Patent Publication Number: US-2011052109-A1

Title: Hydrostatic auxiliary bearing for a turbomachine

Description:
BACKGROUND 
     In turbomachine systems, if a primary bearing (such as a magnetic bearing) fails, the shaft of the turbomachine will generally fall or drop onto the adjacent mechanical surfaces. This drop often causes substantial damage to the shaft and/or the surrounding components. In turbomachine systems that include an auxiliary bearing, the shaft may drop onto the auxiliary bearing without damaging the shaft or surrounding components. 
     There are two common designs for a traditional auxiliary bearing: (a) a dry-lubricated bushing, and (b) one or more rolling element bearing(s) with a clearance between the bearing inner ring(s) and the shaft. 
     The bushing design consists of one or more segments of a material containing a dry lubricant. When a shaft drops onto this bearing, it slides within the bearing as it coasts down from speed. Considerable heat is generated in this process, which limits the time the rotor can spin on the auxiliary bearing. Furthermore, the bearing surface is subject to wear, and the friction forces on the rotor have the potential for sending it into destructive backward whirl. 
     With the rolling element bearing design, the shaft drops onto the inside of the bearing inner ring. In this configuration, the auxiliary bearing is accelerated almost instantaneously to match the shaft speed when a drop occurs. The auxiliary bearing may then be used to allow the shaft to coast down on the auxiliary bearing (maintaining normal operating speed is generally not possible). However, this configuration may cause amplitude backward whirl of the shaft, brinelling of the races from the impact of the shaft, skidding between rolling elements and races due to the high acceleration rate of the bearing to match the shaft speed, high stresses in the cage or separator (if employed), and overheating of the auxiliary bearing. Further, the life of auxiliary bearings in this configuration is often only a few drops of the shaft, and as such, the longevity and reliability are challenges. 
     The lack of a reliable, long-lasting auxiliary bearing technology, also known as “coastdown bearing” or “catcher bearing” technology, has been a barrier to the implementation of magnetic bearings into turbomachines. Magnetic bearings are now being considered for applications where the auxiliary bearing may be required to support the shaft at operating speed for sustained operation when a primary bearing fails (e.g., minutes to days). Thus, there is a need for an auxiliary bearing system or configuration that provides for continued operation of a turbomachine for longer periods of time when a primary bearing fails. 
     SUMMARY 
     Embodiments of the disclosure may provide a system for supporting a rotating shaft, including a primary bearing configured to maintain the shaft within a predetermined range of operational shaft positions; a process fluid source; and a hydrostatic auxiliary bearing having an inner surface manufactured from a self lubricating composite material. The hydrostatic auxiliary bearing is fluidically coupled to the process fluid source and configured to use a pressurized process fluid provided by the process fluid source to maintain the shaft within the predetermined range of operational shaft positions when the primary bearing fails to maintain the shaft within the predetermined range of operational shaft positions when the primary bearing fails. 
     Embodiments of the disclosure may further provide a method for supporting a rotating shaft, including maintaining the shaft within a predetermined range of operational shaft positions using a primary active magnetic bearing. The method may further include maintaining the shaft within the predetermined range of operational shaft positions using a hydrostatic auxiliary bearing when the primary active magnetic bearing fails to maintain the shaft within the predetermined range of operational shaft positions. The hydrostatic auxiliary bearing is configured to support the shaft using a pressurized process fluid from a process fluid source, and an inner surface of the hydrostatic auxiliary bearing comprises a self lubricating composite material. 
     Embodiments of the disclosure may further provide a system for supporting a rotating shaft, including first means for maintaining the shaft within a predetermined range of operational shaft positions. The system may further include a second means for maintaining the shaft within a predetermined range of operational shaft positions when the first means fails to maintain the shaft within the predetermined range of operational shaft positions, wherein the second means uses a pressurized process fluid from a means for storing process fluid. An inner surface of the second means comprises a self lubricating composite material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying Figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  illustrates a schematic view of a turbomachine according to one or more aspects of the present disclosure. 
         FIG. 2A  illustrates a cross-sectional view of an auxiliary or catcher bearing according to one or more aspects of the present disclosure. 
