Note: Descriptions are shown in the official language in which they were submitted.
CA 02590269 2007-05-29
HYDROSTATIC MECHANICAL SEAL
WITH LOCAL PRESSURIZATION OF SEAL INTERFACE
FIELD OF THE INVENTION
The present invention relates generally to hydrostatic mechanical face seals
for
providing, for example, fluid sealing between a housing and a rotating shaft.
This
invention more specifically relates to a hydrostatic mechanical seal assembly
having a
local arrangement for pressurizing fluid near the sealing interface. Although
not limited to
any particular deployment, this invention may be particularly advantageous in
various
downhole drilling tools such as drilling motors, drill bit assemblies, and
rotary steering
tools.
BACKGROUND OF THE INVENTION
Mechanical face seals are used on various types of machines and equipment,
such
as pumps, compressors, and gearboxes, for providing a seal between, for
example, a
rotating shaft and a stationary component such as a housing. Such mechanical
seals
typically include a pair of annular sealing rings concentrically disposed
about the shaft and
axially spaced from each other. Typically, one sealing ring remains stationary
(e.g.,
engaged with the housing) while the other sealing ring rotates with the shaft.
The sealing
rings further include opposing sealing faces that are typically biased towards
one another.
Mechanical seals may be generally categorized as "contacting" or "non-
contacting". In
contacting mechanical seals the biasing force is carried by mechanical contact
between the
annular sealing rings. In non-contacting mechanical seals a pressurized fluid
film between
the annular sealing rings carries the biasing force. Non-contacting mechanical
seals may
be subcategorized as "hydrodynamic pressure lubricated" or "hydrostatic
pressure
lubricated".
In a hydrodynamic pressure lubricated mechanical face seal (also referred to
herein
as a hydrodynamic mechanical seal) the seal faces are provided with features
such as
grooves or vanes. Relative motion of the faces thus tends to draw the
lubricating fluid into
the interface between the seal faces and effectively pressurize the
lubricating fluid film
against the fluid being sealed (e.g., drilling fluid in downhole tools). The
hydrodynamic
lift (separation) of the faces is dependent on rotational speed, fluid
viscosity, and the shape
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of the hydrodynamic features. Fluid viscosity is typically highly dependent on
temperature. Such dependencies on speed and temperature tend to make it
difficult to
design hydrodynamic seals that meet the criteria required for typical downhole
tools.
In hydrostatic pressure lubricated mechanical face seals (also referred to
herein as
hydrostatic mechanical seals) an essentially steady state fluid pressure is
provided to the
interface between the seal faces, for example, by remote pumps or energized
accumulators. In a typical hydrostatic pressure lubricated seal, a radial
taper is formed in
the seal interface. The radial taper typically converges from the higher
pressure fluid to the
lower pressure fluid and acts to maintain a predetermined gap between the seal
faces (the
size of the gap being the primary deterrent to fluid leakage). Hydrostatic
mechanical seals
typically have a broader range of stable operation as compared with
hydrodynamic
mechanical seals. For example, hydrostatic mechanical seals are typically much
less
dependent on rotational speed than hydrodynamic mechanical seals.
In use hydrostatic mechanical seals typically require a stable pressure
differential
from the higher pressure sealed fluid to the lower pressure excluded fluid.
Reversing
pressure may be particularly harmful since it may reverse the direction of
fluid flow. Such
pressure changes may also change the radial taper such that it reverses
convergence,
thereby allowing contaminants into the sealing interface and compromising the
sealing
function. Accumulators, in particular, tend to be subject to sticking or
fouling, which may
cause loss (or reversing of) pressurization in hydrostatic mechanical seals.
Such loss (or
reversing) of pressurization often allows the excluded fluid to enter the seal
interface and
thus may result in premature failure of the seal assembly. In certain downhole
tools, such
as drill bit assemblies, drilling motors, rotational steering tools,
measurement while
drilling tools, turbines, alternators, and production pumps, such failure of
the seal
assembly often results in penetration of drilling fluid into the interior of
the tool, which is
known to have caused serious damage and/or failure of the tool.
