Abstract:
Systems and methods according to these exemplary embodiments provide sealing mechanisms for centrifugal compressors. A sealing mechanism includes first, second and third dry gas seals arranged in series. Each seal receives its own sealing gas and has its own venting mechanism. Sealing gas pressures remain low enough that a dedicated compressor for supplying the sealing gases is not needed. Additionally, the risk of process gas being released into the atmosphere in case of seal failure is limited.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a national stage application under 35 U.S.C. §371(c) of prior-filed, co-pending PCT patent application serial number PCT/EP2010/067456, filed on Nov. 15, 2010, which claims priority to Italian Patent Application Serial No. CO2009A000051, filed on Nov. 23, 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 the provision of dry gas seal systems in compressors. 
         [0004]    2. Description of the Prior Art 
         [0005]    A compressor is a machine which accelerates the particles of a compressible fluid, e.g., a gas, through the use of mechanical energy to, ultimately, 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 can be said to be part of a class of machinery known as “turbo machines” or “turbo rotating machines”. 
         [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 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. Various types of gases are used in centrifugal compressors, some of which are toxic or dangerous to the environment and/or to workers in the plants. Accordingly, centrifugal compressors employ sealing systems, usually placed on the ends of the shaft that supports the impeller(s), to prevent the gas from escaping from the compressor and contaminating the surrounding environment. Single rotor centrifugal compressors are usually provided with two separate seals as part of this sealing system, i.e., one for each end of the shaft, while in a overhung centrifugal compressor it is usually sufficient to seal the shaft end, located immediately downstream of the impeller. 
         [0007]    Recently there has been an increase in the use of so-called “dry” gas seals in sealing systems for centrifugal compressors. Dry gas seals can be described as non-contacting, dry-running mechanical face seals which include a mating or rotating ring and a primary or stationary ring. In operation, grooves in the rotating ring generate a fluid-dynamic force causing the stationary ring to separate and create a gap between the two rings. These seals are referred to as “dry” since they do not require lubricating oil which, among other things, greatly reduces their maintenance requirements. 
         [0008]    For centrifugal compressors, such dry gas seals are available in different configurations, e.g., so-called tandem configurations which are primarily used in compressors that employ toxic or flammable gases as the input or process gas. As shown in  FIG. 1 , a tandem-type dry gas seal system includes a first seal  2  and a second seal  4 , both contained in a single package. During normal operation of the compressor, the first seal  2  operates to contain the total pressure of gas processed, while the second seal  4  acts as a back-up which is designed to operate only if the first seal  2  fails or leaks excessively. Generally a conditioned gas flow coming from compressor discharge is injected upstream of seal  2  to isolate the dry gas seal from process gas. In the applications with highly toxic process gases (e.g., gas having high contents of H 2 S) and high sealing pressure, an external sealing gas source having a low sulfur content, e.g., a so-called “sweet” gas is usually provided to isolate the process gas from the surroundings. Due to the high sealing pressure a dedicated reciprocating compressor  6  that operates independently of the centrifugal compressor is used to feed the sealing gas system. The second seal  4  in the tandem may receive a lower pressure (e.g., below 10 Bar) of nitrogen as secondary sealing gas via a source  8  to ensure that no toxic/flammable gas escapes to the surroundings. 
         [0009]    Centrifugal compressors equipped with these types of dry gas sealing systems thus also require additional compressors whose function is solely to provide the sealing gas, thus making the overall system more complex. In addition to simply adding complexity, reciprocating compressors  6  may have greater maintenance requirements than even the centrifugal compressors which they are intended to serve. Moreover, although the second seal  4  in the tandem configuration does provide a back-up capability, current dry gas seal systems are still not fault free, in which case they may undesirably release a certain amount of sealing gas into the atmosphere. 
