Abstract:
A tandem gas seal assembly is described for sealing between the rotor and the casing of a rotary machine. The assembly comprises two sealing stages  10, 12  mounted axially adjacent to one another on the machine rotor. In the invention, wherein the two stages  10  and  12  are totally separable from one another and each is capable of functioning as a seal when separated from the other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims priority from prior British Patent Application No. 0327671.4, filed on Nov. 28, 2003, which is related to British Patent Application No. 0306402.9, filed on Mar. 20, 2003, the entire disclosure of each reference which are herein incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to seals and more particularly relates to seals for restricting leakage of a fluid about a shaft extending through a housing, such as in a compressor or a turbine, and in particular for restricting leakage of gas at elevated pressure. 
   BACKGROUND OF THE INVENTION 
   Gas seals are currently used widely to seal process gases in large rotary machines such as turbo compressors and associated turbo machinery. The industry has adopted as its standard, an arrangement of sealing cartridge which is comprised of a stacked assembly that can be slipped on to the end of the rotor. 
   The most common type of gas seal in current use is a tandem seal, as shown in section in  FIG. 1  of the accompanying drawings. A similar seal is shown, for example, in FIG. 3 of U.S. Pat. No. 5,412,977. The cartridge comprises two stages, namely an inboard stage  10  and an outboard stage  12 . The inboard stage  10  is used to seal the complete process gas pressure. The outboard stage  12  is also engineered to be capable of sealing the complete process gas pressure. However, during normal running, it is only exposed to around 5–15 bar of gas pressure, its main function being to act as a back up to the inboard stage  10 . Should the inboard stage  10  suffer catastrophic failure during operation, the outboard stage  12  takes on the full sealing function and enables the compressor to be run down, stopped and the process gas pressure vented in a controlled manner. 
   Each stage  10 ,  12  comprises a primary ring  14  mounted in a retainer and balance diameter assembly  18  in the machine casing. A spring  20  within the assembly  18  acts on the primary ring  14  through a pressure plate  22  that is sealed relative to the stationary assembly housing. The spring  20  urges the primary ring towards a mating ring  16  that rotates with the machine rotor  30 . 
   The primary ring  14  has axial end faces which are exposed to the pressure of the process gas and have different surface areas. The primary ring  14  therefore acts as a differential piston on which the pressure of the process gas exerts a force in the direction to compress the spring  20 . In operation, an equilibrium occurs between the gas pressure forces and the spring force when there is a small gap between the primary and the mating rings  14  and  16 . This gap prevents the wear to the surfaces of the rings  14  and  16  while limiting escape of the process gas to a small and acceptable leakage. 
   The manner in which a gas seal operates and the details of its construction, such as the choice of materials to make the rings, are well known in the art and need not therefore be described further in the context of the present invention. 
   In the known seal cartridges, the two sealing stages are mounted on a common support sleeve  40 . The inner surface of the sleeve  40  and the outer surface of the rotor  30  are stepped so as to limit the extent to which the sleeve can move to the left, as viewed in  FIG. 1 , relative to the rotor  30 . A single locknut  42  on the rotor  30  is therefore all that is required to clamp the support sleeve  40  on the rotor  30 . This allows both the sealing stages  10  and  12  to be released from the machine by removal of one locknut  42 . At one end, the support sleeve  40  has a radial flange  44  which carries the mating ring  16  of the inboard stage  10 . The mating ring  16  is held against the flange  44  of the support sleeve  40  by a spacer sleeve  50  which itself has a radial flange  52  which carries the mating ring  16  of the outboard stage. An O-ring seal  54  seals between inner surface of the spacer sleeve  50  and the outer surface of the support sleeve  40 . A locking sleeve  60  clamps the mating ring  16  of the outboard stage  12  against the flange  52  of the spacer sleeve  50  and its inner surface is also suitably sealed relative to the outer surface of the spacer sleeve  50 . 
   The above described construction of a tandem seal is convenient in that it allows for easy replacement of both stages of the seal and it enables a compact construction in that the entire cartridge is held in place by only one locknut. 
   While the tandem seal is useful it has shortcomings and disadvantages. As earlier explained, it is quite common for only one stage to fail but the known construction does not allow the different stages to be worked on independently. Thus, in the embodiment illustrated in  FIG. 1 , in order to access the inboard stage, it is necessary to dismantle the entire outboard stage, which may not itself require attention. Many other problems can be identified, and are discussed in more detail below, which stem from the fact that the two stages cannot function separately. 
   SUMMARY OF THE INVENTION 
   The present invention mitigates the foregoing disadvantages of the prior art tandem seals and provides, a tandem gas seal assembly for sealing between the rotor and the casing of a rotary machine which comprises two sealing stages for mounting axially adjacent to one another on the machine rotor, wherein the two stages are totally separable from one another and each is capable of functioning as a seal when separated from the other. 
