Patent Description:
As is known, mismatch in thermal expansion of neighboring components is a common problem in gas turbine engines. In fact, huge temperature gradients may occur in gas turbine engines both in transient and stationary conditions. Differences in constituent material, shape and exposure to hot gas of components often lead to remarkably different thermal expansion, that, in turn, may cause adverse conditions, such as stress, cracks, deformation or relative displacement of parts. For example, seals may be loosened on account of different thermal expansion and start leaking, thus possibly allowing hot gas to reach cold components, which are not designed to withstand high temperatures.

This is also the case of exhaust gas systems of gas turbine engines, especially when parts operated at relatively low temperature are enclosed in a casing that is closed by a cover directly exposed to hot gas. In particular, the exhaust gas housing of an exhaust gas system normally comprises a central hub, in which the rear bearings of the machine rotor are housed. The hub may be closed immediately downstream of the rotor bearings by a cover directly exposed to hot gas in the exhaust flow path. Even if special materials are used, the static temperature difference in use and thermal cycles may impair the sealing function of the cover, causing leakage and ingestion of hot gas, that may result in damages for the bearings.

<CIT> discloses a hub cover assembly for an exhaust gas housing of a heavy-duty gas turbine engine, comprising an adapter ring, configured to be coaxially fitted to a rear end of an exhaust hub of a heavy-duty gas turbine engine, a spacer ring structure, having a first axial end removably and sealingly connectable to the adapter ring, and an external cover applied to a second axial end of the spacer ring structure, whereby the spacer ring structure and the external cover sealingly delimit a chamber laterally and axially on one side.

Other examples of known hub cover assemblies are disclosed in <CIT>, <CIT>, <CIT> and <CIT>.

It is an aim of the present invention to provide a hub cover assembly for an exhaust gas housing of a heavy-duty gas turbine engine, a heavy-duty gas turbine engine and a method of retrofitting a heavy-duty gas turbine that allow the above limitations to be overcome or at least reduced.

According to the present invention there is provided a hub cover assembly for an exhaust hub of a heavy-duty gas turbine engine, comprising:.

The hub cover assembly may be fitted to a rear end of an exhaust hub of a gas turbine engine to form a sealed cylindrical chamber that protects the shell of the exhaust hub from excessive thermal stress both in new and existing gas turbine engines. The exhaust hub may in fact accommodate structures designed to operate at relatively low temperature, such as rear bearings of the rotor of the gas turbine engine. The sealed cylindrical chamber which may be formed by the hub cover assembly keeps an intermediate temperature between the temperature of the hot gas flow path and the temperature inside the rear end of the exhaust hub and in practice serves as a temperature buffer that reduces the mechanical stress caused by different thermal expansion. The risk of damages of components exposed to extreme temperatures is therefore effectively attenuated. Moreover, the structure of the hub cover assembly is quite simple and is relatively easy to replace in case of ageing. In any case, the hub cover assembly offers an additional protection to components at the rear end of the exhaust hub.

The adapter ring may also have a function of restoring the original circular shape of the rear end of the exhaust hub, in case of deformation because of mechanical stress.

The wedges help insertion and centering of the adapter ring when the latter is fitted to the rear end of the exhaust hub, especially when deformation has occurred on account of thermal stress during operation of the gas turbine engine.

According to an aspect of the invention, at least some of the wedges comprise respective feet extending radially inwards and preferably the wedges have respective radially outer faces inclined inwards away from the adapter ring.

The wedges provided with feet may be used to carry mechanical load in the coupling of the adapter ring to the rear end of the exhaust hub. The other wedges, if any, may be dedicated to favor insertion and centering.

According to an aspect of the invention, the adapter ring comprises a flange and a circular coupling edge extending axially from the flange in a direction opposite to the spacer ring structure.

The flange and a circular coupling edge define different coupling members that may be separately optimized and are dedicated to coupling to the rear end of the exhaust hub and to the spacer ring structure.

According to an aspect of the invention, the spacer ring structure comprises:.

According to an aspect of the invention, the coupling ring extends radially inwards from the ring shell.

The spacer ring structure thus defined has simple construction and the volume of the chamber can be easily and flexibly selected according to design preferences to provide appropriate temperature buffer function.

According to an aspect of the invention, the adapter ring and the spacer ring structure are formed of respective separable halves.

Separation of the halves helps connection of the spacer ring structure to the rear end of the exhaust hub and makes mounting steps simpler.

According to the present invention there is moreover provided a heavy-duty gas turbine engine, comprising an exhaust hub extending along an axis and a hub cover assembly as defined above, sealingly closing a rear end of the exhaust hub.

According to an aspect of the invention, the exhaust hub comprises an inner cover closing the rear end and the adapter ring is coaxially fitted to the rear end of an exhaust hub around the inner cover.