         FIG. 2B  illustrates a partial cross-sectional view of an auxiliary or catcher bearing according to one or more aspects of the present disclosure. 
         FIG. 2C  illustrates a cross-sectional view of another exemplary embodiment of auxiliary or catcher bearing according to one or more aspects of the present disclosure. 
         FIG. 3A  illustrates a schematic view of a turbomachine according to one or more aspects of the present disclosure. 
         FIG. 3B  illustrates a perspective view of a damper seal according to one or more aspects of the present disclosure. 
         FIG. 3C  illustrates a perspective view of a portion of a damper seal according to one or more aspects of the present disclosure. 
         FIG. 4A  illustrates a cross-sectional view of an auxiliary or catcher thrust bearing according to one or more aspects of the present disclosure. 
         FIG. 4B  illustrates a cross-sectional view of a combined auxiliary, or catcher, radial and thrust bearing according to one or more aspects of the present disclosure. 
         FIG. 4C  illustrates a partial cross-sectional view of a combined auxiliary, or catcher, radial and thrust bearing according to one or more aspects of the present disclosure. 
         FIG. 5  illustrates a flowchart of a method for supporting a shaft of a turbomachine according to one or more aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure, however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the exemplary embodiments presented below may be combined in any combination of ways, i.e., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure. 
     Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Further, in the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. 
       FIG. 1  illustrates a turbomachine  100  in accordance with an exemplary embodiment of the present disclosure. The turbomachine  100  may include a turbine, such as a steam turbine. In an exemplary embodiment, the turbomachine  100  may include a compressor, such as a rotary compressor. In other exemplary embodiments, the turbomachine  100  may be any device that generates energy using a process fluid or process gas, including without limitation a turboset. The turbomachine  100  includes a casing  102 , and a shaft  104  positioned within the casing  102 . High pressure steam or air surrounds the shaft  104  at a location  105   a.  Further, steam or air, at low or atmospheric pressure, surrounds the shaft  104  at a location  105   b.    
     Primary bearings  106   a  and  106   b  are coupled to respective interior surfaces of the casing  102  at opposing end portions of the shaft  104 , and provide support for the shaft  104 . Each of the primary bearings  106   a  and  106   b  is an active magnetic bearing that has one or more electromagnets controlled by a magnet control  107 . The magnet control  107  may also be equipped with one or more sensors configured to monitor operating conditions of the primary bearings  106   a  and  106   b.  In an exemplary embodiment, each of the primary bearings  106   a  and  106   b  may be a passive magnetic bearing that only includes permanent magnets. In yet another exemplary embodiment, each of the primary bearings  106   a  and  106   b  may be a suspension bearing. In other exemplary embodiments, each of the primary bearings  106   a  and  106   b  may be any kind of conventional bearing that may fail and cause the shaft to drop or otherwise become positioned out of the normal operation position (axially within the turbomachine). Means for maintaining the shaft within a predetermined range of operational shaft positions may include any of the foregoing embodiments of the primary bearing  106  and any equivalents thereof. 
     A plurality of labyrinth seal assemblies  108   a - i  circumferentially surround the shaft  104 . In an exemplary embodiment, the labyrinth seal assemblies  108   a - i  may include one or more labyrinth seal segments that form one or more labyrinth packing rings. The labyrinth seal segments may be segmented cylindrical-toothed rings. Other designs for labyrinth seal assemblies  108   a - i  that are known in the art are also within the scope of the present disclosure. A leakage recycler  112  is fluidically coupled to a leakage recycler pipe  114 . The leakage recycler pipe  114  is also be fluidically coupled to openings  115   a - c  formed in the casing  102 . In other exemplary embodiments, other configurations for positioning the leakage recycler pipe  114  may also be used without departing from the scope of the present disclosure. 
