Abstract:
A centrifugal compressor includes a rotor assembly with a shaft and a plurality of impellers, bearings located at ends of the shaft and configured to support the rotor assembly, a sealing mechanism disposed between the rotor assembly and the bearings, and a gas bearing disposed between the plurality of impellers for supporting the shaft and receiving a working gas from an impeller downstream from a location of the gas bearing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a national stage application under 35 U.S.C. §371(c) prior-filed, co-pending PCT patent application serial number PCT/EP2010/069347, filed on Dec. 10, 2010, which claims priority to Italian Patent Application No. CO2009A000067, filed on Dec. 17, 2009, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention relate generally to compressors and, more specifically, to a mid-span gas bearing in a multistage compressor. 
         [0004]    2. Description of the Prior Art 
         [0005]    A compressor is a machine which increases the pressure of a compressible fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications and in a large number of industrial processes, including power generation, natural gas liquification and other processes. Among the various types of compressors used in such processes and process plants are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration, for example, by rotating a centrifugal impeller. 
         [0006]    Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of centrifugal stages in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet volute for gas to be compressed, a rotor which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy. 
         [0007]    A multistage compressor  100  is illustrated in  FIG. 1 . Compressor  100  includes a shaft  120  and a plurality of impellers  130 - 136  (only three of the seven impellers are labeled). The shaft  120  and impellers  130 - 136  are included in a rotor assembly that is supported through bearings  150  and  155 . 
         [0008]    Each of the impellers  130 - 136 , which are arranged in sequence, increase the pressure of the process gas. That is, impeller  130  may increase the pressure from that of gas in inlet duct  160 , impeller  131  may increase the pressure of the gas from impeller  130 , impeller  132  may increase the pressure of the gas from impeller  131 , etc. Each of these impellers  130 - 136  may be considered to be one stage of the multistage compressor  100 . 
         [0009]    The multistage centrifugal compressor  100  operates to take an input process gas from inlet duct  160  at an input pressure (P in ), to increase the process gas pressure through operation of the rotor assembly, and to subsequently expel the process gas through outlet duct  170  at an output pressure (P out1 )which is higher than its input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof 
         [0010]    The pressurized working fluid within the machine (between impellers  130  and  136 ) is sealed from the bearings  150  and  155  using seals  180  and  185 . A dry gas seal may be one example of a seal that can be used. Seals  180  and  185  prevent the process gas from flowing through the assembly to bearings  150  and  155  and leaking out into the atmosphere. A casing  110  of the compressor is configured so as to cover both the bearings and the seals, and to prevent the escape of gas from the compressor  100 . 
         [0011]    While additional stages can provide an increase in the ratio of output pressure to input pressure (i.e. between inlet  160  and outlet  170 ), the number of stages cannot simply be increased to obtain a higher ratio. 
         [0012]    An increase in the number of stages in a centrifugal compressor leads to multiple problems. The bearings which support the shaft are outside a sealed area that includes the impellers. An increase in the number of stages necessitates a longer shaft. A longer shaft cannot be safely supported by the bearings for the same operating speed, which become further apart as the shaft length increases making the shaft more flexible. 
         [0013]    As the rotor assembly gets longer, the shaft becomes flexible therefore decreasing the rotor natural frequencies. When operating at higher speeds, the decrease in the fundamental natural frequencies of the rotor assembly tends to make the system more susceptible to rotor-dynamic instability, which can limit the operating speed and output of the machine. 
         [0014]    The other issue is the forced response due to synchronous rotor imbalance. When the operating speed coincides with a rotor natural frequency, the machine is defined to be operating at a critical speed, which is a result of rotor imbalance. The compressor must pass through several of these natural frequencies or critical speeds before reaching the design operating speed. 
         [0015]    As the compressor passes through critical speeds, the vibration amplitude of the rotor must be bounded by damping from bearings. However, with a long shaft, the majority of the rotor-dynamic energy is transferred to bend the rotor instead of energy dissipation at the bearings. This results in low damped rotor modes and high amplification factors at rotor resonances that can lead to casing and impeller rubs and even catastrophic failure of the machine. 
         [0016]    At higher speeds past the rotor critical speeds, fluid induced forces are generated between the rotor assembly and the casing (i.e. fluid induced rotor dynamic instability). These pulsations, stemming from fluid forces can excite destructive or even catastrophic vibrations if not adequately dampened. Rotor-dynamic instability is a different mechanism from critical speeds or imbalance response and often time is much more difficult to address. 
         [0017]    It would be desirable to design and provide a multistage centrifugal compressor which includes additional stages without increasing the diameter of the shaft and other design parameters that would drastically change the size and cost of the machine. 
       BRIEF SUMMARY OF THE INVENTION 
       [0018]    Systems and methods according to these exemplary embodiments provide for an increase in the number of stages in a centrifugal compressor while overcoming problems typically associated with such an increase. 
