Abstract:
A sealing system for a centrifugal compressor includes a stator having a seal, a seal disposed in the seal housing and having an abradable portion along an inner circumference, a rotor having a plurality of rotor teeth configured to rotate within the inner circumference of the seal and configured to create rub grooves within the abradable portion, and a first spring disposed between the stator and the seal and configured to facilitate axial movement of the seal relative to the seal housing.

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/US2010/059258, filed on Dec. 7, 2010, which claims priority to Italian Patent Application No. CO2009A000071, filed on Dec.  22, 2009, the entire contents of which are incorporated herein by reference.    
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Exemplary embodiments relate generally to compressor seals and, more specifically, to the provision of abradable seals with axial offset for reducing leakage. 
         [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 impellers 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 conduit for gas to be compressed, an impeller 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  is included in casing  110  within which is mounted a shaft  120  and a plurality of impellers  130 . The shaft  120  and impellers  130  are included in a rotor assembly that is supported through bearings  190  and  195 . 
         [0008]    The multistage compressor operates to take an input process gas from an inlet duct  160 , to increase the process gas pressure through operation of the rotor assembly, and to subsequently expel the process gas through an outlet duct  170  at an output pressure 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. 
         [0009]    Seals or sealing systems  180 ,  185  and  188  are provided between impellers  130  and bearing  190  and  195  to prevent the process gas from flowing through to the bearings. Seals  188  are impeller eye seals. 
         [0010]    Each of the impellers  130  increases the pressure of the process gas. Each of the impellers  130  may be considered to be one stage of the multistage compressor  100 . Additional stages, therefore, result in an increase in the ratio of output pressure to input pressure. 
         [0011]    The rotor assembly includes stationary portions known as stators and rotating portions known as rotors. The overall operating efficiency of a compressor is adversely affected by leakage flow of the working fluid or gas between the stator and rotor due to differential axial pressure of the rotor. 
         [0012]    In centrifugal compressors, abradable seals can be used to reduce the leakage flow (of the working gas) to improve stage efficiency via clearance reduction without the risk of harming the rotor portion. 
         [0013]    A sealing system using an abradable seal is illustrated in  FIG. 2 . Sealing system  200  includes a rotor  210  (i.e., a rotating portion) and a stator  220  (i.e., a stationary portion). Rotor  210  rotates around a longitudinal axis of the compressor. Rotor  210  includes a plurality of rotor teeth  215 . Rotor teeth  215  can be radial. Stator  220  includes a cavity or housing  230  for housing a stator seal  223 . 
         [0014]    The stator seal includes an abradable portion or coating  225 . Stator seal  223  can be an insert ring with an abradable coating on the inner circumference in some arrangements. In other arrangements, the (entire) insert ring can also be made of an abradable material. 
         [0015]    The stator seal circumferentially encloses the rotor; the rotor can rotate within along the inner circumference of the stator seal. A stator seal is located on either side of each stage of a multistage compressor. 
         [0016]    Rotation of the rotor  210  causes the rotor-mounted radial teeth  215  to create rub grooves  227  along the inner circumference of the stator-mounted abradable seal  223  and can cause debit (i.e., an increase in the leakage flow) to sealing performance. 
         [0017]    In order to reduce leakage flow, a stator seal within housing  230  as illustrated in  FIG. 2  can be offset radially from a top portion  235  of the housing  230  by a spring activated mechanism  240 . The insert ring is typically partitioned into two or four equal sized sections. The spring activated mechanism permits the stator seal to radially move toward or away from the rotor  210 . The two or four equal-sized section of the insert ring facilitate the radial movement. 
         [0018]    Stator seal  223  with the spring activated mechanism may also be referred to as a compliant seal, a spring-backed seal or a spring-energized abradable seal. A compliant seal, when compared to a non-compliant seal (i.e., a seal not having a spring activated mechanism and hence no radial offset) provides a smaller radial clearance and a reduction in the leakage flow. 
         [0019]    It would be desirable to design and provide an improved sealing mechanism for reducing the leakage flow debit further. 
       BRIEF SUMMARY OF THE INVENTION 
       [0020]    Systems and methods according to these exemplary embodiments provide improved sealing characteristics to reduce leakage flow between a stator seal and a rotor of a rotor assembly by introducing a spring mechanism for facilitating axial positioning control over the seal. 