The inner cover and its connections to the exhaust hub, that would be normally exposed to huge temperature gradients because of the different operating conditions between the inside of the exhaust hub and the hot gas flow path, are effectively protected by the hub cover assembly. In fact, the hub cover assembly forms a chamber at intermediate temperature around the inner casing and thus reduces temperature gradients.

According to the present invention there is moreover provided a method of retrofitting a heavy-duty gas turbine engine, comprising an exhaust hub extending along an axis, the method comprising:.

According to an aspect of the invention, the method comprises forming longitudinal cuts in the cylindrical casing, wherein the longitudinal cuts extend to a rear edge of the cylindrical casing and define axial tabs.

The tabs defined by the longitudinal cuts may be deflected by insertion of the wedges, that is therefore favored. Moreover, the longitudinal cuts and the tabs help restoring the original circular shape of the rear end of the exhaust hub, in case of deformation during operation.

The present invention will now be described with reference to the accompanying drawings, which show a number of non-limitative embodiments thereof, in which:.

With reference to <FIG>, number <NUM> defines a gas turbine engine comprising a compressor <NUM>, a combustor assembly <NUM>, a turbine <NUM>, an exhaust gas housing <NUM> and an axial exhaust diffuser <NUM>, all extending about a longitudinal axis, which is indicated by A in <FIG>. In one embodiment (not shown), the diffuser may be a radial diffuser.

The compressor <NUM> feeds the first combustor assembly <NUM> with a flow of compressed air drawn from outside.

In one embodiment, the first combustor assembly <NUM> comprises an annular combustion chamber <NUM> and a plurality of burners <NUM> circumferentially distributed about the longitudinal axis A. However, the combustor assembly could be of a different type, for example a silos combustor, a can or can-annular combustor with single or sequential stages.

The turbine <NUM> receives and expands a flow of hot gas from the combustor assembly <NUM> to extract mechanical work, which is transferred to an external user, typically an electric generator, here not shown.

The hot gas is then conveyed through the exhaust gas housing <NUM> and the exhaust diffuser <NUM>. The exhaust gas housing <NUM> and the exhaust diffuser <NUM> extend along the longitudinal axis A immediately downstream of the turbine <NUM>.

With reference to <FIG> and <FIG>, the exhaust gas housing <NUM> comprises an exhaust hub <NUM> and an outer casing <NUM>, that respectively define an inner boundary and an outer boundary of a portion of an exhaust flow path. Struts <NUM> connect the exhaust hub <NUM> to the outer casing <NUM>.

The exhaust hub <NUM> comprises a bearing casing <NUM> and a cylindrical casing <NUM> arranged concentrically around the bearing casing <NUM> (see the enlarged detail of <FIG>). An annular gap <NUM> is defined between the bearing casing <NUM> and the cylindrical casing <NUM>.

The bearing casing <NUM> accommodates rear bearings <NUM> of a rotor <NUM> (not shown in <FIG>; see <FIG>) of the gas turbine engine <NUM>. The bearing casing <NUM> is closed by an axially inner cover <NUM> at a rear end 11a of the exhaust hub <NUM>. The cylindrical casing <NUM> defines the inner boundary of the flow path in the exhaust gas housing <NUM>.

The rear end 11a of the exhaust hub <NUM> is also sealingly closed by a hub cover assembly <NUM>, which comprises an adapter ring <NUM>, a spacer ring structure <NUM> and an external cover <NUM>. In one embodiment, the adapter ring <NUM> and the spacer ring structure <NUM> are formed of respective separable halves, which join one another at a split plane of the gas turbine engine <NUM>.

The adapter ring <NUM> is coaxially fitted to the rear end 11a of the exhaust hub <NUM> and is removably connectable to the spacer ring structure <NUM>. More precisely, the adapter ring <NUM> has a radial flange <NUM> for connection to the spacer structure <NUM> and a circular coupling edge <NUM> extending axially from the flange <NUM> in a direction opposite to the spacer ring structure <NUM>. A diameter of the circular coupling edge <NUM> is selected to allow insertion of the circular coupling edge <NUM> into the annular gap <NUM>. The adapter ring may be welded to the rear end 11a of the exhaust hub <NUM>, specifically to the cylindrical casing <NUM>.

The adapter ring <NUM> also comprises a plurality of wedges <NUM> circumferentially arranged at a same distance from a center of the adapter ring <NUM>. The wedges <NUM> project axially from the circumferential coupling edge <NUM> in a direction opposite to the spacer ring structure <NUM> and have respective radially outer faces 30a inclined inwards in a direction away from the adapter ring <NUM> (in use, toward the rear end 11a of the exhaust hub <NUM>).

