Patent Description:
As known, a gas turbine assembly for power plants comprises a compressor assembly, a combustor assembly and a turbine assembly. The compressor assembly is configured for compressing incoming air supplied at a compressor inlet. The compressed air leaving the compressor assembly flows into a volume (called "plenum") and from there into the combustor assembly. This combustor assembly comprises usually a plurality of burners configured for injecting fuel (at least one type of fuel) into the compressed air flow. The mixture of fuel and compressed air enters a combustion chamber where this mixture ignites. The resulting hot gas flow leaves the combustion chamber and by passing in the turbine assembly performs a rotating work on the rotor connected to a generator. As known, the turbine assembly comprises a plurality of stages, or rows, of rotating blades that are interposed by a plurality of stages, or rows, of stator vanes. The rotating blades are supported by the rotor whereas the stator vanes are supported by a casing (called "vane carrier") that is concentric and surrounding the turbine assembly.

In order to achieve a high efficiency, the hot gas flow has to have a very high turbine inlet temperature. However, in general this high temperature involves an undesired high NOx emission level. In order to reduce this emission and to increase operational flexibility without decreasing the efficiency, a so called "sequential" gas turbine is particularly suitable. In general, a sequential gas turbine comprises a first and a second combustor or combustion stages wherein each combustor is provided with a plurality of burners in a combustion chamber. Today at least two different kinds of sequential gas turbines are known. According to a first embodiment, the first and the second combustor are annular shaped and are physically separated by a stage of turbine blades, called high pressure turbine. Downstream the second combustor a second turbine unit is present (called low pressure turbine). This kind of gas turbine is produced by the Applicant and is available on the market as "GT26". According to a second embodiment of a sequential gas turbine, the gas turbine is not provided with the high-pressure turbine and the combustor assembly is realized in form of a plurality of can-combustors. Each can-combustor comprises a first combustor and a second combustor arranged directly one downstream the other inside a common can shaped casing. Also, this second kind of a sequential gas turbine is produced by the Applicant and is available on the market as "GT36". These two examples of gas turbine assemblies (GT26 and GT36) have been cited only as non-limiting examples wherein the present invention (i.e. an improved membrane seal device as will be described in the following) can be applied during service or can be incorporated from the beginning.

In almost all kind of gas turbine assemblies, seals are used for sealing for separating the hot gas flow from the cooling air flow. In particular, the sealing of the circumferential interface between the combustor outlet and the first vane row of the turbine is today a challenge because this sealing has to be guaranteed during all operating conditions. Indeed, in this interface the vane usually has to be cooled (the temperature of the hot gas leaving the combustor is very high) and moreover the vane can move in both the axial and radial directions.

According to a first prior art example, the sealing arrangement of the combustor to turbine interface comprises a honeycomb seal arrangement. In this example, each first vane comprises an inner platform. The word "inner" means near to the turbine axis in contrast with the outer end of the vane connected to the outer supporting casing surrounding the turbine. This platform comprises of labyrinth teeth acting against a honeycomb seal fixed to the combustor end (i.e. to the end of a combustor liner segment carrier). The above honeycomb sealing arrangement involves some drawbacks. In particular, a full seal loss during operation may occurs or a simply partial seal delamination can lead to high leakages in this interface.

According to a second prior art example, the sealing arrangement of the combustor to turbine interface comprises a membrane seal arrangement extending radially from the downstream end of the combustor liner segment carrier (in the following combustor casing) to the inner face of the turbine vane platform. In particular, this membrane seal is comprises a membrane connecting an inner enlarged end housed in a seat or groove in the combustor casing and an outer enlarged end housed in a seat or groove in the vane inner platform. This solution allows relative vane movements, particularly in the axial direction. Unfortunately, this prior art solution using a single membrane seal (for instance a "dog-bone" or an asymmetric membrane seal) involves some drawbacks. In particular, the upstream face of the groove in the casing and housing the inner end of the seal is affected by wear due to the above mentioned operational movements and due to the high pressure pushing the small contact surface of the membrane seal against the upstream surface of the casing groove. This wear implies periodical casing repair processes that unfortunately are expensive and not easy to be implemented. This wear also reduces the lifetime of the membrane seal, as well as allowing cooling air to leak past the wear damaged sealing surface reducing gas turbine performance.

<CIT> discloses a gas turbine according to the preamble of claim <NUM>.

