Abstract:
A non-contacting gas compressor seal assembly is disclosed with an intermediate buffer chamber. The process gas is corrosive or otherwise hazardous and is contained from entering the atmosphere by pumping the barrier gas toward the process fluid. The inboard seal of the assembly is designed to maintain a sealing relationship in the event of loss of buffer gas pressure by operating as a non-contacting seal on the process fluid.

Description:
BACKGROUND OF THE INVENTION 
     Non-contacting seals are successfully employed in gas compressors to provide a seal against loss of process gas. Such a seal is shown, for example, in U.S. Pat. No. 4,212,475 to Sedy. 
     In practice, non-contacting seals are often arranged in an assembly having two spaced apart sets of relatively rotating rings which, in some applications, define an intermediate chamber containing a pressurized barrier gas. The seal ring sets each include a mating ring and an axially movable primary ring. Grooves formed on the face of one of the rings of each set communicate with the barrier gas. One seal set pumps gas from the buffer chamber toward the process fluid. The other pumps toward the atmosphere. An example of such a seal is the double Type 28 gas compressor seal manufactured by John Crane Inc., Morton Grove, Ill. 
     Gas compressor seals of the type described are configured such that on loss of buffer or barrier gas, the inboard seal opens and defines a leakage path to the intermediate chamber. The outboard pair of seal rings operates as a non-contacting seal and pumps a controlled amount of the process gas between the faces. However, since loss of buffer gas often results from failure of the outboard seal, opening of the faces of the inboard seal could cause undesirable leakage through the buffer chamber to atmosphere. 
     Non-contacting seals that operate on a film of gas have more recently been employed to seal liquid in pump applications. An example is found in U.S. Pat. No. 5,375,853. There, spaced seal sets define a buffer chamber for gas at a pressure higher than the process. The inboard seal set pumps the gaseous barrier across the relatively rotatable faces toward the process fluid. The outboard set pumps the barrier gas toward the atmosphere. John Crane Inc. manufactures and sells such a seal arrangement for pumps under the designation T-2800. 
     In pump applications, the inboard seal set is configured such that on loss of buffer pressure the inboard seal closes and operates as a contacting seal sufficiently long to permit shut-down of the pump. Such an arrangement would not be feasible in the gas compressor environment because the resulting face contact could affect structural integrity. 
     It has been determined, however, that in gas compressor and similar applications, the process fluid can effectively be contained upon a pressure reversal if the inboard seal ring set were arranged to continue to operate as a non-contacting seal with the process fluid providing the requisite lift. In this way only a small, controlled quantity of process gas would pass to the buffer chamber, thereby, minimizing loss to atmosphere. The present invention is directed to a seal assembly arranged to provide this capability. 
     SUMMARY OF THE INVENTION 
     The present invention provides a non-contacting seal arrangement between a housing and relatively rotatable shaft to contain a process fluid in the housing which, on loss of barrier fluid pressure, the inboard seal continues to operate as a non-contacting seal. The seal arrangement includes a pair of spaced sets of relatively rotating rings defining an intermediate chamber to receive a barrier gas at a pressure exceeding process fluid pressure. Each set includes a non-rotatable ring and a rotatable ring, one of the rings being movable axially relative to the other. Each ring of each set defines a generally radial annular sealing face in relatively rotating sealing relation to the sealing face of the other ring of the set at a sealing interface. One of the rings of at least one set has a pumping mechanism thereon arranged to pump barrier gas from the intermediate chamber between the interface. That set is adapted to be disposed to pump barrier gas toward the process fluid in the housing. The pumping mechanism of the ring is further configured to pump process fluid between the interface toward the intermediate chamber when the process fluid pressure exceeds the pressure of the barrier gas. 
     More particularly, the invention may include a retainer to support the axially movable ring of the set disposed to pump barrier gas toward the process fluid. The retainer and ring define an axially elongated annular pocket. An O-ring seal is disposed in the pocket and provides a secondary seal between the retainer and the ring. It is sized such that it has a cross-sectional diameter that is smaller than both the axial and radial extent of the pocket. 
