Radial flow seals for rotating shafts which deliberately induce turbulent flow along the seal gap

Abstract: A face seal is provided for a rotating shaft for sealing between a normally high pressure region and a normally lower pressure region, having a seal ring shaped to form a gap between the ring and a runner surface on the shaft, which gap converges in the direction of fluid flow and deliberately creates turbulent flow along the seal gap and sufficient clearance between the rotating runner and the seal ring to accommodate distortions in the seal ring which may occur over its lifetime. A servo system is coupled to the seal ring which moves the seal ring away from the runner during low pressure differences between the regions and which restores the sealing function along the seal gap when the pressure difference between the regions increases sufficiently.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to seals for rotating shafts to restrict fluid flow between rotating and stationary members; more particularly, it is related to a face seal which has improved sealing characteristics and which deliberately induces turbulent flow along the seal gap. 2. Description of the Known Art The employment of seals along the rotating shaft for large machines such as gas turbines or steam turbines provides complex sealing problems. Such machines have traditionally utilized labyrinth type seals at critical sealing locations such as at the compressor discharge of a gas turbine or at the ends of a steam turbine shaft. Labyrinth type seals normally result in relatively high leakage, inasmuch as a labyrinth seal is usually designed such that for axial flow the radial clearance is 0.00254cm (1 mil, 0.001 inches) of clearance per 2.54cm (inch) of shaft diameter. Thus, for a 50.8cm (20 inch) diameter shaft, 0.0508cm (20 mils) of clearance would normally be provided in a labyrinth seal. In gas turbines, 50.8 cm (20 inch) diameter shafts are not uncommon, whereas in certain steam turbines, 152.40cm to 203.20cm (60 to 80 inch) diameter shafts require sealing and a .015240cm To .02032cm (60 to 80 mil) clearance may well be provided. On the other hand, seals such as hydrostatic face seal assemblies are normally intentionally designed so that laminar flow occurs in the seal gap. Laminar flow is achieved by providing a converging flow path on the seal from a high pressure region in the machine toward a lower pressure region, with a face clearance on the order of 0.00127cm .0005 inches (.5 mil inches) and smaller. Seals with such minimal face clearances are difficult if not impossible to use in such applications, because such seals are essentially intolerant to distortions in geometry which can occur over a seal lifetime and during transient conditions where, for example, pressure and temperature differences may vary significantly. As a result, hydrostatic face seals have not been employed successfully for such applications. Seal designs such as the seal described in US-A-5284347, which had been proposed for such applications in such seal designs, include rather complex gas bearing faces in flow paths as well as complex centering means to attempt to minimize seal distortion. While not expressly discussed, such seals are not designed to deliberately provide turbulent flow in the seal gap. Further reference is made to prior document US-A-5284347 which discloses a seal having a servo mechanism 9, 10 to open the seal gap in the event of low pressure across the gap, but does not teach deliberately induced turbulent flow in the seal gap. Reference is also made to prior document EP-A-0305941 which discloses a reactor coolant pump seal operating between a high pressure chamber 36 and a lower pressure region (adjacent shaft 34) and shows in Fig. 17 a graph of pressure difference and film thickness which creates only laminar flow in the seal gap either with or without a balancing chamber; thus there is no teaching of deliberately inducing turbulent flow in the seal gap between surfaces 64 and 66. Reference is also made to prior document US-A-4471966 which discloses a seal arrangement between a high pressure chamber and a lower pressure chamber 2 along a converging flow path. However it requires the use of an even higher pressure source to pass fluid into the seal gap by conduits 9, so that flow in the seal gap from opening 8 toward chamber 1 is 70% of the total flow from conduit 9 and that toward chamber 2 is 30% of that flow; hence there is no teaching or inference of a structure which deliberately induces turbulent flow in the seal gap between opening 8 and chamber 2. SUMMARY OF THE INVENTION It is the intention of this invention to permit the use of hydrostatic face seal assemblies in the aforementioned applications by providing a seal assembly which creates a sufficiently large gap or leakage path along the seal