Note: Descriptions are shown in the official language in which they were submitted.
CA 02738354 2011-04-28
MECHANISM FOR SEALING
PFA SEAL BAGS
FIELD OF THE INVENTION
[001] This invention relates generally to the field of submersible pumping
systems, and
more particularly, but not by way of limitation, to a seal section separation
bag for use
with a submersible pumping system.
BACKGROUND
[002] Submersible pumping systems are often deployed into wells to recover
petroleum fluids from subterranean reservoirs. Typically, the submersible
pumping
system includes a number of components, including one or more fluid filled
electric
motors coupled to one or more high performance pumps. Each of the components
and
sub-components in a submersible pumping system must be engineered to withstand
the
inhospitable downhole environment, which includes wide ranges of temperature,
pressure
and corrosive well fluids.
[003] Components commonly referred to as "seal sections" protect the electric
motors
and are typically positioned between the motor and the pump. In this position,
the seal
section provides several functions, including transmitting torque between the
motor and
pump, restricting the flow of wellbore fluids into the motor, protecting the
motor from
axial thrust imparted by the pump, and accommodating the expansion and
contraction of
motor lubricant as the motor moves through thermal cycles during operation.
Many seal
sections employ seal bags to accommodate the volumetric changes and movement
of
fluid in the seal section. Seal bags can also be configured to provide a
positive barrier
between clean lubricant and contaminated wellbore fluid.
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[004] In the past, seal bags have been constructed by sliding an open-ended
bag over
cylindrical mounting blocks. As shown in the Prior Art drawing in FIG. 1, a
prior art seal
bag 10 includes a central portion 12 and two neck portions 14. The prior art
seal bag 10
is installed over cylindrical mounting blocks 16 by fastening the open neck
portions 14 to
the cylindrical mounting blocks 16 with common hose clamps 18. This prior art
design
may be unsuitable in certain applications because the hose clamps 18 tend to
shear the
prior art seal bag 10 after repeated or extensive expansion. Furthermore, the
necessity of
the neck portions 14 in the prior art seal bag 10 decreases the available
length and volume
of the central portion 12.
[005] As the use of downhole pumping systems extends to new applications,
traditional
bladder systems may fail under inhospitable downhole environments. For
example, the
use of downhole pumping systems in combination with steam assisted gravity
drainage
(SAGD) technology exposes bladder components to temperatures in excess of 500
T.
To increase the resistance of the bladder to degradation under these
increasingly hostile
environments, manufacturers have employed durable polymers, including various
forms
of polytetrafluoroethylene (PTFE), as the preferred material of construction.
Although
PTFE is generally resistant to the harsh downhole environment, the use of PTFE
as a
material of construction is discouraged by the need to create the bladder with
a seam-type
design that frustrates efforts to provide an effective seal. There is,
therefore, a need for
an improved seal bag, seal sections and submersible pumping systems that
overcome the
deficiencies of the prior art. It is to this and other needs that the present
invention is
directed.
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SUMMARY OF THE INVENTION
[006] In a preferred embodiment, a seal section for use in a downhole
submersible
pumping system includes a housing, a bag support tube extending through the
housing, a
shaft positioned inside the bag support tube and a bag plate connected to the
bag support
tube. The bag plate preferably includes an end cap secured to the bag support
tube and a
locking collar threadingly engaged with the end cap. A seal bag that is
substantially
cylindrical and includes an open end that is captured between the end cap and
the locking
collar.
[007] In the preferred embodiments, the end cap includes a tapered head that
tapers
from a first diameter that is slightly larger than the inner diameter of the
seal bag to a
second diameter that is slightly smaller than the inner diameter of the seal
bag. The seal
bag is preferably manufactured from perfluoroalkoxy polymer and provided with
sufficient elasticity to permit the stretched placement of the seal bag over
the tapered
head of the end cap. This mechanism for securing the seal bag onto the bag
plate reduces
the dependency on narrow manufacturing tolerances and obviates the need for o-
ring or
other mechanical seals between the seal bag and the bag plate.
BRIEF DESCRIPTION OF THE DRAWINGS
[008] FIG. 1 is a side perspective view of a PRIOR ART seal bag.
[009] FIG. 2 is an elevational view of a submersible pumping system
constructed in
accordance with a presently preferred embodiment.
