Note: Descriptions are shown in the official language in which they were submitted.
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PUMP SEALING SYSTEM WITH THROTTLE BUSHING
Field of the Disclosure
[0001] The disclosure is generally related to the field of positive
displacement pumps,
and more particularly to an improved seal arrangement that improves the mean
time
between failures and reduces cost compared to known technologies.
Background of the Disclosure
[0002] There are a large number of different shaft sealing systems used
with fluid
handling pumps that are very satisfactory for normal operating conditions and
for
conventional pumps. In many applications, pumps are used to pump harsh fluids,
the
leakage of which may be undesirable. Leakage from such pumps can present
hazardous
situations if large scale leakage occurs within the pump casing, or if leakage
outside the
pump casing occurs. Some systems may have shaft seal systems that are designed
to
prevent all leakage. These seal systems require complex combinations of
mechanical
seals, which may require regular replacement. In most applications the seal
replacement
process interferes with the system operations, since the system often must be
taken off
line to complete the seal replacement process. The entire replacement process
can take
several hours or days.
[0003] When using mechanical seals, methods must be employed to ensure
lubrication between mechanical seal faces. This can be accomplished by various
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methods, including using single and double mechanical seal arrangements, using
gas seals,
using a single mechanical seal and pressurizing the inside of the seal with a
barrier fluid, and
by using a single mechanical seal on the process side and lip seal and pumping
ring on the
atmospheric side.
[0004] All of these arrangements seek to prevent all leakage of the
process fluid out
of the pumping chamber. As such, they all suffer from problems associated with
shorter than
desired "mean time between failures." Thus, it would be desirable to provide
an improved
pump seal that increases the associated mean time between failures, that is
simple to install
and operate, and which can be implemented in new pump applications as well as
in retrofit
applications.
Summary of the Disclosure
[0004a] Certain exemplary embodiments can provide a sealing arrangement
for a
rotating shaft, comprising: a pump casing having a process fluid chamber and
an end
region; a shaft rotatably mounted with respect to the pump casing; a
mechanical seal having
a stationary portion and a rotatable portion, the stationary portion
associated with the pump
casing and the rotatable portion associated with the shaft; a throttle bushing
having an outer
surface coupled to the pump casing and an inner surface surrounding a portion
of the shaft,
the throttle bushing inner surface configured to provide a fluid passageway
between the
inner surface of the throttle bushing and an outer surface of the shaft;
wherein the throttle
bushing is spaced along the shaft at an axial distance from the mechanical
seal to form a
barrier fluid space therebetween; wherein the mechanical seal separates the
barrier fluid
space from the process fluid chamber; and wherein the throttle bushing is a
one-piece
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configuration, and the fluid passageway being configured to allow for barrier
fluid to flow
from the barrier fluid space to a bearing assembly disposed between an end
region of the
pump and the throttle bushing.
[0005] A sealing arrangement is disclosed for use with a rotating shaft.
The
sealing arrangement may include a pump casing having a process fluid chamber
and an
end region. A shaft may be rotatably mounted with respect to the pump casing.
A
mechanical seal may have a stationary portion and a rotatable portion, the
stationary
portion associated with the pump casing and the rotatable portion associated
with the
shaft. A throttle bushing may include an outer surface coupled to the pump
casing and an
inner surface surrounding a portion of the shaft. The throttle bushing inner
surface may
be configured to provide a fluid passageway between the inner surface of the
throttle
bushing and an outer surface of the shaft. The throttle bushing may be spaced
along the
shaft at an axial distance from the mechanical seal to form a barrier fluid
space
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therebetween. The mechanical seal may separate the barrier fluid space from
the process
fluid chamber.
[0006] A sealing arrangement is disclosed for a rotating shaft. The sealing
arrangement may include a pump casing having a process fluid chamber and an
end
region, a shaft rotatably mounted within the pump casing, and a mechanical
seal having a
stationary portion associated with the pump casing and a rotatable portion
associated with
the shaft. The sealing arrangement may further include a throttle bushing
having an outer
surface coupled to the pump casing and an inner surface configured to provide
a fluid
passageway between the inner surface of the throttle bushing and an outer
surface of the
shaft. A barrier space may be formed between the mechanical seal, the throttle
bushing,
and an outer surface of the shaft. The barrier space may be coupled to a
source of barrier
fluid. The source of barrier fluid may be configured to maintain barrier fluid
in the
barrier space within a predetermined pressure range. The mechanical seal may
separate
the barrier fluid space from the process fluid chamber.