face to permit the seal to operate without damage due to ambient induced distortion of the seal assembly components. This is achieved by designing the seal so that turbulent flow rather than laminar flow is deliberately induced along the seal gap. Turbulent flow is provided for by selecting a predetermined seal geometry, particularly along the seal face, which creates a seal gap height of a minimum of .00508cm (2 mils) of clearance between the rotating seal runner and the closest part of the seal ring. Turbulent flow is induced in the seal gap by the provision of a convergent flow path in the form of a step, having a height of 0.00254cm or more, in the axial direction. In addition, for start-up and shut-down segments of the operating cycle of the seal, the seal assembly is provided with a servo mechanism which moves the critically machined seal and runner faces substantially away from one another during the low pressure segments of the operating cycle of the machine, as well as during expected and unexpected transients. Reference is made to prior art document US-A-3685839 which discloses a hydrostatic step face seal having a different servomechanism for opening the seal, but has no teaching of a step size that would deliberately induce turbulent flow in the seal gap during seal operation. The present invention is useable not only with compressible fluids such as air, but also with incompressible fluids such as water or oil. It will be understood, however, that turbulent flow in the converging flow path from the high pressure edge to the lower pressure edge along the seal gap is deliberately induced for such medium, and turbulent flow is achieved when the Reynolds Number for the system is in excess of 1800. It will be remembered that the Reynolds Number varies inversely with the viscosity of the medium in which the seal is functioning; for a low viscosity fluid such as air, the step in the seal ring face, is to be in excess of 0.00254cm (1 mil (.001 inches)). For supersaturated steam in a steam turbine application, which acts in many respects the same as air, the viscosity of the medium is somewhat higher; therefore the step height is to be somewhat greater than 0.00254cm (1 mil). For liquid media, the convergence of the seal gap is also in excess of 0.00254cm (1 mil), because the viscosity of the liquid is significantly greater. Thus, to ensure that turbulent flow through the seal gap is being achieved, the geometry of this system, the temperature of the fluid, the viscosity of the fluid, the flow rate along the seal gap and the pressure drop (although to a smaller extent) are taken into consideration as each influences the Reynolds Number for the system. Thus, to employ this invention, the seal system is designed with a step such that the Reynolds Number in the seal gap is in excess of 1800 so that turbulent flow along the seal gap is created. For a given gaseous system, in order to ensure a seal gap of sufficient axial height, the geometry should be such that the axial step of the seal ring face in the seal gap in the downstream direction is at least 0.00508cm (2 mils). BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference may be had to the accompanying drawings in which: Figure 1A is a sectional view of a housing having a rotating shaft therein and having a seal assembly constructed in accordance with the principles of this invention, sealing between a low pressure region and a high pressure region of the housing, with the seal assembly being illustrated in its normal operating position. Figure 1B is a view similar to Figure 1A wherein the seal assembly is depicted in its open or retracted position. Figure 2 is a sectional view of the seal assembly of Figures 1A and 1B taken along the lines II-II of Figure 1A. DETAILED DESCRIPTION Referring now to the embodiment of this invention illustrated in Figures 1A, 1B and 2, there is illustrated a section of a housing 10, for example, the portion of the housing 10 of a gas turbine, downstream of the compressor unit exit, having a high pressure chamber 12 therein and a lower pressure chamber 14. An elongated shaft 16 having a horizontal axis is adapted to rotate at relatively high speed within the housing 10. The shaft 16 has formed either integrally therewith, as illustrated herein (or as attached thereto by means, not shown, which are well known in the art), a radially extending annular seal runner 18 which extends laterally from the axis of the shaft 16. Seal runner 18 has a radially extending, precisely machined, sealing surface 20 of annular configuration formed thereon. The low pressure region 14 within housing 10 is illustrated to be adjacent to rotating shaft 16, while the high pressure region 12 is located more outwardly of the outer edge of seal runner 18. Interposed between high pressure region 12 and low pressure