[010] FIG. 3 is a cross-sectional view of a first preferred embodiment of a
seal section
for use with the submersible pumping system of FIG. 2.
[011] FIG. 4 is a side perspective view of the seal bag of FIG. 3.
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[0121 FIG. 5 is a side cross-sectional view of the first preferred embodiment
of the bag
plate of the seal section of FIG. 3.
[0131 FIG. 6 is a view of the bag plate of FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0141 In accordance with a preferred embodiment of the present invention, FIG.
2
shows an elevational view of a pumping system 100 attached to production
tubing 102.
The pumping system 100 and production tubing 102 are disposed in a wellbore
104,
which is drilled for the production of a fluid such as water or petroleum. As
used herein,
the term "petroleum" refers broadly to all mineral hydrocarbons, such as crude
oil, gas
and combinations of oil and gas. The production tubing 102 connects the
pumping
system 100 to a wellhead 106 located on the surface. Although the pumping
system 100
is primarily designed to pump petroleum products, it will be understood that
the present
invention can also be used to move other fluids. It will also be understood
that, although
each of the components of the pumping system are primarily disclosed in a
submersible
application, some or all of these components can also be used in surface
pumping
operations.
[0151 The pumping system 100 preferably includes some combination of a pump
assembly 108, a motor assembly 110 and a seal section 112. The motor assembly
110 is
preferably an electrical motor that receives power from a surface-mounted
motor control
unit (not shown). When energized, the motor assembly 110 drives a shaft that
causes the
pump assembly 108 to operate. The seal section 112 shields the motor assembly
110
from mechanical thrust produced by the pump assembly 108 and provides for the
expansion of motor lubricants during operation. The seal section 112 also
isolates the
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motor assembly 110 from the wellbore fluids passing through the pump assembly
108.
Although only one of each component is shown, it will be understood that more
can be
connected when appropriate. It may be desirable to use tandem-motor
combinations,
multiple seal sections, multiple pump assemblies or other downhole components
not
shown in FIG. 2.
[016] Referring now to FIG. 3, shown therein is a cross-sectional view of the
seal
section 112. The seal section 112 includes a housing 114, a shaft 116, a seal
bag 118, a
support tube 120 and first and second bag plates 122a, 122b. The seal bag 118
is
configured to prevent the contamination of clean motor lubricants with
wellbore fluids.
The shaft 116 transfers mechanical energy from the motor assembly 110 to the
pump
assembly 108. The bag support tube 120 provides support for the seal bag 118
and
shields the shaft 116 as its passes through the seal bag 118. For the purposes
of the
instant disclosure, the terms "bag seal assembly" will refer to the seal bag
118, the bag
support tube 120 and the first and second bag plates 122a, 122b. In addition
to the bag
seal assembly, the seal section 112 may also include seal guides 124, a
plurality of ports
126 and one or more o-ring seals 128. The o-ring seals 128 are located at
various
positions within the seal section 112 and limit the migration of fluid along
the shaft 116.
[017] For purposes of illustration, the bag seal assembly is disclosed as
contained within
the seal section 112. It will be understood, however, that the bag seal
assembly could be
installed elsewhere in the pumping system 100. For example, it may be
desirable to
integrate the bag seal assembly within the motor assembly 110 or pump assembly
108.
[018] Referring now also to FIG. 4, shown therein is a side perspective view
of a
preferred embodiment of the seal bag 118. The seal bag 118 preferably includes
a central
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portion 130, a first end 132 and a second end 134. The seal bag 118 is
substantially
configured as an elongated cylinder with an inner surface 136 and an outer
surface 138.
By comparing FIGS. 1 and 4, it can be seen that the seal bag 118 provides a
larger
internal capacity. This increased capacity is largely due to the configuration
of the seal
bag 118, which does not include the neck portions 14 that were used to secure
the prior
art seal bag 10 to the mounting blocks 16.
[0191 In the preferred embodiment, the seal bag 118 is fabricated from a
perflouroalkoxy (PFA) flouropolymer, which is commercially available from a
number of
sources, including E.I. du Pont de Nemours and Company and Daikin Industries.
Like
PTFE, PFA exhibits favorable resistance to corrosive chemicals and elevated
temperatures. Unlike PTFE, however, PFA is melt-processable using conventional
injection molding and screw extrusion mechanisms. The ability to extrude or
mold PFA
permits the construction of a seamless, unitary seal bag 118. Thus, as an
advance over
the prior art, the seal bag 118 is a seamless bag that is fabricated using
injection molding
or extrusion techniques.
