Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to compressors and more specifically to seals with swirl brakes. 
     2. Description of the Prior Art 
     A compressor is a machine which accelerates the particles of a compressible fluid, e.g., a gas, through the use of mechanical energy to increase the pressure of that compressible fluid. Compressors are used in a number of different applications, including operating as an initial stage of a gas turbine engine. Among the various types of compressors are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration which accelerates the gas particles, e.g., by rotating a centrifugal impeller through which the gas is passing. More generally, centrifugal compressors are part of a class of machinery known as “turbo machines” or “turbo rotating machines.” 
     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 accelerated, 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. 
     In centrifugal compressors there are rotating elements and static elements. Seals can be used between certain rotating and static elements to prevent undesirable leakage within the centrifugal compressor. For example, labyrinth seals or honeycomb seals can be used as internal seals at, for example, a balance piston and an impeller eye (or each impeller eye in a multi-stage centrifugal compressor). Generally, labyrinth seals use grooves and lands to provide a difficult flow path for a fluid, while honeycomb seals use hexagonal shaped cells to resist the flow of the fluid. Both types of seals allow for a small gap (or an equivalent feature) between a rotating surface and a static surface. Various seal designs have been implemented since the inception of turbo machines. These seals can affect leakage and gas swirl. One example of this for use in a compressor, as shown in  FIG. 1 , is a labyrinth seal  2  with swirl brakes  4  manufactured by Dresser-Rand and seen online at www.dresser-rand.com/literature/services/2035-SwirlBrake.pdf. This particular design of labyrinth seal  2  with swirl brakes  4  is purported to be used to reduce and reverse the swirl entering the labyrinth seal. 
     However, another area of interest associated with swirl brakes in centrifugal compressors is rotordynamic stability under various operating conditions. For example, when a part rotates in the seal area, an undesirable circumferential flow can be induced in a chamber of the seal. Entry swirl can also induce undesirable circumferential elements to the flow in the seal, therefore it is desirable to reduce both entry swirl and the effects of rotation in the seal to improve rotordynamic stability in centrifugal compressors. 
     Accordingly, other systems and methods for seals with swirl brakes for improving rotordynamic stability are desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a centrifugal compressor is disclosed. The centrifugal compressor comprises a first duct configured to receive a process gas, an impeller connected to the first duct and configured to compress the process gas, wherein the impeller has an outer surface, a first seal comprising, a first plurality of swirl brakes configured to have a tooth shape to reduce the inlet seal swirl of a flow of the process gas at an entrance to the first seal, wherein each swirl brake is disposed at a leading edge of the first seal and is configured to have a gap between the first plurality of swirl brakes and the outer surface of the impeller, wherein each swirl brake has a first surface with a first predetermined length, a second surface connected to the first surface and having a predetermined angle with the first surface, a third surface which extends from an end of the second surface to a beginning of the seal section having a second predetermined length, wherein the third surface has a first taper. 
     According to another embodiment of the present invention, a seal including a plurality of swirl brakes for reducing an inlet swirl is disclosed. The swirl brakes comprise the plurality of swirl brakes each configured to have a tooth shape which are disposed at a leading edge of a seal and are configured to have a gap between the plurality of swirl brakes and an outer surface of a rotating part, wherein each tooth has a first surface with a first predetermined length, a second surface connected to the first surface and having a predetermined angle with the first surface, a third surface which extends from an end of the second surface to a beginning of the seal section having a second predetermined length, wherein the third surface has a taper. 