     The turbomachine  100  also includes auxiliary or catcher bearings  116   a  and  116   b  that circumferentially surround the shaft  104 . The auxiliary bearing  116   a  is located between the labyrinth seal assemblies  108   c  and  108   d.  The auxiliary bearing  116   b  is located between labyrinth seal assemblies  108   h  and  108   i.  In an exemplary embodiment, the auxiliary bearings  116   a  and  116   b  may include a hydrostatic bearing. A hydrostatic bearing is also sometimes referred to as a fluid film bearing. In an exemplary embodiment, a clearance  118   a  may be formed between an inner surface  119   a  of the auxiliary bearing  116   a  and the shaft  104 , and a clearance  118   b  may be formed between an inner surface  119   b  of the auxiliary bearing  116   b  and the shaft  104 . 
     In an exemplary embodiment, the inner surface  119  may include material that consists of a durable surface, such as NASA PS300. As disclosed in U.S. Pat. No. 5,866,518, NASA PS300 is a self lubricating, friction-reducing, and wear-reducing composite material. U.S. Pat. No. 5,866,518 is herein incorporated in its entirety to the extent it does not contradict the present disclosure. In another exemplary embodiment, the inner surface  119   a  or  119   b  may include a hard coating such as chrome oxide or titanium nitride, and may include materials such as clutch material, brake material, and sintered material. 
     In an exemplary embodiment, the auxiliary bearings  116   a  and  116   b  include inlets  120   a  and  120   b,  respectively, which are fluidically coupled to a process fluid source  122  via a process fluid pipe  124 . The fluid source  122  may provide boiler feedwater to the inlets  120   a  and  120   b  via the process fluid pipe  124 . Further, the leakage recycler  112  is fluidically coupled to the process fluid source  122  via a recirculation pipe  126 . In other exemplary embodiments, multiple recirculation pipes  126  may fluidically couple the leakage recycler  112  to the process fluid source  122 . Also, a system recycle pipe (not shown) may fluidically couple the leakage recycler  112  to other components of the turbomachine  100  that are not shown in  FIG. 1 . 
     The process fluid pipe  124  is configured to operate with valves  128   a  and  128   b . The valves  128   a  and  128   b  have valve controls  129   a  and  129   b,  respectively, which are configured to control the valves  128   a  and  128   b.  A master control system (not shown) may be communicably coupled to the valve controls  129   a  and  129   b  and the magnet control  107 . One or more wires may facilitate communication between the master control system, the valve controls  129   a  and  129   b,  and the magnet control  107 . 
     Turning now to the operation of the turbomachine  100 , in an exemplary embodiment, the primary bearings  106   a  and  106   b  maintain the shaft  104  within a predetermined range of operational shaft positions. When the magnet control  107  detects that the primary bearing  106   a  or  106   b  is failing to maintain the shaft  104  within the predetermined range of operational shaft positions, the magnet control  107  sends a signal to the valve controls  129   a - b  to open the valves  128   a - b.    
     In another exemplary embodiment, one or more sensors (not shown) may be used to monitor the operation conditions of the turbomachine  100 . The valve controls  129 - b  may be configured to communicate with the sensors, and open the valves  128   a - b  when a predetermined set of operating conditions is detected by the sensors. For example, in one embodiment, the valve controls  129   a - b  cause the valves  128   a - b  to open when a sensor detects that the temperatures of the inner surfaces  119   a - b  of the auxiliary bearings  116   a - b  have reached a predetermined temperature that indicates that the shaft  104  is outside of a predetermined range of operational shaft positions, i.e., that a drop is about to occur or is occurring. 
     In yet another exemplary embodiment, one or more sensors may track the center of the shaft  104 , and may instruct the valve controls  129   a - b  to open the valves  128   a - b  when the sensors detect that the center of the shaft  104  is outside a predetermined range of operational shaft positions. In another exemplary embodiment, the auxiliary bearings  116   a - b  may be configured to idle at a predetermined hydrostatic pressure that may be lower than an operating pressure of the bearings, so that the auxiliary bearings  116   a - b  may become fully operational (brought up to operating pressure) more quickly when the valves  128   a - b  are opened. Other methods for determining when to open the valves  128   a - b  are also within the scope of the present disclosure. 
     In an exemplary embodiment, when the valves  128   a - b  are open, high-pressure process fluid may be provided from the process fluid source  122  to the inlets  120   a - b  via the process fluid pipe  124 . Upon entering the inlets  120   a - b , the process fluid may form a fluid film between the shaft  104  and the inner surfaces  119   a - b . The hydrostatic pressure of the fluid film may maintain the shaft  104  within a predetermined range of operational shaft positions. 
