Apparatus and methods for eliminating cracking in a turbine exhaust shield

Undue flexing and cracking of an internal exhaust shield of a turbine is eliminated by isolating thermally-induced movement in the shield from that of strut apparatus passing therethrough to a surrounding outer exhaust casing. Methods and apparatus are disclosed.

FIELD OF THE INVENTION

This invention relates to jet turbines, more particularly to land-based turbines used, for example in Combined Cycle Power Generation facilities and even more particularly to the exhaust structures of such turbines and the inherent problems therein of cracking due to differentials in thermal generated expansion and contraction of the respective exhaust components.

BACKGROUND OF THE INVENTION

Typically, such turbines include an exhaust structure which comprises an outer exhaust casing in the form of an annulus, and an inner exhaust tunnel defined by an annular exhaust cylinder or shield with an annular dead air space defined between the outer casing and the inner shield. Simply stated, an outer casing or cylinder surrounds an inner shield or cylinder in the exhaust area of a turbine.

The turbine typically includes a cylindrical bearing housing disposed around a central axis of a turbine and located and centered there, preferably by two sets or arrays of radially-extending struts. The respective struts in each array, at about 120 degrees apart, are encased in strut housings, extending radially inwardly to the bearing housing from the outer exhaust casing and through the inner shield to the bearing housing. The strut housings are welded to the inner shield, where they pass outwardly therethrough to the outer casing and inwardly therethrough to the bearing housing. Typically the struts, strut housings and inner shield are made from high heat-resistant materials or alloys, including, for example, an alloy known in the industry as “hastalloy”.

It is in the area of the welded joinery between the strut housings and the inner shield which suffer the principal consequences of differential thermal-caused material expansion and contraction.

As the turbine is operated, it can produce exhaust gases of high temperatures such as 1000 to 1300 degrees Fahrenheit or more. This heat, applied to the strut housing and to the inner shield causes them to expand or move in respective directions in response to this heating. Thus, the strut housing may expand longitudinally (in a radial direction from the bearing housing) while the inner shield moves or expands in other directions, or at least at different rates, all in response to the exhaust heat. Thus, there is a differential of material movement and destructive force at the welded joint between the strut housing and the inner shield. This differential causes the inner shield to flex, then crack or break apart at the intersection area at and around the welded joint to the strut housing. The dead air space between the inner shield and outer casing is thus opened to direct exhaust gas.

In the past, this material failure is cured only by shutting down the turbine, accessing the cracked area, removing the affected parts of the inner shield and welding replacement and reinforcing plates in the shield and to the strut housing. This work is expensive, requires periods of turbine shutdown, is difficult to access, is subject eventually to repeat of the continuing problem and is, for these and other reasons, very problematical.

Such turbines generally experience these adverse thermally-caused movement and force differentials on startup from inoperative conditions. Those cycles occur periodically on even a daily basis or multiple times per day. Thus, every time the turbine is started, the thermal expansion produces the described stress and flexes the inner shield eventually to failure.

Accordingly, it has been one objective of this invention to provide an improved exhaust structure for turbines which is not subject to the failure of materials due to differential thermal expansion and/or contraction.

A further objective of the invention has been to provide an improved exhaust structure for a turbine extending the life and maintenance requirements of such turbines in the exhaust components.

A further objective of the invention has been to provide an improved method for handling turbine exhaust.

SUMMARY OF THE INVENTION

In summary, a preferred embodiment of the invention contemplates the isolation of thermally-caused expansion in the strut apparatus from thermally caused expansion in the shield. By isolating these respective forces, one from the other, undesirable flexing and cracking of the shield caused by these prior interfering differential forces is eliminated.

This method and result is obtained by passing the strut apparatus through the shield such that both these components are slidable or moveable with respect to each other so that the relative movement of one dose not stress or adversely affect the other. A combination of mounting plates, on the shield and stops of the strut apparatus are useful in attaining these results.

These and other objectives will be readily apparent from the following written description and from the drawings in which:

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings,FIG. 1diagrammatically illustrates the invention and its use. A turbine10is oriented about a major axis11. A bearing housing14extends rearwardly from the turbine about axis11and provides support for a rotary turbine shaft (not shown). An outer exhaust casing20in the form of an annulus or cylinder and defining an outer exhaust cylinder extends rearwardly from turbine10. An inner shield30in the form of an annulus or a cylinder and defining an exhaust tunnel also extends rearwardly from turbine10, radially inward from outer casing20. The outer casing20outwardly surrounds inner shield30such that there is an annular dead space40between casing20and shield30. Exhaust from turbine T flows in the direction of arrows E.

