Rotating diffuser for pressure recovery in a steam cooling circuit of a gas turbine

This application is a continuation of application Ser. No. 09/239,276, filed Jan. 29, 1999 (abandoned), the entire content of which is hereby incorporated by reference in this application.

The Government of the United States of America has rights in this invention pursuant to COOPERATIVE AGREEMENT NO. DE-FC21-95MC31176 awarded by the U.S. Department of Energy.

The present invention generally relates to a steam-cooling circuit in a gas turbine and particularly relates to pressure recovery in a spent cooling steam return passage in the steam-cooling circuit of a gas turbine for improving cooling system efficiency. The present invention also relates to pressure recovery in the spent cooling steam return passage of a gas turbine employed in a combined cycle system for improving the performance of the system.

In an advanced gas turbine design, at least certain of the rotating parts carried by the rotor, for example, the first and second-stage buckets, are steam-cooled. Minimization of the pressure drop in the turbine rotor steam cooling circuit is essential to the performance of the design. All components of the steam circuit are therefore designed for minimum pressure drop. In the advanced gas turbine design, a bore tube assembly is provided in which coaxial tubes define cooling steam supply and return passages. The cooling steam supply passage comprises an annular passage about the return passage for supplying cooling steam through a plurality of radially outwardly extending tubes for transmission axially along the rim of the rotor to the first and second-stage buckets. Return of the spent cooling steam from the buckets is provided by a plurality of axially extending circumferentially spaced passages along the rim of the rotor which communicate with the axial return passage via a plurality of radially inwardly extending return tubes. Thus, cooling steam enters the bore tube assembly from an upstream static supply manifold and leaves the rotor exiting the spent cooling steam return passage via a downstream static pipe. The diameter of the static pipe is significantly larger than the return passage in the bore tube assembly. Sudden expansion, however, from the bore tube return passage diameter to the diameter of the larger pipe disadvantageously creates a significant pressure loss.

It will also be appreciated that the advanced gas turbine design may be used in, although is not exclusively for, a combined cycle system. In a combined cycle system, exhaust gases from a gas turbine are used to heat steam in a heat recovery steam generator and which steam is supplied to a steam turbine, typically including high, intermediate and low pressure turbines wherein the gas and steam turbines are coupled on a common shaft or on different shafts to drive one or more generators for the generation of electricity. The cooling steam for the rotating parts of the gas turbine may be supplied from the exhaust of the high pressure steam turbine. The spent cooling steam from the gas turbine is combined with hot reheat steam from the heat recovery steam generator for use in driving the intermediate pressure steam turbine. Accordingly, there is a need for a reduction in the pressure drop in the steam-cooling circuit for the gas turbine.

In accordance with a preferred embodiment of the present invention, a diffuser is provided in the return spent cooling steam passage to reduce the pressure drop and hence recover at least in part some of the axial velocity head in the bore tube assembly through a gradual expansion of the spent cooling steam. The gradual expansion is achieved by providing a diffuser as part of the rotating tube defining the spent cooling steam return passage. The rotating diffuser is located just upstream of the static pipe which receives the spent cooling steam and directs that flow to the intermediate pressure steam turbine in the combined cycle system. That is, the rotating diffuser increases in cross-sectional area in a downstream direction, hence reducing the pressure drop. Given the limited axial length of the bore tube assembly within which the area ratio must be extant, a flow separation or undesirable flow instabilities may occur if the full area ratio is used in the available axial length. Accordingly, the diffuser is preferably cropped in that it does not expand the exit steam to a cross-sectional area corresponding to the cross-sectional area of the static pipe receiving the spent returning cooling steam. Thus, the diffuser expands the steam gradually to an area slightly less than the diameter of the static pipe, followed by a sudden expansion further expanding the steam to the pipe diameter. It will be appreciated that by reducing the pressure drop in the cooling steam return passage, the high pressure steam turbine is enabled to operate at a higher pressure ratio, i.e., a greater expansion of the steam and the high pressure steam turbine is achieved, improving cycle efficiency.

