A radial turbomachine with improved impeller eye seal

Abstract

Disclosed herein is a radial turbomachine including a casing and a rotor arranged for rotation in the casing. The rotor includes at least one impeller with a hub, a shroud, and a plurality of blades between the hub and the shroud. An eye seal is stationarily housed in the casing and surrounds an impeller eye. The impeller eye includes a stepped external surface facing the eye seal. The stepped external surface includes a plurality of cylindrical surface portions. The eye seal in turn includes a plurality of annular fins. Each annular fin projects radially inwardly towards a corresponding surface portion of the impeller eye and ends with a annular fin tip at a clearance distance from the respective cylindrical surface portion of the impeller eye. The annular fins include an end projection at the fin tip, the end projection extending in an axial direction.

Description

A RADIAL TURBOMACHINE WITH IMPROVED IMPELLER EYE SEALDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure relates to radial turbomachines, such as centrifugal compressors and centripetal turbines. Specifically, the present disclosure relates to radial turbomachines comprising at least one shrouded impeller.BACKGROUND ART

[0002] Centrifugal compressors are used in a wide range of applications, for instance in the oil and gas industry, to compress a gaseous fluid. Centrifugal compressors and centripetal turbines comprise a casing and a rotor housed in the casing for rotation around a rotation axis. The rotor comprises in turn one or more impellers, each having an inlet and an outlet. In centrifugal compressors the inlet is oriented axially and the outlet is oriented radially. In centripetal turbines, the inlet is oriented radially and the outlet is oriented axially.

[0003] The impellers can be unshrouded, or shrouded. Unshrouded impellers comprise a hub and blades integral with the hub, which define a plurality of flow paths extending from an impeller inlet to an impeller outlet. Shrouded impellers additionally include a shroud which closes the flow paths formed between the blades, the hub and the shroud. The shroud forms an impeller eye surrounding the axial inlet of the impeller in case of centrifugal compressors, or the axial outlet of the impeller in case of radial, centripetal turbines.

[0004] In centrifugal impellers, process gas flows axially into the impeller through an impeller inlet and exits radially from the impeller outlet towards a diffuser which surrounds the impeller outlet. Mechanical energy applied to the rotor is transferred to the process gas and converted into kinetic energy of the process gas, which accelerates while flowing through the flow paths of the rotating impeller. The accelerated process gas is slowed down in the diffuser, where the kinetic energy of the accelerated process gas is converted into pressure.

[0005] A pressure gradient is therefore generated between the impeller inlet and the impeller outlet, a high-pressure region being located at the impeller outlet and a low- pressure region being located at the impeller inlet.

[0006] To prevent or limit gas leakages from the impeller outlet back towards the impeller inlet, through a leaking gap between an outer surface of the shroud and a stationary bundle of the centrifugal compressor, an eye seal is stationarily housed in the compressor casing. The eye seal extends annularly around the rotation axis of the compressor rotor and surrounds the impeller eye. The eye seal usually includes a plurality of annular fingers, also referred as fins, which extend radially inwardly from a foot or base towards a tip. The impeller eye includes a stepped surface formed by a plurality of mainly cylindrical surface portions, each of which co-acts with a corresponding fin of the eye seal. The fins and the stepped surface form a labyrinth seal aimed at reducing the leakage flow of process gas from the impeller outlet towards the impeller inlet.

[0007] To prevent the eye seal from rubbing against the stepped surface, a clearance is present between the tip of each fin and the respective cylindrical surface portion of the stepped surface of the impeller eye. The clearance must be maintained large enough to prevent a rubbing contact between the eye seal and the impeller eye under any operating condition of the compressor. Consequently, gas leakage through the eye seal cannot be entirely avoided.

[0008] Process gas leaking through the eye seal adversely affects the efficiency of the compressor, since process gas returning from the impeller outlet towards the impeller inlet must be compressed again. The loss of efficiency due to gas leakages is a function of the flow coefficient. This latter is defined aswherein:Qvis the compressor flowrate in m3 / s U is the tip speed of the impeller in m / s R is the outside radius of the impeller in m.

[0009] The lower the flow coefficient of the impeller, the higher the losses.

[0010] Similar issues regarding leakages around the impeller eye arise in radial turbines, specifically in centripetal turbines, where the process gas flows from a radial inlet towards an axial outlet of the turbine impeller. Compressed process gas can flow from the high-pressure region at the radial inlet of the impeller towards low-pressure region at the axial outlet of the impeller, between the outer surface of the impeller shroud and the stationary diaphragm, through the eye seal.