<CIT> discloses a seal between two static turbine parts is provided with a carrier piece comprising a flat metal piece with a middle piece and end pieces each of which is arranged in a groove in the static turbine parts.

<CIT> refers to a method of refurbishing a seal land on a transition piece of a turbomachine by applying a wear strip to a wall surface of the seal land, and covering the wear strip with a slot protector.

A primary object of the present invention is to provide a gas turbine suitable for overcoming the above problems of the prior art. In particular, the primary object of the present invention is to provide a gas turbine having an improved seal arrangement (or seal device) suitable for sealing the combustor to turbine interface and that allow larger displacements of components with low wear, in particular without combustor casing wear. A gas turbine suitable for being provided with the new membrane seal arrangement of the present invention is a gas turbine for power plant as claimed in claim <NUM>. A suitable gas turbine is comprising:.

Although not all components have been listed above, the skilled person in the field of gas turbines for power plants will know that a gas turbine for this purpose is an assembly comprising:.

The words "at least one turbine" mean that a gas turbine according to the present invention may comprise a single-stage turbine or a high-pressure turbine followed by a low-pressure turbine. Also, the combustor assembly may comprise a single-stage combustor or two sequential stages of combustion. In this last case, these two stages of combustion may be divided by a turbine stage (the above high-pressure turbine) or may be connected directly in series in a common can-shaped casing (a so-called can-combustor gas turbine).

As foregoing mentioned, the combustor to turbine interface, i.e. the interface is the passage between the downstream end of the combustor casing (or liner segment carrier) and the first row of turbine vanes. As known, at this interface there is the main hot gas flow, which flows from upstream to downstream into the vane aerofoils and a cooling air flow (for instance compressed air) flowing in a channel or volume below (radially inwards) the vane platform with a seal. Thus, volume on the downstream side of the seal is called "the higher-pressure volume" because is fed with high pressure cooling air and the volume on the upstream side the seal is called "the lower-pressure volume". As common in the field of gas turbines, the words "upstream" and "downstream" refer to the hot gas flow direction. The words "inwards/inner" and "outwards/outer" refer the radial position relative to the axis of the gas turbine. Also, the words "axially", "radially" and "circumferentially" refer to the axis of the gas turbine.

The foregoing cited cooling air flowing below the vane platform (radially inwards and in counter flow) is used for cooling the vane and it is important to avoid leakage of this cooling air toward the hot gas flowing above the vane platform (radially outwards). In view of this problem, as foregoing cited, the closest prior art for the present invention relates to a single membrane seal. This known membrane seal comprises:.

The above definition of the membrane seal refers to the cross-section of the seal that extends circumferentially along the combustor to turbine interface. For instance, the entire circumferential extent of the combustor to turbine interface may be sealed by two half membrane seals. In any case a skilled person very well knows this seal.

Starting from this prior art, instead of the above single membrane seal the present invention provides an improved seal arrangement comprising two pieces; namely a membrane seal (exactly as described and as used today) and a strip seal arranged between the inner end of the membrane seal and the upstream face of the inner groove in the combustor casing. In other words, according to the invention the membrane seal (due to the pressure of the cooling air) does not contact anymore the upstream face of the groove on the combustor casing because the strip seal is provided in between. For this reason, the groove of the combustor inner casing discloses an enlarged seat configured for housing not only the membrane seal but also the strip seal.

In this way no wear occurs on the casing because the upstream face of the inner casing groove is no longer in contact with an operationally moving membrane seal but to the in-operation stationary strip seal. Thus, wear of the casing is eliminated and absorbed by the "sacrificial" strip seal that is easy to replace during an outage, when the gas turbine is disassembled for periodic servicing.

The strip seal halves are circumferentially restrained in position by at least an axial pin inserted in a corresponding hole in the casing.

Preferably, the strip seal and the inner groove are shaped to avoid relative displacements (in particular radial displacements).

Preferably the strip seal may be produced from a material more resistant to wear than the casing material (for instance the same material chosen for the membrane seal).

Preferably, at least the wear (downstream) surface of the strip seal has an anti-wear coating (for instance chrome carbide). Also, the corresponding face of the inner end of the membrane seal may be similarly coated.

Preferably, the strip seal has an L-shaped profile, to replace the entire upstream surface of a worn casing groove.

Preferably, the seal strip and casing groove are provided with an axial retention feature to prevent loss of the strip seal during gas turbine shutdown in the eventuality of a heavily worn interface between the membrane seal and the strip seal. This feature can be reached by providing an angled L-shaped strip seal.