         [0021]    According to an exemplary embodiment, a sealing system for a centrifugal compressor is disclosed. The sealing system includes a stator having a seal housing, a seal disposed in the seal housing and having an abradable portion along an inner circumference, a rotor having a plurality of rotor teeth configured to rotate within the inner circumference of the seal and configured to create rub grooves within the abradable portion, and a first spring disposed between the stator and the seal and configured to facilitate axial movement of the seal relative to the seal housing. 
         [0022]    According to another embodiment, a method for reducing leakage flow between a seal housed within a stator seal housing and a rotor of a centrifugal compressor wherein the rotor rotates within an inner circumference of the seal is disclosed. The method includes biasing the seal toward a high pressure side of the seal housing by a spring load to create an axial gap between the seal and the seal housing, starting the compressor, cutting radial rub grooves in an abradable portion of the seal, increasing a speed of the compressor and moving the seal toward a low pressure side of the seal housing. 
         [0023]    According to a further embodiment, a centrifugal compressor includes a stator having a seal housing, a seal disposed in the seal housing and having an abradable portion along an inner circumference, a rotor having a plurality of rotor teeth configured to rotate within the inner circumference of the seal and configured to create rub grooves within the abradable portion, a first spring disposed between the stator and the seal and configured to facilitate axial movement of the seal, and a secondary seal located between a lower pressure side of the housing and the seal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The accompanying drawings illustrate exemplary embodiments, wherein: 
           [0025]      FIG. 1  illustrates a multistage compressor; 
           [0026]      FIG. 2  illustrates a side view of a rotor/stator sealing assembly utilizing an abradable seal; 
           [0027]      FIG. 3  illustrates a side view of a rotor/stator sealing assembly utilizing an abradable seal according to exemplary embodiments in an initial state; 
           [0028]      FIG. 4  illustrates a side view of a rotor assembly utilizing an abradable seal according to exemplary embodiments in an operating state; and 
           [0029]      FIG. 5  illustrates a method in accordance with exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    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. 
         [0031]    In exemplary embodiments, leakage flow between rotor teeth and stator rub grooves in a rotor assembly may be reduced by employing a pressure activated mechanism to achieve active axial positioning control of the rotor teeth relative to the rub grooves. 
         [0032]    A computational fluid dynamics (CFD) analysis of rub groove impact indicates that teeth/groove axial positioning plays an important role in leakage performance. An axial offsetting of rotor teeth after initial formation of the grooves may reduce the performance debit. 
         [0033]    Exemplary embodiments may utilize a pressure-activating mechanism to achieve active axial positioning control of the rotor teeth relative to the rub groove as illustrated in sealing system  300   FIG. 3 . Sealing system  300  may include rotor  310  and stator  320 . Rotor  310  may include a plurality of rotor teeth  315  which may be radial. Stator  320  may include a stator seal  323  located within a seal housing  330 . 
         [0034]    As described above, the stator seal may include an abradable portion or coating  325 . Stator seal  323  may include an insert ring (two or more portions) with an abradable coating on the inner circumference in some arrangements. The insert ring may be made of steel. The abradable coating on the inner circumference may be a porous metallic material such as porous Aluminum, Cobalt or Nickel-based thermally sprayed coatings; it may alternatively be plastic-based (e.g., Teflon, Polyester). 
         [0035]    In other arrangements, the (entire) insert ring may also be made of an abradable material. The abradable material may be Fluorosint-500, a mica-reinforced Teflon for example. 
         [0036]    Stator seal  323  may be placed within housing  330  and offset radially from a top portion  335  of the housing by a spring activated mechanism  340 . Rotor teeth  315  may create rub grooves  327  in the abradable stator portion  325 . In exemplary embodiments, an axial gap  360  may be created or designed between stator seal  320  and seal housing  330  by the use of an axially oriented spring  350 . 
         [0037]    While being assembled, stator seal  323  may be pushed (or set) to the high(er) pressure side (P+) of housing  330  by the axially oriented spring  350 . Spring  350  may be placed between seal  323  and the low(er) pressure side (P−) of housing  330 . Spring  350  may be in its (original) or slightly compressed state at this time. The axially oriented spring  350  may bias the stator seal  323  toward the higher pressure side. 