     In an exemplary embodiment, the pressure of the process fluid may be less than about 1000 psi. In another exemplary embodiment, the pressure of the process fluid may range from about 500 psi to about 900 psi, and may preferably be about 750 psi. Other process fluid pressure ranges are also within the scope of the present disclosure. The process fluid may be maintained at a temperature that will not flash with a pressure drop that exists outside of the auxiliary bearings  116   a - b.    
     In an exemplary embodiment, the process fluid may be water. A high pressure feed pump or an emergency high-pressure pump may provide the water to the auxiliary bearings  116   a - b  in liquid form. The water may also be made available to the auxiliary bearings  116   a - b  in gaseous form as high-pressure steam. According to an exemplary embodiment, the water may be feedwater from a Rankine cycle turbomachine. In yet another exemplary embodiment, the process fluid may be ethylene glycol. It should be understood that a gas, such as air or an inert gas, may also be used as a process gas instead of using a process fluid. 
     According to an exemplary embodiment, process fluid leakage may pass through the auxiliary bearings  116   a - b  via the clearances  118   a - b . The leakage may be restricted by the labyrinth seal assemblies  108   a - i  and may be directed via the leakage recycler pipe  114  to the leakage recycler  112 . The leakage recycler  112  may recycle the leakage. Recycling the leakage may include cooling the leakage, as well as performing other operations on the leakage so that it may be reused as process fluid. Upon recycling the leakage, a portion of the recycled leakage may be directed to the process fluid source  122  via the recirculation pipe  126 . Further, the recycled leakage may be directed to other components of the turbomachine  100  via a system recycle pipe. In an exemplary embodiment, the leakage recycler  112  may include a condenser configured to cool the leakage. However, other means for recycling leakage are also within the scope of the present disclosure. 
     In several exemplary embodiments, auxiliary bearings that are substantially similar to the bearing  106   a  or  106   b  may circumferentially surround other portions of the shaft  104 . 
     Referring now to  FIG. 2A , the auxiliary bearing  116   a  or  116   b  of  FIG. 1  may be a pocketed auxiliary bearing  200 . In an exemplary embodiment, the pocketed auxiliary bearing  200  includes one or more bearing segments  201   a - d  arranged around the shaft  104 . Each segment  201   a - d  includes an inner surface  202  that faces the shaft  104 . 
     A clearance  203  is formed between the inner surface  202  of the pocketed auxiliary bearing  200  and the shaft  104 . Various clearance  203  widths may be used. A decreased clearance  203  width may result in decreased fluid flow between the outer surface of the shaft  104  and the inner surface  202  of the pocketed auxiliary bearing  200 . In another exemplary embodiment, an increased clearance  203  width may result in increased fluid flow between the shaft  104  and the inner surface  202 . In one exemplary embodiment, the size of the clearance  203  may vary depending on the weight of the shaft  104 . The pocketed auxiliary bearing  200  includes one or more pockets  204   a - d  formed along the inner surface  202 . Each of the pockets  204   a - d  are formed across two adjacent bearing segments  201   a - d.    
     One or more inlets  206  are formed between circumferentially adjacent bearing segments  201   a - d . The inlet  206  may correspond to either inlet  120   a  or inlet  120   b  in  FIG. 1 . One or more orifices  208   a - c  are formed between two adjacent segments  201   a - d.    
     Each pocket  204   a - d  may be fed with a high-pressure process fluid via the inlet  206 . Each orifice  208   a - c  may regulate process fluid flow and prevent the effects of pressure changes in one pocket  204   a - d  from affecting another pocket  204   a - d , as described below. In another exemplary embodiment, other means may also be used to regulate the flow of process fluid and prevent the effects of pressure changes within a pocket  204   a - d  from affecting another pocket  204   a - d.    