Preferably, the sealing surface between the strip seal and the casing is located at the outer radius to protect the groove against oxidation. In operation the seal strip is pushed against this sealing surface by the pressure drop across the membrane seal preventing the eventuality of hot gas entering the groove from the cavity between the turbine vane and combustor.

Preferably, along the entire combustor to turbine circumferential interface two half strip seals are provided (for gas turbine assembly reasons). In particular, the ends of these two strip seal halves may comprise corresponding protruding and recessing portions to allow overlap in operation to reduce leakage at these two positions.

Finally, the present invention refers to a method as claimed in claim <NUM>. The method is for retrofit in a gas turbine wherein the former standard single membrane seal is replaced with a new two-piece seal arrangement according to the present invention. For this reason, the method comprises also the steps of enlarging the casing groove for housing also the strip seal, in particular by removing part of the upstream face of the casing groove, and of arranging the strip seal in the inner groove.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Other advantages and features of the invention will be apparent from the following description, drawings and claims.

The features of the invention believed to be novel are set forth with particularity in the appended claims.

The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes an exemplary embodiment of the invention, taken in conjunction with the accompanying drawings, in which:.

In cooperation with the attached drawings, the technical contents and detailed description of the present invention are described thereinafter according to preferred embodiments, being not used to limit its executing scope. Any modification made according to appended claims is all covered by the claims.

Reference will now be made to the drawing figures to describe the present invention in detail.

Reference is now made to <FIG> that is a schematic view of a first non-limiting example of a gas turbine assembly (or only gas turbine) for power plant <NUM> that can be provided with the new membrane seal device according to the present invention. According to the embodiment of <FIG> this gas turbine is a so called "sequential-combustion gas turbine" provided with a high-pressure and a low-pressure turbine. Following the main gas flow <NUM>, the gas turbine <NUM> of <FIG> comprises a compressor <NUM>, a first combustor <NUM>, a high-pressure turbine <NUM>, a second combustor <NUM> and a low-pressure turbine <NUM>. The compressor <NUM> and the two turbines <NUM>, <NUM> are part of or are connected to a common rotor <NUM> rotating about an axis <NUM> and surrounded by a concentric casing <NUM>. The compressor <NUM> is supplied with air and has rotating blades <NUM> and stator vanes <NUM> configured to compress the air entering the compressor <NUM>. Exiting the compressor, the compressed air flows into a plenum <NUM> and from there into numerous first burners <NUM> of the first combustor <NUM> arranged in a circular pattern around the axis <NUM>. Each first burner <NUM> is configured for injecting at least one type of fuel (connected to at least one first fuel supply <NUM>) into the compressed airflow. Preferably, this first burner <NUM> may be defined as a "premix" burner because is configured for mixing the compressed air and the injected fuel before the ignition. The fuel/compressed air mixture flows into a first combustion chamber <NUM> annularly shaped, where this mixture ignites. During start-up this mixture is initially ignited by an ignitor, for instance by a spark igniter; once ignited the ignition is self-sustaining and the ignitor is turned off. The resulting hot gas leaves the first combustor chamber <NUM> and is partially expanded in the high-pressure turbine <NUM> performing work on the rotor <NUM>. Downstream of the high-pressure turbine <NUM> the partially-expanded hot gas flows into a row of second burners <NUM> where at least one type of fuel is injected by fuel lances <NUM> (each burner has one lance). The partially-expanded gas has a high temperature and contains sufficient oxygen for further combustion that occurs by self-ignition in the second combustion chamber <NUM> arranged downstream of the row second burners <NUM>. These second burners <NUM> are also called "reheat" burners. The reheated hot gas leaves the second combustion chamber <NUM> and flows in the low-pressure turbine <NUM> where it is expanded performing work on the rotor <NUM>. The low-pressure turbine <NUM> comprises numerous stages: rows of rotor blades <NUM> arranged in series in the main flow direction. Such rows of blades <NUM> are interposed by rows of stator vanes <NUM>. The rotor blades <NUM> are connected to the rotor <NUM> whereas the stator vanes <NUM> are connected to a vane carrier <NUM> that is a concentric casing surrounding the low-pressure turbine <NUM>. Although not visible in <FIG>, this gas turbine <NUM> has numerous seals, in particular many membrane seals arranged, for instance, at the interface of the second combustor to first vane row of the low-pressure turbine. In view of the above, a gas turbine as disclosed in <FIG> may be provided with the new membrane seal device of the present invention.