[0011] An improved eye seal aimed at reducing the gas leakages around the impeller eye would therefore be welcomed in the art, both for centrifugal compressors, as well as centripetal turbines.SUMMARY

[0012] Disclosed herein is a radial turbomachine, such as a centrifugal compressor or a radial, centripetal turbine, comprising a casing and a rotor arranged for rotation around a rotation axis in the casing. The rotor in turn includes at least one shrouded impeller having a hub, a shroud, and a plurality of blades between the hub and the shroud. An eye seal is stationarily housed in the casing and surrounds an impeller eye. The eye seal is adapted to prevent or limit process fluid leakage from a high-pressure region to a low-pressure region of the impeller. The impeller eye comprises a stepped external surface facing the eye seal and comprising a plurality of stepwise-arranged surface portions, for instance of cylindrical shape. The eye seal comprises a plurality of annular fins. Each annular fin projects radially inwardly towards a corresponding one of said surface portions of the impeller eye and ends with an annular fin tip at a clearance distance from the respective surface portion of the impeller eye. At least one of said annular fins, and preferably each annular fin, comprises an end projection at the fin tip. The end projection extends in an axial direction, toward the high-pressure region of the impeller.

[0013] In some embodiments, the end projection extending in axial direction and the respective surface portion of the impeller eye are configured such that, in use, a stationary vortex forms in each annular groove between adjacent fins of the eye seal. Each stationary vortex extends in the annular gap formed by the projection of the annular fin positioned at the high-pressure side of the annular groove, and the surface portion of the stepped side surface of the impeller eye. The stationary vortex opposes the leakflow through the eye sael and thus reduces the fluid leakage between each annular fin and the respective surface portion of the impeller eye.

[0014] Further features and embodiments of the turbomachine of the present disclosure are described below and outlined in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Reference is now made briefly to the accompanying drawings, in which:Fig. l is a partial cross-sectional view of a compressor in a plane containing the rotation axis of the compressor rotor; andFig.2 is an enlargement the eye seal of Fig. l; andFig.3 is an enlarged view of portion III of Fig.2.DETAILED DESCRIPTION

[0016] The following description is referred to a compressor including one compressor stage. It shall be understood, however, that the novel features of the eye seal arrangement disclosed herein can be used in a multi-stage compressor comprised of a plurality of impeller, wherein one or more impellers are shrouded impellers and which may include one or more un-shrouded impellers. The novel features disclosed herein can be used for one, some or all shrouded impellers of a multi-stage centrifugal compressor.

[0017] Those skilled in the art will understand that novel features disclosed herein can be beneficial to reduce leakages also in centripetal turbines, wherein the process gas expands in a centripetal impeller while flowing from a radial inlet towards an axial outlet of the impeller, and wherein, therefore, a higher gas pressure is present at the radial inlet of the impeller and a lower gas pressure is present at the impeller eye. The exemplary embodiment described below shall not be construed as limiting the present disclosure to centrifugal compressors.

[0018] Fig. l illustrates a portion of a centrifugal compressor 1, comprising at least one compressor stage. Specifically, Fig. l is a cross-sectional view of the compressor along a plane containing a rotation axis A-A of the compressor and shows only half section, since the machine is substantially axial-symmetric. The compressor 1comprises a casing 3 and a rotor 5 housed in the casing 1 for rotation around the rotation axis A-A. The rotor 5 comprises an impeller 7 constrained for rotation on a rotor shaft 9.

[0019] The impeller 7 is a shrouded impeller and comprises a hub 7.1, a shroud 7.2 and a plurality of blades 7.3 extending between the hub 7.1 and the shroud 7.2. Flow paths 7.4 are formed in the impeller 7 between the hub 7.1, the shroud 7.2 and the blades 7.3. The impeller 7 further includes an axially oriented impeller inlet 7.5 and a radially oriented impeller outlet 7.6. The impeller outlet 7.6 extends annularly around the rotation axis A-A. Reference 7.7 designates an impeller eye, formed by the shroud 7.2 and extending around the rotation axis A-A and around the impeller inlet 7.5.

[0020] In use, the impeller 7 rotates around the rotation axis A-A (arrow f), such that process gas enters the rotating impeller 7 (arrow Fin) through the impeller inlet 7.5 at a lower pressure and speed and exits the impeller 7 (arrow Fout) through the impeller outlet 7.6 at a higher speed. The kinetic energy of the exiting gas is gradually converted into pressure energy by slowing down the gas through a diffuser 11, which surrounds the impeller outlet 7.6. The diffuser 11 can be a bladed diffuser comprised of stationary blades 11.1 to guide the process gas through the diffuser towards a return channel or an outlet scroll (not shown).