region 14 of housing 10 is a seal ring assembly 22 receiving a portion of shaft 16 thereon and positioned to move axially along shaft 16 toward and away from the radial sealing surface 20 of seal runner 18. As will be described in detail hereinafter, fluid flow from the high pressure chamber 12 to the low pressure region 14 within housing 10 is prevented by the seal ring assembly 22 cooperating with other components within housing 10, except along primary seal gap 21 between seal ring assembly 22 and runner sealing surface 20. The seal ring assembly 22 includes an annular seal ring 24 of generally L-shaped cross section which moves toward and away from surface 20 of seal runner 18 to vary the size of seal gap 21. Housing portion 10 includes a thickened housing segment 30 disposed adjacent the outer periphery of housing portion 10 and includes therewithin a stationary frame assembly 28 extending toward shaft 16 which serves to mount the seal ring assembly 22 within housing 10. Frame assembly 28 is of generally annular configuration and includes an annular foot 32 which is positioned to be received in thickened portion 30 and is sealingly secured thereto by suitable means, e.g., an annular retaining ring such as snap ring 34 which is mounted in a complementary recess 35 in thickened portion 30. The frame 28 includes a central hub assembly 36 of generally annular configuration located closer to shaft 16 than foot 32. The central hub 36 has a seal mounting extension 38 of generally L-shaped cross section on which the seal ring assembly 22 is mounted and which serves to accommodate a secondary seal ring 40, to be described. Central hub 36 is disposed at generally the same radial location relative to shaft 16 as the runner 18 and is positioned to the right thereof in the axial direction as illustrated in Figures 1A and 1B so that the hub 36 and seal mounting extension 38 are positioned generally opposite the radial runner sealing surface 20. The seal ring 24 moves axially toward and away from the runner 18 between the hub 36 and runner 18. The seal ring 24 is desirably a circumferentially continuous structure and has a radially extending ring-shaped sealing surface 26 thereon which is precisely machined and lapped and confronts the sealing surface 20 on seal runner 18 creating the primary seal gap 21 therebetween. In this embodiment of the invention, the ring sealing surface 26 is of a flat configuration in the radial direction, and the convergence of seal gap 21 is achieved by a step 42 extending toward runner surface 20 to form a sealing dam 44 on ring 24 adjacent shaft 16. The size of the step 42 is critical to this invention, as it must create a geometry which produces turbulent flow in the seal gap, thereby producing sufficient clearance between the seal ring and seal runner to make a face seal assembly practical for applications and environments described herein. Extending from the outer periphery of the seal ring 24 is an arcuate support arm 46 of continuous, annular configuration which extends to overlie a portion of the outermost surface 50 of the seal runner 18 and a ring-shaped, continuous, auxiliary restrictive tooth 48 of predetermined size extends from the support arm 46 toward the axial outer surface 50 on the seal runner 18 with the spacing B between the tooth 48 and surface 50 also being important to the operation of the seal assembly 22. Extending in the axial direction away from seal runner 18 and from the inner end of the base of the L-shaped seal ring 24 is an annular extension 52, having a sealing surface 54 formed on the outer axial surface thereof to cooperate with secondary seal ring 40 to provide a bore seal along surface 54. Annular extension 54 terminates in a plurality of alignment arms 56 which extend axially outwardly from the right-hand edge thereof (Figures 1A and 1B) and coact with openings or slots 58 formed in a downwardly extending arm 60 in the hub 36 to align the seal ring assembly 24 relative to the hub 36 and the seal runner 18. It will be appreciated that the alignment arms 56 are closely received within openings 58 in the hub arm 60 and further that a plurality of alignment arms are provided circumferentially spaced along the periphery of the seal ring extension 52 so as to support and center adequately the seal ring assembly 24 relative to the shaft 16 and runner 18. A servo mechanism assembly identified generally by the reference character 62 is interposed between seal ring 24 and the seal mounting extension 38 as will be described. More particularly, a plurality of shafts such as the shaft 64 extend axially from the rear surface 66 of the seal ring 24 toward the seal mounting extension 38 and through aligned openings such as opening 68 therein. Shaft 64 extends entirely through mounting extension 38 and terminates in the annular space between