferably constructed of tungsten
carbide. The
grinder is well suited to grind up the soft solids before they enter the pump
impellers, thus
increasing reliability and longevity. Both the stationary plate 40 and the
pump stage
diffusers 30 are held in compression with the pump housing 12. All pump stages
30 must
be constructed so that the impeller hubs 76 are in constant contact and are
spaced to ride
in the middle of each respective diffuser 60. This construction method is
known as
"compression" construction. Each stack of seven impellers 70 are separated by
a bearing
capable of supporting the shaft from lateral movement. The inner race portion
of this
bearing is attached to the central shaft for rotation and the outer race
portion is held by
lateral compression by the pump wall. No impeller 70 or diffuser 60 may touch
in either a
running or non-running mode. This design allows the pump to run through the
full range
of its design curve without damage to the components normally caused by low
flow. With
the combination of materials and design, this design allows the pump to run
without fluid
for substantially longer periods than prior pumps before damage occurs to its
components. The pump is constructed with a variety of stage volume sizes as
dictated by
free gas calculations. The initial stages are of larger volumetric design and
placed below
progressively lower volumetric design stages as the gas-liquid mixture is
compressed.
This design acts as an internal compression device that progressively
compresses free gas
into smaller and smaller bubbles until the pump can efficiently pump the
combination of
liquid and gas. Each stage must be built in a compression configuration
without the aid of
externally used pressure compensation devices.
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It is to be understood that the present invention is not limited to the
embodiment
described above, but encompasses any and all embodiments within the scope of
the
following claims.
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