Brief Description of the Drawings
[0007] By way of example, a specific embodiment of the disclosed device
will now
be described, with reference to the accompanying drawings:
[0008] FIG. 1 is a cross-section view of an exemplary embodiment of the
disclosed
seal arrangement;
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[0009] FIG. 2 is a cross-section view of another embodiment of the
disclosed seal
arrangement;
[0010] FIG. 3 is an isometric view of an exemplary positive displacement
pump
incorporating the disclosed seal arrangement;
[0011] FIG. 4 is a cross-section view of the pump of FIG. 3 taken alone
line 4-4;
[0012] FIG. 5 is a detail view of a portion of FIG. 4 illustrating the
disclosed seal
arrangement;
[0013] FIG. 6 is a partial cutaway view of the pump of FIG. 3; and
[0014] FIGS. 7A-7C are isometric, cross-section, and detail views,
respectively, of
an exemplary throttle bushing for use with the disclosed seal arrangement.
Detailed Description
[0015] The disclosure describes an improved dual shaft seal for use in
positive
displacement pumps. These terminologies, as well as others in this disclosure,
follow
those used in American Petroleum Institute (API) standard 682. It will be
appreciated
that although the description will proceed in relation to sealing of positive
displacement
pump shafts, the disclosed seal arrangement is not so limited, and thus, it
may be applied
to a wide variety of rotating shaft sealing applications.
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[0016] The disclosed seal arrangement addresses the aforementioned
deficiencies in
prior seal arrangements. In some embodiments the seal arrangement includes a
throttle
bushing in combination with a mechanical seal, and a barrier fluid disposed
between the
bushing and the mechanical seal. With the disclosed design, controlled leakage
of barrier
fluid past the throttle bushing is collected at the pump ends and reintroduced
into the
system. In some cases, this collected barrier fluid can function to lubricate
one or more
pump bearings and gears located at or near the pump ends.
[0017] FIG. 1 shows an exemplary implementation of the disclosed seal 1 in
the
context of a pump shaft sealing application. The pump shaft 2 may be rotatable
about its
longitudinal axis A-A, supported at least in part by a bearing 4 coupled
directly or
indirectly to a pump casing 6. The pump shaft 2 may have a mechanical seal 8
having a
stationary portion 8a and a rotating portion 8b. One of the portions will be
coupled to the
shaft 2, while the other portion will be coupled to the pump casing 6. The
rotating
portion 8b of the mechanical seal may be subject to process fluid "PF" (i.e.,
the fluid
being pumped) on one side, while the stationary portion may be subject to a
barrier fluid
"BF," which in one non-limiting embodiment is lubricating oil. As will be
appreciated,
the barrier fluid "BF" may be provided at a desired pressure with respect to
the pressure
of the process fluid "PF." The barrier fluid pressure is adjusted so that it
is higher than
the pressure of the process fluid "PF," thus ensuring clean fluid between the
seal faces.
[0018] It will be appreciated that the presence of liquid between the
bearing surfaces
of the mechanical seal portions 8a, 8b will serve to lubricate the portions to
minimize
wear. For embodiments in which the process fluid "PF" is held at a higher
pressure than
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the barrier fluid "BF," the process fluid "PF" may migrate to the interface
between the
bearing surfaces of the stationary seal portion 8a and the rotating seal
portion 8b. The
presence of process fluid "PF" between these portions may not always serve to
lubricate
their respective bearing surfaces thereby reducing seal life. Alternatively,
for
embodiments in which the barrier fluid "BF" is held at a higher pressure than
the process
fluid "PF," the barrier fluid "BF" may migrate to the interface between the
bearing
surfaces of the seal 8 and serve to lubricate those surfaces with a clean
fluid, increasing
seal life.
[0019] The barrier fluid "BF" may be provided in a cavity 10 in the pump
casing
adjacent to the mechanical seal 8. A circumferential throttle bushing 12 may
be
positioned about the shaft 2 on a side of the cavity 10 opposite the
mechanical seal 8.
The throttle bushing 12 may be coupled directly or indirectly to the pump
casing 6 so that
the shaft rotates with respect to an inner surface of the throttle bushing. In
the illustrated
embodiment, a shaft sleeve 14 is provided between the shaft 2 and the
mechanical seal 8
and between the shaft and the throttle bushing 12. It will be appreciated that
this shaft
sleeve 14 is not required, and the disclosed seal arrangement can be used
equally well for
shaft arrangements that do not include a sleeve.