[0021] The diffuser 11 extends between a stationary portion 13.1 of a compressor bundle which is housed in the casing 1 and comprises a diaphragm 13.2.

[0022] Due to the pressure difference between the diffuser 11 and the impeller inlet 7.5, process gas leaks (arrow Ik) through a gap 16 between the outer surface of the shroud 7.2 and the diaphragm 13.2, from the diffuser 11 towards the impeller eye 7.7.

[0023] To reduce the gas leakages Ik an eye seal 15 is stationarily mounted on the diaphragm 13.2 and surrounds the impeller eye 7.7. The structure of the impeller eye 7.7 and of the eye seal 15 are best shown in Figs. 2 and 3.

[0024] The impeller eye 7.7 comprises an external surface facing the eye seal 15. The external surface is stepped and comprises a plurality of surface portions labeled 7.10, co-axial to rotation axis A-A. In some embodiments, the surface portions are cylindrical. Here below the surface portions 7.10 will be therefore referred to ascylindrical surface portions.

[0025] The diameter of the cylindrical surface portions 7.10 increases from the impeller inlet 7.6 in the direction of flow of the incoming process gas (arrow Fin) through the impeller. Each cylindrical surface portion 7.10 co-acts with an annular fin 17 of the eye seal 15. Each annular fin 17, aka finger, extends radially inwardly towards the rotation axis A-A from a base or foot 17.1 towards a tip 17.2. Annular grooves 18 are formed between pairs of sequentially arranged annular fins or fingers 17.

[0026] Each annular fin 17 has a low-pressure side 17L and a high-pressure side 17H. The low-pressure side 17L faces a lower-pressure fluid domain and the high-pressure side 17H faces a higher-pressure fluid domain. Specifically, the low-pressure side 17L of each annular fin 17 is oriented towards the axial inlet of the impeller 7 and the high- pressure side 17H of each annular fin 17 is oriented opposite the axial inlet of the impeller 7.

[0027] In some embodiments, as shown in Fig.3, the low-pressure side 17L and the high-pressure side 17H are conical and oriented in opposite directions. In the crosssection along a plane containing the rotation axis A-A of the compressor each annular fin 17 has therefore a rectilinear profile.

[0028] Each tip 17.2 comprises an end projection 17.3 (see Fig.3), which extends mainly in an axial direction, i.e., parallel to the rotation axis A-A of the compressor 1, from one of the side surfaces of the respective annular fin 17.

[0029] Specifically, in the embodiment show, each end projection 17.3 extends from the high-pressure side 17.H of the respective annular fin 17, i.e., in a direction concordant with the direction Fin of the process gas flow entering the impeller eye 7. Thus, each projection 17.3 extends in a direction of increasing fluid pressure inside the gap between the impeller shroud 7.2 and the diaphragm 13.2 and therefore in the direction of flow of the gas leakages through the eye seal 15.

[0030] In some embodiments, each end projection 17.3 has a first surface 17.31 oriented radially inwardly and substantially coaxial to the respective cylindrical surface portion 7.10 of the impeller eye 7.7 and therefore coaxial to rotation axis A-A of the compressor 1. If the surface portion 7.10 is cylindrical, as shown in the embodimentof the attached drawings, the first surface 17.31 of the end projection 17.3 is also cilindrical.

[0031] In some embodiments, each end projection 17.3 has a second surface 17.32 oriented radially outwardly and facing the foot or base 17.1 of the respective annular fin 17, i.e., facing opposite the rotation axis A-A. The second surface 17.32 can also have a cylindrical shape and be coaxial to the rotation axis A-A.

[0032] Each end projection 17.3 forms an annular gap, or clearance, 19 between the first cylindrical surface 17.31 and the respective cylindrical surface portion 7.10. The gap 19 has a length L19 in axial direction and a height H19 in radial direction. The height Hl 9 is the clearance distance between the fin and the impeller eye. The height H19, i.e., the clearance distance is determined by design constraints and is such that the eye seal (i.e., each end projection 17.3) does not rub against the impeller eye, i.e., against the respective cylindrical surface portion 7.10 in any operating condition.