         [0010]    Accordingly, it would be desirable to design and provide a low emission, dry gas seal for compressors which overcomes the aforementioned drawbacks of existing sealing systems. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    Exemplary embodiments provide sealing mechanisms usable, e.g., for centrifugal compressors. A sealing mechanism includes first, second and third dry gas seals arranged in series. Each seal receives its own sealing gas and has its own venting mechanism. Sealing gas pressures which separate the process gas from the venting system remain low enough that a dedicated compressor for supplying the sealing gases is not needed. Advantages according to exemplary embodiments described herein include, for example, better control over potentially hazardous process gas and lower complexity and maintenance requirements associated with sealing mechanisms for centrifugal compressors. However, it will be appreciated by those skilled in the art that such advantages are not to be construed as limitations of the present invention except to the extent that they are explicitly recited in one or more of the appended claims. 
         [0012]    According to an exemplary embodiment, a centrifugal compressor includes a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, a stator, a sealing mechanism disposed between the rotor assembly and the bearing, the sealing mechanism including a first dry gas seal, disposed proximate an inboard side of the sealing mechanism, and having a primary seal gas supplied thereto at a first pressure, a second dry gas seal, disposed adjacent to the first dry gas seal and having a primary buffer gas supplied thereto at a second pressure, and a third dry gas seal, disposed adjacent to the second dry gas seal and having a buffer gas supplied thereto at a third pressure. 
         [0013]    According to another exemplary embodiment, a method for sealing a centrifugal compressor having a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, and a stator includes the steps of blocking a process gas, which is pressurized by the centrifugal compressor, from reaching the bearing by using a combination of first, second and third dry gas seals in sequence, supplying the first dry gas seal with a primary seal gas at a first pressure, supplying the second dry gas seal, disposed adjacent to the first dry gas seal, with a primary buffer gas at a second pressure, and supplying the third dry gas seal, disposed adjacent to the second dry gas seal, with a buffer gas at a third pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings illustrate exemplary embodiments, wherein: 
           [0015]      FIG. 1  illustrates a tandem sealing mechanism; 
           [0016]      FIG. 2  is a schematic view of a multistage-type centrifugal compressor, provided with sealing mechanisms according to exemplary embodiments; 
           [0017]      FIG. 3  is a partial sectional view of an exemplary dry gas seal used in sealing mechanisms according to exemplary embodiments; 
           [0018]      FIG. 4  is a sectional view of a sealing mechanism including three dry gas seals according to an exemplary embodiment; 
           [0019]      FIG. 5  illustrates a sealing mechanism including input and output fluid controls according to exemplary embodiments; and 
           [0020]      FIG. 6  is a flowchart illustrating a method for sealing a compressor according to exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    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. 
         [0022]    To provide some context for the subsequent discussion relating to sealing systems according to these exemplary embodiments,  FIG. 2  schematically illustrates a multistage, centrifugal compressor  10  in which such sealing systems may be employed. Therein, the compressor  10  includes a box or housing (stator)  12  within which is mounted a rotating compressor shaft  14  that is provided with a plurality of centrifugal impellers  16 . The rotor assembly  18  includes the shaft  14  and impellers  16  and is supported radially and axially through bearings  20  which are disposed on either side of the rotor assembly  18 . 
         [0023]    The multistage centrifugal compressor operates to take an input process gas from duct inlet  22 , to accelerate the particles of the process gas through operation of the rotor assembly  18 , and to subsequently deliver the process gas through outlet duct  24  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. Between the impellers  16  and the bearings  20 , sealing systems  26  are provided to prevent the process gas from flowing to the bearings  20 . The housing  12  is configured so as to cover both the bearings  20  and the sealing systems  26  to prevent the escape of gas from the centrifugal compressor  10 . Also seen in  FIG. 2  is a balance drum  27  which compensates for axial thrust generated by the impellers  16 , the balance drum&#39;s labyrinth seal  28  and a balance line  29  which maintains the pressure on the outboard side of the balance drum  27  at the same level as the pressure at which the process gas enters via duct  22 . 