   Preferably, mating formations are formed at the adjacent axial ends of the two sealing stages to maintain the two stages in axial alignment with one another. 
   The mating formations may suitably include an annular collar projecting axially from the end of one of the stages and fitting over a cylindrical end region of the other sealing stage. An O-ring is advantageously provided as a means for centering the mating formations of the two sealing stages. 
   Even though the sealing stages are independent of one another, they are preferably stacked one against the other and retained on the rotor by means of a single locknut acting on the outboard stage. There is thus no need to sacrifice the advantages offered by prior art construction with regard to space saving and the ease of mounting and dismounting of the seal assembly on the rotary machine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is as earlier described a section through a prior art tandem seal assembly cartridge, and 
       FIG. 2  is a similar section to  FIG. 1  showing a tandem seal assembly according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   It should be understood that these embodiments are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality. 
   In the present invention, the inboard  10  and outboard  12  stages of a tandem seal are constructed as totally separate modules, each containing components corresponding to one of the sealing stages of the known tandem seal shown in  FIG. 1 . In order to avoid unnecessary repetition, like components have been allocated like reference numerals as previously described in  FIG. 1 , and will not be described a second time. Components that serve the same function, but have been modified, have also been allocated the same reference numerals, but a prime has been added to show that the component has been changed. 
   In general, the components secured to the casing of the rotary machine have not been changed and they operate in exactly the same way as earlier described. The important changes are the following: 
   There are two support sleeves  40 ′ associated with the two stages  10  and  12  instead of a single support sleeve  40  common to the two stages. 
   The radial flange  52 ′ supporting the second mating ring  16  is not formed on a spacer sleeve  50  but directly on the second of the two support sleeves  40 ′. 
   In the prior art tandem seal of  FIG. 1 , spacer sleeve  50  is used to hold the mating ring  16  to the annular flange  44  of the inboard stage  10 . On the other hand, in the tandem gas seal assembly of the present invention shown in  FIG. 2 , no spacer sleeve is used to hold the mating ring  16  to the annular flange  44 ′ of the support sleeve  40 ′ of the inboard stage  10 . In the tandem gas seal assembly of the present invention, the mating ring  16  of the inboard stage  10  is held to the annular flange  44 ′ of the inboard stage  10  by a locking sleeve  60 ′ at the inboard stage  10 . 
   The above modifications result in a total separation of the two sealing stages and each can now function totally independently of the other. In other words, they do not need to be placed next to one another for them both to function normally. It is preferred to mount them next to each other and each sealing stage is fastened to its respective rotating support sleeve  40 ′ with a separate fastener  75  oriented in substantially one direction as illustrated. Further each sealing stage can then be retained using a single locknut  42  on the rotor  30 , as previously described. 
   Though it is possible to rely exclusively on the rotor to maintain the two stages in correct alignment, it assists assembly and improves structural rigidity to provide mating formations on the axial ends of the two stages to maintain them in correct axial alignment and to provide a seal between the two stages. In the illustrated embodiment of the invention, the mating formations comprise an annular collar  70  projecting from the axial end of the support sleeve of the outboard stage surrounding a cylindrical surface defined by the end of the locking sleeve  60 ′ of the inboard stage, the seal between the two being effected by an O-ring  72 . 
   The separation of the sealing stages into separate modules offers several advantages, which will now be discussed. 
   Each of support sleeves is shorter in length for the same given diameter thus providing, improved structural integrity. 
   Separation of the stages allows each to be dynamically balanced independently of the other, improving both the ease and the quality of the balancing. 
   The modular arrangement, enables independent replacement of just one module, be it the inboard or the outboard stage. This significantly reduces down time. 
   Any refurbishment, repair, or replacement need only be done on one module, without having to strip the other. 
   Less metal is required during the manufacture as compared with a conventional tandem seal cartridge. This is because the support sleeve  40 , for example, needs to be machined from a long blank having a diameter greater than that of the radial flange  44  and the spacer sleeve needs to be machined from a second blank of nearly the same size. By contrast, in the seal assembly of the present invention, the two support sleeves  40 ′ can be machined out of a blank of approximately the same size as that required for the support sleeve  40 . Aside from starting with less metal, the manufacturing process also requires less metal removal and is therefore significantly less time consuming and costly. 
   The assembly procedure is simplified, resulting in reduced assembly time and reduced assembly costs. 
   The modular arrangement enables much more rapid fault diagnosis on test, thus further reducing time and costs. The fact that there are no shared components means that a fault can be more easily located in one or other of the modules and each can be tested separately from the other. 
   Although a specific embodiment of the present invention has been disclosed, it will be understood by those having skill in the art that changes can be made to this specific embodiment without departing from the spirit and scope of the present invention. The scope of the present invention is not to be restricted, therefore, to the specific embodiment, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.