A turbocharger has a casing system which houses a centrifugal impeller connected to a turbine by a shaft. The casing system includes an insert casing which forms a duct for feeding air to and through the impeller, and a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing. The casing system further includes a main casing which forms a housing for the shaft and for the shaft-side end of the impeller. The impeller includes a hub which has a radially outer annular rim, and has front and rear faces which converge towards the rim from respectively the inlet side and the shaft side of the hub. The impeller further includes a plurality of circumferentially arranged vanes on the front face of the hub. The impeller further includes a seal formation formed on the rear face at the rim.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

Not applicable.

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NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

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REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft.

Industrial turbochargers, particularly for marine application, are made so that, if the compressor impeller were to burst, the surrounding casings would be capable of containing all the impeller fragments. Marine certification societies dictate that impeller hub burst containment must be demonstrated at turbocharger rotational speeds 20% in excess of the maximum allowable operational speed.

When an impeller bursts, there are two main mechanisms whereby fragments might not be contained by the casings. The first is penetration of the casings by impeller fragments. The second results from failure of fixings holding casings together, allowing gaps to appear between casings, and impeller fragments to escape through the gaps.

The impeller can be designed so that fragments originating from its outermost portion are of low mass (and therefore low energy). Typically, therefore, a state of the art impeller is designed with a relatively thin hub region over its outer portion.

FIG. 1shows schematically a sectioned view through a turbocharger impeller and a casing system housing the impeller. The impeller has a hub1with an outer annular rim2. The hub also has a front face3and a rear face4which converge towards the rim from respectively the inlet side and the shaft side of the hub. An annular balance land5projects from the rear face4of the hub. The casing system includes a volute casing6which forms a volute for receiving compressed air from the impeller, and a separate insert casing7which inserts from the inlet side of the impeller into the volute casing to form a duct for feeding air to and through the impeller. The casing system also includes a main casing8which forms a housing for the shaft and for the shaft-side end of the impeller.

A seal plate9attached to the main casing8forms a labyrinth seal10with the rear face4of the hub1outward of the balance land5and close to the outer face of the rim2. The seal plate also extends radially outwardly to carry the rear wall of an annular passage11, optionally containing diffuser vanes12, which directs compressed air from the impeller to the volute. The balancing land5produces a neck region13of reduced thickness immediately inboard thereof.

During a hub burst, cracks generally initiate in the hub1near the impeller centre-line. As they propagate outwards, cracks also form in the neck region13, allowing the impeller rim2to be shed. Fragments of the rim pass through any diffuser vanes12(which offer little resistance due to their relatively flimsy structure) and then impact on the wall of the volute casing6. This wall is therefore usually thickened to prevent penetration. The remaining larger pieces of the hub1, of relatively small outer diameter once the rim2has been shed, impact the insert casing7immediately surrounding the impeller. Typically this casing shatters, absorbing the energy of the hub fragments.

Increasingly, higher pressure ratios are being demanded from industrial turbochargers. As a result, rotational speeds of impellers are increasing and impeller designs must be altered to allow for consequently increasing stresses. Typically therefore the shape of the impeller hub is made more wedge-shaped (i.e. the angle between its front and rear faces is increased) to support the added centrifugal loads from the impeller rim. This in turn means that the neck region in the impeller is displaced to a higher diameter, and the rim, outboard of the neck, is reduced in size. Impeller designs suitable for higher pressure ratios tend to have a narrower operating range: the usable range of mass flow at a particular pressure ratio is low compared to impellers designed for lower pressure ratios. To overcome this tendency, the wall of the duct which feeds air to and through impeller may incorporate slot-shaped apertures.

Since the rim of the impeller is smaller (albeit rotating at higher speed at failure), the energy that must be dissipated to prevent penetration of rim fragments does not necessarily increase in line with the rotor speed. As a result, only a modest increase in the thickness of the volute casing may be necessary to prevent penetration by such fragments.

However, the remaining hub fragments are larger, extend to a higher radius and are more wedge-shaped. In combination with the higher speed at failure, this results in the remaining hub fragments containing substantially greater amounts of energy. As a consequence, although the shattering of the insert casing absorbs some of their energy, the wedge fragments may not be blunted and may pass intact, still with significant energy, beyond the insert casing, thereby potentially avoiding containment.