     The axially movable ring of the seal set disposed to pump barrier fluid toward the process may include a first portion defining the radially directed sealing face, a second portion supporting the ring for axial movement, and an intermediate portion configured to decouple said first and second portions to ensure a parallel relationship between the relatively rotating sealing faces under varying conditions of operating pressure and temperature. 
     The invention further contemplates the method of sealing using the seal assembly comprising providing a barrier gas in the intermediate chamber at a pressure in excess of the pressure of said process fluid, pumping barrier gas from the intermediate chamber between the interface toward the process fluid when the pressure of the barrier gas exceeds the pressure of the process fluid, and pumping process fluid between the interface toward the intermediate chamber when the process fluid pressure exceeds the pressure of the barrier gas. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional elevational view of an embodiment of a seal assembly illustrative of the present invention. 
     FIG. 2 is a fragmentary plan view of the mating ring face of the outboard seal set of the apparatus of FIG.  1 . 
     FIG. 3 is a fragmentary plan view of the mating ring face of the inboard seal set of the apparatus of FIG.  1 . 
     FIG. 4 is a sectional view of the inboard seal set of the apparatus of FIG. 1, showing a different operational mode. 
     FIG. 5 is an elevational view, in section, of a preferred form of the primary ring of the inboard seal of the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 4 illustrate a dual, non contacting seal assembly generally designated  10  illustrative of the principles of the present invention. It is operatively positioned between a housing  12  of a piece of turbomachinery equipment such as a gas compressor and its rotatable shaft  16 . Housing  12  defines an inner chamber containing process fluid pressurized by the operation of the compressor. The seal assembly  10  contains the gas from passage between the shaft  16  and housing  12  to the surrounding environment. 
     The embodiment shown is illustrative of the principles of the invention, but is not to be considered limiting. The invention could be applied to seal assemblies having, for example, rotating seal heads. It is further contemplated that the invention could be employed in a single non-contacting seal configuration or in an assembly including both a non-contacting and a contacting seal. Also, it is contemplated that the invention could be employed to seal a process fluid such as a high vapor pressure liquid such as liquid propane or the like. 
     The seal assembly  10  is a dual seal arrangement comprised of an inboard seal ring set  22  adjacent to the process gas chamber  18  and an outboard seal ring set  24  adjacent the ambient environment  20  external to housing  12 . The seal ring sets  22  and  24  define an intermediate buffer or barrier fluid chamber  26  that contains a buffer gas such as nitrogen, which is inert to the process gas. The barrier gas is normally maintained at a pressure exceeding the process gas pressure. Chamber  26  is defined by liner  28  which is fixed to housing  12  and attaches the non-rotating elements of the seal assembly to the housing  12 . Sleeve assembly  32  surrounds shaft  16  and secures the rotating components to the shaft. 
     Outboard seal ring set  24  includes a rotating seal ring  36  and a stationary seal ring  38 . Ring  38  is axially movable to accommodate axial translation of the shaft  16  and associated sleeve  32  during compressor operation. Such movement could be as much as one-sixteenth of an inch or more in either axial direction from the nominal position. 
     Seal ring  36  defines an annular generally radial seal face  40  in relatively rotatable sealing relation with an annular, generally radial sealing face  42  of ring  38 . 
     Rotating sealing ring  36 , referred to as the mating ring, is secured to sleeve  32  by collar  44 . It is axially fixed relative to the shaft  16 . An O-ring  45  provides a secondary seal between a back face of ring  36  and an adjacent radial surface formed on radial extension  43  of sleeve  32 . 
     Face  40  of mating ring  36  includes a pattern of depressions and lands forming a pumping mechanism to pump barrier fluid from chamber  26  between the faces toward the surrounding environment. A preferred pumping mechanism is a series of spiral grooves  46  best seen in FIG.  2 . These grooves and lands commence at the radially outer circumferential edge of the interface between the faces  40 - 42  and are open to the chamber  26 . They extend toward an ungrooved area or dam  48  adjacent the radially inner circumferential edge of the interface. During operation, pumping of the barrier gas between the interface  40 - 42  creates lift to separate these faces for non-contacting performance. 