         [0024]    According to exemplary embodiments, each of the sealing systems  26  includes three dry gas seals which together cooperate to seal the process gas from escaping toward the bearings  20 . Generally speaking, each of the three dry gas seals in the sealing system  26  can be implemented as, for example, shown in  FIG. 3 . Therein, a dry gas seal  30  is seated on the compressor shaft  14  to block the flow of the process gas along the gas path  32  from the inboard side to the outboard side of the centrifugal compressor  10 . Each dry gas seal  30  includes a rotating seat  34  which rotates with the compressor shaft and mates with a stationary ring  36 . During operation, grooves formed in at least one of the rotating seat  34  and the stationary ring  36  have a pressurized fluid pumped therein which has the effect of generating a fluid dynamic force which causes the stationary ring  36  to separate from the rotating seat  34 . This creates a gap between the two rings, the combination of which operates as a seal that generally prevents leakage of the process gas, without friction between the rotating seat  34  and the stationary ring  36 . Examples of these types of dry gas seals can be found in U.S. Pat. Nos. 5,492,341 and 5,529,315, the disclosures of which are incorporated here by reference. 
         [0025]      FIG. 4  illustrates a portion of a rotary machine, e.g., a centrifugal compressor, having a triple dry gas seal system  26  according to exemplary embodiments. Therein, the triple dry gas seal system  26  includes three individual dry gas seals  40 ,  42  and  44  arranged in series along the compressor shaft  14 . In this exemplary embodiment, a labyrinth seal  45  is disposed upstream of the triple dry gas seal system  26  (on the inboard side proximate the impellers) and a barrier seal  48  is disposed downstream of the triple dry gas seal system  26  on (i.e., on the outboard side proximate the bearings  20 ), although those skilled in the art will appreciate that this particular configuration is purely illustrative and that the labyrinth and/or barrier seals may be omitted from other embodiments. Each of the three dry gas seals  40 ,  42  and  44  has respective inlets and outlets for their respective sealing gases. More specifically, the first stage (primary) seal  40  has an inlet  46  and an outlet  48 , the second stage (secondary) seal  42  has an inlet  50  and an outlet  52 , and the third stage (tertiary) seal  44  has an inlet  54  and an outlet  56 . Similarly, each of the three dry gas seals  40 ,  42  and  44  has rotating seats  58 ,  62 ,  66  and stationary rings  60 ,  64  and  68 , respectively, and each of the three dry gas seals  40 ,  42  and  44  is designed to handle the maximum sealing pressure associated with the process gas. 
         [0026]      FIG. 5  depicts a triple dry gas sealing system  26  according to exemplary embodiments from a fluid pressure point of view. Therein, according to this exemplary embodiment, the primary seal stage  40  is provided with conditioned (i.e., appropriately filtered, heated and controlled) process gas as the sealing gas. This sealing gas may, for example, be delivered at a pressure of 70-400 Bar, and provided to the first seal  40  at a pressure higher than process gas of, for example, 300 Bar via a pressure control valve (PCV)  70  and associated gas conditioning elements  72  (which are not necessarily limited to the heater and filter indicated in  FIG. 5 , e.g., cooling elements or other gas conditioning elements could be provided as additional or alternative elements). The seal gas is automatically controlled in flow or in differential pressure by the PCV  70  in order to ensure the flow of conditioned gas in all operating conditions (e.g., pressurization, start-up, normal operation, shut down etc.). 
         [0027]    According to this exemplary embodiment, the secondary seal stage  42  is provided with fuel gas or other suitable sweet gas source as a primary buffer gas, which is provided to the dry gas seal  42  at, for example, 20 Bar via pressure control valve  74  and associated gas conditioning elements  76 . The primary buffer gas (normally sweet fuel gas or other suitable gas that is available in the plant) is injected into the compressor  10  (e.g., via port  50  in  FIG. 4 ) via the PCV  74  in a manner which ensures a positive separation between sour and sweet seal gas. Similarly, the tertiary seal stage  44  may be provided with nitrogen as its buffer gas from a source which delivers the gas at a pressure of 4-10 Bar, e.g., higher than the flare system pressure, and which can be controllably provided to the third dry gas seal  44  at 4 Bar by PCV  78  and associated gas conditioning elements  80 . Note, however, that the provision of nitrogen to this third dry gas seal  44  is optional and, therefore, gas pathway elements  79  to the third dry gas seal  44  may be omitted. Moreover, the buffer gas (nitrogen in this example) may also be supplied to the barrier seal  81 . 