         [0019]    According to an exemplary embodiment, a centrifugal compressor includes a rotor assembly having a shaft and a plurality of impellers, a pair of bearings located at ends of the shaft and configured to support the rotor assembly, a sealing mechanism disposed between the rotor assembly and the bearings, and a first gas bearing disposed between the plurality of impellers and configured to support the shaft. The first gas bearing receives a working gas from an impeller located downstream from the location of the first gas bearing. 
         [0020]    According to another exemplary embodiment, a method of processing a working gas in a centrifugal compressor includes providing the working gas to an inlet duct of the compressor, processing the gas through a plurality of compression stages with each stage increasing the speed of the gas, bleeding a portion of the accelerated gas after a stage that is downstream from a midway point of the compression stages, providing the bled gas to a bearing, reintroducing the gas from the bearing to the working gas flowing in the compressor, and expelling the working gas from an outlet duct of the compressor. 
         [0021]    According to a further embodiment, a centrifugal compressor includes a rotor assembly having a shaft and a plurality of impellers, a pair of bearings located at ends of the shaft and configured to support the rotor assembly, a sealing mechanism disposed between the rotor assembly and the bearings, and a plurality of gas bearings disposed between the plurality of impellers and configured to support the shaft. The gas bearings receive a working gas from respective impellers located downstream from a location of the gas bearings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The accompanying drawings illustrate exemplary embodiments, wherein: 
           [0023]      FIG. 1  illustrates a multistage centrifugal compressor; 
           [0024]      FIG. 2  illustrates a multistage centrifugal compressor according to exemplary embodiments; and 
           [0025]      FIG. 3  illustrates a method in accordance with exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
         [0027]    In exemplary embodiments, a mid-span bearing may be utilized to provide additional stiffness to the rotor assembly with a longer shaft to overcome the critical speed issue highlighted above. Such a bearing makes the rotor assembly less flexible and therefore allows the rotor-dynamic energy (due to synchronous rotor imbalance forces) to be transmitted to the bearings. 
         [0028]    This three-bearing configuration increases the damping in the rotor modes and lowers amplification factors as the rotor traverses through the critical speed allowing for safe operation of the rotor assembly. A mid-span bearing may, therefore, be provided within the casing for facilitating an increased number of stages (i.e. longer shaft) and overcoming the rotor dynamic instability. 
         [0029]    Surface speed of a shaft (such as shaft  120 ) is a function of its diameter. The diameter in the middle portion of the shaft is greater than the diameter at the end portions. The difference in speeds between these portions (i.e. between middle and end) may be in the order of 2 to 3 times. Therefore, the surface speed of a shaft is greater (by a factor of 2 to 3) at the center portion of the shaft than it is at the end portions. 
         [0030]    Bearings, such as bearing  150  and  155  of  FIG. 1 , may typically be oil bearings. Oil bearings, however, are limited to usage where surface speed is typically closer to the surface speed at end portions of the shaft. 
         [0031]    A mid-span bearing according to exemplary embodiments may be a gas bearing. Gas bearings can be used where surface speed is closer to the surface speeds at middle portions of a shaft. 
         [0032]    In existing systems, highly corrosive working fluids such as hydrogen disulfide can damage conventional oil lubricated journal bearings. Such damage, greatly limits the life of the machine as oil lubricated bearings are not resistant to corrosive gases. A process gas lubricated bearing, however, does not require such sealing and can operate even in this corrosive environment while maintaining the life of the machine. 
         [0033]    In addition to having ultra high surface speed viscous fluid capability, there is negligible power loss with gas bearings relative to oil bearings. Oil bearings also require sealing systems for preventing leakage of oil into the gas being processed by the compressor. Gas bearings obviate this need for sealing systems. 
         [0034]      FIG. 2  illustrates a compressor according to exemplary embodiments. Compressor  200  includes a shaft  220 , a plurality of impellers  230 - 239  (only some of these impellers are labeled), bearings  250  and  255 , seals  280  and  285 , inlet duct  260  for taking an input process gas at an input pressure (P in ) and outlet duct  270  for expelling the process gas at an output pressure (P out2 ). A casing  210  of the compressor  200  covers both the bearings and the seals and prevents the escape of gas from the compressor  200 . 
         [0035]    Compressor  200  also includes bearing  290 . Bearing  290  may be located near the middle between the first and last impellers  230  and  239  in exemplary embodiments. The number of impellers  230 - 239  may be increased with the mid-span bearing according to exemplary embodiments than is currently possible for the additional reasons described herein further. 
         [0036]    Currently, a limiting factor in the number of stages that can be included in a compressor is the ratio between the length and the diameter of a shaft. This ratio is referred to as the flexibility ratio. In order to operate effectively, a compressor may have a maximum flexibility ratio. This ratio can be increased with a longer shaft and a mid-span gas bearing according to exemplary embodiments. 
         [0037]    The gas used in gas bearing  290  may be the gas being processed by compressor  200 . The placement of gas bearing  290  may be at a location where the rotor displacement for a nearest natural frequency may be most pronounced. This location may be of optimal effectiveness from a rotor dynamic point of view. 