mounting extension 38 and hub 36. A cup-shaped member 70 having an outwardly extending flange 72 on the end thereof remote from opening 68 is secured to the right-hand end of the shaft 64. Interposed between the flange 72 and the mounting extension 38 for each shaft 64 is a coiled spring 74 which urges the seal ring 24 away from runner 18 and toward the central hub 36. In order to ensure that fluid flow from higher pressure chamber 12 to lower pressure chamber 14 passes only along the seal gap 21, the secondary seal means such as assembly 40 is interposed to provide not only a bore seal between the outward circumferential surface 54 of the seal ring extension 52, but also a face seal between the surface 76 on the central hub 32 and the opposed radial surface on secondary seal 40. It will be seen that the surface 76 is disposed in opposition to (i.e., facing) sealing surface 20 on runner 18. The secondary or bore seal on surface 54 is in alignment with the step 42 so that surface 54 forms the balance diameter of seal ring assembly 22. Secondary seal assembly 40 may comprise one of many known seal arrangements such as a piston ring or a segmented annular seal ring. Alternatively, an annular bellows seal may be interposed directly between seal ring 24 and hub surface 76 to achieve the sealing function. Illustrated in Figures 1A, 1B and 2 as nearly one example known in the art, is a segmented annular seal ring with only one of the segments 77 being shown. For a more detailed understanding of the secondary seal ring 40 illustrated in Figures 1 and 2, reference may be had to Stein United States Letters patent 4,082,296, issued on April 4, 1978, and assigned to Stein Seal Company. Of course, the seal of US patent 4,082,296 would not employ its negative hydrodynamic lift feature since it is being used with non-rotating parts, but is otherwise usable as an example of a secondary seal. The secondary seal assembly 40 is designed to complement the application in which the primary seal assembly 22 is being employed; thus, for a gas face seal, a different secondary seal may be employed from an application wherein the seal assembly 22 and secondary seal 40 are in a liquid medium. As pointed out, the secondary seal assembly does not form a part of this invention other than to ensure that a secondary sealing function occurs. Thus, as an example, employing the seal ring of Figures 1 through 4 of Stein US patent 4,082,296 for the secondary seal assembly 40, it will be seen that a garter spring 78 urges the seal ring segments 77, only one of which is illustrated, into engagement with the sealing surface 54 on seal ring extension 52 to provide the bore seal. At the same time, a plurality of symmetrically-spaced axial coiled springs, one of which is coil spring 80, are interposed between the seal ring segments 77 and the opposing face 82 on seal support 38 to move the segments 77 toward face 76 to form a face seal. Coil springs 80 are received in opposed recesses 83 and 85 formed respectively in mounting extension 38 and segments 77 to position springs 80. Axially-extending spaced recesses 79 are formed in the seal ring segment 77, creating wear pads for the bore seal, and radial recesses 81 are formed in the radial outer sealing surface which join a circumferential groove 83 of segment 77 to prevent binding of this segment 77 to face 76 of hub 36. NORMAL OPERATION It will be seen that the high pressure from chamber 12 exerts a closing force on the back face 66 of the seal ring 24 when a high pressure difference exists between region 12 and region 14, urging the seal ring 24 toward the sealing surface 20 on seal runner 18. This movement is resisted by the force of the several coil springs 74 which are urging the seal ring 24 away from runner 18. The opposing force of springs 74 is overcome by the high pressure, and seal ring 24 moves toward runner surfaces to close seal gap 21. Seal ring extension 52 concurrently moves axially with respect to shaft 16, and surface 54 slides relative to the secondary seal assembly 40 to maintain the secondary bore seal and face seal. The size of the gap B between restrictor tooth 48 and end surface 50 of runner 18 is, during normal operation of the seal, significantly larger than the seal gap 21 between sealing dam 44 and circumferential surface 20 on runner 18. The dimension C of the seal gap 21 is determined by the axial position of the nose of seal dam 44 relative to the runner surface 20. Thus, during normal operation of the seal, high pressure from chamber 12 is exerted on the back surface 66 of the seal ring 24 urging the seal ring toward the runner 18, and the high pressure is prevented from moving from chamber 12 to lower pressure region 14 by secondary seal 40 except through the space B between restricted tooth 48 and surface 50 and the opposed seal gap 21. The