Furthermore, remote pressurizing devices tend to be slow to respond to
external
pressure variations, for example, drilling fluid pressure spikes in a downhole
drilling
environment. Such pressure spikes have been observed to cause a pressure
reversal in
hydrostatic mechanical seals and therefore may also allow excluded fluid, such
as drilling
fluid, to penetrate into the interior of the tool.
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Therefore, there exists a need for an improved hydrostatic mechanical seal
assembly, in particular, an improved hydrostatic mechanical seal assembly
including a
pressure generating device that might provide improved robustness for use in
downhole
tools.
SUMMARY OF THE INVENTION
The present invention addresses one or more of the above-described drawbacks
of
prior art hydrostatic mechanical sealing assemblies. Aspects of this invention
include a
hydrostatic mechanical seal assembly comprising a locally deployed pump for
pressurizing a lubricant fluid between the opposing faces of a mating ring and
a sealing
ring. In one embodiment, such pressurization may be achieved via a device that
converts
the rotational motion of a drive shaft into fluid pressure. For example, a
helical groove
pump may be deployed integral with a sealing ring carrier. Alternatively, a
cam driven
piston pump may be deployed, for example, about a rotating shaft in close
proximity with
the mating and sealing rings. Other alternative embodiments of hydrostatic
mechanical
sealing assemblies according to this invention may include, for example,
piston, vane,
gear, positive displacement, electromechanical, and/or centrifugal pumps, and
the like
deployed locally with the seal assembly.
Exemplary embodiments of the present invention advantageously provide
several technical advantages. In particular, embodiments of this invention may
provide a
stable positive pressure on the sealing interface between the mating and
sealing rings. As
a result, various embodiments of the hydrostatic mechanical sealing system of
this
invention may exhibit improved sealing characteristics, especially in
demanding downhole
environments. Tools embodying this invention may thus display improved
reliability and
prolonged service life as compared to tools utilizing conventional hydrostatic
mechanical
sealing assemblies. The local pressurization provided by this invention also
obviates the
need for remote pumps and/or energized accumulators typically used in
conjunction with
conventional hydrostatic mechanical seals.
In one aspect this invention includes a hydrostatic mechanical face seal
assembly. The assembly includes a mating ring having a first sealing face and
a sealing
ring having a second sealing face, the first and second sealing faces being
biased towards
one another. The sealing ring is deployed substantially coaxially with the
mating ring and
further disposed to rotate relative to the mating ring. The assembly further
includes a
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pump disposed to pressurize a lubricating fluid at an interface between the
first and second
sealing faces. The pump is deployed locally with the mating ring and the
sealing ring. In
one exemplary embodiment of this invention the mating ring is coupled to a
mating ring
carrier, the sealing ring is coupled to a sealing ring carrier, and the pump
is deployed on a
member selected from the group consisting of the sealing ring, the sealing
ring carrier, the
mating ring, and the mating ring carrier.
In another aspect, this invention includes a tool having a rotatable drive
shaft
deployed in a substantially non rotating tool housing and a hydrostatic
mechanical face
seal assembly disposed to seal a contaminant fluid. The seal assembly includes
a mating
ring having a first sealing face, the mating ring deployed substantially
coaxially about the
drive shaft; the mating ring being substantially non rotational relative to
the tool housing.
The seal assembly also includes a sealing ring having a second sealing face,
the sealing
ring deployed substantially coaxially about and coupled with the drive shaft,
the sealing
ring and the mating ring disposed to rotate relative to one another, the first
face and the
second face biased towards one another. The seal assembly further includes a
pump
disposed to pressurize a lubricating fluid at an interface between the first
and second
sealing faces, the pump deployed locally with the seal assembly.
The foregoing has outlined rather broadly the features and technical
advantages
of the present invention in order that the detailed description of the
invention that follows
may be better understood. Additional features and advantages of the invention
will be
described hereinafter which form the subject of the claims of the invention.
