Patent Abstract:
A seal assembly forms a barrier between a compressor&#39;s interior and exterior regions. The seals assembly includes a primary seal stage and a secondary seal stage. The primary seal stage is formed of materials chosen to effectively block flow of gas through the seal assembly. The secondary seal stage is formed of materials chosen to survive a failure of the primary seal stage.

Full Description:
TECHNICAL FIELD 
     The present disclosure generally pertains to a seal between relatively movable parts and more particularly to a tandem dry gas seal suitable for use with a centrifugal compressor. 
     BACKGROUND 
     Seal systems are used in a wide variety of rotary shaft devices, such as blowers, compressors, and pumps, which have critical sealing requirements. Dry gas seal systems provide a barrier between the gas in the working chamber, or process gas, and the external environment to minimize the loss of process gas to the environment. Seal systems may include two stages of seals arranged in tandem to improve reliability. Mosley and Haynes, in European Patent Application publication EP 0 701 074 A1, describe a dry gas seal with two face seal stages of the same construction. 
     Dry gas seals operate with very small gaps or separations between opposed sealing surfaces. Brittle materials such silicon or tungsten carbide are used for some sealing surfaces to provide precise surfaces for small separations between the opposed sealing surfaces. Such materials may, however, fail and a failure can be catastrophic. 
     The present disclosure is directed toward overcoming one or more of the problems discussed above as well as additional problems discovered by the inventor. 
     SUMMARY OF THE DISCLOSURE 
     A seal assembly includes a primary seal stage and a secondary seal stage. The primary seal stage includes a primary ring arranged to be coupled to a housing and a mating ring arranged to be coupled to a rotating shaft. The primary ring and the mating ring of the primary seal stage are formed materials chosen to effectively block flow of gas through the seal assembly. The secondary seal stage is coaxially positioned with respect to the primary seal stage and includes a primary ring arranged to be coupled to the housing and a mating ring arranged to be coupled to the rotating shaft. The primary ring and the mating ring of the secondary seal stage are formed of materials chosen to survive a failure of the primary seal stage. The seal assembly may be used in a compressor for sealing a penetration of the compressor&#39;s shaft through the compressor&#39;s housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cutaway illustration of an exemplary centrifugal compressor. 
         FIG. 2  is a cross-sectional view of a seal assembly according to an exemplary disclosed embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cutaway illustration of an exemplary centrifugal compressor  100 . Process gas enters the centrifugal compressor  100  at a suction port  112  formed on a housing  110 . The process gas is compressed by one or more centrifugal impellers  122  mounted to a shaft  120 . The compressed process gas exits the centrifugal compressor  100  at a discharge port  114  that is formed on the housing  110 . 
     The shaft  120  and attached elements such as the centrifugal impellers  122  are supported by bearings  132  installed on axial ends of the shaft  120 . Seal assemblies  142  are disposed about the shaft  120  inward of the bearings  132 . The seal assemblies  142  seal high pressure inside the centrifugal compressor  100 . Different designs may use more or fewer seal assemblies  142 . 
     The seal assemblies  142  include primary and secondary seal stages. The primary seal stage normally operates to block the flow of the process gas out of the compressor. The secondary seal stage may be considered a backup to block the flow of the process gas out of the compressor in the event of failure or malfunction of the primary seal stage. In an embodiment, the secondary and primary seal stages are substantially identical but formed of different materials. 
       FIG. 2  is a cross-sectional view of a seal assembly  142 . The elements of the seal assembly are generally ring shaped or radially disposed about a central axis of the seal assembly.  FIG. 2  illustrates a cross-section of one side of a symmetrical seal assembly. The seal assembly may be used as the seal assemblies  142  of the centrifugal compressor  100  of  FIG. 1 . The seal assembly of  FIG. 2  includes a primary seal stage  30  and a secondary seal stage  50 . The primary seal stage  30  is disposed at an inner, or process gas, end of the seal assembly  142 . The secondary seal stage  50  is disposed at an outer, or bearing, end of the seal assembly  142 . 
     The seal assembly is illustrated in  FIG. 2  adjacent to a buffer seal  20 . The buffer seal  20  includes segmented carbon rings  21 ,  22  held in a buffer seal housing  24 . Radial passages in the buffer seal housing  24  provide a purge inlet  15 . A secondary vent  17  is disposed between the buffer seal  20  and the secondary seal stage  50 . A primary vent  13  is disposed between the primary seal stage  30  and the secondary seal stage  50 . A primary inlet  11  is disposed on the process gas end of the primary seal stage  30 . 
     When the seal assembly illustrated in  FIG. 2  is used in a compressor, various gas flows exist during operation. In an embodiment, filtered process gas  12  flows into the primary inlet  11 . Some of the filtered process gas leaks through the primary seal stage  30 . The filtered process gas that leaks through the primary seal stage  30  passes out the primary vent  13  as a primary vent gas  14  which may then be collected or, for example, for natural gas, flared off. A purge gas  16 , such as nitrogen, flows into the purge inlet  15 . Some of the purge gas flows past the segmented carbon ring  22  and out the secondary vent  17  as a secondary vent gas  18 . Some of the filtered process gas that leaked through the primary seal stage  30  also leaks through the secondary seal stage  50  and out the secondary vent  17 . 