[0033] In some embodiments, the axial length L19 is larger than the clearance distance H19. In some embodiments, the axial length L19 is at least twice or at least three times the clearance distance Hl 9. In some embodiments, the axial length L19 is equal to or less than ten times the clearance distance Hl 9, for instance equal to or less than five times the clearance distance Hl 9.

[0034] The axial length L 19 can be equal to or larger than the thickness of the annular projection 17.3. For instance, the axial length L19 can be at least twice, and for instance less than four times the thickness in radial direction of the projection 17.3.

[0035] Each projection 17.3, which extends in the axial direction and faces the respective cylindrical surface portion 7.10 of the impeller eye, forms an aerodynamic barrier, which reduces process gas leakages through the eye seal. As pictorially represented by lines V (for the sake of clarity shown for only one annular groove 18 in Fig.3), a stationary vortex forms in each annular groove 18 between a first, upstream annular fin 17 and a second, downstream annular fin 17. “Upstream” and “downstream” are referred in this case to the direction of flow of the leaking gas Ik. Thus, the upstream annular fin 17 is the one positioned in a region of the gap between the shroud 7.2 and the diaphragm 13.2 where the leaking gas has a higher pressure. The stationaryvortex V extends in the annular gap formed by the projection 17.3 of the upstream annular fin 17. The stationary vortexes extending into the annular gaps reduce gas leakages between each annular fin 17 and the respective cylindrical surface portion 7.10, such that a higher overall performance of the compressor 1 can be achieved with the same clearance distance Hl 9 compared to compressors of the current art.

[0036] Conversely, a larger clearance distance Hl 9 can be foreseen without increasing the leakage flow, thus ensuring safer operation of the compressor without rubbing between eye seal and impeller also under critical operating conditions. This can be particularly beneficial in case of high-temperature and high-speed centrifugal com- pressors, for instance.

[0037] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A radial turbomachine comprising: a casing; a rotor arranged for rotation around a rotation axis in the casing and comprising at least one impeller with a hub, a shroud, and a plurality of blades between the hub and the shroud; an eye seal stationarily housed in the casing and surrounding an impeller eye, the eye seal being adapted to prevent or limit fluid leakage from a high-pressure region to a low-pressure region of the impeller; wherein: the impeller eye comprises a stepped external surface facing the eye seal; the stepped external surface comprises a plurality of stepwise-arranged surface portions; the eye seal comprises a plurality of annular fins; each annular fin projects radially inwardly towards a corresponding one of said surface portions of the impeller eye and ends with a annular fin tip at a clearance distance from the respective surface portion of the impeller eye; and at least one of said annular fins comprises an end projection at the fin tip, the end projection extending in an approximately axial direction.

2. The radial turbomachine of claim 1, wherein each annular fin comprises a respective end projection.

3. The radial turbomachine of claim 1 or 2, wherein the surface portions of the impeller eye are cylindrical and coaxial to the rotor.

4. The radial turbomachine of any one of the preceding claims, wherein each end projection has a first surface oriented radially inwardly and substantially coaxial to the respective surface portion of the impeller eye.

5. The radial turbomachine of claim 4, wherein the first surface of each end projection is cylindrical.

6. The radial turbomachine of any one of the preceding claims, wherein each end projection has a second surface oriented radially outwardly, opposite theimpeller eye.

7. The radial turbomachine of claim 6, wherein the second surface of said end projections is cylindrical.

8. The radial turbomachine of any one of the preceding claims, wherein each end projection has an axial extension which is greater than the clearance distance and less than ten time the clearance distance.

9. The radial turbomachine of any one of the preceding claims, wherein each annular fin has a low-pressure side and a high-pressure side, and wherein each end projection extends from the high-pressure side of the respective annular fin.

10. The radial turbomachine of claim 9, wherein the high-pressure side and the low-pressure side of each annular fin are conical and extend from a base of the annular fin towards the tip of the annular fin.

11. The radial turbomachine of any one of the preceding claims, wherein the diameter of the surface portions of the stepped external surface of the impeller eye increases from an axial aperture toward a radial aperture of the impeller.

12. The radial turbomachine of any one of the preceding claims, wherein the turbomachine is a centrifugal compressor having an axial inlet surrounded by the impeller eye, and a radial outlet, the low-pressure region of the impeller being at the axial inlet and the high-pressure region of the impeller being at the radial outlet.

13. The radial turbomachine of any one of the preceding claims, wherein the turbomachine is a centripetal turbine, having a radial inlet, and axial outlet surrounded by the impeller eye, the low-pressure region of the impeller being at the axial inlet and the high-pressure region of the impeller being at the radial outlet.

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