It should be
appreciated by those skilled in the art that the conception and the specific
embodiment
disclosed may be readily utilized as a basis for modifying or designing other
structures for
carrying out the same purposes of the present invention. It should also be
realized by
those skilled in the art that such equivalent constructions do not depart from
the spirit and
scope of the invention as set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages
thereof, reference is now made to the following descriptions taken in
conjunction with the
accompanying drawings, in which:
FIGURE 1 depicts a downhole tool including an exemplary hydrostatic mechanical
seal assembly embodiment according to the present invention.
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FIGURE 2 depicts, in cross section, an exemplary hydrostatic mechanical seal
assembly according to this invention.
FIGURE 3 depicts, in cross section, a portion of the embodiment shown on
FIGURE 2.
FIGURE 4 depicts, in cross section, another exemplary embodiment of a
hydrostatic mechanical seal assembly according to this invention.
DETAILED DESCRIPTION
Referring to FIGURES 1 through 3, it will be understood that features or
aspects of
the embodiments illustrated may be shown from various views. Where such
features or
aspects are common to particular views, they are labeled using the same
reference
numeral. Thus, a feature or aspect labeled with a particular reference numeral
on one view
in FIGURES 1 through 3 may be described herein with respect to that reference
numeral
shown on other views.
FIGURE 1 schematically illustrates one exemplary embodiment of a hydrostatic
mechanical seal assembly 10 according to this invention in use in a downhole
tool,
generally denoted 100. Downhole tool 100 may include substantially any tool
used
downhole in the drilling, testing, and/or completion of oilfield wells,
although the
invention is expressly not limited in this regard. For example, as shown in
FIGURE 1,
downhole tool 100 may include a three-dimensional rotary steering tool (3 DRS)
in which
the seal assembly 10 provides a sealing function between an inner rotating
shaft (or
cylinder) 120 and an outer housing I10. In such a configuration, the housing
110 and
force application members 115 are typically substantially non-rotational
relative to the
well bore during the drilling operation. Downhole tool 100 may be configured
for
mounting on a drill string and thus include conventional threaded or other
known
connectors on the top and bottom thereof, such as drill bit receptacle 125. In
other
exemplary embodiments downhole tool 100 may include drilling motors, drill bit
assemblies, stabilizers, measurement while drilling tools, logging while
drilling tools,
other steering tools, turbines, alternators, production pumps, under-reamers,
hole-openers,
turbine-alternators, downhole hammers, and the like.
Although the deployments and embodiments described herein are directed to
subterranean applications, it will be appreciated that hydrostatic mechanical
seal
assemblies according to the present invention are not limited to downhole
tools, such as
CA 02590269 2007-05-29
that illustrated on FIGURE 1, or even to downhole applications. Rather,
embodiments of
the invention may be useful in a wide range of applications requiring one or
more
mechanical seals, such as for example, pumps, compressors, turbines, gear
boxes,
motorized vehicles, engines, electric power generation equipment, boats,
household
appliances, agricultural and construction equipment, and the like.
With reference now to FIGURE 2, a cross sectional schematic of one exemplary
embodiment of a hydrostatic mechanical seal assembly 10 is shown. Seal
assembly 10
includes a mating ring 20 having a sealing face 22 and a sealing ring 30
having a sealing
face 32. Seal assembly 10 further includes a biasing member 42 (such as a
metal bellows,
a spring member, or another suitable equivalent), which resiliently preloads
(i.e., biases)
the face 32 of sealing ring 30 towards the face 22 of mating ring 20. It will
be appreciated
that while the biasing member 42 is shown biasing the sealing ring 30 towards
the mating
ring 20 on FIGURE 2, the biasing member 42 may be alternatively disposed to
bias the
mating ring 20 towards the sealing ring 30. Moreover, one or more biasing
members 42
may also simultaneously bias faces 22 and 32 towards one another. Seal
assembly 10
further includes a pressure generating device 60 (e.g., a pump) deployed
locally with the
seal assembly 10, as described in more detail below with respect to FIGURES 2
and 3. It
will be appreciated that deploying the pressure generating device 60 locally
with the seal
assembly includes deploying the pressure generating device 60 integrally with,
resident
on, adjacent to, and in close proximity to one or more members of the
hydrostatic
mechanical seal assembly.