BRIEF SUMMARY OF THE INVENTION

The present invention is at least partly based on a recognition that a danger of such uncontained fragments is that they may wedge apart the main and volute casings, causing the failure of any fixings and allowing the escape of burst fragments.

Accordingly, in a first aspect, the present invention provides a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft;the casing system including:an insert casing which forms a duct for feeding air to and through the impeller,a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, anda main casing which forms a housing for the shaft and for the shaft-side end of the impeller;the impeller including:a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub, anda plurality of circumferentially arranged vanes on the front face of the hub, anda seal formation formed on the rear face at the rim; andthe casing system further including:a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation, the seal plate extending inboard from the rim to an attachment flange which attaches to the main casing, and extending outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute;wherein the thickness of the seal plate is narrowed in an annular waist region adjacent the rim, the thickness of the waist region being less than 2.5 times the minimum thickness (and preferably less than two times the minimum thickness), as measured in the axial direction, of the rim.

By providing such a waist region of narrowed thickness, cracking of the seal plate around the waist region can be promoted during an impeller burst. The cracking absorbs energy, but in addition the parts of the seal plate outboard of the waist region (including, for example, the part carrying or forming the rear wall of the annular passage) are more likely to remain intact, acting as a penetration barrier to large fragments.

The thickness of the waist region can be greater than 1.5 times the minimum thickness, as measured in the axial direction, of the rim.

The waist region may be substantially cylindrical in shape.

The annular waist region can be located between the attachment flange and the seal plate's seal formation.

Optionally, each vane extends to a respective vane exit edge at its radially outward end; and the volute casing axially overlaps the main casing and the seal plate at a location which is outboard of the vane exit edges, the overlap having an axial extent that is at least three times the span (and preferably four or five times the span), as measured in the axial direction, of the vane exit edges. Hub fragments may attempt to escape along an interface between the volute and main casings. However, by providing the axial overlap between the casings, the fragments can be blocked at the overlap, which is of such an axial extent that it is maintained even if the fragments are successful in partially wedging the casings apart.

More generally, in a second aspect, the present invention provides a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft;the casing system including:an insert casing which forms a duct for feeding air to and through the impeller,a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, anda main casing which forms a housing for the shaft and for the shaft-side end of the impeller; andthe impeller including:a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub, anda plurality of circumferentially arranged vanes on the front face of the hub, each vane extending to a respective vane exit edge at its radially outward end;wherein the volute casing axially overlaps the main casing at a location which is outboard of the vane exit edges, the overlap having an axial extent that is at least three times the span (and preferably four or five times the span), as measured in the axial direction, of the vane exit edges.

In respect of the second aspect, the impeller may further include a seal formation formed on the rear face at the rim. The casing system may then further include a seal plate having a corresponding seal formation which sealingly interacts with the impeller seal formation. The seal plate can extend outboard from the rim to carry or form a rear wall of an annular passage directing compressed air from the impeller to the volute (and can typically also extend inboard from the rim to an attachment flange which attaches to the main casing). In respect of the first or second aspect, the seal formations may together form a labyrinth seal. Alternatively, however, the seal formations may just be facing locations of the impeller and the seal plate that are in close proximity with each other. When the impeller includes a seal formation, the volute casing may axially overlap the seal plate as well as the main casing. In this situation, the seal plate can be considered as part of the main casing for the determination of the axial extent of the overlap. That is, the total axial extent of the overlap of the main casing and the seal plate by the volute casing should be at least three times the span, as measured in the axial direction, of the vane exit edges.

Optionally, in respect of the first or second aspect, each vane extends from a respective vane entry edge at its radially inward end to a respective vane exit edge at its radially outward end; and the insert casing has a wall which forms the duct, the duct wall being shaped to provide a close clearance with the vanes from their entry edges to their exit edges, and containing a slot which encircles the vanes adjacent the entry edges, the thickness of the duct wall increasing gradually and continuously from the slot to a position adjacent the exit edges such that at the exit edges the duct wall is at least 1.3 times thicker than at the slot (and preferably at least two or three times thicker than at the slot). The thickened duct wall means that, when the insert casing fractures during an impeller burst, more impeller energy can be absorbed by the insert casing, reducing the likelihood of fragments escaping beyond the insert casing.