     Seal ring  38 , usually referred to as the primary ring, includes an inner cylindrical surface  50 . It also has a series of drive grooves about its outer periphery and has a radial back face  52 . 
     Seal ring  38  is supported on a retainer  54 , which is fixed to housing liner  28 . Retainer  54  defines an outer cylindrical surface  56  of a diameter slightly smaller than inner cylindrical surface  50  of ring  38 . This relationship permits axial translation of ring  38 . 
     The ring  38  is held against rotation by interengagement of one or more drive lugs  57  on retainer  54  with the drive grooves formed about outer diameter of ring  38 . This relationship is such as to preclude rotation of ring  38  but permit axial movement. 
     A series of axially directed compression coil springs  58  are positioned in pockets  60  formed in retainer  54 . A spring disc  62  is disposed between springs  58  and rear face  52  of the primary ring  38 . The disc  62  receives the axial force imparted by the springs  58 , to urge it toward the rear face  52  of the primary ring  38 . 
     Disc  62  forms a pocket adjacent rear face  52  of primary ring  38 . The pocket includes an axial surface  66  and a radial surface  67  which define an O-ring receptacle. O-ring  68  is disposed in the pocket. It is compressed between radial surface  67  of disc  62  and back radial face  52  of the primary ring  38 . It provides a secondary seal to preclude passage of gas between the back of ring  38  and disc  62 . The O-ring  68  is sized to contact outer cylindrical surface  56  of retainer  54  but permit axial movement of the primary ring  38 , disc  62  and O-ring  68  to accommodate axial translation of shaft  16  relative to housing  12 . 
     Inboard seal ring set  22  includes a rotating seal ring  136  and a stationary seal ring  138 . Ring  138  is axially movable to accommodate axial translation of shaft  16  and associated sleeve  32 . 
     Seal ring  136  defines an annular, generally radial seal face  140  in relatively rotatable sealing relation with annular, generally radial sealing face  142  of ring  138 . 
     Rotating sealing ring  136  is secured to sleeve  32  by collar  144 . It is axially fixed relative to the shaft  16 . An O-ring  145  provides a secondary seal between a back face of ring  136  and an adjacent radial surface formed on radial extension  43  of sleeve  32 . 
     Face  140  of mating ring  136  includes a pattern of depressions and lands forming a pumping mechanism to pump barrier fluid from chamber  26  between the faces toward the process gas. Best seen in FIG. 3, the preferred pumping mechanism is a radially outer pattern of spaced spiral grooves  146  and associated lands  147 . The grooves are open to the chamber  26  and extend radially inward toward an ungrooved area or dam  148 . During operation, pumping of the barrier gas between the interface  140 - 142  to process chamber  18  creates the requisite lift to separate these faces for non-contacting performance. 
     Referring to FIG. 3, for clarity the radial extent and position of the interface between face  140  of mating ring  136  and face  142  of primary ring  138  is illustrated by dashed lines. The groove and land pattern  146 - 147  commences at the outer circumferential periphery of mating ring  136  and extends inwardly toward the process chamber  18 . The pattern terminates short of the inner circumferential periphery of the interface  140 - 142  to define dam  148 . 
     The face pattern of grooves described above with respect to the mating ring  136  of inboard seal ring set  22  is commonly used in gas compressor seals such as the T-28 double seal manufactured by John Crane Inc. The pattern of the grooves  46  and lands  47  on the face  40  of mating ring  36  of outboard seal ring set  24  would be essentially the same spiral groove and land pattern. However, the angle of spiral is in the opposite direction of that formed on ring  136 . 
     In accordance with the present invention, the pumping mechanism formed on radial face  140  of mating ring  136  includes a pattern of radially inner spiral grooves  149  separated by associated ungrooved lands  143 . These grooves communicate with the process fluid in chamber  18  and extend radially outwardly from the inner circumferential periphery of interface  140 - 142  toward the radial inward terminus of spiral grooves  146 . The grooves  149  terminate short of the radially inner terminus of grooves  146 . The land between these groove patterns defines a continuous ungrooved annular dam  148 . 