[0020] In operation, the throttle bushing 12 may allow a small quantity of
barrier
fluid "BF" to migrate between the throttle bushing 12 and the shaft 2 (or
between the
throttle bushing 12 and the shaft sleeve 14, if one is used). In the
illustrated embodiment
barrier fluid flows past the throttle bushing is in a direction indicated by
arrows "B."
Once past the throttle bushing 12, the migrated fluid "BF" can be collected at
a suitable
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location, such as the pump end. In some embodiments the barrier fluid "BF" is
lubricating oil, and thus the migrated barrier fluid can serve to lubricate
one or more shaft
bearings 4 located between the throttle bushing 12 and the pump end. Once
collected, the
barrier fluid "BF" may be reused. For example, the barrier fluid "BF" can be
drained or
pumped to an external lubricating oil tank (not shown). The collected barrier
fluid "BF"
may be cooled, filtered, and returned to the cavity 10 under pressure in the
manner
previously described.
[0021] In one embodiment, a fluid loop is provided to direct the collected
barrier
fluid "BF" to a lubricating oil tank, and then back to the cavity 10. A
pressure regulator
(not shown) can be provided in this fluid loop to ensure that the barrier
fluid "BF" in the
cavity 10 is maintained within a desired pressure range. The barrier fluid
"BF" is
maintained at a pressure that is about 25 pounds per square inch gauge (psig)
higher than
the pressure of the process fluid "PF." In a non-limiting example, the process
fluid "PF"
may be from about 0 psig to about 100 psig, while the barrier fluid "BF" may
be from
about 25 psig to about 125 psig. It will be appreciated that these pressure
ranges are not
critical, and that the disclosed seal arrangement can be used at other
pressures.
[0022] FIG. 2 illustrates an embodiment of the disclosed seal in which the
throttle
bushing 12 allows for the controllable migration of barrier fluid "BF" between
the shaft
sleeve 14 and the bushing (again, in the direction of arrow "B"). In this
embodiment the
barrier fluid "BF" lubricates the bearing 4 within bearing housing 16, and is
collected in
the pump end, which is enclosed by an end cap 18. The fluid "BF" (which may be
the
same as the lubricating oil used to lubricate the components of the pump end)
may then
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be directed to a return loop (in the direction of arrow "C") and reused in the
manner
described in relation to the embodiment of FIG. 1.
[0023] FIGS. 3-6 illustrate an exemplary application of the disclosed seal
arrangement in a positive displacement pump 20. The illustrated pump 20 is a
two-screw
pump having a long shaft screw 22 and an intermeshing short shaft screw 24
which, when
rotated, draw process fluid "PF" from suction 26 and discharge the pressurized
process
fluid via discharge 28. The pump 20 may have a casing 30 including a pump head
32
through which the long shaft screw 22 protrudes, and a gear housing 34 which
helps
support the screws 22, 24 at an end opposite that of the pump head 32.
[0024] The long and short shaft screws 22, 24 are supported in the casing
30 by a
plurality of bearing sets 36a, 36b, 36c which, in the illustrated embodiment,
are roller
bearing sets. A pair of mechanical seals 38, 40 are provided for each of the
shaft screws
22, 24 to seal the process fluid "PF" from bearing sets 36a and other internal
portions of
the pump 20. The mechanical seals 38, 40 may be any of a variety of seal
types.
[0025] Each of the shaft screws 22, 24 may be provided with a throttle
bushing 42
positioned between each of the mechanical seals 38, 40 and the adjacent
bearing sets 36a.
The throttle bushings 42 may each have an inside diameter "ID" (FIG. 7B) for
engaging
an outer diameter of the respective shaft screw 22, 24. The throttle bushings
42 may also
have an outside diameter "OD" with a recess 44 formed therein for receiving an
o-ring
46. The o-ring may be formed of a polymer that is compatible the barrier fluid
"BF," a
non-limiting example of which is Viton. The o-ring may be sized so that its
outer surface
protrudes from the recess 44 so that it engages the front bearing assembly 46
to fix the
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throttle bushing 42 in place, and to prevent barrier fluid "BF" from passing
through the
interface between the bearing assembly and the "OD" of the throttle bushing. A
similar
throttle bushing 42 arrangement (including o-ring 46) is provided adjacent to
the bearing
sets 36b of the rear bearing assembly 48.
[0026] In the instant pump 20, process fluid "PF" is disposed in the
chamber region
50, and is sealed off from the pump head 32 by the combination of the
mechanical seals
38, 40 and the associated throttle bushings 42. Barrier fluid "BF" is supplied
in the space
between the mechanical seals 38, 40 and the throttle bushings 42 at a pressure
that is
higher than the pressure of the process fluid "PF," for the reasons previously
described in
relation to the embodiments of FIGS. 1 and 2.