[0201 Turning now to FIGS. 5 and 6, shown therein is a close-up, partial cross-
sectional
and cut-away view of the engagement between the seal bag 118, the support tube
120 and
bag plate 122a. Although only bag plate 122a is shown in FIGS. 5 and 6, it
will be
appreciated that bag plate 122b makes use of the same components on the
opposite end of
the seal bag 118 (as shown in FIG. 3). Unless otherwise specified, the various
components within the bag plates 122a, 122b of the preferred embodiments in
FIGS. 5
and 6 are the same.
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[021] In the presently preferred embodiments, the bag plate 122a includes an
end cap
140 and a locking collar 142. The end cap 140 preferably includes a stem 144
and a
tapered head 146. In a preferred embodiment, the end cap threadingly engaged
with the
bag support tube 120. Alternatively, the end cap 140 can be configured to rest
on a
shoulder formed on the bag support tube 120. The preferred embodiment
illustrated in
FIG. 5 illustrates both forms of engagement between the end cap 140 and the
bag support
tube 120.
[022] The head 146 of the end cap 140 is preferably tapered from a first outer
diameter
slightly larger than the inner diameter of the seal bag 118 to a second out
diameter that is
slightly less than the inner diameter of the seal bag 118. In this way, the
first end 132 of
the seal bag 118 can be pushed onto the head 146 of the end cap 140. The
elasticity of
the PFA material allows the seal bag 118 to stretch to conform to the shape of
the head
146 of the end cap 140. The engagement between the seal bag 118 and the head
146 is
superior to prior methods of securing seal bags because the elastic nature of
the seal bag
118 reduces the dependency on tight manufacturing tolerances otherwise
required
between sealing surfaces. Accordingly, the ability of the seal bag 118 to
conform to the
contour of the head 146 of the end cap 140 reduces the risk of a seal failure
and obviates
the need for o-rings or other mechanical seals found in prior designs.
[023] The seal bag 118 is held in place over the head 146 by the locking
collar 142,
which applies a compressive force on the end portion 132 of the seal bag 118.
The
locking collar 142 includes a contact ring 148 that includes a tapered
interior surface 150.
The tapered interior surface 150 is preferably oriented at a different angle
than the
tapered surface of the head 146. The different angles ensure that an area of
contact or
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pressure point will be present between the tapered interior contact surface
150 and
tapered head 146. The compressive force of the locking collar 142 further
improves the
sealed engagement between the seal bag 118 and the end cap 140. The locking
collar 142
is preferably configured for threaded engagement over the outside of the end
cap 140.
[0241 In a preferred method of installation, the end cap 140 of one of the bag
plates 122
is secured to bag support tube 120. The seal bag 118 is then inserted over the
bag support
tube 120 and stretched onto the tapered head 146 of the end cap 140. Next,
locking collar
142 is extended over the free end of the seal bag 118 and moved into position
adjacent
end cap 140. The locking collar 142 is then rotated into threaded engagement
with the
end cap 140 until the tapered interior surface 150 of the contract ring 148
applies
sufficient compression to the seal bag 118. Due to the linear movement of the
locking
collar 142 as it is threaded onto end cap 140, the compression exerted between
the offset
angled tapered head 146 and the tapered interior contact surface 150 can be
controlled by
adjusting the extent of engagement between the locking collar 142 and end cap
140. The
installation of the seal bag 118 to the second bag plate 122 is accomplished
in the same
manner, except that the second locking collar 142 must be temporarily
positioned over
the seal bag 118 before it is attached to the end cap 140 of the second bag
plate 122.
[0251 It is to be understood that even though numerous characteristics and
advantages of
various embodiments of the present invention have been set forth in the
foregoing
description, together with details of the structure and functions of various
embodiments
of the invention, this disclosure is illustrative only, and changes may be
made in detail,
especially in matters of structure and arrangement of parts within the
principles of the
present invention to the full extent indicated by the broad general meaning of
the terms in
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which the appended claims are expressed. It will be appreciated by those
skilled in the
art that the teachings of the present invention can be applied to other
systems without
departing from the scope and spirit of the present invention.
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