         [0028]    Those skilled in the art will appreciate that the specific gas pressures described above and illustrated in  FIG. 5  are purely exemplary and that other pressures may be used. More generally, the gas pressures through the sealing system  26  should generally be set such that P 1 &gt;P 2 &gt;P 4 &gt;P 3 &gt;P 6 &gt;P 5 &gt;P 7 , referring to the pressure zones illustrated in the figure, such that a stepwise decrease in pressure is exhibited through the zones. Note, however, that although these pressure values are exemplary, they are sufficiently low that none of the sealing gas supply sources require the provision of an auxiliary (e.g., reciprocating) compressor to supply the sealing gas, e.g., as a non-limiting example supply pressures ranging from 1-50 Bar or, stated somewhat differently, lower than 51 Bar. This, in turn, renders exemplary embodiments more cost efficient and requires less maintenance than conventional compressor systems. 
         [0029]      FIG. 5  also illustrates pressure controlled venting mechanisms for each of the first two of the three sealing stages of the sealing system  26 . For example, the primary seal  40  includes a venting mechanism  82  which returns process gas that escapes from the primary seal  40  back to a recovery system. The venting mechanism  82  includes, among other things, an optional PCV  84  set to an appropriate pressure level given the sealing gas pressure, in this example 10 Bar. The recovered seal gas venting mechanism  82  is also equipped with flow and pressure monitoring instrumentation which can monitor variations of flow and pressure (higher or lower) along the return path, which parameters may be indicative of a malfunction of the seals. These values are detected and can be used to generate system alarm or shutdown signals. The recovered process gas is then routed to a recovery system and injected into the process gas loop. 
         [0030]    Similarly, the secondary seal  42  is equipped with a venting mechanism  86 . The primary vent is equipped according to this exemplary embodiment, and like the recovered gas vent, with flow and pressure monitoring instrumentation, as well as with a PCV  88  to keep the pressure within a defined range. This pressure can be set to be higher than a pressure used in the plant flare system, to which venting mechanism  86  vents. The variation of flow and pressure (higher or lower) can also be used to detect and generate alarm or shutdown signals in the secondary seal venting system  86 . The tertiary seal  44  also has a venting mechanism  90  which is sized to avoid high back pressure in case of a failure of the sealing mechanism  26 , and which vents the nitrogen (or primary buffer gas) to the atmosphere. 
         [0031]    Thus, according to one exemplary embodiment, a method for sealing a centrifugal compressor having a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, and a stator, includes the method steps illustrated in the flowchart of  FIG. 6 . Therein, at step  100 , a process gas, which is pressurized by the centrifugal compressor, is blocked from reaching the bearing by using a combination of first, second and third dry gas seals in sequence. This further involves supplying the first dry gas seal with a primary seal gas at a first pressure (step  102 ), supplying the second dry gas seal, disposed adjacent to the first dry gas seal, with a primary buffer gas at a second pressure (step  104 ), and supplying the third dry gas seal, disposed adjacent to the second dry gas seal, with a buffer gas at a third pressure. 
         [0032]    Thus, based on the foregoing, it will be seen that exemplary embodiments provide for a sealing mechanism for a centrifugal compressor which is capable of preventing, or at least make it unlikely, that the potentially hazardous process gases will be released into the atmosphere. This is particularly useful, for example, in the presence of process gases such as hydrogen sulfide (H2S). In addition, these exemplary embodiments create seal mechanisms which are substantially impervious to dry gas for a centrifugal compressor that does not require the presence of another compressor which is dedicated to the generation of a highly pressurized sealing gas. Moreover, although sealing mechanisms as shown and described in the above exemplary embodiments have three dry gas seals, it will be appreciated that four or more dry gas seals provided in sequence could also be used according to other exemplary embodiments. 
         [0033]    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.