With continued reference to FIGURE 2, in exemplary embodiments of seal
assembly 10, mating ring 20 is substantially stationary (i.e., non-rotating)
and coupled to
(e.g., sealingly engaged with) a mating ring carrier 25, which may, for
example, be
coupled to a tool housing 110. Mating ring 25 may further include a dynamic
seal 27 with
the drive shaft 120 (or a shaft sleeve 122). Sealing ring 30 may be coupled to
(e.g.,
sealingly engaged with) a sealing ring carrier 35, for example via biasing
member 42,
which as described above resiliently preloads the face 32 of sealing ring 30
towards the
face 22 of mating ring 20. Sealing ring carrier 35 may be sealingly engaged
via a static
seal 37, for example, to a drive shaft 120 (or a shaft sleeve 122) that
rotates relative to the
housing. One or more radial bearings 50 may be utilized to maintain precise
alignment
between the rotating and non-rotating components. In the exemplary embodiments
shown
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on FIGURE 2, the pressure generating device 60 is deployed integrally with
ring carrier 35
and is configured to provide pressurized lubricant fluid from, for example, a
fluid reservoir
70, to the interface 24 between mating ring 20 and sealing ring 30. In various
exemplary
embodiments, pressure generating device 60 is configured to utilize the
rotational motion
of drive shaft 120 to pressurize the lubricating fluid.
The mating ring 20 and sealing ring 30 may be made from substantially any
suitable material. For downhole deployments of the invention, it may be
advantageous to
fabricate the mating ring and/or the sealing ring from ultra-hard materials to
combat the
hard abrasive solids found in certain drilling fluids. A typical ultra-hard
mating ring
and/or sealing ring might optimally be made from a material having a Rockwell
hardness
value, Rc, greater than about 65. Such ultra-hard materials include, for
example, tungsten
carbide, silicon carbide, boron containing steels (boronized steels), nitrogen
containing
steels (nitrided steels), high chrome cast iron, diamond, diamond like
coatings, cubic
boron nitride, ceramics, tool steels, stellites, and the like. It will be
appreciated that while
ultra-hard materials may be advantageous for certain exemplary embodiments,
this
invention is not limited to any particular mating ring and/or sealing ring
materials. In
applications where hard abrasive solids need not be combated, conventional
carbon
graphite may be used as a material from which to manufacture the mating ring
and/or
sealing ring.
With continued reference to FIGURE 2, and further reference now to FIGURE 3,
one exemplary embodiment of a pressure generating device 60 is described in
further
detail. As described above, seal assembly 10 includes a pressure generating
device 60
(such as a pump) deployed locally with the seal assembly 10. In various
exemplary
embodiments, the pressure generating device 60 may be integral with one or
more
members of the seal assembly. For example, the ring carrier 35 may be fitted
with a
helical groove pump (also referred to as a screw pump) as shown on FIGURE 3.
In the
embodiment shown, the outer surface 64 of ring carrier 35 is fitted with one
or more
helical grooves 62 that serve to pump fluid (thereby increasing the pressure)
towards 68
sliding interface 24 upon rotation of the drive shaft 120. It will be
appreciated that while
the embodiment shown on FIGURE 3 includes a helical groove pump deployed on
the
sealing ring carrier 35, the pressure generating device 60 may be deployed
substantially
anywhere in or about the seal assembly 10. For example, a helical groove pump
(e.g., one
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or more helical grooves such as grooves 62 in sealing ring carrier 35) may
likewise be
deployed on the inner surface of a housing or mating ring (e.g., mating ring
25) adjacent
carrier ring 35, on the outer surface 34 of the sealing ring 30, on the inner
surface 28 of the
mating ring carrier 25 adjacent the sealing ring 30, or substantially any
other suitable
location. Likewise, it will further be appreciated that substantially any
suitable pressure
generating device may be utilized in embodiments of this invention. For
example, various
alternative embodiments may include piston, vane, gear, positive displacement,
electromechanical, and/or centrifugal pumps.