Reference is now made to <FIG> that is a schematic view of a second not-limiting example of a gas turbine that can be provided with the new membrane seal device according to the present invention. This gas turbine <NUM> is also a "sequential-combustion gas turbine" that can be provided with the innovative features according to the present invention. In particular, <FIG> shows a partial view of a gas turbine <NUM> with a compressor <NUM>, a turbine <NUM> and a sequential combustor <NUM>. The sequential combustor <NUM> of <FIG> has numerous so-called can combustors, i.e. numerous bolt-on casings wherein there are numerous of first burners <NUM>, for instance four first burners <NUM>, a first combustion chamber <NUM>, numerous second burners <NUM>, and a second combustion chamber <NUM>. Upstream of the second burner <NUM> an mixer may be provided for adding air into the hot gas leaving the first combustion chamber <NUM> and creating turbulence in the air/hot gas mixture. The sequential combustor arrangement is at least in part housed in an outer casing <NUM> supporting the individual can combustors <NUM> arranged in a circular pattern around the turbine axis <NUM>. At least one type of fuel is introduced via a first fuel injector (not shown) into the first burners <NUM> wherein the fuel is mixed with the compressed air supplied by the compressor <NUM>. Also, each of the first burners <NUM> of this embodiment is a "premix" burner configured for generating a premixed flame. When the hot gas leaves the second combustion chamber <NUM> it then expands in the turbine <NUM> performing work on a rotor <NUM>. Although not visible in <FIG>, this gas turbine <NUM> comprises numerous seals, including membrane seals, which could be arranged for instance at the can combustors to first turbine vane interface. In view of the above, a gas turbine as disclosed in <FIG> may be provided with the new membrane seal of the present invention.

Reference is made to <FIG> which is a schematic enlarged view of the portion of <FIG> labelled as III. In particular <FIG> discloses a first prior art practice for sealing the interface between the second combustor and the first row of vanes of the low pressure turbine. This gas turbine <NUM> comprises a combustor schematically represented with reference <NUM> producing a hot gas flow <NUM>. Moreover, <FIG> shows:.

As described in the prior art chapter, this prior art practice has drawbacks, particularly the limited capacity to accommodate operational movement of the vane <NUM>. For this reason, an alternative solution of the prior art practice is disclosed in <FIG>. According to this example, the sealing arrangement comprises a single "dog-bone" membrane seal <NUM>. This membrane seal <NUM> has an enlarged upper end <NUM> and a enlarged lower end <NUM> and a middle a straight central membrane <NUM> (preferably provided with a purge air hole <NUM>). The inner end <NUM> is housed in a circumferential recess or groove <NUM> obtained in the downstream end of the casing <NUM>. This casing <NUM> may be part of a combustor liner or of a rotor cover. The outer end <NUM> is housed in a circumferential recess or groove <NUM> obtained in the inner face of the vane platform <NUM>. The two ends involve a continuous circumferential contact line with the corresponding grooves (positions <NUM> and <NUM>) to minimize cooling-air <NUM> leakage into the main flow hot gas <NUM>. The volume <NUM> upstream the seal <NUM> is a lower-pressure volume whereas the downstream volume <NUM> (fed with counter flow cooling air) is a higher-pressure volume. <FIG> shows the assembled position. In operation, the vane platform <NUM> has a large radially inwards and axially downstream movement relative to fixed parts of the system. <FIG> discloses this operation condition wherein the high-pressure cooling air creates a high force at the contact point <NUM> shown in <FIG>, i.e. against the upstream face of the casing groove <NUM>. This contact force at <NUM>, and the operational movements of the membrane seal, cause wear on the upstream face of the casing groove <NUM>. This wear in turn requires periodical repair.