convergence of the seal ring sealing surface 20 is formed in the example of Figures 1 and 2 by step 42 to create a converging flow path in the direction of fluid flow from the outer edge of surface 20 to the inner edge thereof. The degree of convergence, i.e. the height of step 42, is deliberately in excess of 0.00254cm (0.001 inches (1 mil)), thereby creating a Reynolds Number for the system in excess of 1800 so that turbulent flow is induced along the seal gap 21. In this way, the clearance C between the seal dam 44 and the runner surface 20 is thus created to be, in this example, on the order of 2 milli-inches (0.00508cm) in height so as to provide a greater clearance than that which would be achieved by designing a seal system having only laminar flow through the seal gap 21. In this example, the degree of convergence of the surface 26 on the seal (i.e. the height of step 42) is sized to be about the same as the spacing during normal operation between the seal ring dam 44 and seal runner surface 20. With the seal being designed to create a deliberate turbulent flow region along the seal gap, the seal will operate with sufficient clearances to minimize seal wear in the event of thermal distortion, vibration and the like, to which the seal ring assembly 22 is subjected during operation and during transients. This permits the provision of a sealing assembly of high reliability and long life with relatively low leakage when compared to other sealing systems employed for these applications. Of course, seals constructed in accordance with this invention will have greater leakage rates than seals which have a geometry permitting only laminar flow, but the latter seals will be unreliable and/or have shorter operating lifetimes for many important sealing applications. TRANSIENT OPERATION During transient operations such as start-up of a gas or steam turbine, before high pressure is created in the chamber 12, it will be seen that there is an absence of high pressure which would urge surface 66 of seal ring 24 toward runner 18. Thus, the seal ring assembly 22 is moved to the right of the seal runner 18 and axially parallel to the axis of shaft 16 to the position illustrated in Figure 1B, due to the influences of the several coil springs 74 which react between flange 72 and the mounting extension 38. While the seal assembly is in the position illustrated in Figure 1B, as pressure in chamber 12 increases, the flow from chamber 12 to chamber 14 is limited by the clearance B between the aspirator tooth 48 and surface 50 on runner 18, whereby a pressure drop is created between chambers 12 and 14 but of significantly smaller magnitude than that during normal operation of the seal assembly 22. As pressure in chamber 12 increases, the aspirator tooth 48 causes pressure in the seal gap 21 to be lower than the pressure being exerted by the fluid in chamber 12 on the back surface 66 of seal ring 24, and the higher pressure in surface 66 urges the seal ring 24 toward the runner 18 in opposition to the spring forces created by springs 74. Pressure continuing to build up moves seal ring 24 closer to surface 20 until the seal gap C between the seal ring dam 44 and runner 18 becomes smaller than the restrictor tooth gap B between tooth 48 and runner surface 50. Seal ring assembly 22 slides toward the runner 18 against the forces of springs 74 and against frictional forces created by the bore seal of secondary seal ring 40 on surface 54, and turbulent flow in the seal gap 21 begins. The seal assembly 22 remains centered by the alignment arm 56 being closely received within opening 58 in leg 60 of the central hub 36, and normal operation of the seal is achieved where pressure increases in chamber 12 are accommodated by the seal gap C to essentially maintain the pressure difference between chambers 12 and 14. In the event of a sudden large drop in pressure in chamber 12, the forces on surface 66 are overcome by the spring forces 74 moving the seal ring assembly 22 away from runner 18 and increasing the seal gap C to a point where it becomes larger than the restrictor tooth gap B, and the seal assembly moves toward the position illustrated in Figure 1B. In the embodiment of Figure 4, it will be seen that seal runner 18" and seal ring 24" are provided, together with continuous, annular support arm 46", which supports annular, continuous restrictor tooth 48" disposed outwardly of runner outer surface 50" in runner 18 to form gap B". In this embodiment, however, the converging flow path between seal ring 24" and seal runner 18" is created by a combination of a step 42" and a tapered section 43". The alternate converging flow path seal ring may be employed in order to fit the seal ring constructed in accordance with this invention to given system parameters so that the turbulent flow along the seal gap is ensured.