More generally, in a third aspect, the present invention provides a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft;the casing system including:an insert casing having a wall which forms a duct for feeding air to and through the impeller,a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, anda main casing which forms a housing for the shaft and for the shaft-side end of the impeller; andthe impeller including:a hub which has a radially outer annular rim, and has a front face and a rear face which converge towards the rim from respectively the inlet side and the shaft side of the hub,a plurality of circumferentially arranged vanes on the front face of the hub, each vane extending from a respective vane entry edge at its radially inward end to a respective vane exit edge at its radially outward end;wherein the duct wall is shaped to provide a close clearance with the vanes from their entry edges to their exit edges, and contains a slot which encircles the vanes adjacent the entry edges, the thickness of the duct wall increasing gradually and continuously from the slot to a position adjacent the exit edges such that at the exit edges the duct wall is at least two times thicker than at the slot (and preferably at least three times thicker than at the slot).

Optionally, in respect of any one of the first to third aspects, the insert casing has: a wall which forms the duct, upstream and downstream annular structures for joining the insert casing to the volute casing at respectively the upstream and downstream ends of the duct, and a plurality of T- or Y-shaped pillar formations which hold the duct wall relative to the annular structures, each pillar formation having a first pillar which extends to the duct wall, a second pillar which extends to the downstream annular structure, and a third pillar which extends to the upstream annular structure, the minimum cross-sectional area of the first pillar being greater than the minimum cross-sectional area of the second pillar, and the minimum cross-sectional area of the second pillar being greater than the minimum cross-sectional area of the third pillar. Such a progression of cross-sectional areas of the pillars increases absorption of energy on insert casing distortion and fracture. It can also help to prevent the insert casing being driven axially from the impeller.

More generally, in a fourth aspect, the present invention provides a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft;the casing system including:an insert casing having a wall which forms a duct for feeding air to and through the impeller,a volute casing which forms a volute for receiving compressed air from the impeller, the insert casing inserting into the volute casing, anda main casing which forms a housing for the shaft and for the shaft-side end of the impeller;wherein the insert casing has:upstream and downstream annular structures for joining the insert casing to the volute casing at respectively the upstream and downstream ends of the duct, anda plurality of T- or Y-shaped pillar formations which hold the duct wall relative to the annular structures, each pillar formation having a first pillar which extends to the duct wall, a second pillar which extends to the downstream annular structure, and a third pillar which extends to the upstream annular structure, the minimum cross-sectional area of the first pillar being greater than the minimum cross-sectional area of the second pillar, and the minimum cross-sectional area of the second pillar being greater than the minimum cross-sectional area of the third pillar.

Further aspects of the present invention provide: (i) the casing system of any one of the first to fourth aspects, (ii) the seal plate of the first aspect, and (iii) the insert casing of any one of the first to fourth aspects.

Further optional features of the invention will now be set out. Unless specified otherwise, these are applicable singly or in any combination with any aspect of the invention.

Typically, the insert casing inserts into the volute casing from the inlet side of the impeller.

Typically, the main casing also forms a housing for the shaft-side end of the turbine.

The impeller may further include an annular balance land which projects from the rear face of the hub. In relation to the first aspect of the invention, the seal formation of the impeller may then be outboard of the balance land, and the waist region of the seal plate may be radially located between that seal formation and the balance land.

The rear face of the hub may lie on a conical surface having an internal half angle which is less than 80°, and preferably less than 76°.

Further optional features of the invention are set out below.

DETAILED DESCRIPTION OF THE INVENTION

A turbocharger impeller and a casing system housing the impeller according to an embodiment of the present invention are shown inFIGS. 2 and 3.

The impeller has a hub21with an outer annular rim22. The hub also has a front face23and a rear face24which converge towards the rim from respectively the inlet side and the shaft side of the hub. A plurality of circumferentially arranged vanes36are provided on the front face of the hub. Each vane extends from a respective vane entry edge37at its radially inward end to a respective vane exit edge38at its radially outward end. An annular balance land25projects from the rear face24of the hub. The casing system includes a volute casing26which forms a volute for receiving compressed air from the impeller, and a separate insert casing27which inserts from the inlet side of the impeller into the volute casing. The insert casing27has a wall39which forms a duct for feeding air to and through the impeller. The casing system also includes a main casing28which forms a housing for the shaft and for the shaft-side end of the impeller.