         [0038]    The gas being processed may be “bled” from an output of an impeller that is “downstream” from gas bearing  290  using known elements/components and methods. The term downstream is used in this case as it relates to the direction of the gas flow and higher pressure in the case of compressors. That is, pressure is higher downstream and lower upstream relative to a particular location. For example, as illustrated in  FIG. 2 , gas bearing  290  is “upstream” relative to impeller  235  but is “downstream” relative to impeller  234 . 
         [0039]    The pressure of the working gas coming into bearing  290  has to be at a higher pressure than the pressure of the working gas in “bounding” or “adjacent” stages to the gas bearing so that the gas flow is out of the bearing pad and not into the bearing pads. 
         [0040]    The working gas, therefore, has to be bled from a stage that is beyond the location of gas bearing  290 . If bearing  290  is placed after five stages (i.e. impeller  234 ) for example, then the working gas has to be bled from a stage after the sixth stage (i.e. impeller  235 ). In one embodiment, the working gas may be bled from at least two stages downstream from the location of the mid-span gas bearing (i.e. after impeller  236 ). The high pressure is needed by bearing  290  to work in a stable manner. 
         [0041]    The working gas that is bled from a downstream compression stage may be processed through filter  240  and provided to gas bearing  290  in some embodiments. Filter  240  may remove any impurities and particulates in the gas being processed. The rotor assembly may be flushed with gas via gas bearing  290  to remove heat from the assembly. The percent of working gas mass flow going to the bearing  290  may be less than 0.1% of the core flow. 
         [0042]    Small bore channels may be provided between bearing  290  and the working flow path. The gas from bearing  290  may be lead into the flow path by the bore channel to the proper pressure. 
         [0043]    An increase in the length of the shaft leads to an increase in a ratio of the length to the diameter of the compressor bundle/casing. This facilitates the addition of compression stages within the same casing. 
         [0044]    Thus, according to an exemplary embodiment, a method for processing a gas  300  through a multistage compressor having a mid-span gas bearing includes the method steps in the flowchart of  FIG. 3 . At  310 , a working gas may be supplied to an inlet duct of a compressor. The working gas may be processed by a plurality of compression stages to increase the pressure (and speed) at  320 . A portion of the working gas may be bled from its flow through the compression stages after it has been processed by a number of compression stages at  330 . This number of stages may be greater than one half of the compression stages in the compressor. 
         [0045]    The gas may be supplied to a gas bearing at  340  to flush and remove heat from the rotor assembly, the gas bearing being located upstream of the filter. The gas supplied to the gas bearing may be reintroduced into the flow of the working gas at  350 . Gas from the final stage of compression may be expelled via the outlet duct at  360 . In some embodiments, the gas that has been bled may be processed by a filter to remove any impurities before being provided to the gas bearing. 
         [0046]    The number of mid-span gas bearings may be greater than one. Additional (or, multiple) mid-span gas bearings may be included in some embodiments utilizing the principles described above. Also, a mid-span bearing may not be exactly in the center—it may be offset depending on the particular design and specifications such as having an odd number of stages. Each of the multiple gas bearings may receive working gas from a separate impeller downstream. 
         [0047]    If multiple gas bearings are implemented within a compressor, the number of (compression) stages between the input and the first of the gas bearings may be the same as the number stages between the last of the gas bearings and the output. The multiple gas bearings may also be spaced apart by the same number of stages. Therefore, the number of stages between the input and the first gas bearing may be the same as the number stages between the first and the second gas bearings (and between each of the subsequent gas bearings) which may also be the same as the number of stages between the last gas bearing and the output, etc. 
         [0048]    A first of the gas bearings may receive compressed gas from a stage that is both downstream from the first gas bearing and upstream from a second of the gas bearings. That is, the first gas bearing may receive compressed gas from a stage that is between the first and the second gas bearings. 
         [0049]    Those skilled in the art will appreciate that the specific number of impellers described above and illustrated in  FIG. 2  are purely exemplary and that other number of impellers may be used. There may be a greater or a lesser number impellers depending on the application. The shaft may be a single shaft. 
         [0050]    Exemplary embodiments as described herein provide multiple advantages over compressors that are in use at present. Additional impellers (and longer rotor assembly) may be placed within one casing as opposed to having a series of casings for increasing pressure. Efficiency within each casing (having longer rotor assembly for example) is increased as well. Space requirements for compressors to achieve a particular ratio of output pressure to input pressure are reduced. The flexibility ratio is increased to facilitate additional impellers. 
         [0051]    Length (L 2 ) of shaft  220  in compressor  200  ( FIG. 2 ) according to exemplary embodiments is greater than the length (L 1 ) of shaft  120  in compressor  100  ( FIG. 1 ). 
         [0052]    In addition, the use of gas bearings also obviates the need for elaborate sealing systems within the casing as oil does not enter the casing. The cost is also dramatically reduced as a result of the design as described. 
         [0053]    The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.