     The spiral grooves  149  are angled oppositely from the grooves  146 . They, therefore, are arranged to pump from the process chamber  18  toward the intermediate or barrier gas chamber  26 . The grooves  146  and lands  147  are equal in circumferential extent. The grooves  149  have a circumferential extent of one half the circumferential extent of each associated land. The spiral grooves  146  and associated lands  147  span about 55-60% of the radial extent of the interface  140 - 142 , preferably about 58%. The spiral grooves  149  and associated lands  143  span about 10 to 15%, preferably about 13%, of the radial extent of the interface. The intermediate dam spans the remainder. 
     The grooves  149  are also shallower than the grooves  146 . Grooves  146  have a depth of about 0.0005 inches. Grooves  149  have a depth of about 0.0002 inches. The grooves are at an angle of 15° to a tangent to the circumference from which they extend. The radially inner tip of each groove  149  is aligned on a radial line with the radially inner tip of every other groove  146 . 
     Seal ring  138  is axially elongated as compared to seal ring  38  of outboard seal set  24 . It includes a first inner cylindrical surface  150  and a second inner cylindrical surface  151  which is of a diameter smaller than first inner cylindrical surface  150 . A radial sealing surface  153  extends from second inner cylindrical surface  151  and joins first inner cylindrical surface  150  with a radius or fillet. Ring  138  also has a series of drive grooves about its outer periphery and a radial back face  152 . 
     Primary ring  138  is a unitary component made from a single blank of material. As best understood with reference to FIG. 5, ring  138  is configured to decouple an outboard end portion  138   a , which defines radial face  142  from an inboard end portion  138   c  which supports the ring in the assembly  10 . An intermediate portion  138   b  connects the end portions  138   a  and  138   c . This configuration permits the radial faces  140  and  142  to remain essentially parallel over the range of pressures and temperatures experienced during operation. 
     Seal ring  138  is supported on a retainer  154  fixed to housing  12  by liner  28 . Retainer  154  defines a first outer cylindrical surface  155  supporting surface  150 . It is of a diameter slightly smaller than the first inner cylindrical surface  150 . Retainer  154  includes a second outer cylindrical surface  156  supporting surface  151  of ring  138 . It is of a diameter smaller than the second inner cylindrical surface  151  of ring  138 . This relationship permits axial translation of ring  138  relative to retainer  154 . 
     A radial sealing surface  159  extends radially inwardly from first outer cylindrical surface  155  and joins second outer cylindrical surface  156  at axially extending conical ramp  161 . Ramp  161  extends radially outwardly at a 20° angle to the horizontal from its intersection with second outer cylindrical surface  156  to its joinder with radial sealing surface  159 . 
     The ring  138  is held against rotation by interengagement of one or more drive lugs  157  on retainer  154  with drive grooves formed about outer diameter of ring  138 . This relationship is such as to preclude rotation of ring  138  but permit axial movement. 
     A series of axially directed compression coil springs  158  are positioned in pockets  160  formed in retainer  154 . A spring disc  162  is disposed between springs  158  and rear face  152  of the primary ring  138 . The disc  162  receives the axial force imparted by the springs  158  and transfers it to a rear face  152  of primary ring  138 . 
     A secondary seal in the form of an O-ring  168  prevents passage of gas between retainer  154  and primary ring  138 . The first inner cylindrical surface  150  of primary ring  138 , and radial sealing surface  153  of primary ring  138 , second outer cylindrical surface  156  of retainer  154 , conical ramp  161  and radial sealing surface  159  of retainer  154  define an axially elongate annular O-ring pocket surrounding secondary seal O-ring  168 . The pocket has an axial extent between radial sealing surface  153  of ring  138  and radial sealing surface  159  of retainer  154  that exceeds the cross-sectional diameter of the O-ring. The ring  168  is, therefore, free to move axially within the pocket as the shaft  16  translates axially relative to housing  12 . First inner cylindrical surface  150  of ring  138  overlies second outer cylindrical surface  156  of retainer  154 . These surfaces define the radial extent of the annular pocket. 