     The flows through or pressures in the primary inlet  11 , the primary vent  13 , the purge inlet  15 , and the secondary vent  17  are monitored to control operation of the seal. The monitoring can also be used to detect a malfunction or abnormal operation of the seal. A system monitoring the seal can shut down the compressor when abnormal operation is detected. 
     The primary seal stage  30  includes a sleeve  5 . The sleeve  5  may be coupled to the shaft of a compressor. The sleeve  5  may be formed of a stainless steel. A mating ring  32  is disposed in an opening of the sleeve  5 . A sleeve O-ring  33  is disposed in a slot in the opening of the sleeve  5 . The sleeve O-ring  33  provides a static seal between the sleeve  5  and the mating ring  32 . The sleeve O-ring  33  may be made of a polymer, for example, polytetrafluoroethylene (PTFE). 
     The primary seal stage  30  also includes a primary ring  31  disposed in an opening of a retainer  34 . The retainer  34  may be formed of a stainless steel. The retainer  34  may be coupled to the housing of a compressor. The primary ring  31  and the mating ring  32  include corresponding opposing faces. 
     A spring  35  biases the primary ring  31  towards the mating ring  32 . Although one spring is illustrated in  FIG. 2 , the primary seal stage  30  may have multiple springs circumferentially distributed around the central axis of the seal assembly  142 . The spring  35  may be formed of a superalloy. A spring plate  36  is disposed between the spring  35  and the primary ring  31 . A retainer O-ring  37  is disposed between the spring plate  36  and the retainer  34  and provides a static seal between the spring plate  36  and the retainer  34 . The retainer O-ring  37  may be made of a polymer, for example, PTFE. 
     The mating ring  32  of the primary seal stage  30  is made of a brittle material. In an embodiment, the primary ring  31  of the primary seal stage  30  is also made of a brittle material. The primary ring  31  and the mating ring  32  may be made of the same material or different materials. The primary ring  31  and mating ring  32  of the primary seal stage  30  may be coated with additional materials, for example, the rings may be diamond coated. In another embodiment, the primary ring  31  is made of a more flexible material, such as a carbon composite. Brittle materials provide precise shapes that experience limited distortion during operation at high gas pressures, for example, 1000 PSI, high rotational speeds, for example, 20,000 RPM, and high temperatures, for example, 400° C. 
     Ductile and brittle materials are distinguished by the relationships between stresses and strains in the materials. Ductile materials can withstand relatively large strains before failure. Objects made of either type of material exhibit elastic deformation in response to initial stresses. When stresses are removed after elastic deformation, the objects return to their initial shapes. 
     Objects made of ductile materials exhibit plastic deformation in response to stresses greater than an elasticity limit. When stresses are removed after plastic deformation, the objects do not return to their initial shapes. Plastic deformation can result in a large deformation in a ductile material, for example, 15%, before the material fractures. An example ductile material is steel. A material may be considered ductile when it can be deformed more than 5% in plastic deformation. 
     Objects made of brittle materials do not exhibit large plastic deformations. Objects made of brittle materials abruptly fracture in response to stresses greater than a fracture limit. Example brittle materials include tungsten carbide and silicon carbide. A material may be considered brittle when it can be deformed less than 5% before fracture. 
     The secondary seal stage  50  includes a portion of the sleeve  5  in the embodiment of  FIG. 2 . In other embodiments, the secondary seal stage  50  may include a separate sleeve. The secondary seal stage  50  includes a mating ring  52  disposed in an opening of the sleeve  5 . A sleeve O-ring  53  is disposed in a slot in the opening of the sleeve  5 . The sleeve O-ring  53  provides a static seal between the sleeve  5  and the mating ring  52 . The sleeve O-ring  53  may be made of a polymer, for example, PTFE. 
     The secondary seal stage  50  also includes a primary ring  51  disposed in an opening of a retainer  54 . The retainer  54  may be formed of a stainless steel. The retainer  54  may be coupled to the housing of a compressor. The primary ring  51  and the mating ring  52  include corresponding opposing faces. 
     A spring  55  biases the primary ring  51  towards the mating ring  52 . Although one spring is illustrated, the secondary seal stage  50  may have multiple springs circumferentially distributed around the central axis of the seal assembly  142 . The spring  55  may be formed of a superalloy. A spring plate  56  is disposed between the spring  55  and the primary ring  51 . A retainer O-ring  57  is disposed between the spring plate  56  and the retainer  54  and provides a static seal between the spring plate  56  and the retainer  54 . The retainer O-ring  57  may be made of a polymer, for example, PTFE. 