     According to another embodiment of the present invention, a method of manufacturing a seal including a plurality of swirl brakes for use in reducing an inlet seal swirl in a centrifugal compressor is disclosed. The method comprises procuring the seal of interest, machining the plurality of swirl brakes to each have a tooth shape on the seal leading edge, machining a first surface of a swirl brake with a first predetermined length for each swirl brake, machining a second surface of the teeth connected to the first surface having a predetermined angle with the first surface for each tooth; and machining a third surface of the teeth which extends from an end of the second surface to a beginning of a seal section having a second predetermined length for each tooth, wherein the third surface has a first taper configured to reduce an inlet seal swirl. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings illustrate exemplary embodiments, wherein: 
         FIG. 1  depicts a labyrinth seal with a swirl brake; 
         FIG. 2  illustrates a multi-stage centrifugal compressor according to exemplary embodiments of the present invention; 
         FIG. 3  shows a final impeller stage and a balance piston within the multi-stage centrifugal compressor according to exemplary embodiments of the present invention; 
         FIG. 4  illustrates a labyrinth seal with a plurality of swirl brakes prior to final shaping according to exemplary embodiments of the present invention; 
         FIG. 5  shows a plurality of tooth shaped swirl brakes prior to final shaping according to exemplary embodiments of the present invention; 
         FIG. 6  illustrates a schematic of a gap between a statoric part and a rotoric part according to exemplary embodiments of the present invention; 
         FIG. 7  illustrates a seal and a swirl brake in a first orientation according to exemplary embodiments of the present invention; 
         FIG. 8  shows a seal and a swirl brake in a second orientation according to exemplary embodiments of the present invention; 
         FIG. 9  depicts a shape of a swirl brake according to exemplary embodiments of the present invention; and 
         FIG. 10  is a flowchart showing a method of manufacturing a seal including a plurality of swirl brakes for use in reducing an inlet seal swirl in a centrifugal compressor according to exemplary embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to exemplary embodiments, seals with swirl brakes can be manufactured and installed to improve rotordynamic stability. The improvement is created by the swirl brakes driving inlet seal entry swirl to substantially zero. This can reduce the amount of undesirable circumferential forces in the seal during operation of centrifugal compressors. 
     To provide some context for the subsequent discussion relating to seals with swirl brakes according to these exemplary embodiments,  FIG. 2  illustrates a multi-stage, centrifugal compressor  6  in which such seals with swirl brakes may be employed. Therein, the centrifugal compressor  6  includes a housing  8  within which is mounted a rotating compressor shaft  10  that is provided with a plurality of centrifugal impellers  12 . The centrifugal compressor  6  takes an input process gas from duct inlet  14 , accelerates the particles of the process gas through operation of the impellers  12 , and subsequently delivers the process gas through outlet duct  16  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. A balance piston  18  (also known as a balance drum) is also shown. The balance piston is used to compensate for axial thrust generated by the impellers  12 . Seals  20  and  21 , e.g., labyrinth and/or honeycomb seals, are used to prevent leakage and to maintain pressure. Additionally, a balance line  22  is shown. The balance line maintains pressure on the outboard side of the balance piston  18  at the same level as the pressure at which the process gas enters via duct  14 . 
       FIG. 3  shows an exploded view of the last impeller  12  in contact with the balance piston  18 . The arrows show the primary flow direction of the process gas used in the centrifugal compressor  6 . Seals  20  and  21  prevent (or greatly reduce the amount of) leakage from a higher pressure side to a lower pressure side of the process gas. Additionally, according to exemplary embodiments, the seals  20  and  21  can include swirl brakes (described below in more detail) which can reduce swirl of the flow, e.g., drive the value of swirl towards zero, and improve the rotordynamics of compressor  6 . 
     According to exemplary embodiments, seal  20  (and seal  21 ) can include swirl brakes  24  as shown in  FIGS. 4 and 5 . The swirl brakes  24  shown in  FIGS. 4 and 5  are tooth shaped and can be machined from the base stock material of the seal  20 .  FIG. 5  shows the exploded section A from  FIG. 4 . The plurality of swirl brakes  24  shown in  FIG. 5  are shown in a basic tooth form prior to being machined to desired final dimensions which are described, according to exemplary embodiments, in more detail below. Arrow  26  represents a longitudinal axis of the seal  20  and area  32  represents the portion of seal  20  that can be a labyrinth, a honeycomb or a pocket damper seal section. For reference, the labels of pitch, length, thickness and height for the swirl brakes  24  are shown in  FIG. 5 . When considering the direction of flow of the process gas, the main flow will occur roughly parallel to surface  28 . The flow of gas that is to be minimized, e.g., blocked between the higher and lower pressure zones, will occur roughly parallel to surface  30 . 