     O-ring  168  is sized to define an inner peripheral surface that slightly contacts second outer cylindrical surface  156  of retainer  154 . As illustrated in FIG. 1, at ambient design temperature of 70° F. (Fahrenheit), it has cross-sectional diameter such that the outer peripheral surface is slightly spaced from first inner cylindrical surface  150  of primary ring  138 . This relationship of the cross-sectional diameter of the O-ring  168  to the radial extent of the O-ring seal pocket results from the need to accommodate axial translation of the primary ring  138  under all conditions of elevated operating temperature. 
     Complication arises from the different rates of thermal expansion of the materials used in the various seal components. Typically, the mating rings  36  and  136  are silicon carbide or tungsten carbide. The primary rings  38  and  138  are carbon. The secondary seal O-rings  68  and  168  and other O-ring seals are a polymeric material such as Kalrez, a fluoroelastomer manufactured by E.I. duPont &amp; Company. Other fluoroelastomers could be used, depending on compressor operating temperatures. The remaining metal parts, such as retainers  54  and  154 , are stainless steel, such as 410 stainless or Hastelloy C. 
     Operating temperatures range from ambient, which, for design purposes, is 70° F. or higher. At operating temperatures, about 325° F., the radial extent of the O-ring pocket is smaller than it is at ambient or other temperatures below operation. A cross-section of O-ring  168 , sized to fit the largest radial extent, would experience excessive radial load at operating temperature. Hence, it is necessary to size the O-ring  168  to accommodate all conditions of operation. 
     In this instance the O-ring  168  is configured for ambient temperature of 70° F. to define an internal circumference sufficient to expand slightly onto the second outer circumferential surface  156  of retainer  154 . To avoid excessive radial compression within the pocket at operating temperature, 325° F., the diameter of the cross-section of the O-ring  168  is smaller at ambient temperature of 70° F. than the radial distance between first inner cylindrical surface  150  of primary ring  138  and second outer cylindrical surface  156  of retainer  154 . As a result, on pressure reversal at ambient temperature, an effective secondary seal between the primary ring  138  and retainer  154  could not be assured. Conical ramp  161  on second outer cylindrical surface  156  of retainer  154  solves this problem. 
     In operation, barrier gas in chamber  26  is maintained at a pressure that exceeds the process pressure generated by the compressor operation. Shaft  16  and sleeve  32  rotate at operating speed rotating mating rings  36  and  136 . The pumping mechanisms on the faces  40  and  140 , in particular the spiral grooves exposed at the radial outer periphery of the interface of faces  40 - 42  and  140 - 142 , pump barrier gas between the seal faces causing lift and resulting in non-contacting operation. 
     In the event of a loss of barrier gas pressure, the process pressure in chamber  18  exceeds the pressure in the barrier chamber  26 . Because the radially inner spiral grooves  149  are exposed to the process gas and the inner periphery of the seal ring interface  140 - 142 , process gas is pumped between the faces to provide lift and permit continued non-contacting operation of the inner seal ring set  22 . 
     Secondary O-ring seals  45  and  68  in outboard seal ring set  24  separate the barrier gas chamber  26  from the surrounding environment  20 . Secondary seals  145  and  168  in inboard seal ring set  22  separate the barrier gas chamber  26  from the process gas chamber  18 . 
     In normal operation conditions, the barrier gas is at a pressure that exceeds the process gas pressure. The O-ring  168  is, therefore, urged toward radial sealing surface  153  in the O-ring seal pocket and seats against the radial sealing surface  153  of primary ring  138  and the second outer cylindrical surface  156  of retainer  154 . 
     FIG. 4 is an enlarged sectional view of the seal sets. As illustrated in FIG. 4, a pressure reversal causes the O-ring  168  to be urged axially toward radial sealing surface  159  of retainer  154 . To effect a sealing relationship, it is necessary that the O-ring  168  engage both the radial sealing surface  159  of retainer  154  and first inner cylindrical surface  150  of primary ring  138 . At certain operating conditions, for example, ambient temperature of 70° F., the size of the outer circumference of O-ring  168  and the radial distance between second outer cylindrical surface  156  of retainer  154  and first inner cylindrical surface  150  of primary ring  138  are such that sealing engagement with first inner cylindrical surface  150  would not occur. Inclined conical ramp  161 , however, causes the inner circumference of O-ring  168  to expand radially as the ring moves toward radial sealing surface  159  of retainer  154 . 