A series of radially-extending struts50extends from outer casing20to bearing housing14, through shield30at intersection35. Struts50are encased in strut housing60as shown. Struts50and housings60are attached, as by welding, to the bearing housing14and to outer casing20where strut housing end60ais attached as by welding to casing20. However, according to the invention and advantageously, neither the housings60or the struts50are welded to inner shield30. The intersection35or joint area between strut housing60and inner shield30are shown in more detail in Detail A ofFIG. 1and in the other Figures.

More specifically, and according to the invention, the intersection of strut housing60to inner shield30is defined as a sliding, relatively moveable or extensible intersection35which alleviates any undesirable interconnection between these components. In other words, and in accordance with the invention, any thermal differential expansion, between inner shield30on one hand and the housing60and struts50, are accommodated by the relative movement between and at the intersection of the shield30and housing60. These parts are thus free to move responsive to thermal variations without exerting structural impact or force on the other component. As a result, thermal expansions of these components are accommodated without creation of stress on the other component. Cracking due to the differentiation of respective thermal expansion/contraction is thus eliminated, significantly reducing structural failure, maintenance schedules, downtime and expense previously associated with the exhaust structures of turbines.

More particularly, the FIGS.1A and2A-6illustrate the construction steps and final configuration of the interface components of the moving intersection between each strut50,60and the inner shield30.

Preferably, the shield30is provided with an opening70, larger than but preferably complementary to the elongated, oval-like cross-sectional shape of the strut housing60. Accordingly, there is a clearance (at71) between the strut housing60and the shield30. Two pairs of complementary mounting plates72,73and74,75are arranged about the housing60and welded to the shield30with a small clearance74A between the formed apertures in plates72,73and the opposed faces of plates74,75with housing60. Plates74,75are welded along the plates72,73. Two elongated angle filler plates76,77are inserted between plates72,73and housing60and are welded to plates72,73but not to housing60. These elongated filler plates76,77extend between ends of plates74,75. Accordingly, the housing60is not attached to, but extends slidably through shield30at intersection35so the housing60can elongate in thermal response to exhaust heat, but without transferring any of that elongation force to shield30. Likewise, shield30can move in response to thermal stimulus without resulting in undue stress from attachment to the strut apparatus. Housing60is thus slidable with respect to filler plates76,77.

A pair of stops78,79in the form of elongated bars are welded to housing60as shown radially inwardly with respect to shield30and angle plates76,77.

Likewise, a pair of two, relatively shorter angle-shaped stops80,81are welded to strut housing60radially outwardly with respect to shield30for non-attached engagement with outer surfaces of shield30. The distance between the relative shield-facing stop surfaces of stops78,79and80,81, respectively are at least slightly greater than the combined thicknesses of shield30and the respective plates72-75, such that there can be relative motion between strut housing60and shield30in at least a radial direction along strut60(and preferably some at least slight motion normal to the radial extension of strut housing60).

Thus, it will be appreciated that the relative radial position of shield30on strut housing60is variable between interior stops78,79and outer stops80,81.

In another aspect of the invention, an alternative embodiment of the invention is illustrated inFIG. 7. This embodiment provides similar function of the preceding embodiment but with a modified structure.

Referring toFIG. 7, it should be noted that stops78,79ofFIGS. 1-6have been omitted. Also, the angled filler plates76,77are omitted. Components ofFIG. 7which are like those inFIGS. 1-6bear the same numbers. New components with specific new numbers are described below.

In the embodiment ofFIG. 7, a collar90is provided about the strut housing60, and is slidable thereon. Collar90is welded to shield30at an end91of collar90(or to mounting plates such as plates72-75, if used in this embodiment).

In place of stops78,79ofFIGS. 1-6, there are welded angle stops92,93along the sides of strut housing60.

Collar90is provided with two opposed slots94,95on opposite sides of the collar90.

Pins96,97are attached to strut housing60by welding or any suitable expedient, on opposite sides thereof in a position such that each extends through a respective slot94,95, respectively.

Washers98,99of larger extend than slots94,95are secured to pints96,97respectively, so collar90is captured thereby on housing60.

The length of slots94,95is long enough to accommodate translation or movement of collar90along strut housing60between stops92,93radially inward of shield30, and stops80on housing60, radially outward of shield30.

It will be appreciated that collar90extends much further along strut housing60than did filler plates76,77, thereby providing a more efficient sliding contact between the collar90and housing60than did filler plates76,77.

Also, the interaction of pins96,97and slots94,95can define relative parameters of motion of collar90on strut60as desired.

Accordingly, the actual thermal responsive movement of shield30and strut housing60in both embodiments do not adversely transfer between these components. Material flex and failure due to thermal expansion or contraction is eliminated, and the exhaust structure for turbine10is vastly improved.

Other structures may be used to produce the same or similar isolation of thermal caused movement, reducing and eliminating cracking or damage.

These and other modifications and advantages will become readily apparent to those of ordinary skill in the art to which this invention pertains, without departing from the scope of the invention, and applicant intends to be bound only by the claims appended hereto.