In the embodiment of <FIG>, some of the wedges <NUM>, for example two upper and two lower wedges <NUM>, comprise respective feet 30b extending radially inwards. The wedges <NUM> equipped with feet 30b carry the mechanical load of the adapter ring <NUM>, while the other wedges <NUM> may provide a centering function only and help fitting the adapter ring <NUM> into the gap <NUM>. In other embodiments, however, the different wedge or all or none of the wedges may have feet.

The spacer ring structure <NUM> has a first axial end removably and sealingly connectable to the adapter ring <NUM>, whereas a second axial end is closed by the external cover <NUM>. Thus, the spacer ring structure <NUM> and the external cover <NUM> sealingly delimit a cylindrical chamber <NUM> laterally and axially on one side. The chamber <NUM> is axially delimited by the rear end 11a of the exhaust hub <NUM> on the opposite side. The spacer ring structure <NUM> and the chamber <NUM> need not be cylindrical or circular. For example, the spacer ring structure may have oval shape or frustoconical shape, either right or oblique. The chamber, as it is laterally defined by the spacer ring structure, will have the same shape. Likewise, in some embodiments the external cover <NUM> may not be coaxial with the rear end 11a of the exhaust hub <NUM> and the adapter ring <NUM>. For example, if the spacer ring structure has oblique frustoconical shape, the external cover <NUM> is not coaxial.

Specifically, the spacer ring structure <NUM> comprises a ring shell <NUM>, a coupling ring <NUM> at the first axial end and a cover ring <NUM> at the second axial end.

The ring shell <NUM> axially extends from the first axial end to the second axial end and laterally delimits the chamber <NUM>. The coupling ring <NUM> and the cover ring <NUM> may be formed monolithically with the ring shell <NUM>; as an alternative the coupling ring <NUM> and the cover ring <NUM> may be formed as separate components and subsequently welded to opposite ends of the ring shell <NUM>.

The coupling ring <NUM> is removably connected to the adapter ring <NUM>, e.g. by bolts <NUM>. The connection also ensures that the chamber <NUM> is sealed and ingestion of hot gas is prevented. In one embodiment, the coupling ring <NUM> extends radially inwards from the ring shell <NUM>.

The external cover <NUM> is secured to the cover ring <NUM> by bolts <NUM>.

The hub cover assembly <NUM> may be natively included in the gas turbine engine <NUM> or may be subsequently applied to the rear end 11a of the exhaust hub <NUM>.

In order to apply the hub cover assembly <NUM>, the rear end 11a of the exhaust hub <NUM> may be prepared by forming longitudinal cuts <NUM> in the cylindrical casing <NUM>. The longitudinal cuts <NUM> extend to the rear edge of the cylindrical casing and define axial tabs <NUM>. Preparation is not strictly necessary and is particularly advantageous in case the rear end 11a of the exhaust hub <NUM> is severely deformed because of thermal and mechanical stress during use. If the shape of the rear end 11a has not substantially changed, the preparation may be omitted.

The adapter ring <NUM> is then coaxially fitted to the rear end 11a of the exhaust hub <NUM> around the inner cover <NUM>. Specifically, the circular coupling edge <NUM> and the wedges <NUM> of the adapter ring <NUM> are inserted in the gap <NUM> between the bearing casing <NUM> and the cylindrical casing <NUM>. Axial tabs <NUM>, if present, help insertion of the wedges <NUM> and of the circular coupling edge <NUM>. In turn, the adapter ring <NUM>, once fitted to the rear end 11a of the exhaust hub <NUM> helps restore its original circular shape, especially for the cylindrical casing <NUM>.

The adapter ring <NUM> is then welded to the cylindrical casing <NUM> through a welding cord, not shown.

Then, the coupling ring <NUM> at the first axial of the spacer ring structure <NUM> is removably and sealingly connected to the adapter ring <NUM> and the external cover <NUM> is applied to the second axial end of the spacer ring structure <NUM>, thus forming the cylindrical chamber <NUM> at the rear end of the exhaust hub on a second side.

Claim 1:
A hub cover assembly for an exhaust gas housing of a heavy-duty gas turbine engine, comprising:
an adapter ring (<NUM>), configured to be coaxially fitted to a rear end (11a) of an exhaust hub (<NUM>) of a heavy-duty gas turbine engine;
a spacer ring structure (<NUM>), having a first axial end removably and sealingly connectable to the adapter ring (<NUM>); and
an external cover (<NUM>) applied to a second axial end of the spacer ring structure (<NUM>), whereby the spacer ring structure (<NUM>) and the external cover (<NUM>) sealingly delimit a chamber (<NUM>) laterally and axially on one side;
characterized in that the adapter ring (<NUM>) comprises a plurality of wedges (<NUM>) arranged at a same distance from a center of the adapter ring (<NUM>) and projecting axially in a direction opposite to the spacer ring structure (<NUM>).