     The mating ring  52  of the secondary seal stage  50  is made of a ductile material, for example, steel. In an embodiment, the primary ring  51  of the secondary seal stage  50  is also made of a ductile material. The primary ring  51  and the mating ring  52  may be made of the same material or different materials. The primary ring  51  and the mating ring  52  of the secondary seal stage  50  may be strengthened by surface treatment, for example, using induction heating. In another embodiment, the primary ring  51  is made of a more flexible material, such as a carbon composite. 
     INDUSTRIAL APPLICABILITY 
     The rate that gases leak between the sealing faces of the primary ring  31  and the mating ring  32  is decreased when the faces are closely spaced. The primary ring  31  and the mating ring  32  may be spaced, for example, by a few microns. The components of the seal assembly  142  are subject to shape distortion by thermal changes, gas pressures, and rotational forces. 
     Prior seal assemblies have used primary and secondary seal stages made of the same materials. Early seal assemblies used mating rings, in both primary and secondary seal stages, made of steel, a ductile material. The seal assemblies used primary rings, in both primary and secondary seal stages, made of a carbon composite material. The carbon composite used is relatively flexible (having a low modulus of elasticity) and low strength compared to the mating ring. The carbon composite is also quite brittle. The carbon composite, because of its low strength, is generally not used as for the mating ring, which rotates. 
     For use at higher pressures, prior seal assemblies use mating rings, in both primary and secondary seal stages, made of tungsten carbide or silicon carbide, brittle materials. The relatively flexible primary rings conformed against the much stiffer mating rings creating the desired small spacing between the faces of the primary and mating rings. For use at still higher pressures, other prior seal assemblies use mating rings and primary rings, in both primary and secondary seal stages, made of tungsten carbide or silicon carbide. 
     A seal assembly using a carbide mating ring and a carbon primary ring can fail when the highly stressed mating ring develops cracks due to thermal, rotational, and pressure induced stresses. When the mating ring fails, the carbide material can break up into pieces with jagged edges. With rotation, these pieces can cut into and break up the carbon primary ring causing destruction of the primary ring. 
     The carbon primary ring is not typically considered the initiator of a failure. If the carbon primary ring were to crack first, since it has low strength, it would not cause another ring to crack and break up. Although the gas flow would increase due to the cracks in the carbon ring, the flow would still be low compare to when pieces of the rings are liberated opening up large flow paths. 
     A seal assembly using a carbide mating ring and a carbide primary ring can fail in the same manner. Breakup of one of the carbide rings liberates hard pieces which can cause the other carbide ring to fail. 
     The present seal assembly  142  uses materials in the primary seal stage  30  and the secondary seal stage  50  selected for the distinct functions of the stages. The seal assembly is both very effective at blocking the flow of gases and very rugged. The primary seal stage  30  is effective at blocking flow of gases. The primary seal stage  30  may [add example of seal performance]. The ruggedness of the secondary seal stage  50  can allow it to survive a failure of the primary seal stage. 
     The materials used in the primary ring  31  and the mating ring  32  of the primary seal stage  30  are selected for their superior performance as a gas seal. For intermediate to high gas pressures at least one of the rings is a rigid material like silicon carbide or tungsten carbide. In some embodiments, both the primary ring  31  and the mating ring  32  are made of these types of materials. Although these materials provide superior seal performance at elevated pressures, in the event of a failure, fracturing and liberation of pieces of these rigid, brittle materials often results in large openings within the seal assembly, which causes excessive amounts of pressurized gas to escape. 
     The materials used in the primary ring  51  and the mating ring  52  of the secondary seal stage  50  are selected for their ruggedness in the event of a failure of the primary seal stage  30  in addition to performance as a gas seal. The use of a ductile material, like steel, in the highly stressed rotating mating ring  52  mitigates the possibility of pieces of the mating ring  52  being liberated as in the case of a brittle material failure. In various embodiments, the primary ring  51  is made from a ductile material or a carbon material, which is a relatively flexible although somewhat brittle. These materials result in the primary ring remaining more intact and in place after a failure than rings made of the materials used in the primary seal stage. 
     The disclosed seal assembly embodiments may be suited for any number of industrial applications, such as various aspects of the oil and natural gas industry. For example, applications for compressors with the disclosed seal assemblies may include transmission, gathering, storage, withdrawal, and lifting of oil and natural gas. 
     The seal assemblies discussed above may be used in servicing a compressor in the field. An existing seal assembly may be removed and replaced with a new seal assembly. The new seal assembly is of a type disclosed above. 
     The preceding detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. The described embodiments are not limited to use in conjunction with a particular type of compressor. Hence, although the present disclosure, for convenience of explanation, depicts and describes a seal assembly for a centrifugal compressor, it will be appreciated that seal assemblies in accordance with this disclosure can be implemented in various other configurations and used in other types of machines. Furthermore, there is no intention to be bound by any theory presented in the preceding background or detailed description. It is also understood that the illustrations may include exaggerated dimensions to better illustrate the referenced items shown, and are not consider limiting unless expressly stated as such.

Technology Classification (CPC): 5