     Process pressure, acting on O-ring  168 , causes it to travel axially from second outer cylindrical surface  156  of retainer  154  to a position overlapping inclined conical ramp  161  where it is also pressed against radial sealing surface  159 . The conical ramp causes the inner circumference of O-ring  168  to expand sufficiently to ensure sealing engagement of the outer circumferential periphery of the O-ring  168  with second inner cylindrical surface  150  of primary ring  138 . The O-ring  168  also contacts the radial surface  159  of retainer  154  in sealing relation. Thus, even at ambient design temperature of 70° F., an effective secondary seal is accomplished which continues to separate the process chamber  18  from the barrier gas chamber  26 . 
     Seal balance ratio is the ratio of the amount of force from fluid pressure acting on the back of the axially movable seal ring tending to close the faces divided by the forces between the faces tending to open them. It is measured by the ratio of areas exposed to such pressure causing such closing and opening forces. 
     It should be noted that in the seal of FIG. 1 the inboard seal set is configured to change the balance on pressure reversal, thereby maintaining a sufficient balance to ensure that the faces  140 - 142  remain in an operational relationship. 
     In this regard, under normal operation, the barrier gas pressure exceeds the process gas pressure. O-ring  168  is seated against second outer cylindrical surface  156  which determines the area of back face  152  of primary ring  138  exposed to pressure in the barrier chamber  26 . The circumference of second outer cylindrical surface  156  is the balance diameter. 
     On a pressure reversal, O-ring  168  is urged against radial sealing surface  159  and first inner cylindrical surface  150  of primary ring  138 . The balance diameter shifts to the circumference of first inner cylindrical surface  150  with those radial surfaces of primary ring  138  radially inward of the circumference representing the area subjected to the higher pressure of the process fluid. With such a shift in balance diameter, balance may be maintained at levels in excess of 0.5 regardless of the location of higher pressure. 
     Balance under normal conditions of a barrier gas pressure in excess of process pressure can be about 0.85. On reversal conditions, with the process pressure higher than the pressure in the barrier gas chamber  28 , a balance can be about 0.65. It should be noted that balance in either direction can be increased by decreasing the diameter of the outer circumferential periphery of the interface  140 - 142  of rings  136 - 138 . 
     The reverse pumping grooves  149  produce lift that counteracts the closing force and avoids damage to primary ring  138  and mating ring  136  due to hard contact on a pressure reversal. The grooves are sized to produce lift such that, on pressure reversal, the faces  140  and  142  operate with no contact or slight contact. Hard contact due to pressure reversal is avoided. 
     As previously explained, the primary ring  138  of inboard seal set  22  is axially elongated with that portion  138   a  defining the radial sealing face  142  decoupled from that portion  138   c  supported on retainer  154 . The intermediate portion  138   b  defines a flexible transition. 
     FIG. 5 is an enlarged sectional view and shows a cross-section of primary ring  438  illustrative of a preferred configuration for primary ring  138 . It is a unitary ring made of a single bland of carbon material. 
     Ring  438  includes portion  438   a  that defines a radial sealing face  442  for relatively rotating sealing relation with a sealing face  140  of a mating ring  136 . Axial extent of portion  438   a  between face  442  and a back wall  467  is about 17% of the total axial extent of the ring. It includes inner cylindrical surface  454  of a diameter smaller than second outer cylindrical surface  156  of retainer  154 . It also includes an outer cylindrical surface  455 . 
     Portion  438   c  defines first inner cylindrical surface  450  adapted to be supported on first outer cylindrical surface  155  of retainer  154 . It includes outer cylindrical surface  457  having drive notches for engagement with lugs  157  of retainer  154 . Axially, portion  438   c  extends about the same distance as the distance between back face  452  and radial sealing surface  453 . It represents about 24% of the axial extent of the ring  438 . 