A seal plate29attaches to the main casing28at a radially inward annular attachment flange34. The seal plate forms a labyrinth seal30with the rear face24of the hub21outward of the balance land25and close to the outer face of the rim22. The seal plate also extends radially outwardly to form the rear wall of an annular passage31, containing optional diffuser vanes32, which directs compressed air from the impeller to the volute. The balance land25produces a neck region33of reduced thickness immediately inboard thereof.

The impeller is designed for operation at high pressure ratios. Consequently, the shape of the impeller hub is relatively wedge-shaped to support the added centrifugal loads from the impeller rim22. As better shown inFIG. 3, which is a longitudinal cross-section through the turbocharger ofFIG. 2, this wedge shape is produced by the rear face of the hub lying on a conical surface having an internal half angle α which is less than 80°

Between the attachment flange34and the labyrinth seal30, the seal plate29has a substantially cylindrical waist region35which is or reduced thickness relative to the other parts of the plate. As shown inFIG. 3, the thickness W of the waist region is between 2.5 times and 1.5 times the minimum thickness R, as measured in the axial direction, of the rim22. As the energy of the hub fragments during an impeller burst tend to scale with R, the upper limit for W helps to ensure that the seal plate cracks around the waist region35. This absorbs energy, and also makes it more likely that the parts of the seal plate29outboard of the waist region remain intact, allowing them to act as a penetration barrier to large fragments of the hub1. The lower limit for W helps to ensure the integrity of the seal plate29during normal operation.

A further barrier to escape of hub fragments during an impeller burst is provided by the large axial overlap A between the volute casing and the main casing and the seal plate. This overlap is about 5.5 times the span S, as measured in the axial direction, of the vane exit edges38. Hub fragments which attempt to escape along the interface between the volute casing26and the main casing28may partially wedge the casings apart. However, even if they are successful in this, they do not eliminate the overlap, and can thus be retained within the casing system. The extent of the overlap can be varied, depending on the expected energy of the hub fragments, but preferably A/S should be at least about three.

The combination of the waist region35and the axial overlap A can be particularly effective at containing hub fragments, the outer parts of the seal plate29preventing many fragments from reaching the interface between the volute casing26and the main casing28, and the overlap A preventing any fragments that do reach the interface from escaping further.

The insert casing27is configured to further improve the energy absorption capability of the casing system, and reduce the likelihood of hub fragments escaping. As a result, any fragments which reach the seal plate29and/or the interface between the volute casing26and the main casing28having passed through the insert casing can be reduced in energy.

The duct wall39of the insert casing27is shaped to provide a close clearance with the vanes36from their entry edges37to their exit edges38. The duct wall39also contains upstream40and downstream41circumferential slots. These are provided to increase the usable range of mass flows of the impeller. The downstream slot41encircles the vanes36adjacent their entry edges37. The thickness of the duct wall39increases gradually and continuously from the slot41to a position adjacent the exit edges38such that at the exit edges the duct wall is about 1.3 times thicker than at the slot. When the insert casing27fractures during an impeller burst, this thickening increases the amount of impeller energy that is absorbed by the insert casing27.

The insert casing27also has upstream42and downstream43annular structures which join the insert casing to the volute casing26at respectively the upstream and downstream ends of the duct. The upstream annular structure is a flange42which is joined to the volute casing26by fixing bolts. The downstream annular structure is an abutment surface43which abuts the volute casing without mechanical fasteners, although a ring seal can be provided at the interface of the abutment surface43and the volute casing26to improve the seal therebetween. T-shaped or Y-shaped pillar formations retain the duct wall39relative to the structures42,43. Each pillar formation has a first pillar44awhich extends to the duct wall39, a second pillar44bwhich extends to the downstream abutment surface43, and a third pillar44cwhich extends to the flange42. The minimum cross-sectional area of the first pillar44ais greater than the minimum cross-sectional area of the second pillar44b. In addition, the minimum cross-sectional area of the second pillar44bis greater than the minimum cross-sectional area of the third pillar44c. This progression of cross-sectional areas increases the absorption of energy when the insert casing27distort and fractures. It can also help to prevent the insert casing being driven axially from the impeller.