Turning now to FIGURE 4, one alternative embodiment of a sealing assembly
according to this invention is shown. Downhole tool 200 includes rotor 290 and
stator 295
assemblies of a downhole turbine deployed in a downhole tool body 210 and
coupled to a
drive shaft 218 and alternator 280. In the embodiment shown, drilling fluid
(drilling mud)
is pumped down through annular region 215 to power the turbine. The sealing
assembly is
similar to that described above with respect to FIGURE 2 in that it includes
mating 220
and sealing 230 rings having adjacent sealing faces. Coil springs 242 are
disposed to bias
sealing ring 230 towards mating ring 220. In the embodiment shown, mating ring
220 is
substantially stationary (i.e., non-rotating), while sealing ring 230 and coil
spring 242 are
disposed to rotate with the drive shaft 220.
In the exemplary embodiment shown on FIGURE 4, a piston pump 260 is
deployed substantially adjacent to sealing ring 230. The piston pump 260 is
driven by an
eccentric diameter cam 262 formed in the drive shaft 220 and is disposed to
provide
pressurized fluid from a fluid reservoir 272 to the pump 260 through
passageway 265 and
on to the interface between the mating 220 and sealing 230 rings via
passageway 264. The
piston pump 260 includes a dynamic seal 263 with the drive shaft 220 to
prevent pressure
loss in the pressurized fluid (i.e., to separate the high and lower pressure
fluid). The tool
200 may optionally include a bladder 275 (e.g., an elastomeric boot) disposed
in the fluid
reservoir 272 for providing pressure equalization between drilling fluid in
annular region
215 and lubricating fluid in the fluid reservoir 272. Use of the bladder 275
advantageously tends to equalize pressure spikes between the drilling fluid
and sealed
fluid and therefore tends to reduce the likelihood of pressure reversals at
the interface
between the mating 220 and sealing 230 rings.
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As described above, the exemplary embodiments shown on FIGURES 2 and 4
include pumps 60 and 260 deployed locally with the sealing members. In the
embodiment
shown on FIGURE 2, the pump 60 is deployed integrally with the sealing ring
carrier 35.
In the exemplary embodiment shown on FIGURE 4, pump 260 is deployed in close
proximity to mating 220 and sealing 230 rings. In this exemplary embodiment,
pump 260
is deployed about 6 inches above the mating 220 and sealing 230 rings. Of
course, the
invention is not limited in these regards. Rather, these exemplary embodiments
shown on
FIGURES 2 and 4 are intended to illustrate what is meant by "local deployment"
of the
pumping mechanism. In the exemplary embodiments shown, the pumps 60 and 260
are
deployed near enough to the respective sealing interfaces so that there is
substantially no
pressure loss in the lubricating fluid between the pumps 60 and 260 and the
sealing
interfaces. This is in contrast to prior art arrangements in which remote
deployment of the
pump and/or accumulator often results in a pressure loss (drop) in the
lubricating fluid
between the pump and the sealing interface. Such pressure losses are typically
due to both
the distance between the pump and the sealing interface and the tortuous fluid
flow path
therebetween. As described above in the Background Section, such pressure
drops and/or
spikes are known to result in premature seal failure, especially in downhole
tools. In many
prior art arrangements the pump and/or accumulator is deployed 2 feet or more
above or
below the sealing members.
Although the present invention and its advantages have been described in
detail, it
should be understood that various changes, substitutions and alternations can
be made
herein without departing from the spirit and scope of the invention as defined
by the
appended claims.
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