[0027] The "ID" surfaces of the throttle bushings 42 may be carefully
selected so that
a controlled amount of fluid will pass between the bushings and the shafts at
a particular
differential pressure across the bushings. In one non-limiting exemplary
embodiment,
the "ID" surfaces of the throttle bushings may comprise one or more grooves to
improve
sealing performance. In some embodiments these grooves may comprise labyrinth
grooves. As such, during operation, a controlled amount of barrier fluid "BF"
may be
allowed to migrate between the shaft screws 22, 24 and the associated throttle
bushings
42. As previously noted, this fluid may lubricate the adjacent bearing sets
36a, and may
be collected in the pump head 32.
[0028] A similar arrangement may exist in relation to the gear housing 34.
Barrier
fluid "BF" is supplied to the chamber region 50 side of the throttle bushings
42
associated with the shaft screws 22, 24. As shown in more detail in FIG. 5,
the barrier
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fluid "BF" is supplied to the space 45 between the throttle bushings 42 and
the
mechanical seal 43 (a portion of which is shown in FIG. 5). During operation,
barrier
fluid "BF" migrates between the shaft screws 22, 24 and the associated
throttle bushings
42 in a manner previously described, lubricating bearing sets 36b. The fluid
may then be
collected in the gear housing 34. Although FIG. 5 shows this arrangement in
relation to
long shaft screw 22, it will be appreciated that a similar arrangement will
exist for short
shaft screw 24.
[0029] As can be seen in FIG. 6, tube connections 52, 54 are provided in
the gear
housing 34 and pump head 32, respectively to allow for the coupling of tubing
(not
shown) to draw away the bearing fluid "BF" collected in the pump head 32 and
gear
housing 34. The collected fluid may be recirculated to an exterior taffl(
where it can be
cooled, filtered, and reintroduced into the regions between the mechanical
seals and the
throttle bearings in the manner previously described. As previously noted,
this
recirculation scheme can include a pressure regulator (not shown) to regulate
the pressure
of the barrier fluid "BF" that is being reintroduced into the pump.
[0030] FIGS. 7A-C show an exemplary throttle bushing 42 for use in the
disclosed
seal arrangement. The throttle bushing may have an inside diameter "ID," and
outside
diameter "OD," and a recess 44 disposed about the outer perimeter of the
bushing for
receiving an appropriately sized o-ring or other sealing member to seal the
bushing with
respect to the pump casing. Leakage of the barrier fluid "BF" between the ID
of the
throttle bushing 42 and the outside diameter of the respective shaft 22, 24
can be
controlled by adjusting the clearance between the two surfaces, as well as the
length
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"BL" (FIG. 7C) of the bushing. The throttle bushing may be made from carbon,
bronze,
polymer, or the like. In the illustrated embodiment a desired flow across the
bushing is
achieved through careful selection of the bushing "ID" in relation to the
outer diameter of
the associated pump shaft. In other embodiments, to increase the pressure drop
across the
bushing, the inner surface "IS" of the bushing 42 may include one or more
grooves. In
one embodiment, the grooves may comprise a labyrinth arrangement.
[0031] As will be appreciated, the disclosed seal arrangement may result in
increased
seal life and/or reduced cost as compared to prior seal designs. The disclosed
seal
arrangement can also occupy less space than current, more complex, seal
arrangements.
The disclosed seal arrangement includes dual use of barrier and lubrication
fluid instead
of separate systems. As such, it does not waste, barrier fluid.
[0032] Based on the foregoing information, it will be readily understood by
those
persons skilled in the art that the present invention is susceptible of broad
utility and
application. Many embodiments and adaptations of the present invention other
than
those specifically described herein, as well as many variations,
modifications, and
equivalent arrangements, will be apparent from or reasonably suggested by the
present
invention and the foregoing descriptions thereof, without departing from the
substance or
scope of the present invention. Accordingly, while the present invention has
been
described herein in detail in relation to its preferred embodiment, it is to
be understood
that this disclosure is only illustrative and exemplary of the present
invention and is made
merely for the purpose of providing a full and enabling disclosure of the
invention. The
foregoing disclosure is not intended to be construed to limit the present
invention or
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otherwise exclude any such other embodiments, adaptations, variations,
modifications or
equivalent arrangements; the present invention being limited only by the
claims appended
hereto and the equivalents thereof Although specific terms are employed
herein, they
are used in a generic and descriptive sense only and not for the purpose of
limitation.
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