As disclosed in the summary of the invention chapter, the present invention offers a solution for overcoming the above drawbacks (casing groove wear). According to the general definition of the invention, the new solution proposed is to replace the single membrane seal with a seal arrangement comprising two pieces. The first piece is a membrane seal, i.e. a dog-bone or an asymmetric membrane seal that it could be also made according to the prior art. The second piece is a strip seal interposed between the inner end of the membrane seal and the upstream face of the casing groove to prevent wear of this last component. <FIG> discloses a cross-section of an embodiment of the membrane seal arrangement according to the present invention. In this <FIG> some numerical references used in <FIG> have been reproduced because a lot of components are in common. Indeed, in summary, the main difference between the prior art and this invention is to provide the above mentioned strip seal interposed between the upstream face of the inner end of the membrane seal and the upstream face of the casing groove. Of course, for this reason the shape of the casing groove has to be modified to also house the additional strip seal. In <FIG> the new references <NUM> and <NUM> respectively refer to the new added strip seal and to the upstream face of the casing groove wherein the casing groove has been upstream enlarged. According to the example of <FIG>, the strip seal <NUM> has an L-shaped cross section, in particular the strip seal <NUM> has an upstream face with a radial retention feature <NUM> that fits into the upstream recess <NUM> in the casing groove <NUM>. In operation, the downstream face <NUM> of the strip seal is in contact with the inner end <NUM> of the membrane seal <NUM> at le line <NUM>. Therefore, preferably, at least at the line of contact <NUM> between the membrane seal and strip seal has an anti-wear coating is provided (preferably the downstream face <NUM> of the strip seal has an anti-wear coating).

<FIG> refer to the strip seal <NUM>. In particular, <FIG> discloses the circumferential extent of a strip seal half <NUM> (approximately <NUM>° preferably) and <FIG> disclose the two ends of each strip seal half <NUM>. According to <FIG>, the end <NUM> discloses a protruding portion whereas the opposite end <NUM> discloses a corresponding recess so that two strip seal <NUM> may overlap in operation sealing entire circumferential extent of the interface to reduce leakage. <FIG> discloses a middle portion of the strip seal half <NUM> wherein there is an axial hole <NUM> to allow assembly of a fixation pin (reference <NUM> in <FIG>) configured to restrain the strip seal half <NUM> inside the groove <NUM> in the correct circumferential position.

Finally, <FIG> disclose in detail the seal strip and the groove <NUM> as it has been modified for being suitable for housing a two-piece seal arrangement according to the present invention, i.e. the upstream side of the groove has been enlarged to house the additional strip seal. <FIG> discloses the operating positions of pressure loaded membrane seal <NUM> (pressure direction <NUM>) and seal strip <NUM> in the casing groove <NUM>. The upstream side of inner end <NUM> of the membrane seal <NUM> is pressed into line contact <NUM> against the downstream face <NUM> of the seal strip <NUM>. Thus, the upstream side <NUM> of the strip seal <NUM> is in turn pressed into contact with the upstream face <NUM> of the casing groove <NUM>. Axial retaining features <NUM><NUM> are provided between the strip seal <NUM> and casing groove recess in order to prevent loss of the seal strip <NUM> during gas turbine shutdown if the seals mutual contact surfaces should become heavily worn. Finally, <FIG> discloses the axial hole <NUM> obtained in the casing <NUM> for allowing the assembly of the circumferential fixation pin <NUM> passing also in the hole <NUM> of the seal strip <NUM>.

Claim 1:
A gas turbine for power plant; the gas turbine (<NUM>, <NUM>) comprising:
- at least a turbine (<NUM>, <NUM>, <NUM>, <NUM>) provided with a first row of stator vanes (<NUM>, <NUM>) configured for guiding an hot gas flow (<NUM>); each vane (<NUM>, <NUM>) comprising a platform (<NUM>);
- a combustor comprising an inner casing (<NUM>);
- a membrane seal device extending from the combustor casing (<NUM>) to the vane platform (<NUM>) for isolating the hot gas flowing above the vane platform (<NUM>) from a higher-pressure cooling air flowing below the vane platform (<NUM>);
the seal arrangement comprises a membrane seal (<NUM>);
wherein the membrane seal (<NUM>) comprises:
- an inner end (<NUM>) housed in a groove (<NUM>) obtained in the combustor casing (<NUM>), the groove (<NUM>) being provided with a upstream face (<NUM>);
- an outer end (<NUM>) housed in a groove (<NUM>) obtained in the vane platform (<NUM>);
- a membrane (<NUM>) connecting the inner end (<NUM>) to the outer end (<NUM>);
characterized in that the seal arrangement comprises a strip seal (<NUM>) housed in the casing groove (<NUM>) and arranged between the inner end (<NUM>) of the membrane seal (<NUM>) and the upstream face (<NUM>) of the casing groove (<NUM>);
wherein an axial pin (<NUM>) is provided configured for keeping in circumferential position the strip seal (<NUM>) inside the casing groove (<NUM>); the casing (<NUM>) and the strip seal (<NUM>) comprising axial holes (<NUM>, <NUM>) for housing the pin (<NUM>).