Abstract:
A hydrostatic mechanical seal assembly includes a locally deployed pump for pressurizing a lubricant fluid between the opposing faces of a mating ring and a sealing ring. In one exemplary embodiment, such pressurization may be achieved via a device that converts the rotational motion of a drive shaft into fluid pressure. The locally deployed pump is intended to advantageously provide a stable positive pressure on the sealing interface between the mating and sealing rings, which may provide improved sealing characteristics, especially in demanding downhole environments.

Description:
FIELD OF THE INVENTION 
       [0001]    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 
       [0002]    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”. 
         [0003]    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 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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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 
       [0008]    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. 
         [0009]    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. 
         [0010]    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 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. 
         [0011]    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. 
         [0012]    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 
         [0013]    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: 
           [0014]      FIG. 1  depicts a downhole tool including an exemplary hydrostatic mechanical seal assembly embodiment according to the present invention. 
           [0015]      FIG. 2  depicts, in cross section, an exemplary hydrostatic mechanical seal assembly according to this invention. 
           [0016]      FIG. 3  depicts, in cross section, a portion of the embodiment shown on  FIG. 2 . 
           [0017]      FIG. 4  depicts, in cross section, another exemplary embodiment of a hydrostatic mechanical seal assembly according to this invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIGS. 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  FIGS. 1 through 3  may be described herein with respect to that reference numeral shown on other views. 
         [0019]      FIG. 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  FIG. 1 , downhole tool  100  may include a three-dimensional rotary steering tool (3DRS) in which the seal assembly  10  provides a sealing function between an inner rotating shaft (or cylinder)  120  and an outer housing  110 . 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. 
         [0020]    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 that illustrated on  FIG. 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. 
         [0021]    With reference now to  FIG. 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  FIG. 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  FIGS. 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. 
         [0022]    With continued reference to  FIG. 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 on  FIG. 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. 
         [0023]    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. 
         [0024]    With continued reference to  FIG. 2 , and further reference now to  FIG. 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  FIG. 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  FIG. 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 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. 
         [0025]    Turning now to  FIG. 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  FIG. 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 . 
         [0026]    In the exemplary embodiment shown on  FIG. 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. 
         [0027]    As described above, the exemplary embodiments shown on  FIGS. 2 and 4  include pumps  60  and  260  deployed locally with the sealing members. In the embodiment shown on  FIG. 2 , the pump  60  is deployed integrally with the sealing ring carrier  35 . In the exemplary embodiment shown on  FIG. 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  FIGS. 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. 
         [0028]    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.