In a preferred embodiment according to the present invention, there is provided in a combined cycle system having a gas turbine including rotating steam-cooled parts, a steam turbine, and a heat recovery steam generator in heat transfer relation with hot exhaust gases of the gas turbine for heating steam for flow to the steam turbine, a steam supply conduit for flowing steam exhausted from the steam turbine to the gas turbine for cooling the rotating parts and a return conduit for flowing spent cooling steam from the gas turbine to the steam turbine, a diffuser in the return conduit for reducing pressure drop and increasing combined cycle efficiency.

In a further preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor rotatable about an axis and carrying steam-cooled parts, a bore tube assembly forming part of the rotor, comprising cooling steam supply and spent cooling steam return passages concentrically disposed about the axis with the supply passage forming an annular passage about the return passage and separated therefrom by a rotatable bore tube defining the return passage, the bore tube including a diffuser having an upstream cross-sectional area less than a downstream cross-sectional area thereof for recovering axial velocity head in the return tube through expansion of the returning spent cooling steam.

In a still further preferred embodiment according to the present invention, there is provided in a combined cycle system having a gas turbine including rotating steam-cooled parts, high and intermediate pressure steam turbines, and a heat recovery steam generator in heat transfer relation with hot exhaust gases from the gas turbine for heating steam for flow to the high pressure and intermediate pressure steam turbines, the gas turbine including a rotor rotatable about an axis and having a bore tube assembly including cooling steam supply and spent cooling steam return passages concentrically disposed about the axis with the supply passage forming an annular passage about the return passage and separated therefrom by a bore tube defining the return passage and rotatable with the rotor about the axis, a method of increasing combined cycle efficiency including expanding the returning spent cooling steam along the return passage by disposing a diffuser in the return passage with an upstream cross-sectional area less than a downstream cross-sectional area thereof.

FIG. 1 is a fragmentary cross-sectional view of a portion of a gas turbine illustrating a turbine section and a bore tube assembly defining supply and return passages for cooling steam;

FIG. 2 is an enlarged view of the bore tube assembly illustrated in FIG. 1; and

FIG. 3 is a schematic illustration of a combined cycle system employing the present invention.

Referring now to the drawings, there is illustrated a portion of a turbine, including a turbine rotor, generally designated 10 , comprised of stacked elements, for example, the rotor wheels 12 , 14 , 16 and 18 which form portions of a four-stage exemplary turbine rotor with spacers 20 , 22 and 24 alternating between the wheels. The wheel and spacer elements are held together in the rotor by a plurality of elongated, circumferentially extending bolts, only one of which is illustrated at 26 . The wheels 12 , 14 , 16 and 18 mount a plurality of circumferentially spaced turbine buckets 12 a , 14 a , 16 a and 18 a , respectively. Nozzles 30 , 32 , 34 and 36 form stages with the buckets 12 a , 14 a , 16 a and 18 a , respectively. An aft shaft wheel 42 forms part of the rotor 10 and is bolted to the stacked wheels and spacers.

In an advanced gas turbine design of the assignee hereof, the aft shaft 44 houses a bore tube assembly described and illustrated in detail in co-pending U.S. patent application Ser. No. 09/216,363. Particularly, the bore tube assembly includes outer and inner tubes 48 and 50 , respectively, defining an annular steam-cooling passage 52 and a spent steam-cooling return passage 54 . The passages 52 and 54 communicate steam to and from the outer rim of the rotor through sets of radially extending conduits or tubes 56 and 58 , respectively, which in turn communicate with longitudinally extending tubes spaced circumferentially about the rim of the rotor. The steam supplied through the steam supply passage 52 and radial tubes 56 supply cooling steam to buckets 12 a and 14 a of the first and second stages, respectively, while the radial tubes 58 and return passage 54 receive the spent cooling steam from the buckets for return to a stationary or static pipe 40 (FIG. 2 ). It will be appreciated that the bore tubes 48 and 50 are part of and rotate with the rotor 10 .