Claims: A hydrostatic face seal assembly used to restrict fluid flow from a relatively higher pressure region (12) in a housing (10) to a relatively lower pressure region (14) in the housing (10) at a juncture of a stationary housing member (36) and a rotating shaft member (16) rotating about its axis, said shaft having a radially extending annular seal runner (18) having a sealing surface (20) thereon, said runner (18) mounted for rotation therewith, said assembly including a face seal ring (22) mounted on said shaft and having a primary annular sealing face (26), positioned for movement toward and away from said seal runner surface (20) to break down the pressure radially across the ring, the outer edge (adjacent 46) of said sealing face extremity being exposed to the higher pressure region (12) and the inner edge (adjacent 44) of said sealing face (26) being exposed to the lower pressure region (14) said sealing face (26) being formed to converge (adjacent 42) toward said runner surface (20) in the direction from the outer edge (adjacent 46) thereof to the inner edge (adjacent 44) thereof to provide a converging flow path (21) in the radial direction from said outer edge (adjacent 46) toward said inner edge (adjacent 44), characterized in that the geometry of the flow path (21) created by said sealing face (26) and said runner (18) is shaped to converge in the direction of fluid flow by providing a step (42) at least 0.00254cm in height facing said outer edge of said flow path (21) to deliberately induce turbulent flow along said flow path (21), by creating a larger clearance (C) between said sealing face (26) and said seal runner surface (20) than would exist for laminar flow along said flow path (21), thereby creating a Reynolds Number of more than 1800 in said flow path (21) during normal seal operation. The hydrostatic face seal assembly of Claim 1, having a servomechanism (46, 48, 66, 74) including resilient means (74) extending between the housing (38) and the seal ring (22) which are biased to move the seal ring away from the seal runner (18) to open the seal gap (21) when insufficient pressure difference exists between the higher pressure region (12) and the lower pressure region (14), said seal ring (22) having a surface (66) always exposed to the pressure in the higher pressure region (12) and positioned to apply force on said seal ring (22) to move the seal ring (22) toward the seal runner (18) against force exerted by said resilient means (74) when sufficient pressure difference exists between the higher pressure region (12) and the lower pressure region (14) to reduce the size of the seal gap (21, C). The fluid seal assembly of Claim 1, wherein said seal ring (22) has an elongated annular extension (52) thereon concentric with said shaft (16) which extends outwardly from said seal ring (22) and receiving said shaft (16) in the direction away from said seal runner (18) therein, and additional seal means (40) coacting between said extension (52) and said housing (36). The fluid seal assembly of Claim 3, wherein said housing (36) has a plurality of circumferentially spaced openings (58) therein symmetrically disposed about said shaft (16), and said seal ring extension (52) having a plurality of alignment means (56) extending from the outer end of said extension (52) and slidably received in said openings (58), respectively, to maintain said seal ring (22) in axial alignment with said shaft (16) and said runner sealing surface (20). The fluid seal assembly of Claim 2 wherein said servomechanism (46, 48, 66, 74) comprises an annular restrictor tooth (48) located closely adjacent said runner (18) and positioned upstream of said outer edge of said seal ring sealing face (26), said restrictor tooth (48) being spaced from said runner at predetermined distance (B) which distance (B) is greater than the widest height of the seal gap (C) during normal pressurized operation of said seal ring (22), said restrictor tooth (48) being mounted such that its spacing (B) from the runner (18) does not vary even when pressure in said pressure chamber is eliminated, and means (46) connecting said restrictor tooth (48) and said seal ring (22) to prevent flow into said seal gap (21) except through the said spacing (B) between said restrictor tooth (48) and said runner (18). The fluid seal assembly of Claim 5 having resilient means (74) coacting between said seal ring (22) and said housing (at 38) and urging said seal ring sealing face (26) away from said runner surface (20), whereby said resilient means (74) and said restrictor tooth (48) cooperate to open said seal gap (C) when pressure in said higher pressure region (12) is significantly reduced and cooperating with said seal ring surface (66) to move said seal ring (22) toward said runner surface (20) to restore said seal gap (21) without causing frictional engagement of said seal ring (22) and said runner (18) when the pressure in said higher pressure region (12) is increased significantly. The fluid seal assembly of Claim 1 having a servomechanism (46, 48, 66, 74) cooperating with said seal ring (22) which opens the gap (C) between said seal ring (22) and said runner surface (20) when pressure in said higher pressure region (12) is significantly reduced and which moves said seal ring (22) closer to said runner surface (20) without causing frictional engagement of said seal ring (22) and said runner (18), in response to a return of high pressure to said higher pressure region (12). The fluid seal assembly of Claim 6 wherein said resilient means (74) comprises a plurality of springs (74) symmetrically spaced about said seal ring (22).

Inventors: POPE ADAM NELSON, POPE, ADAM NELSON
Applicants: STEIN SEAL CO, STEIN SEAL COMPANY
Application year: 1996
Main classification: F16J15
IPC codes: F16J15