     In an exemplary embodiment, when the pocketed auxiliary bearing  200  receives process fluid, the resulting fluid film may cause the shaft  104  to adjust its position within a predetermined range of operational shaft positions. During operation of the pocketed auxiliary bearing  200 , radial and/or axial forces may cause the shaft  104  to move out of a predetermined range of operational shaft positions. For example, if the shaft  104  moves toward the pocket  204   c,  such movement may cause a reduction in process fluid leakage out of the pocket  204   c  via the orifice  208   b.  Such reduction in leakage may cause the pressure within the pocket  204   c  to rise. As a result, the clearance  203  between the pocket  204   a  and shaft  104  may increase. A balancing reaction may cause leakage out of the pocket  204   a  via the orifice  208   a  to increase. This increase in leakage out of the pocket  204   a  may counteract the increased pressure in pocket the  204   c.  Similar balancing reactions occurring at each of the pockets  204   a - d  may maintain the shaft  104  within the predetermined range of operational shaft positions. 
       FIG. 2B  shows a partial cross-sectional view of the inner surface  202  of the pocketed auxiliary bearing  200 . If the pocketed auxiliary bearing  200  were to be unrolled and laid flat, the inner surface  202  of the pocketed auxiliary bearing  200  may appear similar to the view shown in  FIG. 2B . 
     Referring now to  FIG. 2C , illustrated is another exemplary embodiment  250  of the previously described pocketed auxiliary bearing  200 . In the illustrated embodiment, an inner surface  253  of the pocketed auxiliary bearing  250  may form a uniform clearance  254  with respect to the shaft  104 . A series of tilting pads  252  may be positioned within the clearance  254 , and the series of tilting pads  252  may form one or more pockets  256   a - d . The series of tilting pads  252  may be evenly spaced, and may circumferentially surround the shaft  104 . Each tilting pad  252  may include either an orifice  258  and/or an inlet  260  that is fluidically communicable with each of the pockets  256   a - d . In an exemplary embodiment, the pocketed auxiliary bearing  250  may be commercially available through the Waukesha Bearing Corporation. 
     The operation of the pocketed auxiliary bearing  250  may be similar to the operation of the pocketed auxiliary bearing  200 , except that the tilting pads  252  may adjust its position within the clearance  254  to help maintain the shaft  104  within a predetermined range of operational shaft positions. 
     Referring now to  FIG. 3A , with continued reference to  FIG. 1 , a turbomachine  300  is shown. In the turbomachine  300 , pairs of bushings  302  serve as auxiliary bearings and may be referred to as “axially-fed auxiliary bearings.” Each of the bushings  302  may have a smooth or patterned inner surface  304 . 
     In an exemplary embodiment, each of the bushings  302  may define a pattern of small apertures on its inner surface  304 . The small holes may either be circular, honeycomb-shaped, or any other shape. Such bushings  302  may be referred to as “damper seals.” 
     In an exemplary embodiment, the bushings  302  circumferentially surround the shaft  104 . The bushings  302  may be installed in a back-to-back arrangement between, for example, the labyrinth seal assemblies  108   c  and  108   d,  and between the labyrinth seal assemblies  108   h  and  108   i,  as illustrated in  FIG. 3A . Inlets  310   a  and  310   b  are formed between the respective bushings  302  along the shaft  104 . The inlets  310   a - b  are fluidically coupled to the process fluid source  122  containing process fluid. Respective clearances  316  may exist between the respective inner surfaces  304  of the bushings  302 , and the outer surface of the shaft  104 . 
     With continuing reference to  FIG. 3A , according to an exemplary embodiment of operation, if the primary bearing  106   a  or  106   b  fails, the control system  107  instructs the valve controls  129   a - b  to open the valves  128   a - b . When the valves  128   a - b  are open, the process fluid source  122  supplies a process fluid to the inlets  310   a - b.    
     When the process liquid has entered the inlets  310   a - b , the process fluid axially flows across the bushings  302  towards regions of lower pressure. Leakage may be restricted by the labyrinth seal assemblies  108   a - i , and may be directed to the leakage recycler  112  via the leakage recycler pipe  114 . When leakage reaches the leakage recycler  112 , the leakage recycler  112  recycles the leakage and directs the recycled leakage to the process fluid source  122  via the recirculation pipe  126 . In an exemplary embodiment, the leakage recycler  112  may include a condenser configured to cool the leakage. Furthermore, a system recycle pipe (not shown) may fluidically couple the leakage recycler  112  to other components of the turbomachine  300  that are not shown in  FIG. 3A . 