More particularly, the cooling steam is supplied from a plenum 60 to the annular passage 52 via an annular entrance opening 62 . Various labyrinth seals 64 are provided on opposite sides of the plenum 60 . Note that the inner tube 50 comprises a substantially constant diameter from the axial position at which it receives spent returning cooling steam from tubes 58 to an axial position adjacent the location of the plenum 60 . In accordance with a preferred embodiment of the present invention, the tube 50 extends from that axial location in an aft direction to include a diffuser 70 . The diffuser 70 terminates at its aft end substantially coincident with the aft end of the rotating rotor and lies in communication with the static pipe 40 (FIG. 2 ). Diffuser 70 , of course, increases from a cross-sectional area at its upstream end corresponding to the cross-sectional area of the tube 50 to an enlarged cross-sectional area at its downstream aft end. It will be appreciated, therefore, that the employment of the diffuser 70 decreases the pressure drop along the tube 50 and recovers a portion of the axial velocity head in the bore tube through a gradual expansion to the aft end of the diffuser 70 .

As illustrated in FIG. 2, the aft end of diffuser 70 is reduced in cross-sectional area as compared with the cross-sectional area of the static pipe 40 . Because of the possibility of undesirable flow instabilities or separation, diffuser 70 is cropped in relation to the static pipe 40 such that the exit of the bore tube including diffuser 70 does not expand gradually to the static piping diameter. The returning cooling steam is thus expanded gradually through the diffuser to a cross-sectional area slightly less than that of the static pipe 40 and a sudden expansion takes place along the step between the aft end of diffuser 70 and static pipe 40 , further expanding the steam to the pipe diameter. As a consequence of the diffuser in the cooling steam return passage, increased cooling system efficiency by decreasing pressure drop is accomplished.

Referring to FIG. 3, the gas turbine disclosed in FIGS. 1 and 2 may be used in a combined cycle system illustrated in FIG. 3 . In the system of FIG. 3, the gas turbine 10 includes a compressor 80 , and a combustor 82 whereby hot gases of combustion are supplied turbine 10 . The combined cycle system illustrated includes high pressure, intermediate pressure and low pressure steam turbines 84 , 86 and 88 , respectively. As illustrated, the gas and steam turbines lie on a single shaft for driving a generator G, although it will be appreciated that the steam and gas turbines may lie on different shafts for driving different generators.

A heat recovery steam generator 90 (HRSG) is also illustrated. As in conventional combined cycle systems, the hot exhaust gases from the gas turbine 10 flow through the HRSG 90 via a conduit 92 whereby steam is generated and reheated in the HRSG for supply to the steam turbines and to the cooling circuit for the gas turbine. Particularly, superheated steam is supplied from the HRSG 90 via conduit 94 to the high pressure steam turbine 84 . Hot reheat steam and low pressure steam are supplied to the intermediate pressure and low pressure steam turbines 86 and 88 , respectively, via respective conduits 96 and 98 . Cooling steam is supplied from the high pressure steam turbine exhaust via conduits 100 and 102 to the steam supply plenum 60 of the gas turbine 10 for supplying cooling steam to the rotating parts of the gas turbine. Cooling steam is also provided to other parts, i.e., static parts of the gas turbine via conduit 104 in communication with conduit 102 . Spent return cooling steam from the gas turbine is supplied via tube 50 to the static pipe 40 and via a conduit 108 to the hot reheat steam conduit 96 where it joins the hot reheat steam for supplying and driving the intermediate pressure steam turbine 86 .

It will be appreciated that with the reduced pressure drop afforded by the diffuser 70 in the gas turbine, a higher pressure ratio may be provided in the high pressure steam turbine. The pressure drop also affords the capacity to increase the cooling steam flow to the buckets or to provide a higher pressure ratio in the high pressure steam turbine or both.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.