     According to exemplary embodiments, the tooth shaped swirl brakes  24  can be tapered as shown in  FIG. 6 .  FIG. 6  shows the statoric part, e.g., seal  20 , the rotoric part  34 , e.g., the impeller  12  or the balance piston  18 , the main flow direction  36  for the process gas, the attempted flow path  38  through the seal, a leading edge clearance  40  and a trailing edge clearance  42  between the statoric part and the rotating part. The leading edge clearance  40  is larger than the trailing edge clearance  42 . Additionally, the leading edge clearance  40  is larger, relatively, than the clearance found in conventional swirl brakes where it has typically been believed that increasing the upstream clearance  40  deteriorates swirl control. According to an exemplary embodiment, the leading edge clearance  40  can be approximately double the trailing edge clearance  42 , however other values of the clearances could be used. While the rotoric part  34  can be either the impeller  12  or the balance piston  18 , in  FIG. 6  the rotoric part  34  is the impeller  12 . For the case of the balance piston  18  being the rotoric part  34 , would have an opposite orientation from the orientation shown in  FIG. 6  as can be seen from seals  20  and  21  in  FIG. 3 . 
     According to exemplary embodiments, the swirl brakes  24  can be machined to reduce swirl and improve rotordynamic efficiency.  FIGS. 7 and 8  show a diaphragm  46  attached to the seals  20  and  21  which includes the swirl brake  24 . The two different orientations shown in  FIGS. 7 and 8  correspond to the orientations of the seals  20  and  21  in  FIG. 3 . An expanded view of the swirl brake  24  shown as section B in  FIG. 7  is shown in  FIG. 9  and will now be explained. 
     According to exemplary embodiments, the swirl brakes  24  can be machined to have a shape as shown in  FIG. 9 . Each swirl brake  24  can have a first machined surface  48 , a second machined surface  50  and a third machined surface  52 . The first surface  48  is a portion of surface  28  shown in  FIG. 5 . The third surface  52  is a portion of surface  30  as shown in  FIG. 5 . The third surface can have a predetermined taper over its predetermined length b, with a predetermined height change c. According to exemplary embodiments, the predetermined height change c of the taper can be in the range of 0-0.1 mm, however other height change ranges can be used. The angle of the taper is shown as α 1 . The second surface  50  connects the first surface  48  to the third surface  52 , and the second surface has an angle of α 2 . According to exemplary embodiments, the tooth structure can provide the rotordynamic efficiency improvements under a complete range of operating conditions for centrifugal compressors. 
     According to another exemplary embodiments, similar tooth shaped swirl brakes  24 , as shown in  FIG. 9  and described above, can be used in various centrifugal compressors. The dimensions of the surfaces, e.g., surfaces  48 ,  50  and  52 , as well as the values of the angles α 1  and α 2  can vary depending upon the characteristics of the centrifugal compressor, its operating conditions in order to still obtain a minimized or zero inlet swirl amount, etc. 
     Utilizing the above-described exemplary systems according to exemplary embodiments, a method for manufacturing a seal including a plurality of swirl brakes for use in reducing an inlet seal swirl in a centrifugal compressor is shown in the flowchart of  FIG. 10 . The method includes: a step  1002  of procuring the seal of interest; a step  1004  of machining the plurality of swirl brakes to each have a tooth shape on the seal leading edge; a step  1006  of machining a first surface of a swirl brake with a first predetermined length for each swirl brake; a step  1008  of machining a second surface of the teeth connected to the first surface having a predetermined angle with the first surface for each tooth; and a step  1010  of machining a third surface of the teeth which extends from an end of the second surface to a beginning of a seal section having a second predetermined length for each tooth, wherein the third surface has a first taper configured to reduce an inlet seal swirl. 
     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. For example, the centrifugal compressor can be a single stage compressor or a multistage compressor. 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. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Technology Category: 2