     Embodiments of an axially-fed auxiliary bearing may be less efficient than embodiments of the pocketed bearing  200 . However, the use of design tools combined with application experience may increase the efficiency of an embodiment of the axially-fed auxiliary bearing  302 . 
     According to an exemplary embodiment, shaft resonance may determine whether a auxiliary bearing has the radial stiffness and damping required for use in the turbomachine  100  or  300 . During the operation shaft on the auxiliary bearings the natural frequencies of the rotor-bearing system, and therefore the critical speeds, may change because both the location and stiffnesses of the rotor supports change. Likewise, the dynamic amplification factors for vibration at a critical speed or resonant frequency also may change due to the changes in support location, stiffness and damping. Due to these factors, it may be desirable or necessary to avoid operating at or near a critical speed while the rotor is supported on the auxiliary bearings. Therefore, during the operation of the turbomachine  100  or  300 , the shaft  104  may be maintained at an operational speed that does not either surpass or encompass a critical speed where shaft resonance will exist. 
       FIG. 3B  shows a perspective view of a damper seal according to one or more aspects of the present disclosure. More specifically,  FIG. 3B  shows the stator part of a hole-pattern damper seal  305  according to an exemplary embodiment. The damper seal  305  may be made of aluminum. In other exemplary embodiments, other materials may be used depending on operating temperature and the nature of the process fluid used in the turbomachine  100  or  300 . For example, the damper seal  305  may be made of graphitic cast iron and various polymers. Further, the damper seal  305  may be made of Hastalloy or stainless steel. A damper seal  305  may develop substantial radial stiffness when a large pressure drop occurs axially across the damper seal. As shown in  FIG. 3B , the damper seal  305  includes a portion  350  and a portion  360  coupled thereto.  FIG. 3C  shows a perspective view of the portion  350  of the damper seal  305  according to an exemplary embodiment. 
     Referring now to  FIG. 4A , illustrated is an exemplary embodiment of a hydrostatic auxiliary thrust bearing  400 , which includes a plurality of segments  402 . According to an exemplary embodiment, the segments  402  may be evenly spaced, and the segments  402  may be adapted to circumferentially surround the shaft  104 . The segments  402  may be in the shape of a trapezoid. In other exemplary embodiments, the segments  402  may include any other shapes. Each segment  402  includes an inner surface  404 , one or more pockets  406 , one or more orifices  408 , and one or more inlets  410 . In an exemplary embodiment, the segments  402  may be positioned around the shaft  104  so that a top portion of each of the segments  402  forms an equidistant clearance  412  from the outer surface of the shaft  104 . In operation, an auxiliary thrust bearing may be placed along other portions of the shaft  104 , so long as its position allows the auxiliary thrust bearing  400  to counteract axial forces that may exist when the primary bearing  106  fails. The operation of an auxiliary thrust bearing  400  may be similar to the operation of the pocketed auxiliary bearing  200  described above, except that the auxiliary thrust bearing  400  will work to maintain the position of the shaft  104  within predetermined operating ranges with respect to axial forces, rather than radial forces. In an exemplary embodiment, each of the inner surfaces  404  of the hydrostatic auxiliary thrust bearing  400  may include any material that forms a durable surface, such as NASA PS300. 
     In an exemplary embodiment, the auxiliary bearing  116   a  and/or  116   b  of the turbomachine  100  includes a combined auxiliary radial and thrust bearing.  FIG. 4B  illustrates a schematic view of a combined auxiliary radial and thrust bearing  440  according to one or more aspects of the present disclosure. As shown in  FIG. 4B , the combined auxiliary radial and thrust bearing  440  includes the surface  404 , the side pocket  406 , the orifice  408 , the inlet  410 , a radial pocket  450 , and a drain groove  452 . The combined auxiliary radial and thrust bearing  440  is positioned within a thrust collar  460 . The combined auxiliary radial and thrust bearing  440  supports both radial and thrust loads.  FIG. 4C  illustrates a partial cross-sectional view of the combined auxiliary radial and thrust bearing  440 ; if the combined auxiliary radial and thrust bearing  440  were to be unrolled and laid flat, the bearing  440  may appear similar to the view shown in  FIG. 4C . As shown in  FIG. 4C , the bearing  440  includes an inner surface  454 , an inlet  456  and an orifice  458 . 