         [0038]    During the usage, at start-up (i.e., when the pressure ratio is low), rotor teeth  315  may cut into abradable portion  325  during critical vibration and create rub grooves  327 . At start up speeds, the difference in pressure (between P+ and P−) is relatively low making it possible to maintain the position of the seal at the higher pressure side. Rotor teeth  315  may ride in the middle of rub grooves  327  at these speeds and stator seal position. 
         [0039]    At design speed, the pressure differential (between P+ and P−) increases. As illustrated in  FIG. 4 , the increased pressure ratio across seal  323  from the high pressure (or P+) side may force seal  323  to move axially towards the low pressure (or P−) side. Spring  350  may start to contract (i.e., not in its original state) as the pressure increases. 
         [0040]    This axial movement (from the higher pressure side P+ to the lower pressure side P−) forms a smaller or tighter gap between rotor teeth  315  and rub grooves  327 . Rotor teeth  315  are not riding in the middle of the rub grooves  327  as illustrated. 
         [0041]    The spacing between the (tips of the) rotor teeth and the part of the abradable portion facing the rotor teeth of  FIG. 4  is smaller in relation to the spacing between the rotor teeth and the middle of rub grooves of  FIG. 3 . The amount of leakage may be decreased as a result. The leakage can be controlled by spring stiffness and operating pressure ratio. 
         [0042]    A secondary seal may also be included between seal  323  and seal housing  330  in some embodiments to provide a further reduction in leakage as illustrated in  FIGS. 3 and 4 . Referring to  FIG. 4 , secondary seal  370  may block leakage between seal  323  and seal housing  330  at design speed. Secondary seal  370 , therefore, further reduces the leakage. 
         [0043]    While the axial offset as described in exemplary embodiments may be additional to the radial offset, the axial offset may be implemented by itself (i.e., without the radial offset) in some embodiments. The axial offset reduces the radial gap and reduces leakage debit in either (i.e., with or without radial offset) arrangements. 
         [0044]    A method  500  in accordance with exemplary embodiments may be described with reference to  FIG. 5 . During assembly, an axial gap may be created between a seal and seal housing of a centrifugal compressor at  510  by moving the seal toward a high pressure side of the housing utilizing an axially oriented spring (in its original or slightly compressed state). The compressor may be started at  520 . Rotor teeth may cut rub grooves in the stator seal abradable portion at  530 . 
         [0045]    The compressor speed (and hence, pressure) may be increased at  540 . As the compressor operates toward the design speed, the seal spring may become compressed with increased pressure and the seal moves toward the low pressure side at  550 . The gap between rotor teeth and the abradable portion may decrease and secondary seal may provide additional leakage protection as described above. 
         [0046]    CFD analysis demonstrates that axial rub-groove positioning of the abradable seal, achieved by pressure-activating mechanism in exemplary embodiments, increases sealing performance. 
         [0047]    Exemplary embodiments are directed to axial positioning control of the abradable seal design. While previous efforts have focused on minimizing radial clearance between rotor teeth and abradable seal, embodiments as described herein introduce a pressure-activated axial positioning mechanism, which leads to minimizing the sealing gap independent of compressor critical vibration or thermal transients. 
         [0048]    Exemplary embodiments as described herein provide multiple advantages. A sealing system according to exemplary embodiments reduces leakage caused by rub grooves to a non-compliant abradable seal. The system also optimizes performance of the compliant (spring-energized) abradable seal for an impeller eye of a centrifugal compressor. 
         [0049]    Centrifugal compressor stage efficiency can be improved by providing an active axial-positioning mechanism as described in exemplary embodiments to achieve a minimum sealing gap. 
         [0050]    In some embodiments, a plurality of axial positioning mechanisms may be provided in the stator seal. The plurality of axial positioning mechanisms may be distributed (or spaced apart) equally along the seal for example. 
         [0051]    The axial-positioning mechanism according to exemplary embodiments is not limited to implementation in new compressors. Existing compressors with seal rings can be retrofitted with the axial-positioning mechanism. Furthermore, while the active-positioning mechanism of exemplary embodiments has been described with respect to centrifugal compressors, it may be equally applicable to turbo machines in general. 
         [0052]    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.