     Radial sealing surface  453  extends radially outward from a second inner cylindrical surface  451  and connects to first inner cylindrical surface  450  by a fillet or radius. Second inner cylindrical surface  451  is adapted to be supported for axial translational movement on second outer cylindrical surface  156  of retainer  154  shown in FIG.  1 . First inner cylindrical surface  450  is adapted to define an O-ring pocket with radial sealing surface  453 , second outer cylindrical surface  156  of retainer  154  and a radial sealing surface  159  on the retainer such as illustrated in FIG.  1 . 
     The axial outer portions  438   a  and  438   c  are connected by intermediate portion  438   b  which provides the flexibility necessary to structurally decouple the end portions. Intermediate portion  438   b  is about 40% of the axial extent of ring  438 . It is comprised of two portions  438 (1) adjacent portion  438   a.    
     Portion  438   b (2) comprises about 20% of the axial length of ring  438 . It is defined by the inner cylindrical surface  451  which is adapted to be supported on second outer cylindrical surface  156  of retainer  154  and the radial sealing, surface  453 . The diameter of second inner cylindrical surface  451  is larger than the diameter of inner cylindrical surface  454  of portion  438   a . Portion  438   b (2) includes an outer cylindrical surface  459  which is of a diameter smaller than the diameter of outer cylindrical surface  455  of portion  438   a.    
     Portion  438   b (1) is the most flexible portion of ring  438 . It comprises about 40% of the axial extent of the ring  438 . It includes an inner cylindrical surface  461  having a diameter equal to the diameter of inner cylindrical surface  454  of portion  438   a . The outer surface of portion  438   b (1) is of a compound shape. It includes a conical surface  463  that extends from portion  438   b (2) commencing at a diameter about equal to that of first inner cylindrical surface  450  at an angle radially inwardly of 11° to the horizontal to a semi-circular groove  465  formed adjacent commencement of portion  438   a . The radius of the groove is about 4% of the axial extent of ring  454 . 
     The radial extent of the various portions of ring  438  in reference to the radial extent of portion  438   a  are as follows. The radial extent of portion  438   c  relative to the radial extent of portion  438   a  is 70%. The radial extent of portion  438   b (2) relative to portion  438   a  is 66%. The radial extent of the portion  438   b (1) at the groove  465  relative to the radial extent of portion  438   a  is 28%. 
     The ring  438  described above provides the strength necessary to operate at the pressures and temperatures experienced in the compressor environment and the flexibility to ensure that the surfaces of the relatively rotating faces of the inboard seal set remain parallel over the operating range. The portions  438   a  and  438   c  have relatively large mass to withstand these operational conditions. Portion  438   b  provides the requisite flexibility to the structure. 
     Seals have been manufactured incorporating the present invention. Two sizes have been made; for a 7.625 inch shaft and a 5.250 inch shaft. 
     For the 7.625 inch shaft size, the primary ring portion  438   a  defining the relatively rotatable sealing face  442  had an outer diameter of 8.572 inches. The seal ring interface  140 - 142  had an axial extent of 0.414 inches commencing at an outer circumferential diameter of 4.340 inches. Outer cylindrical surface  457  had a diameter of 8.852 inches. The surface  454  was 7.446 inches in diameter. The overall axial length of ring  454  was 1.250 inches. First inner cylindrical surface  450  had a diameter of 8.052 inches. Second inner cylindrical surface  451  had a diameter of 7.651 inches. 
     These seals were designed to experience a maximum process pressure of 200 psig (pounds per square inch gauge). The barrier gas pressure employed was 250 psig. 
     The O-ring  168  for the 7.625 inch seal had a cross-section of 0.205 to 0.215 inches. It had an inside circumferential diameter of 7.430 to 7.520 inches. 
     First outer cylindrical surface  155  had a diameter of 7.993 inches. The second outer cylindrical surface  156  had a diameter of 7.627 inches and a length of 0.513 inches including a 0.093 inch by 30° chamfer. The conical ramp  161  was 0.100 inches in axial extent. 
     Various features of the invention have been described in connection with the illustrated embodiment of the invention. Various modifications may be made without departing from the scope of the invention.