     Referring now to  FIG. 5 , illustrated is a flowchart that describes a method for supporting a shaft of a turbomachine according to an exemplary embodiment of the present disclosure. The method includes maintaining a position of a shaft  104  within a predetermined range of operational shaft positions using a primary bearing configured to support the shaft, as shown in step  502 . The primary bearing may be a magnetic bearing. 
     If the primary bearing fails to maintain the shaft within the predetermined range of operational shaft positions, such as when the shaft drops as a result of a failed primary bearing (step  504 ), a control system may immediately engage a hydrostatic auxiliary bearing before the shaft drops onto the inner surface of the hydrostatic auxiliary bearing, so as to maintain the position of the shaft within the predetermined range of operational shaft positions, as shown in step  506 . In one exemplary embodiment, the auxiliary bearing may be configured to idle at a predetermined pressure during the operation of the primary bearing, so that the auxiliary bearing may become fully operational more quickly when the primary bearing fails. 
     The hydrostatic auxiliary bearing may be configured to support the shaft using a pressurized process fluid, such as water or ethylene glycol, that may be provided by a process fluid source. It should be understood that a gas, such as air or an inert gas, may also be used as a process gas instead of using a process fluid. Means for maintaining the shaft within a predetermined range of operational shaft positions when the primary bearing fails to maintain the shaft within the predetermined range of operational shaft positions, may include the auxiliary bearings  116   a  and  116   b,  the pocketed auxiliary bearings  200  and  250 , axially-fed auxiliary bearings, such as the bushings  302 , and any equivalents of the foregoing. 
     Any leakage resulting from the pressurized process fluid used to support the shaft may be recycled, as shown at step  508 . In an exemplary embodiment, the leakage recycler may include a condenser configured to cool the leakage before directing the leakage to the process fluid source. 
     Generally, hydrostatic bearings have not been utilized as auxiliary bearings in conventional technology, because the lubrication systems (e.g., oil tank, coolers pumps, etc.) that are necessary to provide lubrication to a hydrostatic auxiliary bearing are usually not included in turbomachinery that uses magnetic bearings. Instead of utilizing traditional lubrication systems, the exemplary embodiments set forth in the present disclosure utilize process fluid that is readily available. Further, in the exemplary embodiments disclosed herein, the process fluid may be recycled for use in other areas of a turbomachine. Taking advantage of a readily available process fluid and recycling the process fluid for other uses may result in substantial cost savings. 
     Potential advantages of the auxiliary bearing embodiments described above over conventional technology may include simplified integration into established bearing technology. The current availability of design tools is one factor that makes this possible. Another potential advantage of the exemplary embodiments described herein may be longer system operating life. The exemplary embodiments of the present disclosure may also exhibit increased load capacity potential. 
     Turbomachinery implementing the exemplary embodiments disclosed herein may be more compact than conventional turbomachinery using hydrostatic auxiliary bearings, because a variety of process fluids may be used as bearing lubricant. Exemplary embodiments of the present disclosure may be physically smaller and less complex than conventional technology, because traditional lubricant pumping technology is not necessary when process fluid is used as the bearing lubricant. Further, the exemplary embodiments disclosed in the present disclosure allow for process fluid to be recycled after the process fluid is used as bearing lubricant. Recycling process fluid may improve efficiency and lower turbomachinery operating costs. 
     Although the present disclosure has described embodiments relating to specific turbomachinery, it is understood that the apparatus, systems and methods described herein could applied to other environments. For example, according to another exemplary embodiment, rotating machinery that is driven by a turbomachine may be configured to use embodiments of the auxiliary bearings described above. However, in such applications, the machinery may have to be modified to ensure that the process fluid leaks do not adversely affect the machine. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.