Patent Application
20100303619
The invention relates generally to steam turbines and more specifically to lower exhaust hoods for the steam turbines.
The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing and acts as the supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The hood for large double flow low-pressure steam turbines are of substantial dimensions and weight and usually are assembled only in the field. In many steam turbines, the inner case of the steam turbine, for example a double flow down exhaust unit has an encompassing exhaust hood split vertically and extending along opposite sides and ends of the turbine. This large, box-like structure houses the entire low-pressure section of the turbine. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely. Thus the prior exhaust hoods for steam turbines constitute a large rectilinear structure at the exit end of the conical section for turning and diffusing the steam flow at right angles.
The lower half of the exhaust hood, split vertically from the upper half, directs the exhaust flow of steam to a condenser usually located generally beneath the exhaust hood. The lower exhaust hood typically supports the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. The lower exhaust hood shell is generally of fabricated construction with carbon-steel plates. Typical sidewalls for the lower exhaust hood are flat and vertically oriented. To provide resistance to the inward deflection of the sidewalls under vacuum loading, the lower exhaust hood traditionally has included internal transverse and longitudinal plates and struts. These internal transverse and longitudinal plates and struts form a web, generally underneath the turbine casing and extending to the sidewalls. Vertical sidewalls result in a stagnant flow region underneath the inner casing. Flat walled hoods require flow plates. Flow plates are used to prevent the rapid expansion of the exhaust steam after passing through a horizontal joint restriction between the inner casing and the exhaust hood.
The internal hood stiffeners and flow plates are costly. Further, the thick-walled plate used for the sidewalls is also costly. Prior attempts to stiffen exhaust hoods have focused on different combinations of internal stiffeners (pipe struts, plates) and wall thicknesses.
The sidewalls 45 and end walls 50 may be constructed of flat metal plates 60 (
The problem previously has been addressed by putting transverse and stiffening plates through out the hood. The methodology heretofore followed has been to make hood stiff enough by adding material so as to avoid excess deflection. The problem is that to control the side and end wall deflections of the hood, transverse and stiffeners are required inside of the hood. The existence of these transverse and struts increases the complexity of the hood, increases the weight of the hood and creates aero blockages resulting in aero performance losses.
Accordingly, it may be desirable to provide an alternate hood structure that reduces cost, complexity and improves flow distribution.
The present invention relates to an arrangement and method for providing a stiffened lower exhaust hood for a steam turbine. Stiffening may be provided by stiffening bends, stiffening curvature, and external stiffening beams on the wall plate of the sidewalls of the lower exhaust hood.
Briefly in accordance with one aspect of the present invention, a steam turbine exhaust hood is provided. The steam turbine exhaust hood includes a lower exhaust hood joined at a horizontal joint with an upper exhaust hood section. A chute section is provided within the lower exhaust hood. Opposing sidewalls on the chute section include an inward taper below the horizontal joint. The opposing sidewalls include stiffening means.
According to a further aspect of the present invention, a method is provided for stiffening sidewalls of a lower exhaust hood of a steam turbine exhaust hood. The method includes tapering the sidewalls inward on a chute section below a horizontal joint of the lower hood and providing a stiffening means on the opposing sidewalls
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The following embodiments of the present invention have many advantages, including improving both the stiffening of the sidewalls of the lower exhaust hood and the flow distribution in the chute region of the lower exhaust hood. Improved sidewall stiffening is achieved via bends, curvature and external support beams in the sidewall, used alone or in combination. Flow distribution is improved via the inward (non-vertical) orientation of the sidewalls to direct exhaust flow underneath the inner casing, making use of the formerly stagnant region.
In the present invention, means for stiffening the large-expanse, flat sidewall(s) are provided. Stiffening means may include any combination of bends, curvature and beams in the opposing sidewalls, thereby reducing or eliminating the need for internal stiffeners such as plates and pipe struts. With improved sidewall stiffening, thinner plate for the sidewall can also be considered. Further, the sidewalls are inward-oriented so as to push exhaust steam flow towards the center, increasing usage of the stagnant region underneath the inner casing thereby reducing or eliminating the need for internal flow plates.
Any combination of bends, curvature and beams in sidewall(s) may reduce or eliminate the need for internal stiffeners and thick walls, reducing hood cost. Removal of internal stiffeners also reduces flow blockage, improving aerodynamic performance. The sidewalls are oriented to manage steam expansion within the chute, also improving aerodynamic performance. Better flow management within the chute, to make better use of the stagnant region underneath the inner casing, reduces the need for costly flow plates. In addition, it allows a smaller condenser opening, reducing overall plant cost.
In a further aspect of the present invention, a method is provided for stiffening sidewalls of a lower exhaust hood of a steam turbine exhaust hood. The method includes tapering the sidewalls inward on a chute section below a horizontal joint of the lower hood; and providing stiffening means on the opposing sidewalls. One embodiment of the method may further include forming at least one stiffening bend on the inward tapering sidewalls, stiffening bends being adapted to reduce or eliminate the use of internal transverse stiffeners. A second embodiment of the method may include forming at least one stiffening curvature on the inward tapering sidewalls, the stiffening curvature being adapted to reducing or eliminating the need for internal transverse stiffeners. The method for providing curvature of the sidewalls according may include providing a simple curvature or a complex curvature. A third embodiment of the method for stiffening sidewalls may include applying one or more external stiffening beam axially along the sidewalls, the stiffening beams adapted to reduce or eliminate the use of internal transverse stiffeners.
In further embodiments of the method for stiffening sidewalls, combinations of one or more of forming at least one stiffening bend on the inward tapering sidewalls; forming one or more stiffening curvatures on the inward tapering sidewalls; and one or more of applying external stiffening beams on the exterior of the inward tapering sidewalls may be employed. Herein, the combinations of forming the stiffening bends, forming the stiffening curvatures and applying the external stiffening beams are adapted to reduce or eliminate internal transverse stiffeners.
While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.
